Check Out NASA’s New Instrument that will Look for Life on Enceladus

Ever since the Cassini mission entered the Saturn system and began studying its moons, Enceladus has become a major source of interest. Once the probe detected plumes of water and organic molecules erupting from the moon’s southern polar region, scientists began to speculate that Enceladus may possess a warm-water ocean in its interior – much like Jupiter’s moon Europa and other bodies in our Solar System.

In the future, NASA hopes to send another mission to this system to further explore these plumes and the interior of Enceladus. This mission will likely include a new instrument that was recently announced by NASA, known as the Submillimeter Enceladus Life Fundamentals Instrument (SELFI). This instrument, which was proposed by a team from the NASA Goddard Space Flight Center, recently received support for further development.

Prior to the Cassini mission, scientists thought that the surface of Enceladus was frozen solid. However, Cassini data revealed a slight wobble in the moon’s orbit that suggested the presence of an interior ocean. Much like Europa, this is caused by tidal forces that cause flexing in the core, which generates enough heat to hold liquid water in the interior. Around the southern pole, this results in the ice cracking open and forming fissures.

Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

The Cassini mission also discovered plumes emanating from about 100 different fissures which continuously spew icy particles, water vapor, carbon dioxide, methane, and other gases into space. To study these more closely, NASA has been developing some ambitious instruments that will rely on millimeter-wave or radio frequency (RF) waves to determine their composition and learn more about Enceladus’ interior ocean.

According to SELFI Principal Investigator Gordon Chin, SELFI represents a significant improving over existing submillimeter-wavelenght devices. Once deployed, it will measure traces of chemicals in the plumes of water and icy parties that periodically emanated from Enceladus’ southern fissures, also known as “Tiger Stripes“.  In addition to revealing the chemical composition of the ocean, this instrument will also indicate it’s potential for supporting life.

On Earth, hydrothermal vents are home to thriving ecosystems, and are even suspected to be the place where life first emerged on Earth. Hence why scientists are so eager to study hydrothermal activity on moons like Enceladus, since these could represent the most likely place to find extra-terrestrial life in our Solar System. As Chin indicated in a NASA press statement:

“Submillimeter wavelengths, which are in the range of very high-frequency radio, give us a way to measure the quantity of many different kinds of molecules in a cold gas. We can scan through all the plumes to see what’s coming out from Enceladus. Water vapor and other molecules can reveal some of the ocean’s chemistry and guide a spacecraft onto the best path to fly through the plumes to make other measurements directly.”

The “Tiger Stripes” of Enceladus, as pictured by the Cassini space probe. Credit: NASA/JPL/ESA

Molecules like water, carbon dioxide and other elements broadcast specific radio frequencies, which submillimeter spectrometers are sensitive to. The spectral lines are very discrete, and the intensity at which they broadcast can be used to quantify their existence. In other words, instruments like SELFI will not only be able to determine the chemical composition of Enceladus’ interior ocean, but also the abundance of those chemicals.

For decades, spectrometers have been used in space sciences to measure the chemical compositions of planets, stars, comets and other targets. Most recently, scientists have been attempting to obtain spectra from distant planets in order to determine the chemical compositions of their atmospheres. This is crucial when it comes to finding potentially-habitable exoplanets, since water vapor, nitrogen and oxygen gas are all required for life as we know it.

Performing scans in the submillimeter band is a relatively new process, though, since submillimeter-sensitive instruments are complex and difficult to build. But with help of NASA research-and-development funding, Chin and his colleagues are increasing the instrument’s sensitivity using an amplifier that will boost the signal to around 557 GHz. This will allow SELFI to detect even minute traces of water and gases coming from the surface of Enceladus.

Other improvements include a more energy-efficient and flexible radio frequency data-processing system, as well as a sophisticated digital spectrometer for the RF signal. This latter improvement will employ high-speed programmable circuitry to convert RF data into digital signals that can be analyzed to measure gas quantities, temperatures, and velocities from Enceladus’ plumes.

Possible spectroscopy results from one of Europa’s water plumes. This is an example of the data the Webb telescope could return. Credit: NASA-GSFC/SVS, Hubble Space Telescope, Stefanie Milam, Geronimo Villanueva

These enhancements will allow SELFI to simultaneously detect and analyze 13 different types of molecules, which include various isotopes of water, methanol, ammonia, ozone, hydrogen peroxide, sulfur dioxide, and sodium chloride (aka. salt). Beyond Enceladus, Chin believes the team can sufficiently improve the instrument for proposed  future missions. “SELFI is really new,”he said. “This is one of the most ambitious submillimeter instruments ever built.”

For instance, in recent years, scientists have spotted plume activity coming from the surface of Europa. Here too, this activity is believed to be the result of geothermal activity, which sends warm water plumes from the moon’s interior ocean to the surface. Already, NASA hopes to examine these plumes and those on Enceladus using the James Webb Space Telescope, which will be deploying in 2019.

Another possibility would be to equip the proposed Europa Clipper – which is set to launch between 2022 and 2025 – with an instrument like SELFI. The instrument package for this probe already calls for a spectrometer, but an improved submillimeter-wave and RF device could allow for a more detailed look at Europa’s plumes. This data could in turn resolve the decades-old debate as to whether or not Europa’s interior is capable of supporting life.

In the coming decades, one of the greatest priorities of space exploration is to investigate the Solar System’s “Ocean Worlds” for signs of life. To see this through, NASA and other space agencies are busy developing the necessary tools to sniff out all the chemical and biological indicators. Within a decade, with any luck, we might just find that life on Earth is not the exception, but part of a larger norm.

Further Reading: NASA

How Do We Terraform Saturn’s Moons?

Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present our guide to terraforming Saturn’s Moons. Beyond the inner Solar System and the Jovian Moons, Saturn has numerous satellites that could be transformed. But should they be?

Around the distant gas giant Saturn lies a system of rings and moons that is unrivaled in terms of beauty. Within this system, there is also enough resources that if humanity were to harness them – i.e. if the issues of transport and infrastructure could be addressed – we would be living in an age a post-scarcity. But on top of that, many of these moons might even be suited to terraforming, where they would be transformed to accommodate human settlers.

As with the case for terraforming Jupiter’s moons, or the terrestrial planets of Mars and Venus, doing so presents many advantages and challenges. At the same time, it presents many moral and ethical dilemmas. And between all of that, terraforming Saturn’s moons would require a massive commitment in time, energy and resources, not to mention reliance on some advanced technologies (some of which haven’t been invented yet).

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

100,000 Ice Blocks Mapped Out at the South Pole … of Enceladus

Ever since the Cassini space probe conducted its first flyby of Enceladus in 2005, the strange Saturnian moon has provided us with a treasure trove of images and scientific wonders. These include the jets of icy water vapor periodically bursting from its south pole, the possibility of an interior ocean – which may even harbor life – and the strange green-blue stripes located around the south pole.

These stripes are essentially four fractures bounded on either side by ridges that appear to be composed of mint-green-colored ice. Known unofficially as “tiger stripes”,  these surface fractures have become a source of interest for astronomers since they appear to be the youngest features in the region.

Recently, between these stripes, over 100,000 ice blocks were observed, and they are a further source of wonder. Scientists with the Division of Geological and Planetary Sciences at the California Institute of Technology were able to map out the locations of these blocks in the hopes of determining just how they got there.

Their findings, which are scheduled to appear in the January 2015 issue of Icarus (vol. 245), constitute the first quantitative estimates of ice-block number and density in Enceladus’ southern polar region in relation to major geological features.

Elevated View of Enceladus' South Pole. Credit: NASA/JPL
Elevated View of Enceladus’ South Pole. Credit: NASA/JPL

The preliminary results of their work reveal that ice blocks in the southern hemisphere are most concentrated within the geologically active South Polar Terrain (SPT) and chiefly concentrated within 20 km of the tiger-stripe fractures. They found further that the ice blocks are concentrated just as heavily between tiger-stripe fractures as on the directly adjacent margins.

To ascertain just how these ice-blocks formed and evolved, and how they came to be distributed in the southern region, the team considered various mechanisms. These included the well-known aspects and features of the moon – namely its seismic activity, impacts by meteors, and volcanic eruptions – but also the possible roles of tectonic disruption of the icy surface mantle and ice slides.

Ultimately, they concluded that impact cratering as well as slides, perhaps triggered by seismic events, could account for a majority of ice-block features within the inner SPT.

However, they also noted that cryovolcanic activity – i.e., the ejection of icy material caused by sub-surface volcanic eruption, and the condensation of ice around the eruption vents – could not be ruled out.

They noted that the pervasiveness of fracturing on many size scales, the sheer number of ice blocks in the inner SPT, and the occurrence of linear block arrangements that parallel crack networks along the flanks of tiger stripes, would seem to indicate that tectonic deformation also played an important role.

Gravity measurements by NASA's Cassini spacecraft and Deep Space Network suggest that Saturn's moon Enceladus, which has jets of water vapor and ice gushing from its south pole, also harbors a large interior ocean beneath an ice shell, as this illustration depicts. Image Credit:  NASA/JPL-Caltech
Artists’ concept of Enceladus’ interior ocean. Image Credit: NASA/JPL-Caltech

Furthermore, they postulated that nearer to the warm tiger-stripe fractures, sublimation likely leads to erosion and disaggregation, which plays a role as well.

Last, they noted that the relative scarcity of blocks beyond the bounds of the SPT, particularly on old, cratered terrains, may be attributed to ice grains accumulating on the surface over time rather than the same causal factors that led to the 100,000 blocks observed around in the southern region.

In short, the CIT team believes that the unusual ice-block formation around Enceladus’ south pole is chiefly the result of impacts from meteors or comets and seismic activity, but that the peculiar activity in this young region of the planet – such as volcanic eruptions from the hypothesized interior-ocean – may also play a role.

The SPT ice-blocks were observed at very high resolution during Cassini’s July 14th flyby, when it observed the “blue ice” tiger stripe” around the south pole and noticed an area of extreme tectonic deformation. The blocks were manually identified and mapped from twenty of the highest resolution photos taken by Cassini’s Imaging Science Subsystem (ISS) and rendered using ArcGIS software.

Recently, other researchers also mapped out the location of 101 geysers in this moon’s south polar region.

Cassini continues to study Enceladus, and in fact the spacecraft conducted its latest flyby of Enceladus today at 15:23 (03:23 pm UTC) and its next scheduled flyby will be taking place on Dec. 19th, 2015, at 17:49 (05:49 pm UTC).

Further Reading: Abstract: Spatial distribution of ice blocks on Enceladus… NASA/JPL

Scientists Discover 101 Geysers Erupting at Saturn’s Intriguing Icy Moon Enceladus

Scientists analyzing the reams of data from NASA’s Cassini orbiter at Saturn have discovered 101 geysers erupting from the intriguing icy moon Enceladus and that the spewing material of liquid water likely originates from an underground sea located beneath the tiny moons ice shell, according to newly published research.

The geysers are composed of tiny icy particles, water vapor and trace amounts of simple organic molecules. They were first sighted in Cassini imagery snapped during flyby’s of the 310-mile-wide (500 kilometers wide) moon back in 2005 and immediately thrust Enceladus forward as a potential abode for alien life beyond Earth and prime scientific inquisition.

Liquid water, organic molecules and an energy source are the key requirements for life as we know it.

The eruptions emanated from a previously unknown network of four prominent “tiger stripe” fractures, named Damascus, Baghdad, Cairo and Alexandria sulci, located at the south polar region of Saturn’s sixth largest moon.

Using imagery gathered over nearly seven years of surveys by Cassini’s cameras, researchers generated a survey map of the 101 geysers erupting from the four tiger strips.

This artist's rendering shows a cross-section of the ice shell immediately beneath one of Enceladus' geyser-active fractures, illustrating the physical and thermal structure and the processes ongoing below and at the surface.  Image Credit:  NASA/JPL-Caltech/Space Science Institute
This artist’s rendering shows a cross-section of the ice shell immediately beneath one of Enceladus’ geyser-active fractures, illustrating the physical and thermal structure and the processes ongoing below and at the surface. Image Credit: NASA/JPL-Caltech/Space Science Institute

The new findings and theories on the physical nature of how the geysers erupt have been published in two articles in the current online edition of the Astronomical Journal.

Scientists had initially postulated that the origin of the geysers could be frictional heating generated from back and forth rubbing of the opposing walls of the tiger stripe fractures that converted water ice into liquids and vapors. Another theory held that the opening and closing of the fractures allowed water vapor from below to reach the surface.

The geysers locations was eventually determined to coincide with small local hot spots erupting from one of the tiger stripe fractures after researchers compared low resolution thermal emission maps with the geysers’ locations and found the greatest activity at the warmest spots.

After later high-resolution data was collected in 2010 by Cassini’s heat-sensing instruments the geysers were found to coincide with small-scale hot spots, measuring only a few dozen feet (or tens of meters) across.

“Once we had these results in hand we knew right away heat was not causing the geysers, but vice versa,” said Carolyn Porco, leader of the Cassini imaging team from the Space Science Institute in Boulder, Colorado, and lead author of the first paper. “It also told us the geysers are not a near-surface phenomenon, but have much deeper roots.”

This graphic shows a 3-D model of 98 geysers whose source locations and tilts were found in a Cassini imaging survey of Enceladus' south polar terrain by the method of triangulation. While some jets are strongly tilted, it is clear the jets on average lie in four distinct "planes" that are normal to the surface at their source location. Image credit: NASA/JPL-Caltech/Space Science Institute
This graphic shows a 3-D model of 98 geysers whose source locations and tilts were found in a Cassini imaging survey of Enceladus’ south polar terrain by the method of triangulation. While some jets are strongly tilted, it is clear the jets on average lie in four distinct “planes” that are normal to the surface at their source location. Image credit: NASA/JPL-Caltech/Space Science Institute

“Thanks to recent analysis of Cassini gravity data, the researchers concluded the only plausible source of the material forming the geysers is the sea now known to exist beneath the ice shell. They also found that narrow pathways through the ice shell can remain open from the sea all the way to the surface, if filled with liquid water,” according to a NASA press release.

These are very exciting results in the search for life beyond Earth and clearly warrant a follow up mission.

“In casting your sights on the geysering glory of Enceladus, you are looking at frozen mist that originates deep within the solar system’s most accessible habitable zone,” writes Porco in her Captain’s Log summary of the new findings.

Surveyor's Map of Enceladus' Geyser Basin - On this polar stereographic map of Enceladus' south polar terrain, all 100 geysers have been plotted whose source locations have been determined in Cassini's imaging survey of the moon's geyser basin. Credit: NASA/JPL-Caltech/SSI
Surveyor’s Map of Enceladus’ Geyser Basin – On this polar stereographic map of Enceladus’ south polar terrain, all 101 geysers have been plotted whose source locations have been determined in Cassini’s imaging survey of the moon’s geyser basin. Credit: NASA/JPL-Caltech/SSI

The Cassini-Huygens mission is a cooperative project between NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI). Cassini was launched by a Titan IV rocket in 1997 and arrived at Saturn in 2004.

In 2005 Cassini deployed the Huygens probe which landed on Titan, Saturn’s largest moon sporting oceans of organic molecules and another prime location in the search for life.

The Cassini mission will conclude in 2017 with an intentional suicide dive into Saturn to prevent contamination on Titan and Enceladus – but lots more breathtaking science will be accomplished in the meantime!

Stay tuned here for Ken’s Earth & Planetary science and human spaceflight news.

Ken Kremer

How Many Moons Does Saturn Have?

Saturn is well known for being a gas giant, and for its impressive ring system. But would it surprise you to know that this planet also has the second-most moons in the Solar System, second only to Jupiter? Yes, Saturn has at least 150 moons and moonlets in total, though only 62 have confirmed orbits and only 53 have been given official names.

Most of these moons are small, icy bodies that are little more than parts of its impressive ring system. In fact, 34 of the moons that have been named are less than 10 km in diameter while another 14 are 10 to 50 km in diameter. However, some of its inner and outer moons are among the largest and most dramatic in the Solar System, measuring between 250 and 5000 km in diameter and housing some of greatest mysteries in the Solar System.

Saturn’s moons have such a variety of environments between them that you’d be forgiven for wanting to spend an entire mission just looking at its satellites. From the orange and hazy Titan to the icy plumes emanating from Enceladus, studying Saturn’s system gives us plenty of things to think about. Not only that, the moon discoveries keep on coming. As of April 2014, there are 62 known satellites of Saturn (excluding its spectacular rings, of course). Fifty-three of those worlds are named.

The Cassini spacecraft observes three of Saturn's moons set against the darkened night side of the planet. Credit: NASA/JPL/Space Science Institute
The Cassini spacecraft observes three of Saturn’s moons set against the darkened night side of the planet. Credit: NASA/JPL/Space Science Institute

Discovery and Naming:

Prior to the invention of telescopic photography,  eight of Saturn’s moons were observed using simple telescopes. The first to be discovered was Titan, Saturn’s largest moon, which was observed by Christiaan Huygens in 1655 using a telescope of his own design. Between 1671 and 1684, Giovanni Domenico Cassini discovered the moons of Tethys, Dione, Rhea and Iapetus – which he collectively named the “Sider Lodoicea” (Latin for “Louisian Stars”, after King Louis XIV of France).

n 1789, William Herschel discovered Mimas and Enceladus, while father-and-son astronomers W.C Bond and G.P. Bond discovered Hyperion in 1848 – which was independently discovered by William Lassell that same year. By the end of the 19th century, the invention of long-exposure photographic plates allowed for the discovery of more moons – the first of which Phoebe, observed in 1899 by W.H. Pickering.

In 1966, the tenth satellite of Saturn was discovered by French astronomer Audouin Dollfus, which was later named Janus. A few years later, it was realized that his observations could only be explained if another satellite had been present with an orbit similar to that of Janus. This eleventh moon was later named Epimetheus, which shares the same orbit with Janus and is the only known co-orbital in the Solar System.

Saturn and its moons. Image credit: NASA/JPL/SSI
Collage of Saturn and its largest moons. Credit: NASA/JPL/SSI

By 1980, three additional moons were discovered and later confirmed by the Voyager probes. They were the trojan moons (see below) of Helene (which orbits Dione) as well as Telesto and Calypso (which orbit Tethys).

The study of the outer planets has since been revolutionized by the use of unmanned space probes. This began with the arrival of the Voyager spacecraft to the Cronian system in 1980-81, which resulted in the discovery of three additional moons – Atlas, Prometheus, and Pandora – bringing the total to 17. By 1990, archived images also revealed the existence of Pan.

This was followed by the Cassini-Huygens mission, which arrived at Saturn in the summer of 2004. Initially, Cassini discovered three small inner moons, including Methone and Pallene between Mimas and Enceladus, as well as the second Lagrangian moon of Dione – Polydeuces. In November of 2004, Cassini scientists announced that several more moons must be orbiting within Saturn’s rings. From this data, multiple moonlets and the moons of Daphnis and Anthe have been confirmed.

The study of Saturn’s moons has also been aided by the introduction of digital charge-coupled devices, which replaced photographic plates by the end of the 20th century. Because of this, ground-based telescopes have begun to discovered several new irregular moons around Saturn. In 2000, three medium-sized telescopes found thirteen new moons with eccentric orbits that were of considerable distance from the planet.

The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Some small moons are also shown. All to scale. Credit: NASA/JPL/Space Science Institute
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute

In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons. In 2006, astronomers using Japan’s Subaru Telescope at Mauna Kea reported the discovery of nine more irregular moons. In April of 2007, Tarqeq (S/2007 S 1) was announced, and in May of that same year, S/2007 S 2 and S/2007 S 3 were reported.

The modern names of Saturn’s moons were suggested by John Herschel (William Herschel’s son) in 1847. In keeping with the nomenclature of the other planets, he proposed they be named after mythological figures associated with the Roman god of agriculture and harvest – Saturn, the equivalent of the Greek Cronus. In particular, the seven known satellites were named after Titans, Titanesses and Giants – the brothers and sisters of Cronus.

In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. When in the 20th century, the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology, or giants from other mythologies. All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and Norse ice giants.

Saturn’s Inner Large Moons:

Saturn’s moons 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.

Enceladus. Credit: NASA/JPL/Space Science Institute
Saturn’s moon of Enceladus. Credit: NASA/JPL/Space Science Institute

Saturn’s Inner Large Moons, which orbit within the E Ring (see below), include 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. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.

Some people jokingly call Mimas the “Death Star” moon because of the crater in its surface that resembles the machine from the Star Wars universe. The 140 km (88 mi) Herschel Crater is about a third the diameter of the moon itself, and could have created fractures (chasmata) on the moon’s opposing side. There are in fact craters throughout the moon’s small surface, making it among the most pockmarked in the Solar System.

Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 km and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved and roughly parallel faults within the moon’s southern polar latitudes.

Large geysers have also been observed in the southern polar region that periodically release plumes of water ice, gas and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath it’s icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core.

Dione's trailing hemisphere, showing the patches of "whispy terrain". Credit: NASA/JPL
Dione’s trailing hemisphere, showing the patches of “whispy terrain”. Credit: NASA/JPL

The moon has at least five different kinds of terrain, a “young” geological surface of less than 100 million years. With a geometrical albedo of more than 140%, which is due to it being composed largely of water ice, Enceladus is one of the brightest known objects in the Solar System.

At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep and 2,000 km long.

With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.

It’s covered in canyons, cracking and craters and is coated from dust in the E-ring that originally came from Enceladus. The location of this dust has led astronomers to theorize that the moon was spun about 180 degrees from its original disposition in the past, perhaps due to a large impact.

Saturn’s Large Outer Moons:

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. Of these, Rhea is the second largest – measuring 1,527 km in diameter and 23×1020 kg in mass – and the ninth largest moon of the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons, and takes 4.5 days to complete an orbit.

Views of Saturn's moon Rhea. Credit: NASA/JPL/Space Science Institute
Views of Saturn’s moon Rhea. Credit: NASA/JPL/Space Science Institute

Like other Cronian satellites, Rhea has a rather heavily cratered surface, and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and an as-yet unnamed crater – that measure 400 and 500 km across, respectively.

Rhea has at least two major sections, the first being bright craters with craters larger than 40 km (25 miles), and a second section with smaller craters. The difference in these features are believed to be evidence of a major resurfacing event at some time in Rhea’s past.

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 cryovolcanoes, 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.

ASA's Cassini spacecraft looks toward the night side of Saturn's largest moon and sees sunlight scattering through the periphery of Titan's atmosphere and forming a ring of color. Credit: NASA/JPL-Caltech/Space Science Institute
Image of Titan’s taken by the Cassini spacecraft, showing light passing through the periphery of the moon’s atmosphere. Credit: NASA/JPL-Caltech/Space Science Institute

Titan is also distinguished for being the only Cronian moon that has ever had a probe land on it. This was the Huygens lander, which was carried to the hazy world by the Cassini spacecraft. Titan’s “Earth-like processes” and thick atmosphere are among the things that make this world stand out to scientists, which include its ethane and methane rains from the atmsophere and flows on the surface.

With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn, and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.

Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge).  The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.

At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons, and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.

The two sides of Iapetus. Credit: NASA/JPL
The two sides of Iapetus, Saturn’s “yin-yang moon”. Credit: NASA/JPL

Saturn’s Irregular Moons:

Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large-radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.

The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.

The Gallic group are a group of four prograde outer moons named for characters in Gallic mythology -Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.

Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second largest, Ymir, measures 18 km across.

Saturns rings and moons Credit: NASA
Saturn’s rings and moons Credit: NASA

Within the Inner and Outer Large Moons, there are also those belonging to Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn.  Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.

Moon Formation:

It is thought that Saturn’s moon of Titan, its mid-sized moons and rings developed in a way that is closer to the Galilean moons of Jupiter. In short, this would mean that the regular moons formed from a circumplanetary disc, a ring of accreting gas and solid debris similar to a protoplanetary disc. Meanwhile, the outer, irregular moons are believed to have been objects that were captured by Saturn’s gravity and remained in distant orbits.

However, there are some variations on this theory. In one alternative scenario, two Titan-sized moons were formed from an accretion disc around Saturn; the second one eventually breaking up to produce the rings and inner mid-sized moons. In another, two large moons fused together to form Titan, and the collision scattered icy debris that formed to create the mid-sized moons.

However, the mechanics of how the moon’s formed remains a mystery for the time being. With additional missions mounted to study the atmospheres, compositions and surfaces of these moons, we may begin to understand where they truly came from.

Much like Jupiter, and all the other gas giants, Saturn’s system of satellites is extensive as it is impressive. In addition to the larger moons that are believed to have formed from a massive debris field that once orbited it, it also has countless smaller satellites that were captured by its gravitational field over the course of billions of years. One can only imagine how many more remain to be found orbiting the ringed giant.

We have many great articles on Saturn and its moon’s here at Universe Today. For example, here’s How Many Moons Does Saturn Have? and Is Saturn Making a New Moon?

Here’s an article about the discovery of Saturn’s 60th moon, and another article about how Saturn’s moons could be creating new rings.

Want more information about Saturn’s moons? Check out NASA’s Cassini information on the moons of Saturn, and more from NASA’s Solar System Exploration site.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.


Enceladus’ Jets Reach All the Way to its Sea

Thanks to the Cassini mission we’ve known about the jets of icy brine spraying from the south pole of Saturn’s moon Enceladus for about 8 years now, but this week it was revealed at the 44th Lunar and Planetary Science Conference outside Houston, Texas that Enceladus’ jets very likely reach all the way down to the sea — a salty subsurface sea of liquid water that’s thought to lie beneath nearly 10 kilometers of ice.

Enceladus’ jets were first observed by the Cassini spacecraft in 2005. The jets constantly spray fine particles of ice into space which enter orbit around Saturn, creating the hazy, diffuse E ring in which Enceladus resides.

Emanating from deep fissures nicknamed “tiger stripes” that gouge the 512-km (318-mile) -wide moon’s south pole the icy jets — and the stripes — have been repeatedly investigated by Cassini, which has discovered that not only do the ice particles contain salts and organic compounds but also that the stripes are surprisingly warm, measuring at 180 Kelvin (minus 135 degrees Fahrenheit) — over twice as warm as most other regions of the moon.

Read more: Enceladus’ Salty Surprise

Where the jets are getting their supply of liquid water has been a question scientists have puzzled over for years. Is friction caused by tidal stresses heating the insides of the stripes, which melts the ice and shoots it upwards? Or do the fissures actually extend all the way down through Enceladus’ crust to a subsurface ocean of liquid water, and through tidal pressure pull vapor and ice up to the surface?

"Baghdad Sulcus," one of many tiger stripe fissures on Enceladus (NASA/JPL/SSI)
“Baghdad Sulcus,” one of many tiger stripe fissures on Enceladus (NASA/JPL/SSI)

Researchers are now confident that the latter is the case.

In a presentation at the Lunar and Planetary Science Conference titled “How the Jets, Heat and Tidal Stresses across the South Polar Terrain of Enceladus Are Related” (see the PDF here) Cassini scientists note that the amount of heating due to tidal stress seen along Enceladus’ tiger stripes isn’t nearly enough to cause the full spectrum of heating observed, and the “hot spots” that have been seen don’t correlate with the type of heating caused by shear friction.

Instead, the researchers believe that heat energy is being carried upwards along with the pressurized water vapor from the subsurface sea, warming the areas around individual vents as well as serving to keep their channels open.

With 98 individual jets observed so far on Enceladus’ south polar terrain and surface heating corresponding to each one, this scenario, for lack of a better term… seems legit.

What this means is that not only does a moon of Saturn have a considerable subsurface ocean of liquid water with a heat source and Earthlike salinity (and also a bit of fizz) but also that it’s spraying that ocean, that potentially habitable environment, out into local space where it can be studied relatively easily — making Enceladus a very intriguing target for future exploration.

“To touch the jets of Enceladus is to touch the most accessible salty, organic-rich, extraterrestrial body of water and, hence, habitable zone, in our solar system.”

– Cassini imaging team leader Carolyn Porco et al.

Enceladus is actively spraying its habitable zone out into space (NASA/JPL/SSI)
Enceladus is actively spraying its habitable zone out into space (NASA/JPL/SSI)

Research notes via C. Porco, D. DiNino, F. Nimmo, CICLOPS, Space Science Institute at Boulder, CO, and Earth and Planetary Sciences at UC Santa Cruz, CA.

Top image: color-composite of Enceladus made from raw Cassini images acquired in 2010. The moon is lit by reflected light from Saturn while the jets are backlit by the Sun.