How Do We Colonize Jupiter’s Moons?

Welcome back to our series on Colonizing the Solar System! Today, we take a look at the largest of the Jovian Moons – Io, Europa, Ganymede and Callisto!

In 1610, Galileo Galilei became the first astronomer to discover the large moons of Jupiter, using a telescope of his own design. As time passed, these moons – Io, Europa, Ganymede and Callisto – would collectively come to be referred to as the Galilean Moons, in honor of their discoverer. And with the birth of space exploration, what we’ve come to know about these satellites has fascinated and inspired us.

For example, ever since the Pioneer and Voyager probes passed through the system decades ago, scientists have suspected that moons like Europa might be our best bet for finding life in our Solar System beyond Earth. And because of the presence of water ice, interior oceans, minerals, and organic molecules, it has been speculated that humanity might establish colonies on one or more of these worlds someday.

Examples in Fiction:

The concept of a colonized Jovian system is featured in many science fiction publications. For instance, Robert A. Heinlein’s novel Farmer in the Sky (1953) centers on a teenage boy and his family moving to Ganymede. The moon is in the process of being terraformed in the story, and farmers are being recruited to help turn it into an agricultural colony.

Cover of Arthur C. Clarke's 1982 novel, 2010: Odyssey Two. Credit: Public Domain
Cover of Arthur C. Clarke’s 1982 novel, 2010: Odyssey Two. Credit: Public Domain

In the course of the story, it is mentioned that there are also efforts to introduce an atmosphere on Callisto. Many of his Heinlein’s other novels include passing mentions of a colony on Ganymede, including The Rolling Stones (1952), Double Star (1956), I Will Fear No Evil (1970), and the posthumously-written Variable Star (2006).

In 1954, Poul Anderson published a novella titled The Snows of Ganymede (1954). In this story, a party of terraformers visit a settlement on Ganymede called X, which was established two centuries earlier by a group of American religious fanatics.

In Arthur C. Clarke’s Space Odyssey series, the moon of Europa plays a central role. In 2010: Odyssey Two (1982) an ancient race of advanced aliens are turning the moon into a habitable body by converting Jupiter into a second sun. The warmth of this dwarf star (Lucifer) causes the surface ice on Europa to melt, and the life forms that are evolving underneath are able to emerge.

In 2061: Odyssey Three, Clarke also mentions how Lucifer’s warmth has caused Ganymede’s surface to partially sublimate, creating a large equatorial lake. Isaac Asimov also used the moons of Jupiter in his stories. In the short stories “Not Final!” (1941) and “Victory ‘Unintentional'” (1942), a conflict arises between humans living on Ganymede and the inhabitants of Jupiter.

In Philip K. Dick’s short-story The Mold of Yancy (1955), Callisto is home to a colony where the people conform to the dicates of Yancy, a public commentator who speaks to them via public broadcasts. In Bruce Sterling’s Schismatrix (1985), Europa is inhabited by a faction of genetically-engineered posthumans which are vying for control of the Solar System.

Alastair Reynolds’s short story “A Spy in Europa” depicts colonies built on the underside of Europa’s icy surface. Meanwhile, a race of genetically-altered humans (called the “Denizens”) are created to live in the subsurface ocean, close to the core-mantle boundary where hydrothermal vents keep the water warm and the native life forms live.

Kim Stanley Robinson’s novels Galileo’s Dream (2009) and 2312 (2012) feature colonies on Io, where settlements are adapted to deal with the volcanically active, hostile surface. The former novel is partly set on Callisto, where a massive city called Valhalla is built around the concentric rings of the moon’s giant crater (also mentioned in 2312).

In Robinson’s The Memory of Whiteness (1985), the protagonists visit Europa, which hosts large human colonies who live around pools of melted ice. And in his novel Blue Mars (1996), Robinson makes a passing description of a flourishing colony on Callisto.

A "family portrait" of the four Galilean satellites around Jupiter taken by the New Horizons spacecraft and released in 2007. From left, the montage includes Io, Europa, Ganymede and Callisto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
A “family portrait” of the four Galilean satellites (Io Europa, Ganymede and Callisto) around Jupiter, taken by the New Horizons spacecraft and released in 2007. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Proposed Methods:

Since the Voyager probes passed through the Jovian system, several proposals have been made for crewed missions to Jupiter’s moons and even the creation of settlements. For instance, in 1994, the private spaceflight venture known as the Artemis Project was established with the intent of colonizing the Moon in the 21st century.

However, in 1997, they also drafted plans to colonize Europa, which called for igloos to be established on the surface. These would serve as based for scientists who then drill down into the Europan ice crust and explore the sub-surface ocean. This plan also discussed the possible use of “air pockets” in the ice sheet for long-term human habitation.

In 2003, NASA produced a study called Revolutionary Concepts for Human Outer Planet Exploration (HOPE) which addressed future exploration of the Solar System. Because of its distance from Jupiter, and therefore low exposure to radiation, the target destination in this study was the moon Callisto.

The plan called for operations to begin in 2045. These would begin with the creation of a base on Callisto, where science teams would be able to teleoperate a robotic submarine that would be used to explore Europa’s internal ocean. These science teams would also excavate surface samples near their landing site on Callisto.

Last, but not least, the expedition to Callisto would establish a reusable surface habitat where water ice could be harvested and converted into rocket fuel. This base could therefore serve as a resupply base for all future exploitation missions in the Jovian system.

Also in 2003, NASA reported that a manned mission to Callisto might be possible in the 2040s. According to a joint-study released by the Glenn Research Center and the Ohio Aerospace Institute, this mission would rely on a ship equipped with Nuclear-Electric Propulsion (NEP) and artificial gravity, which would transport a crew on a 5-year mission to Callisto to establish a base.

In his book Entering Space: Creating a Spacefaring Civilization (1999), Robert Zubrin advocated mining the atmospheres of the outer planets – including Jupiter – to obtain Helium-3 fuel. A base on one or more of the Galilean moons would be necessary for this. NASA has also speculated on this, citing how it could yield limitless supplies of fuel for fusion reactors here on Earth and anywhere else in the Solar System where colonies exist.

In the 2000s, the Lifeboat Foundation – a non-profit organization dedicated to the preservation of humanity – was established. In 2012, they released a study titled “Colonizing Jupiter’s Moons: An Assessment of Our Options and Alternatives“, which considered the colonization of the Galilean moons as a potential alternative to colonies on the Moon or Mars.

In October of 2012, Elon Musk unveiled his concept for an Mars Colonial Transporter (MCT), which was central to his long-term goal of colonizing Mars. At the time, Musk stated that the first unmanned flight of the Mars transport spacecraft would take place in 2022, followed by the first manned MCT mission departing in 2024.

In September 2016, during the 2016 International Astronautical Congress, Musk revealed further details of his plan, which included the design for an Interplanetary Transport System (ITS) and estimated costs. This system, which was originally intended to transport settlers to Mars, had evolved in its role to transport human beings to more distant locations in the Solar System – including Europa and other Jovian moons.

Potential Benefits:

Establishing colonies on the Galilean moons has many potential benefits for humanity. For one, the Jovian system is incredibly rich in terms of volatiles – which include water, carbon dioxide, and ammonia ices – as well as organic molecules. In addition, it is believed that Jupiter’s moons also contain massive amounts of liquid water.

For example, volume estimates placed on Europa’s interior ocean suggest that it may contain as much as 3 × 1018 m – three quadrillion cubic kilometers, or 719.7 trillion cubic miles – of water. This is slightly more than twice the combined volume of all of Earth’s oceans. In addition, colonies on the moons of Jupiter could enable missions to Jupiter itself, where hydrogen and helium-3 could be harvested as nuclear fuel.

Based on new evidence from Jupiter's moon Europa, astronomers hypothesize that chloride salts bubble up from the icy moon's global liquid ocean and reach the frozen surface where they are bombarded with sulfur from volcanoes on Jupiter's innermost large moon Io. The new findings propose answers to questions that have been debated since the days of NASA's Voyager and Galileo missions. This illustration of Europa (foreground), Jupiter (right) and Io (middle) is an artist's concept. Image Credit: NASA/JPL-Caltech
Illustration of Europa (foreground), Jupiter (right) and Io (middle) showing water plumes that reach the surface. Credit: NASA/JPL-Caltech
Colonies established on Europa and Ganymede would also allow for multiple exploration missions to be mounted to the interior oceans that these moons are believed to have. Given that these oceans are also thought to be some of the most likely locations for extra-terrestrial life in our Solar System, the ability to examine them up close would be a boon for scientific research.

Colonies on the moons of Io, Europa, Ganymede and Callisto would also facilitate missions farther out into the Solar System. These colonies could serve as stopover points and resupply bases for missions heading to and from the Cronian system (Saturn’s system of moons) where additional resources could be harvested.

In short, colonies in the Jovian system would provide humanity with access to abundant resources and immense research opportunities. The chance to grow as a species, and become a post-scarcity one at that, are there; assuming that all the challenges can be overcome.

Challenges:

And of course, these challenges are great in size and many in number. They include, but are not limited to, radiation, the long-term effects of lower gravity, transportation issues, lack of infrastructure, and of course, the sheer cost involved. Considering the hazard radiation poses to exploration, it is appropriate to deal with this aspect first.

The magnetic field of Jupiter and co-rotation enforcing currents. Credit: Wikipedia Commons
The magnetic field of Jupiter and co-rotation enforcing currents. Credit: Wikipedia Commons

Io and Europa, being the closest Galileans to Jupiter, receive the most radiation of any of these moons. This is made worse by the fact that neither have  a protective magnetic field and very tenuous atmospheres. As such, Io’s surface receives an average of about 3,600 rems per day, while Europa receives about 540 per day.

For comparison, people here on Earth are exposed to less than 1 rem a day (0.62 for those living in developed nations). Exposure to 500 rems a day is likely to be fatal, and exposure to roughly 75 in a period of a few days is enough to cause severe health problems and radiation poisoning.

Ganymede is the only Galilean moon (and only non-gas giant body other than Earth) to have a magnetosphere. However, it is still overpowered by Jupiter’s powerful magnetic field. On average, the moon receives about 8 rads of radiation per day, which is the equivalent of what the surface of Mars is exposed to in an average year.

Only Callisto is far enough from Jupiter that it is not dominated by its magnetic environment. Here, radiation levels only reach about 0.01 rems per day, just a fraction of what we are exposed to here on Earth. However, its distance from Jupiter means that it experiences its fair share of problems as well (not the least of which is a lack of tidal heating in its interior).

Artist's impression of a base on Callisto. Credit: NASA
Artist’s impression of a base on Callisto. Credit: NASA
Another major issue is the long-term effects the lower gravity on these moons would have on human health. On the Galilean moons, the surface gravity ranges from 0.126 g (for Callisto ) to 0.183 g (for Io). This is comparable to the Moon (0.1654 g), but substantially less than Mars (0.376 g). And while the effects of low-g are not well-understood, it is known that the long-term effect of microgravity include loss of bone density and muscle degeneration.

Compared to other potential locations for colonization, the Jovian system is also very far from Earth. As such, transporting crews and all the heavy equipment necessary to build a colony would be very time-consuming, as would missions where resources were being transported to and from the Jovian moons.

To give you a sense of how long it would take, let’s consider some actual missions to Jupiter. The first spacecraft to travel from Earth to Jupiter was NASA’s Pioneer 10 probe, which launched on March 3rd, 1972, and reached the Jupiter system on December 3, 1973 – 640 days (1.75 or years) of flight time.

Pioneer 11 made the trip in 606 days, but like its predecessor, it was simply passing through the system on its way to the Outer planets. Similarly, the Voyager 1 and 2 probes, which were also passing through the system, took 546 days and 688 days, respectively. For direct missions, like the Galileo probe and the recent Juno mission, the travel time was even longer.

Artist's concept of a Bimodal Nuclear Thermal Rocket in Low Earth Orbit. Credit: NASA
Artist’s concept of a Bimodal Nuclear Thermal Rocket in Low Earth Orbit. Credit: NASA

In the case of Galileo, the probe left Earth on October 18th, 1989, and arrived at Jupiter on December 7th, 1995. In other words, it took 6 years, 1 month, and 19 days to make it to Jupiter from Earth without flying by. Juno, on the other hand, launched from Earth on Aug. 5th, 2011, and achieved orbit around Jupiter on July 5th, 2016 – 1796 days, or just under 5 years.

And, it should be noted, these were uncrewed missions, which involved only a robotic probe and not a vessel large enough to accommodate large crews, supplies and heavy equipment. As a result, colony ships would have to be much larger an heavier, and would require advanced propulsion systems – like nuclear-thermal/nuclear-electr ic engines – to ensure they made the trip in a reasonable amount of time.

Missions to and from the Jovian moons would also require bases between Earth and Jupiter in order to provide refueling and resupplying, and cut down on the costs of individual missions. This would mean that permanent outposts would need to be established on the Moon, Mars, and most likely in the Asteroid Belt before any missions to Jupiter’s moons were considered feasible or cost-effective.

These last two challenges raise the issue of cost. Between building ships that have the ability to make the trip to Jupiter in a fair amount of time, established the bases needed to support them, and the cost of establishing the colonies themselves, the colonization of the Jovian moons would be incredibly expensive! Combined with the hazards of doing so, one has to wonder if its even worth it.

SpaceX's newly revealed Interplanetary Transit System will make travel to Mars, and other destinations in our Solar System, possible. Image: SpaceX
SpaceX’s newly revealed Interplanetary Transit System will make travel to Mars, and other destinations in our Solar System, possible. Credit: SpaceX

On the other hand, in the context of space exploration and colonization, the idea of establishing permanent human outposts on Jupiter’s moons makes sense. All of the challenges can be addressed, provided the proper precautions are taken and the right kind of resources are committed. And while it will have to wait until after similar colonies/bases are established on the Moon and Mars, it is not a bad idea as far as “next steps” go.

With colonies on any of the Galilean moons, humanity will have a foothold in the outer Solar System, a stopover point for future missions to Saturn and beyond, and access to abundant resources. Again, it all comes down to how much we are willing to spend.

We have written many interesting articles on colonization here at Universe Today. Here’s Why Colonize the Moon First?, How Do We Colonize Mercury?, How Do We Colonize Venus?, Colonizing Venus with Floating Cities, Will We Ever Colonize Mars?, and The Definitive Guide to Terraforming.

Astronomy Cast also has some interesting episodes on the subject. Check out Episode 95: Humans to Mars, Part 2 – Colonists, Episode 115: The Moon, Part 3 – Return to the Moon, Episode 381: Hollowing Asteroids in Science Fiction.

Sources:

How Many Moons are in the Solar System?

For millennia, human beings stared up at the night sky and were held in awe by the Moon. To many ancient cultures, it represented a deity, and its cycles were accorded divine significance. By the time of Classical Antiquity and the Middle Ages, the Moon was considered to be a heavenly body that orbited Earth, much like the other known planets of the day (Mercury, Venus, Mars, Jupiter, and Saturn).

However, our understanding of moons was revolutionized when in 1610, astronomer Galileo Galilei pointed his telescope to Jupiter and noticed ” four wandering stars” around Jupiter. From this point onward, astronomers have come to understand that planets other than Earth can have their own moons – in some cases, several dozen or more. So just how many moons are there in the Solar System?

In truth, answering that question requires a bit of a clarification first. If we are talking about confirmed moons that orbit any of the planets of the Solar System (i.e. those that are consistent with the definition adopted by the IAU in 2006), then we can say that there are currently 173 known moons. If, however, we open the floor to dwarf planets that have objects orbiting them, the number climbs to 182.

The moons, several minor planets and comets of the Solar System, shown to scale. Credit: Antonio Ciccolella
The moons, several minor planets and comets of the Solar System, shown to scale. Credit: Antonio Ciccolella

However, over 200 minor-planet moons have also been observed in the Solar System (as of Jan. 2012). This includes the 76 known objects in the asteroid belt with satellites, four Jupiter Trojans, 39 near-Earth objects (two with two satellites each), 14 Mars-crossers, and 84 natural satellites of Trans-Neptunian Objects. And some 150 additional small bodies have been observed within the rings of Saturn. If we include all these, then we can say that the Solar System has 545 known satellites.

Inner Solar System:

The planets of the Inner Solar system – Mercury, Venus, Earth and Mars – are all terrestrial planets, which means that they are composed of silicate rock and minerals that are differentiated between a metallic core and a silicate mantle and crust. For a number of reasons, few satellites exist within this region of the Solar System.

All told, only three natural satellites exist orbiting planetary bodies in the Inner Solar System – Earth and Mars. While scientist theorize that there were moons around Mercury and Venus in the past, it is believed that these moons impacted on the surface a long time ago. The reason for this sparseness of satellites has a lot to do with the gravitational influence of the Sun.

Both Mercury and Venus are too close to the Sun (and in Mercury’s case, too weak in terms of its own gravitational pull) to have grabbed onto a passing object, or held onto rings of debris in orbit that could have coalesced to form a satellite over time. Earth and Mars were able to retain satellites, but mainly because they are the outermost of the Inner planets.

Earth has only the one natural satellite, which we are familiar with – the Moon. With a mean radius of 1737 km and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive, which is quite large for a satellite. It is also the second densest moon in our Solar System (after Io), with a mean density of 3.3464 g/cm³.

Several theories have been proposed for the formation of the Moon. The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.

Mars, meanwhile, has two moons – Phobos and Deimos. Like our own Moon, both of the Martian moons are tidally locked to Mars, so they always present the same face to the planet. Compared to our Moon, they are rough and asteroid-like in appearance, and also much smaller. Hence the prevailing theory that they were once asteroids that were kicked out of the Main Belt by Jupiter’s gravity, and were then acquired by Mars.

The larger moon is Phobos, whose name comes from the Greek word which means “fear” (i.e. phobia). Phobos measures just 22.7 km across and has an orbit that places it closer to Mars than Deimos. Compared to Earth’s own Moon — which orbits at a distance of 384,403 km away from our planet — Phobos orbits at an average distance of only 9,377 km above Mars.

Phobos and Deimos, photographed here by the Mars Reconnaissance Orbiter, are tiny, irregularly-shaped moons that are probably strays from the main asteroid belt. Credit: NASA - See more at: http://astrobob.areavoices.com/2013/07/05/rovers-capture-loony-moons-and-blue-sunsets-on-mars/#sthash.eMDpTVPT.dpuf
Phobos and Deimos, photographed here by the Mars Reconnaissance Orbiter. Credit: NASA

Mars’ second moon is Deimos, which takes its name from the Greek word for panic. It is even smaller, measuring just 12.6 km across, and is also less irregular in shape. Its orbit places it much farther away from Mars, at a distance of 23,460 km, which means that Deimos takes 30.35 hours to complete an orbit around Mars.

These three moons are the sum total of moons to be found within the Inner Solar System (at least, by the conventional definition). But looking further abroad, we see that this is really just the tip of the iceberg. To think we once believed that the Moon was the only one of its kind!

Outer Solar System:

Beyond the Asteroid Belt (and Frost Line), things become quite different. In this region of the Solar System, every planet has a substantial system of Moons; in the case of Jupiter and Saturn, reaching perhaps even into the hundreds. So far, a total of 170 moons have been confirmed orbiting the Outer Planets, while several hundred more orbit minor bodies and asteroids.

Due to its immense size, mass, and gravitational pull, Jupiter has the most satellites of any planet in the Solar System. At present, the Jovian system includes 67 known moons, though it is estimated that it may have up to 200 moons and moonlets (the majority of which are yet to been confirmed and classified).

The four largest Jovian moons are known as the Galilean Moons (named after their discoverer, Galileo Galilei). They include: Io, the most volcanically active body in our Solar System; Europa, which is suspected of having a massive subsurface ocean; Ganymede, the largest moon in our Solar System; and Callisto, which is also thought to have a subsurface ocean and features some of the oldest surface material in the Solar System.

Illustration of Jupiter and the Galilean satellites. Credit: NASA
Illustration of Jupiter and the Galilean satellites. Credit: NASA

Then there’s the Inner Group (or Amalthea group), which is made up of four small moons that have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree. This groups includes the moons of Metis, Adrastea, Amalthea, and Thebe. Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system.

Jupiter also has an array of Irregular Satellites, which are substantially smaller and have more distant and eccentric orbits than the others. These moons are broken down into families that have similarities in orbit and composition, and are believed to be largely the result of collisions from large objects that were captured by Jupiter’s gravity.

Similar to Jupiter, it is estimated that Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.

Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.

A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute

The Inner Large Moons, which orbit within the E Ring, includes the larger satellites Mimas Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock.

At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.

The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.

Uranus has 27 known satellites, which are divided into the categories of larger moons, inner moons, and irregular moons (similar to other gas giants). The largest moons of Uranus are, in order of size, Miranda, Ariel, Umbriel, Oberon and Titania. These moons range in diameter and mass from 472 km and 6.7 × 1019 kg for Miranda to 1578 km and 3.5 × 1021 kg for Titania. Each of these moons is particularly dark, with low bond and geometric albedos. Ariel is the brightest while Umbriel is the darkest.

A montage of Uranus's moons. Image credit: NASA
A montage of Uranus’s moons (from left to right) – Ariel,  Credit: NASA

All of the large moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation, or resulted from the large impact suffered by Uranus early in its history. Each one is comprised of roughly equal amounts of rock and ice, except for Miranda which is made primarily of ice.

The ice component may include ammonia and carbon dioxide, while the rocky material is believed to be composed of carbonaceous material, including organic compounds (similar to asteroids and comets). Their compositions are believed to be differentiated, with an icy mantle surrounding a rocky core.

Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (except for S/2004 N 1, which is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.

Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.

Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS

In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.

With a mean diameter of around 2700 km ( mi) and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.

These moons make up the lion’s share of natural satellites found in the Solar System. However, thanks to ongoing exploration and improvements made in our instrumentation, satellites are being discovered in orbit around minor bodies as well.

Dwarf Planets and Other Bodies:

As already noted, there are several dwarf planets, TNOs, and other bodies in the Solar System that also have their own moons. These consist mainly of the natural satellites that have been confirmed orbiting Pluto, Eris, Haumea and Makemake. With five orbiting satellites, Pluto has the most confirmed moons (though that may change with further observation).

The largest, and closest in orbit to Pluto, is Charon. This moon was first identified in 1978 by astronomer James Christy using photographic plates from the United States Naval Observatory (USNO) in Washington, D.C. Beyond Charon lies the four other circumbinary moons – Styx, Nix, Kerberos, and Hydra, respectively.

A portrait from the final approach of the New Horizons spacecraft to the Pluto system on July 11, 2015. Pluto and Charon display striking color and brightness contrast in this composite image. Credit: NASA-JHUAPL-SWRI.
A portrait from the final approach of the New Horizons spacecraft to the Pluto system on July 11th, 2015. Credit: NASA-JHUAPL-SWRI.

Nix and Hydra were discovered simultaneously in 2005 by the Pluto Companion Search Team using the Hubble Space Telescope. The same team discovered Kerberos in 2011. The fifth and final satellite, Styx, was discovered by the New Horizons spacecraft in 2012 while capturing images of Pluto and Charon.

Charon, Styx and Kerberos are all massive enough to have collapsed into a spheroid shape under their own gravity. Nix and Hydra, meanwhile, are oblong in shape. The Pluto-Charon system is unusual, since it is one of the few systems in the Solar System whose barycenter lies above the primary’s surface. In short, Pluto and Charon orbit each other, causing some scientists to claim that it is a “double-dwarf system” instead of a dwarf planet and an orbiting moon.

In addition, it is unusual in that each body is tidally locked to the other. Charon and Pluto always present the same face to each other; and from any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its common center of gravity.

In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. This would seem indicate that Pluto does have a subsurface ocean that is warm in temperature, and that the core is geologically active. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.

Comparison of Sedna with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon
Comparison of Pluto with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon

Coming in second is Haumea, which has two known moons – Hi’iaka and Namaka – which are named after the daughters of the Hawaiian goddess. Both were discovered in 2005 by Brown’s team while conducting observations of Haumea at the W.M. Keck Observatory. Hi’iaka, which was initially nicknamed “Rudolph” by the Caltech team, was discovered January 26th, 2005.

It is the outer and – at roughly 310 km in diameter – the larger and brighter of the two, and orbits Haumea in a nearly circular path every 49 days. Infrared observations indicate that its surface is almost entirely covered by pure crystalline water ice. Because of this, Brown and his team have speculated that the moon is a fragment of Haumea that broke off during a collision.

Namaka, the smaller and innermost of the two, was discovered on June 30th, 2005, and nicknamed “Blitzen”. It is a tenth the mass of Hiiaka and orbits Haumea in 18 days in a highly elliptical orbit. Both moons circle Haumea is highly eccentric orbits. No estimates have been made yet as to their mass.

Eris has one moon called Dysnomia, which is named after the daughter of Eris in Greek mythology, which was first observed on September 10th, 2005 – a few months after the discovery of Eris. The moon was spotted by a team using the Keck telescopes in Hawaii, who were busy carrying out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris) at the time.

This is an artist's concept of Kuiper Belt object Eris and its tiny satellite Dysnomia. Eris is the large object at the bottom of the illustration. A portion of its surface is lit by the Sun, located in the upper left corner of the image. Eris's moon, Dysnomia, is located just above and to the left of Eris. The Hubble Space Telescope and Keck Observatory took images of Dysnomia's movement from which astronomer Mike Brown (Caltech) precisely calculated Eris to be 27 percent more massive than Pluto. Artwork Credit: NASA, ESA, Adolph Schaller (for STScI)
Artist’s concept of the dwarf planet Eris and its only natural satellite, Dysnomia. Credit: NASA, ESA, Adolph Schaller (for STScI)

In April of 2016, observations using the Hubble Space Telescope‘s Wide Field Camera 3 revealed that Makemake had a natural satellite – which was designated S/2015 (136472) 1 (nicknamed MK 2 by the discovery team). It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake.

Largest and Smallest Moons:

The title for largest moon in the Solar System goes to Ganymede, which measures 5262.4 kilometers (3270 mi) in diameter. This not only makes it larger than Earth’s Moon, but larger even than the planet Mercury – though it has only half of Mercury’s mass. As for the smallest satellite, that is a tie between S/2003 J 9 and S/2003 J 12. These two satellites, both of which orbit Jupiter, measure about 1 km (0.6 mi) in diameter.

An important thing to note when discussing the number of known moons in the Solar System is that the key word here is “known”. With every passing year, more satellites are being confirmed, and the vast majority of those we now know about were only discovered in the past few decades. As our exploration efforts continue, and our instruments improve, we may find that there are hundreds more lurking around out there!

We have written many interesting articles about the moons of the Solar System here at Universe Today. Here’s What is the Biggest moon in the Solar System? What are the Planets of the Solar System?, How Many Moons Does Earth Have?, How Many Moons Does Mars Have?, How Many Moons Does Jupiter Have?, How Many Moons Does Saturn Have?, How Many Moons Does Uranus Have?, How Many Moons Does Neptune Have?

For more information, be sure to check out NASA’s Solar System Exploration page.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

Sources:

What is Galileo’s Telescope?

In 1610, Italian astronomer Galileo Galilei looked up at the heavens using a telescope of his making. And what he saw would forever revolutionize the field of astronomy, our understanding of the Universe, and our place in it. Centuries later, Galileo’s is still held in such high esteem; not only for the groundbreaking research he conducted, but because of his immense ingenuity in developing his own research tools.

And at the center of it all is Galileo’s famous telescope, which still inspires curiosity centuries later. How exactly did he invent it. How exactly was it an improvement on then-current designs? What exactly did he see with it when he looked up at the night sky? And what has become of it today? Luckily, all of these are questions we are able to answer.

Description:

Galileo’s telescope was the prototype of the modern day refractor telescope. As you can see from this diagram below, which is taken from Galileo’s own work – Sidereus Nuncius (“The Starry Messenger”) – it was a simple arrangement of lenses that first began with optician’s glass fixed to either end of a hollow cylinder.

Diagram of Galileo's telescope, taken from Sidereus Nuncius. Credit: hps.cam.ac.uk
Diagram of Galileo’s refractor telescope, taken from Sidereus Nuncius (1610). Credit: hps.cam.ac.uk

Galileo had no diagrams to work from, and instead relied on his own system of trial and error to achieve the proper placement of the lenses. In Galileo’s telescope the objective lens was convex and the eye lens was concave (today’s telescopes make use of two convex lenses). Galileo knew that light from an object placed at a distance from a convex lens created an identical image on the opposite side of the lens.

He also knew that if he used a concave lens, the object would appear on the same side of the lens where the object was located. If moved at a distance, it appeared larger than the object. It took a lot of work and different arrangements to get the lens the proper sizes and distances apart, but Galileo’s telescope remained the most powerful and accurately built for a great many years.

History of Galileo’s Telescope:

Naturally, Galileo’s telescope had some historical antecedents. In the late summer of 1608, a new invention was all the rage in Europe – the spyglass. These low power telescopes were likely made by almost all advanced opticians, but the very first was credited to Hans Lippershey of Holland. These primitive telescopes only magnified the view a few times over.

Much like our modern times, the manufacturers were quickly trying to corner the market with their invention. But Galileo Galilei’s friends convinced his own government to wait – sure that he could improve the design. When Galileo heard of this new optical instrument he set about engineering and making improved versions, with higher magnification.

Galileo Galilei showing the Doge of Venice how to use the telescope by Giuseppe Bertini (1858). Credit: gabrielevanin.it
Galileo Galilei showing the Doge of Venice how to use the telescope by Giuseppe Bertini (1858). Credit: gabrielevanin.it

Galileo’s telescope was similar to how a pair of opera glasses work – a simple arrangement of glass lenses to magnify objects. His first versions only improved the view to the eighth power, but Galileo’s telescope steadily improved. Within a few years, he began grinding his own lenses and changing his arrays. Galileo’s telescope was now capable of magnifying normal vision by a factor of 10, but it had a very narrow field of view.

However, this limited ability didn’t stop Galileo from using his telescope to make some amazing observations of the heavens. And what he saw, and recorded for posterity, was nothing short of game-changing.

What Galileo Observed:

One fine Fall evening, Galileo pointed his telescope towards the one thing that people thought was perfectly smooth and as polished as a gemstone – the Moon. Imagine his surprise when found that it, in his own words, was “uneven, rough, full of cavities and prominences.” Galileo’s telescope had its flaws, such as a narrow field of view that could only show about one quarter of the lunar disk without repositioning.

Nevertheless, a revolution in astronomy had begun! Months passed, and Galileo’s telescope improved. On January 7th, 1610, he turned his new 30 power telescope towards Jupiter, and found three small, bright “stars” near the planet. One was off to the west, the other two were to the east, and all three were in a straight line. The following evening, Galileo once again took a look at Jupiter, and found that all three of the “stars” were now west of the planet – still in a straight line!

Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without "a true physical demonstration that the sun does not circle the earth but the earth circles the sun". Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth. This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea. For Galileo, the tides were caused by the sloshing back and forth of water in the seas as a point on the Earth's surface sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616, addressed to Cardinal Orsini. His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. If this theory were correct, there would be only one high tide per day. Galileo and his contemporaries were aware of this inadequacy because there are two daily high tides at Venice instead of one, about twelve hours apart. Galileo dismissed this anomaly as the result of several secondary causes including the shape of the sea, its depth, and other factors. Against the assertion that Galileo was deceptive in making these arguments, Albert Einstein expressed the opinion that Galileo developed his "fascinating arguments" and accepted them uncritically out of a desire for physical proof of the motion of the Earth. Galileo dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides. He also refused to accept Kepler's elliptical orbits of the planets, considering the circle the "perfect" shape for planetary orbits.Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without "a true physical demonstration that the sun does not circle the earth but the earth circles the sun". Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth. This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea. For Galileo, the tides were caused by the sloshing back and forth of water in the seas as a point on the Earth's surface sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616, addressed to Cardinal Orsini. His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. If this theory were correct, there would be only one high tide per day. Galileo and his contemporaries were aware of this inadequacy because there are two daily high tides at Venice instead of one, about twelve hours apart. Galileo dismissed this anomaly as the result of several secondary causes including the shape of the sea, its depth, and other factors. Against the assertion that Galileo was deceptive in making these arguments, Albert Einstein expressed the opinion that Galileo developed his "fascinating arguments" and accepted them uncritically out of a desire for physical proof of the motion of the Earth. Galileo dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides. He also refused to accept Kepler's elliptical orbits of the planets, considering the circle the "perfect" shape for planetary orbits.
Galileo’s Sidereus Nuncius (“Starry Messenger”) shared the discoveries he made of Jupiter with his telescope. Credit and Copyright: brunelleschi.imss.fi.it

And there were more discoveries awaiting Galileo’s telescope: the appearance of bumps next to the planet Saturn (the edges of Saturn’s rings), spots on the Sun’s surface (aka. Sunspots), and seeing Venus change from a full disk to a slender crescent. Galileo Galilei published all of these findings in a small book titled Sidereus Nuncius (“The Starry Messenger”) in 1610.

While Galileo was not the first astronomer to point a telescope towards the heavens, he was the first to do so scientifically and methodically. Not only that, but the comprehensive notes he took on his observations, and the publication of his discoveries, would have a revolutionary impact on astronomy and many other fields of science.

Galileo’s Telescope Today:

Today, over 400 years later, Galileo’s Telescope still survives under the constant care of the Istituto e Museo di Storia della Scienza (renamed the Museo Galileo in 2010) in Italy. The Museum holds exhibitions on Galileo’s telescope and the observations he made with it. The displays consist of these rare and precious instruments – including the objective lens created by the master and the only two existing telescopes built by Galileo himself.

Thanks to Galileo’s careful record keeping, craftsmen around the world have recreated Galileo’s telescope for museums and replicas are now sold for amateurs and collectors as well. Despite the fact that astronomers now have telescopes of immense power at their disposal, many still prefer to go the DIY route, just like Galileo!

A replica of the earliest surviving telescope attributed to Galileo Galilei, on display at the Griffith Observatory. Credit: Wikipedia Commons/Mike Dunn
A replica of the earliest surviving telescope attributed to Galileo Galilei, on display at the Griffith Observatory. Credit: Wikipedia Commons/Mike Dunn

Few scientists and astronomers have had the same impact Galileo had. Even fewer are regarded as pioneers in the sciences, or revolutionary thinkers who forever changed humanity’s perception of the heavens and their place within it. Little wonder then why his most prized instrument is kept so well preserved, and is still the subject of study over four centuries later.

We have written many interesting articles on Galileo here at Universe Today. Here’s

Astronomy Cast also has an interesting episode on telescope making – Episode 327: Telescope Making, Part I

For more information, be sure to check out the Museo Galileo‘s website.

How Do We Terraform Jupiter’s Moons?

Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present to our guide to terraforming Jupiter’s Moons. Much like terraforming the inner Solar System, it might be feasible someday. But should we?

Fans of Arthur C. Clarke may recall how in his novel, 2010: Odyssey Two (or the movie adaptation called 2010: The Year We Make Contact), an alien species turned Jupiter into a new star. In so doing, Jupiter’s moon Europa was permanently terraformed, as its icy surface melted, an atmosphere formed, and all the life living in the moon’s oceans began to emerge and thrive on the surface.

As we explained in a previous video (“Could Jupiter Become a Star“) turning Jupiter into a star is not exactly doable (not yet, anyway). However, there are several proposals on how we could go about transforming some of Jupiter’s moons in order to make them habitable by human beings. In short, it is possible that humans could terraform one of more of the Jovians to make it suitable for full-scale human settlement someday.

Continue reading “How Do We Terraform Jupiter’s Moons?”

Ten Interesting Facts About Jupiter

Jupiter was appropriately named after the king of the gods. It’s massive, has a powerful magnetic field, and more moons that any planet in the Solar System. Though it has been known to astronomers since ancient times, the invention of the telescope and the advent of modern astronomy has taught us so much about this gas giant.

In short, there are countless interesting facts about this gas giant that many people just don’t know about. And we here at Universe Today have taken the liberty of compiling a list of ten particularly interesting ones that we think will fascinate and surprise you. Think you know everything about Jupiter? Think again!

Continue reading “Ten Interesting Facts About Jupiter”

Io, Jupiter’s Volcanic Moon

This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Credit: NASA

Exploring the Solar System is like peeling an onion. With every layer removed, one finds fresh mysteries to ponder over, each one more confounding than the last. And this is certainly the case when it comes to Jupiter’s system of moons, particularly its four largest – Io, Europa, Ganymede and Callisto. Known as the Galilean Moons, in honor of their founder, these moons possess enough natural wonders to keep scientists busy for centuries.

As Jupiter’s innermost moon, it is also the fourth-largest moon in the Solar System, has the highest density of any known moon, and is the driest known object in the Solar System. It is also one of only four known bodies that experiences active volcanism and – with over 400 active volcanoes – it is the most geologically active body in the Solar System.

Continue reading “Io, Jupiter’s Volcanic Moon”

Jupiter’s Moon Europa

Europa

Jupiter‘s four largest moons – aka. the Galilean Moons, consisting of Io, Europa, Ganymede and Callisto – are nothing if not fascinating. Ever since their discovery over four centuries ago, these moons have been a source of many great discoveries. These include the possibility of internal oceans, the presence of atmospheres, volcanic activity, one has a magnetosphere (Ganymede), and possibly having more water than even Earth.

But arguably, the most fascinating of the Galilean Moons is Europa: the sixth closest moon to Jupiter, the smallest of the four, and the sixth largest moon in the Solar System. In addition to having an icy surface and a possible warm-water interior, this moon is considered to be one of the most-likely candidates for possessing life outside of Earth.

Continue reading “Jupiter’s Moon Europa”

The Planet Jupiter

Jupiter and Io. Image Credit: NASA/JPL

Ever since the invention of the telescope four hundred years ago, astronomers have been fascinated by the gas giant known as Jupiter. Between its constant, swirling clouds, its many, many moons, and its Giant Red Spot, there are many things about this planet that are both delightful and fascinating.

But perhaps the most impressive feature about Jupiter is its sheer size. In terms of mass, volume, and surface area, Jupiter is the biggest planet in our Solar System by a wide margin. And since people have been aware of its existence for thousands of years, it has played an active role in the cosmological systems many cultures. But just what makes Jupiter so massive, and what else do we know about it?

Size, Mass and Orbit:

Jupiter’s mass, volume, surface area and mean circumference are 1.8981 x 1027 kg, 1.43128 x 1015 km3, 6.1419 x 1010 km2, and 4.39264 x 105 km respectively. To put that in perspective, Jupiter diameter is roughly 11 times that of Earth, and 2.5 the mass of all the other planets in the Solar System combined.

But, being a gas giant, it has a relatively low density – 1.326 g/cm3 – which is less than one quarter of Earth’s. This means that while Jupiter’s volume is equivalent to about 1,321 Earths, it is only 318 times as massive. The low density is one way scientists are able to determine that it is made mostly of gases, though the debate still rages on what exists at its core (see below).

Jupiter orbits the Sun at an average distance (semi-major axis) of 778,299,000 km (5.2 AU), ranging from 740,550,000 km (4.95 AU) at perihelion and 816,040,000 km (5.455 AU) at aphelion. At this distance, Jupiter takes 11.8618 Earth years to complete a single orbit of the Sun. In other words, a single Jovian year lasts the equivalent of 4,332.59 Earth days.

However, Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours (9 hours, 55 minutes and 30 seconds to be exact. Therefore, a single Jovian year lasts 10,475.8 Jovian solar days. This orbital period is two-fifths that of Saturn, which means that the two largest planets in our Solar System form a 5:2 orbital resonance.

Structure and Composition:

Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons
This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons

The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds as well as trace amounts of benzene and other hydrocarbons. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. Crystals of frozen ammonia have also been observed in the outermost layer of the atmosphere.

The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.

In 1997, the existence of the core was suggested by gravitational measurements, indicating a mass of from 12 to 45 times the Earth’s mass, or roughly 4%–14% of the total mass of Jupiter. The presence of a core is also supported by models of planetary formation that indicate how a rocky or icy core would have been necessary at some point in the planet’s history in order to collect all of its hydrogen and helium from the protosolar nebula.

However, it is possible that this core has since shrunk due to convection currents of hot, liquid, metallic hydrogen mixing with the molten core. This core may even be absent now, but a detailed analysis is needed before this can be confirmed. The Juno mission, which launched in August 2011 (see below), is expected to provide some insight into these questions, and thereby make progress on the problem of the core.

The temperature and pressure inside Jupiter increase steadily toward the core. At the “surface”, the pressure and temperature are believed to be 10 bars and 340 K (67 °C, 152 °F). At the “phase transition” region, where hydrogen becomes metallic, it is believed the temperature is 10,000 K (9,700 °C; 17,500 °F) and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K (35,700 °C; 64,300 °F) and the interior pressure at roughly 3,000–4,500 GPa.

Jupiter’s Moons:

The Jovian system currently includes 67 known moons. The four largest are known as the Galilean Moons, which are named after their discoverer, Galileo Galilei. They include: Io, the most volcanically active body in our Solar System; Europa, which is suspected of having a massive subsurface ocean; Ganymede, the largest moon in our Solar System; and Callisto, which is also thought to have a subsurface ocean and features some of the oldest surface material in the Solar System.

Then there’s the Inner Group (or Amalthea group), which is made up of four small moons that have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree. This groups includes the moons of Metis, Adrastea, Amalthea, and Thebe. Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system.

Jupiter also has an array of Irregular Satellites, which are substantially smaller and have more distant and eccentric orbits than the others. These moons are broken down into families that have similarities in orbit and composition, and are believed to be largely the result of collisions from large objects that were captured by Jupiter’s gravity.

Illustration of Jupiter and the Galilean satellites. Credit: NASA
Illustration of Jupiter and the Galilean satellites. Credit: NASA

Atmosphere and Storms:

Much like Earth, Jupiter experiences auroras near its northern and southern poles. But on Jupiter, the auroral activity is much more intense and rarely ever stops. The intense radiation, Jupiter’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere create a light show that is truly spectacular.

Jupiter also experiences violent weather patterns. Wind speeds of 100 m/s (360 km/h) are common in zonal jets, and can reach as high as 620 kph (385 mph). Storms form within hours and can become thousands of km in diameter overnight. One storm, the Great Red Spot, has been raging since at least the late 1600s. The storm has been shrinking and expanding throughout its history; but in 2012, it was suggested that the Giant Red Spot might eventually disappear.

Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. These clouds are located in the tropopause and are arranged into bands of different latitudes, known as “tropical regions”. The cloud layer is only about 50 km (31 mi) deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region.

There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter, which would be caused by the water’s polarity creating the charge separation needed for lightning. Observations of these electrical discharges indicate that they can be up to a thousand times as powerful as those observed here on the Earth.

A color composite image of the June 3rd Jupiter impact flash. Credit: Anthony Wesley of Broken Hill, Australia.
A color composite image of the June 3rd Jupiter impact flash. Credit: Anthony Wesley

Historical Observations of the Planet:

As a planet that can be observed with the naked eye, humans have known about the existence of Jupiter for thousands of years. It has therefore played a vital role in the mythological and astrological systems of many cultures. The first recorded mentions of it date back to the Babylon Empire of the 7th and 8th centuries BCE.

In the 2nd century, the Greco-Egyptian astronomer Ptolemy constructed his famous geocentric planetary model that contained deferents and epicycles to explain the orbit of Jupiter relative to the Earth (i.e. retrograde motion). In his work, the Almagest, he ascribed an orbital period of 4332.38 days to Jupiter (11.86 years).

In 499, Aryabhata – a mathematician-astronomer from the classical age of India – also used a geocentric model to estimate Jupiter’s period as 4332.2722 days, or 11.86 years. It has also been ventured that the Chinese astronomer Gan De discovered Jupiter’s moons in 362 BCE without the use of instruments. If true, it would mean that Galileo was not the first to discovery the Jovian moons two millennia later.

In 1610, Galileo Galilei was the first astronomer to use a telescope to observe the planets. In the course of his examinations of the outer Solar System, he discovered the four largest moons of Jupiter (now known as the Galilean Moons). The discovery of moons other than Earth’s was a major point in favor of Copernicus’ heliocentric theory of the motions of the planets.

The first star party? Galileo shows of the sky in Saint Mark's square in Venice. Note the lack of adaptive optics. (Illustration in the Public Domain).
Galileo shows of the sky in Saint Mark’s square in Venice. Note the lack of adaptive optics. Credit: Public Domain

During the 1660s, Cassini used a new telescope to discover Jupiter’s spots and colorful bands and observed that the planet appeared to be an oblate spheroid. By 1690, he was also able to estimate the rotation period of the planet and noticed that the atmosphere undergoes differential rotation. In 1831, German astronomer Heinrich Schwabe produced the earliest known drawing to show details of the Great Red Spot.

In 1892, E. E. Barnard observed a fifth satellite of Jupiter using the refractor telescope at the Lick Observatory in California. This relatively small object was later named Amalthea, and would be the last planetary moon to be discovered directly by visual observation.

In 1932, Rupert Wildt identified absorption bands of ammonia and methane in the spectra of Jupiter; and by 1938, three long-lived anticyclonic features termed “white ovals” were observed. For several decades, they remained as separate features in the atmosphere, sometimes approaching each other but never merging. Finally, two of the ovals merged in 1998, then absorbed the third in 2000, becoming Oval BA.

Beginning in the 1950s, radiotelescopic research of Jupiter began. This was due to astronomers Bernard Burke and Kenneth Franklin’s detection of radio signals coming from Jupiter in 1955. These bursts of radio waves, which corresponded to the rotation of the planet, allowed Burke and Franklin to refine estimates of the planet’s rotation rate.

Infrared image of Jupiter from SOFIA’s First Light flight composed of individual images at wavelengths of 5.4 (blue), 24 (green) and 37 microns (red) made by Cornell University’s FORCAST camera. A recent visual-wavelength picture of approximately the same side of Jupiter is shown for comparison. The white stripe in the infrared image is a region of relatively transparent clouds through which the warm interior of Jupiter can be seen. (Visual image credit: Anthony Wesley)
Infrared image of Jupiter from SOFIA’s First Light flight composed of individual images at wavelengths made by Cornell University’s FORCAST camera. Credit: Anthony Wesley/Cornell University

Over time, scientists discovered that there were three forms of radio signals transmitted from Jupiter – decametric radio bursts, decimetric radio emissions, and thermal radiation. Decametric bursts vary with the rotation of Jupiter, and are influenced by the interaction of Io with Jupiter’s magnetic field.

Decimetric radio emissions – which originate from a torus-shaped belt around Jupiter’s equator – are caused by cyclotronic radiation from electrons that are accelerated in Jupiter’s magnetic field. Meanwhile, thermal radiation is produced by heat in the atmosphere of Jupiter. Visualizations of Jupiter using radiotelescopes have allowed astronomers to learn much about its atmosphere, thermal properties and behavior.

Exploration:

Since 1973, a number of automated spacecraft have been sent to the Jovian system and performed planetary flybys that brought them within range of the planet. The most notable of these was Pioneer 10, the first spacecraft to get close enough to send back photographs of Jupiter and its moons. Between this mission and Pioneer 11, astronomers learned a great deal about the properties and phenomena of this gas giant.

Artist impression of Pioneer 10 at Jupiter. Image credit: NASA/JPL
Artist impression of Pioneer 10 at Jupiter. Image credit: NASA/JPL

For example, they discovered that the radiation fields near the planet were much stronger than expected. The trajectories of these spacecraft were also used to refine the mass estimates of the Jovian system, and radio occultations by the planet resulted in better measurements of Jupiter’s diameter and the amount of polar flattening.

Six years later, the Voyager missions began, which vastly improved the understanding of the Galilean moons and discovered Jupiter’s rings. They also confirmed that the Great Red Spot was anticyclonic, that its hue had changed sine the Pioneer missions – turning from orange to dark brown – and spotted lightning on its dark side. Observations were also made of Io, which showed a torus of ionized atoms along its orbital path and volcanoes on its surface.

On December 7th, 1995, the Galileo orbiter became the first probe to establish orbit around Jupiter, where it would remain for seven years. During its mission, it conducted multiple flybys of all the Galilean moons and Amalthea and deployed an probe into the atmosphere. It was also in the perfect position to witness the impact of Comet Shoemaker–Levy 9 as it approached Jupiter in 1994.

On September 21st, 2003, Galileo was deliberately steered into the planet and crashed in its atmosphere at a speed of 50 km/s, mainly to avoid crashing and causing any possible contamination to Europa – a moon which is believed to harbor life.

Artist impression of New Horizons with Jupiter. Image credit: NASA/JPL/JHUAPL
Artist impression of New Horizons with Jupiter. Image credit: NASA/JPL/JHUAPL

Data gathered by both the probe and orbiter revealed that hydrogen composes up to 90% of Jupiter’s atmosphere. The temperatures data recorded was more than 300 °C (570 °F) and the wind speed measured more than 644 kmph (400 mph) before the probe vaporized.

In 2000, the Cassini probe (while en route to Saturn) flew by Jupiter and provided some of the highest-resolution images ever taken of the planet. While en route to Pluto, the New Horizons space probe flew by Jupiter and measured the plasma output from Io’s volcanoes, studied all four Galileo moons in detail, and also conducting long-distance observations of Himalia and Elara.

NASA’s Juno mission, which launched in August 2011, achieved orbit around the Jovian planet on July 4th, 2016. The purpose of this mission to study Jupiter’s interior, its atmosphere, its magnetosphere and gravitational field, ultimately for the purpose of determining the history of the planet’s formation (which will shed light on the formation of the Solar System).

As the probe entered its polar elliptical orbit on July 4th after completing a 35-minute-long firing of the main engine, known as Jupiter Orbital Insertion (or JOI). As the probe approached Jupiter from above its north pole, it was afforded a view of the Jovian system, which it took a final picture of before commencing JOI.

Illustration of NASA's Juno spacecraft firing its main engine to slow down and go into orbit around Jupiter. Lockheed Martin built the Juno spacecraft for NASA's Jet Propulsion Laboratory. Credit: NASA/Lockheed Martin
Illustration of NASA’s Juno spacecraft firing its main engine to slow down and go into orbit around Jupiter. Credit: NASA/Lockheed Martin

On July 10th, the Juno probe transmitted its first imagery from orbit after powering back up its suite of scientific instruments. The images were taken when the spacecraft was 4.3 million km (2.7 million mi) from Jupiter and on the outbound leg of its initial 53.5-day capture orbit. The color image shows atmospheric features on Jupiter, including the famous Great Red Spot, and three of the massive planet’s four largest moons – Io, Europa and Ganymede, from left to right in the image.

The next planned mission to the Jovian system will be performed by the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022, followed by NASA’s Europa Clipper mission in 2025.

Exoplanets:

The discovery of exoplanets has revealed that planets can get even bigger than Jupiter. In fact, the number of “Super Jupiters” observed by the Kepler space probe (as well as ground-based telescopes) in the past few years has been staggering. In fact, as of 2015, more than 300 such planets have been identified.

Notable examples include PSR B1620-26 b (Methuselah), which was the first super-Jupiter to be observed (in 2003). At 12.7 billion years of age, it is also the third oldest known planet in the universe. There’s also HD 80606 b (Niobe), which has the most eccentric orbit of any known planet, and 2M1207b (Lerna), which orbits the brown dwarf Fomalhaut b (Illion).

Here’s an interesting fact. Scientist theorize that a gas gain could get 15 times the size of Jupiter before it began deuterium fusion, making it a brown dwarf star. Good thing too, since the last thing the Solar System needs is for Jupiter to go nova!

Jupiter was appropriately named by the ancient Romans, who chose to name after the king of the Gods (also known as Jove). The more we have come to know and understand about this most-massive of Solar planets, the more deserving of this name it appears.

We have many interesting articles on Jupiter here at Universe Today. Here are some articles on the color and gravity of Jupiter, how it got its name, and how it shaped our Solar System.

Got questions about Jupiter’s greater mysteries? Then here’s Does Jupiter Have a Solid Core?, Could Jupiter Become a Star?, Could We Live on Jupiter?, and Could We Terraform Jupiter?

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast.

Seeing in Triplicate: Catching a Rare Triple Shadow Transit of Jupiter’s Moons

The planet Jupiter is always fascinating to watch. Not only do surface features pop in and out of existence on its swirling cloud tops, but its super fast rotation — once every 9.9 hours — assures its face changes rapidly. And the motion of its four large Galilean moons is captivating to observe as well. Next week offers a special treat for well-placed observers: a triple shadow transit of the moons Callisto, Europa and Ganymede on the evening of June 3rd.

The view at 19:00 UT/3:00 PM EDT on June 3rd. Credit: Starry Night Education Software.
The view at 19:00 UT/3:00 PM EDT on June 3rd. Credit: Starry Night Education Software.

Now for the bad news: only a small slice of the planet will witness this rare treat in dusk skies. This is because Jupiter starts the month of June 40 degrees east of the Sun and currently sets around 11 PM local, just 3 hours after local sunset. Never fear, though, it may just be possible to spy a portion of this triple transit from North American longitudes with a little careful planning.

The action begins on June 3rd at 15:20 Universal Time as Callisto’s shadow slides on to the disk of Jupiter, to be followed by Europa and Ganymede’s shadow in quick succession hours later. All three shadows are cast back onto the disk of Jupiter from 18:05 to 19:53 UT, favoring European and African longitudes at sunset.  The final shadow, that of Ganymede, moves off the disk of Jupiter at 21:31 UT.

The hemisphere of the Earth facing towards Jupiter from the beginning of the triple shadow transit to the end. the red line marks the day/night terminator. Credit: Stellarium.
The hemisphere of the Earth facing towards Jupiter from the beginning of the triple shadow transit to the end. the red line marks the day/night terminator. Credit: Stellarium.

The following video simulation begins at around 15:00 UT just prior to the ingress of Callisto’s shadow and runs through 22:00 UT:

Triple shadow transits of Jupiter’s moons are fairly rare: the last such event occurred last year on October 12th, 2013 favoring North America and the next won’t occur until January 24th, 2015. Jean Meeus calculated that only 31 such events involving 3 different Jovian moons either transiting Jupiter and/or casting shadows onto its disk occur as seen from Earth between 1981 and 2040. The June 3rd event is also the longest in the same 60 year period studied.

The 1:2:4 orbital resonance of the Jovian moons Io, Europa and Ganymede. Credit: Wikimedia Commons.
The 1:2:4 orbital resonance of the Jovian moons Io, Europa and Ganymede. Credit: Wikimedia Commons.

Can four shadow transits occur at once? Unfortunately, the answer is no. The inner three moons are in a 1:2:4 resonance, meaning that one will always be left out of the picture when two are in front. This also means that Callisto must be included for any triple shadow transit to occur. Next week’s event sees Callisto, Europa and Ganymede crossing in front of Jupiter and casting shadows onto its disk while Io is hidden behind Jupiter in its enormous shadow. Callisto is also the only one of the four large Jovian moons that can “miss” the disk of Jupiter on certain years, owing to the slight inclination of its orbit to the ecliptic. Callisto thus doesn’t always cast a shadow onto the disk of Jupiter, and we’re currently in the middle of a cycle of Callisto shadow transits that started in July of 2013 and runs through July 2016. These “Callisto transit seasons” occur twice during Jupiter’s 11.8 year orbit, and triple shadow transits must also occur within these periods.

So, what’s a North American observer to do? Well, it is possible to spot and track Jupiter with a telescope in the broad daylight. Jupiter rises at around 9:20 AM local in early June, and the waxing crescent Moon passes 5.4 degrees south of it on June 1st. The Moon stands 30 degrees from the planet on June 3rd, and it may be juuusst possible to use it as a guide to the daytime event. A “GoTo” telescope with precise pointing will make this task even easier, allowing you to track Jupiter and the triple shadow transit across the daytime sky from North American longitudes. But be sure to physically block the blazing June Sun behind a building or structure to avoid accidentally catching its blinding glare in the eyepiece!

The orientation of Jupiter the Moon and the Sun at 4PM EDT on June 3rd. Credit: Stellarium.
The orientation of Jupiter, the Moon and the Sun at 4PM EDT on June 3rd. Credit: Stellarium.

Do the shadows of the moons look slightly different to you? A triple shadow transit is a great time to compare them to one another, from the inky hard black dot of the inner moons Europa and Io, to the diffuse large shadow of Callisto. With practice, you can actually identify which moon is casting a shadow during any transit just by its size and appearance!

A study of three multi-shadow transits: last year's (upper left) a double shadow transit from early 2014 (upper right) and 2004 (bottom. Photos by author.
A study of three multi-shadow transits: last year’s (upper left) a double shadow transit from early 2014 (upper right) and 2004 (bottom). Photos by author.

Shadow transits of Jupiter’s moons also played an interesting role in the history of astronomy as well. Danish astronomer Ole Rømer noted that shadow transits were being observed at slightly different times than predicted depending on the distance of Jupiter and the Earth, and made the first rough calculation of the speed of light in 1676 based on this remarkable insight. Celestial navigators were also intrigued for centuries with the idea of using the phenomena of Jupiter’s moons as a natural clock to gauge longitude. It’s a sound idea in theory, though in practice, it proved tough to make accurate observations from the pitching deck of a ship at sea.

Jupiter captured near the daytime Moon. Photo by author.
Jupiter captured near the daytime Moon. Photo by author.

Miss the June 3rd event? There’s still two fine opportunities to see Jupiter do its impression of the Earth-Moon system and appear to have only one satellite – Callisto – on the evenings of May 30th and June 7th.

From there, Jupiter slides lower into the dusk as June progresses and heads towards solar conjunction on July 24th.

Let us know if you manage to catch sight of this rare event!

-Send those shadow transit pics in to Universe Today at our Flickr forum.

New Molecules Detected in Io’s Atmosphere

Io – Jupiter’s innermost Galilean moon – is the most geologically active body in the Solar System. With over 400 active volcanic regions, plumes of sulfur can climb as high as 300 miles above the surface.  It is dotted with more than 100 mountains, some of which are taller than Mount Everest. In between the volcanoes and mountains there are extensive lava flows and floodplains of liquid rock.

Intense volcanic activity leads to a thin atmosphere consisting mainly of sulfur dioxide (SO2), with minor species including sulfur monoxide (SO), sodium chloride (NaCl), and atomic sulfur and oxygen. Despite Io’s close proximity to the Earth the composition of its atmosphere remains poorly constrained. Models predict a variety of other molecules that should be present but have not been observed yet.

Recently a team of astronomers from institutions across the United States, France, and Sweden, set out to better constrain Io’s atmosphere. They detected the second-most abundant isotope of sulfur (34-S) and tentatively detected potassium chloride (KCl). The latter is produced in volcanic plumes – suggesting that these plumes continuously contribute to Io’s atmosphere.

Expected yet undetected molecular species include potassium chloride (KCl), silicone monoxide (SiO), disulfur monoxide (S2O), and various isotopes of sulfur. Most of these elements emit in radio wavelengths.

“Depending on their geometry, some molecules emit at well known frequencies when they change rotational state,” Dr. Arielle Moullet, lead author on the study, told Universe Today. “These spectral features are called rotational lines and show up in the (sub)millimeter spectral range.”

These observations were therefore obtained at the Atacama Pathfinder Experiment (APEX) antenna – a radio telescope located 16,700 feet above sea level in northern Chile. The main dish has a diameter of 12 meters, and is a prototype antenna for the Atacama Large Millimeter Array (ALMA).

The Atacama Pathfinder (APEX) antenna. Image Credit: ESO
The Atacama Pathfinder (APEX) antenna. Image Credit: ESO

Following 16.5 hours of total observation time and months of data reduction and analysis, Moullet et al. made a tentative detection of potassium chloride (KCl). Io’s volcanic ejecta produce a large plasma torus around Jupiter, which inlcudes many molecular species including potassium.  This detection is therefore considered the “missing link” between Io and this plasma torus.

The team also made the first detection of one of Sulfur’s isotopes known as 34-S. Sulfur has 25 known isotopes – variants of sulfur that still have 16 protons but differ in their number of neutrons. 34-S is the second most abundant isotope with 18 neutrons.

Previously, the first-most abundant isotope of sulfur, 32-S with 16 neutrons, had been detected. Surprisingly the ratio between the two (34/32 S) is twice as high as the solar system reference, suggesting that there is an abundance of 34-S. A fraction this high has only been reported before in a distant quasar – an early galaxy consisting of an intensely luminous core powered by a huge black hole.

“This result tells us that there probably is some fractionation process that we haven’t yet identified, which is happening either in the magma, at the surface, or in the atmosphere itself,” explains Dr. Moullet.  Something somewhere is producing an unexplained abundance of this isotope.

Other expected yet undetected molecules including silicone monoxide and disulfur monoxide remain undetected. It is possible that these molecules are simply not present, but more likely that the observations are not sensitive enough to detect them.

“To perform a deeper spectral search with a better sensitivity, our group has been awarded observation time with the Atacama Large Millimeter Array, a cutting edge interferometric facility in Chile, which will eventually include more than fifty 12-meter wide dishes,” explains Dr. Moullet.  “We are in the process of analyzing our first dataset obtained with sixteen antennas, which is already much more sensitive than the APEX data.”

While Io is certainly an extreme example, it will likely help us characterize volcanism in general – providing a better understanding of volcanism here on Earth as well as outside the Solar System.

The paper has been accepted for publication in The Astrophysical Journal and is available for download here.