While NASA’s much-lauded Space Launch System stands ready for its maiden flight later this month with the goal of sending astronauts back to the Moon in the next few years, our gazes once again turn to the stars as we continue to ask the question that has plagued humankind since time immemorial: Are we alone? While there are several solar system locales that we can choose from to conduct our search for life beyond Earth, to include Mars and Saturn’s moons, Titan and Enceladus, one planetary body orbiting the largest planet in the solar system has peaked the interest of scientists since the 1970s.
In a recent study published in the Journal of High Energy Physics, two researchers from Brown University demonstrated how data from past missions to Jupiter can help scientists examine dark matter, one of the most mysterious phenomena in the universe. The reason past Jupiter missions were chosen is due to the extensive amount of data gathered about the largest planet in the solar system, most notably from the Galileo and Juno orbiters. The elusive nature and composition of dark matter continues to elude scientists, both figuratively and literally, because it does not emit any light. So why do scientists continue to study this mysterious—and completely invisible—phenomena?
This artist’s conception shows how double ridges on the surface of Jupiter’s moon Europa may form over shallow, refreezing water pockets within the ice shell. This mechanism is based on the study of an analogous double ridge feature found on Earth’s Greenland Ice Sheet. (Image credit: Justice Blaine Wainwright)
Beneath the surface of Jupiter’s icy moon Europa, there’s an ocean up to 100 km (62 mi) deep that has two to three times the volume of every ocean on Earth combined. Even more exciting is how this ocean is subject to hydrothermal activity, which means it may have all the necessary ingredients for life. Because of this, Europa is considered one of the most likely places for extraterrestrial life (beyond Mars). Hence, mission planners and astrobiologists are eager to send a mission there to study it closer.
Unfortunately, Europa’s icy surface makes the possibility of sampling this ocean rather difficult. According to the two predominant models for Europa’s structure, the ice sheet could be a few hundred meters to several dozen kilometers thick. Luckily, new research by a team from Stanford University has shown that Europa’s icy shell may have an abundance of water pockets inside, as indicated by features on the surface that look remarkably like icy ridges here on Earth.
Tara Regio is the yellowish area to left of center, in this NASA Galileo image of Europa’s surface. This region of geologic chaos is the area researchers identified an abundance of sodium chloride. Image Credit: NASA/JPL/University of Arizona
Jupiter’s moon Europa is an intriguing world. It’s the smoothest body in the Solar System, and the sixth-largest moon in the Solar System, though it’s the smallest of the four Galilean moons. Most intriguing of all is Europa’s subsurface ocean and the potential for habitability.
Artist’s illustration of Jupiter and Europa (in the foreground) with the Galileo spacecraft after its pass through a plume erupting from Europa’s surface. Credits: NASA/JPL-Caltech/Univ. of Michigan
Jupiter’s moon Europa continues to fascinate and amaze! In 1979, the Voyager missions provided the first indications that an interior ocean might exist beneath it’s icy surface. Between 1995 and 2003, the Galileo spaceprobe provided the most detailed information to date on Jupiter’s moons to date. This information bolstered theories about how life could exist in a warm water ocean located at the core-mantle boundary.
Even though the Galileo mission ended when the probe crashed into Jupiter’s atmosphere, the spaceprobe is still providing vital information on Europa. After analyzing old data from the mission, NASA scientists have found independent evidence that Europa’s interior ocean is venting plumes of water vapor from its surface. This is good news for future mission to Europa, which will attempt to search these plumes for signs of life.
The study which describes their findings, titled “Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures“, recently appeared in the journal Nature Astronomy. The study was led by Xianzhe Jia, a space physicist from the Department of Climate and Space Sciences and Engineering at the University of Michigan, and included members from UCLA and the University of Iowa.
Artist’s concept of the Galileo space probe passing through the Jupiter system. Credit: NASA
The data was collected in 1997 by Galileo during a flyby of Europa that brought it to within 200 km (124 mi) of the moon’s surface. At the time, its Magnetometer (MAG) sensor detected a brief, localized bend in Jupiter’s magnetic field, which remained unexplained until now. After running the data through new and advanced computer models, the team was able to create a simulation that showed that this was caused by interaction between the magnetic field and one of the Europa’s plumes.
This analysis confirmed ultraviolet observations made by NASA’s Hubble Space Telescope in 2012, which suggested the presence of water plumes on the moon’s surface. However, this new analysis used data collected much closer to the source, which indicated how Europa’s plumes interact with the ambient flow of plasma contained within Jupiter’s powerful magnetic field.
In addition to being the lead author on this study, Jia is also the co-investigator for two instruments that will travel aboard the Europa Clipper mission – which may launch as soon as 2022 to explore the moon’s potential habitability. Jia’s and his colleagues were inspired to reexamine data from the Galileo mission thanks to Melissa McGrath, a member of the SETI Institute and also a member of the Europa Clipper science team.
During a presentation to her fellow team scientists, McGrath highlighted other Hubble observations of Europa. As Jiang explained in a recent NASA press release:
“The data were there, but we needed sophisticated modeling to make sense of the observation. One of the locations she mentioned rang a bell. Galileo actually did a flyby of that location, and it was the closest one we ever had. We realized we had to go back. We needed to see whether there was anything in the data that could tell us whether or not there was a plume.”
Artist’s impression of a water vapor plume on Europa. Credit: NASA/ESA/K. Retherford/SWRI
When they first examined the information 21 years ago, the high-resolution data obtained by the MAG instrument showed something strange. But it was thanks to the lessons provided by the Cassini mission, which explored the plumes on Saturn’s moon Enceladus, that the team knew what to look for. This included material from the plumes which became ionized by the gas giant’s magnetosphere, leaving a characteristic blip in the magnetic field.
After reexamining the data, they found that the same characteristic bend (localized and brief) in the magnetic field was present around Europa. Jia’s team also consulted data from Galileo’sPlasma Wave Spectrometer (PWS) instrument to measure plasma waves caused by charged particles in gases around Europa’s atmosphere, which also appeared to back the theory of a plume.
This magnetometry data and plasma wave signatures were then layered into new 3D modeling developed by the team at the University of Michigan (which simulated the interactions of plasma with Solar system bodies). Last, they added the data obtained from Hubble in 2012 that suggested the dimensions of the potential plumes. The end result was a simulated plume that matched the magnetic field and plasma signatures they saw in the Galileo data.
As Robert Pappalardo, a Europa Clipper project scientist at NASA’s Jet Propulsion Laboratory (JPL), indicated:
“There now seem to be too many lines of evidence to dismiss plumes at Europa. This result makes the plumes seem to be much more real and, for me, is a tipping point. These are no longer uncertain blips on a faraway image.”
Artist’s concept of a Europa Clipper mission, which will study Europa in 2022-2025 to search for signs of life. Credit: NASA/JPL
The findings are certainly good news for the Europa Clipper mission, which is expected to make the journey to Jupiter between 2022 and 2025. When this probe arrives in the Jovian system, it will establish an orbit around Jupiter and conduct rapid, low-altitude flybys of Europa. Assuming that plume activity does take place on the surface of the moon, the Europa Clipper will sample the frozen liquid and dust particles for signs of life.
“If plumes exist, and we can directly sample what’s coming from the interior of Europa, then we can more easily get at whether Europa has the ingredients for life,” Pappalardo said. “That’s what the mission is after. That’s the big picture.”
At present, the mission team is busy looking at potential orbital paths for the Europa Clipper mission. With this new research in hand, the team will choose a path that will take the spaceprobe above the plume locations so that it is in an ideal position to search them for signs of life. If all goes as planned, the Europa Clipper could be the first of several probes that finally proves that there is life beyond Earth.
And be sure to check out this video of the Europa Clipper mission, courtesy of NASA:
Jupiter’s south pole. captured by the JunoCam on Feb. 2, 2017, from an altitude of about 62,800 miles (101,000 kilometers) above the cloud tops.
Credits: NASA/JPL-Caltech/SwRI/MSSS/John Landino
On July 4th, 2016, the Juno mission established orbit around Jupiter, becoming the second spacecraft in history to do so (after the Galileo probe). Since then, the probe has been in a regular 53.4-day orbit (known as perijove), moving between the poles to avoid the worst of its radiation belts. Originally, Juno’s mission scientists had been hoping to reduce its orbit to a 14-day cycle so the probe could make more passes to gather more data.
To do this, Juno was scheduled for an engine burn on Oct. 19th, 2016, during its second perijovian maneuver. Unfortunately, a technical error prevented this from happening. Ever since, the mission team has been pouring over mission data to determine what went wrong and if they could conduct an engine burn at a later date. However, the mission team has now concluded that this won’t be possible.
The technical glitch which prevented the firing took place weeks before the engine burn was scheduled to take place, and was traced to two of the engines helium check valves. After the propulsion system was pressurized, the valves took several minutes to open – whereas they took only seconds during previous engine burns. Because of this, the mission leaders chose to postpone the firing until they could get a better understanding of why the glitch happened.
This amateur-processed image was taken on Dec. 11th, 2016, at 9:27 a.m. PST (12:27 p.m. EST), as NASA’s Juno spacecraft performed its third close flyby of Jupiter. Credits: NASA/JPL-Caltech/SwRI/MSSS/Eric Jorgensen
And after pouring over mission data from the past few months and performing calculations on possible maneuvers, Juno’s science team came to the conclusion that an engine burn might be counter-productive at this point. As Rick Nybakken, the Juno project manager at NASA’s Jet Propulsion Laboratory (JPL), explained in a recent NASA press release:
“During a thorough review, we looked at multiple scenarios that would place Juno in a shorter-period orbit, but there was concern that another main engine burn could result in a less-than-desirable orbit. The bottom line is a burn represented a risk to completion of Juno’s science objectives.”
However, this is not exactly bad news for the mission. It’s current perijove orbit takes it from one pole to the other, allowing it to pass over the cloud tops at a distance of around 4,100 km (2,600 mi) at its closest. At its farthest, the spacecraft reaches a distance of 8.1 million km (5.0 million mi) from the gas giant, which places it far beyond the orbit of Callisto.
During each pass, the probe is able to peak beneath the thick clouds to learn more about the planet’s atmosphere, internal structure, magnetosphere, and formation. And while a 14-day orbital period would allow for it to conduct 37 orbits before its mission is scheduled to wrap up, its current 53.4-day period will allow for more information to be collected on each pass.
And as Thomas Zurbuchen, the associate administrator for NASA’s Science Mission Directorate in Washington, declared:
“Juno is healthy, its science instruments are fully operational, and the data and images we’ve received are nothing short of amazing. The decision to forego the burn is the right thing to do – preserving a valuable asset so that Juno can continue its exciting journey of discovery.”
In the meantime, the Juno science team is still analyzing the returns from Juno’s four previous flybys – which took place on August 27th, October 19th, December 11th, and February 2nd, 2017, respectively. With each pass, more information is revealed about the planet’s magnetic fields, aurorae, and banded appearance. The next perijovian maneuver will take place on March 27th, 2017, and will result in more images and data being collected.
Before the mission concludes, the Juno spacecraft will also explore Jupiter’s far magnetotail, its southern magnetosphere, and its magnetopause. The mission is also conducting an outreach program with its JunoCam, which is being guided with assistance of the public. Not only can people vote on which features they want imaged with every flyby, but these images are accessible to “citizen scientists” and amateur astronomers.
Under its current budget plan, Juno will continue to operate through to July 2018, conducting a total of 12 science orbits. At this point, barring a mission extension, the probe will be de-orbited and burn up in Jupiter’s outer atmosphere. As with the Galileo spacecraft, this will be as to avoid any possibility of impact and biological contamination with one of Jupiter’s moons.
NASA's Juno spacecraft launched on August 6, 2011 and should arrive at Jupiter on July 4, 2016. Credit: NASA / JPL
Ever since Galileo first observed it through a telescope in 1610, Jupiter and its system of moons have fascinated humanity. And while many spacecraft have visited the system in the past forty years, the majority of these missions were flybys. With the exception of the Galileo space probe, the visits of these spacecraft to the Jupiter system were one of several intended objectives, taking place before they made their way deeper into the Solar System.
Having launched on August 5th, 2011, NASA’s Juno spacecraft has a different purpose in mind. Using a suite of scientific instruments, Juno will study Jupiter’s atmosphere, magnetic environment, weather patterns, and shed light on the history of its formation. In essence, it will be the first probe since the Galileo mission to orbit Jupiter, where it will spend the next two years sending information about the gas giant back to Earth.
If successful, Juno will prove to be the only other long-term mission to Jupiter. However, compared to Galileo – which spent seven years in orbit around the gas giant – Juno’s mission is planned to last for just two years. However, its improved suite of instruments are expected to provide a wealth of information in that time. And barring any mission extensions, its targeted impact on the surface of Jupiter will take place in February of 2018.
Juno will dive between the planet and its intense belts of charged particle radiation, coming within 5,000 kilometers (about 3,000 miles) from the cloud tops. Credit: NASA/JPL-Caltech
Background:
As part of the NASA’s New Frontiers program, the Juno mission is one of several medium-sized missions intended to explore the various bodies of the Solar System. It is currently one of three probes that NASA is operating, or in the process of building. The other two are the New Horizons probe (which flew by Pluto in 2015) and OSIRIS-REx, which is expected to fly to asteroid 101955 Bennu in 2020 and bring samples back to Earth.
During a 2003 decadal survey – titled “New Frontiers in the Solar System: An Integrated Exploration Strategy” – The National Research Council discussed destinations that would serve as the source for the first competition for the New Frontiers program. A Jupiter orbiter was identified as a scientific priority, which it was hoped would address several unanswered questions pertaining to the gas giant.
These included whether or not Jupiter had a central core (the research of which would help establish how the planet was formed), the water content of Jupiter’s atmosphere, how its weather systems can remain stable, and what the nature of the magnetic field and plasma surrounding Jupiter are. In 2005, Juno was selected for the New Frontiers program alongside New Horizons and OSIRIS-REx.
Though it was originally intended to launch in 2009, NASA budget restrictions forced a delay until August of 2011. The probe was named in honor of the Roman goddess Juno, the wife of Jupiter (the Roman equivalent of Zeus) who was able to peer through a veil of clouds that Jupiter drew around himself. The name was previously a backronym which stood for JUpiter Near-polar Orbiter as well.
Mission Profile:
The Juno mission was created for the specific purpose of studying Jupiter for the sake of learning more about the formation of the Solar System. For some time, astronomers have understood that Jupiter played an important role in the development Solar System. Like the other gas giants, it was assembled during the early stages, before our Sun had the chance to absorb or blow away the light gases in the huge cloud from which they were born.
As such, Jupiter’s composition could tell us much about the early Solar System. Similarly, the gas giants are believed to have played a major role in the process of planet formation because their huge masses allowed them to shape the orbits of other objects – planets, asteroids and comets – in their planetary systems.
However, for astronomers and planetary scientists, much still remains unknown about this massive gas giant. For instance, Jupiter’s interior structure and composition, as well as what drives its magnetic field, are still the subject of theory. Because Jupiter formed at the same time as the Sun, their chemical compositions should be similar, but research has shown that Jupiter has more heavy elements than our Sun (such as carbon and nitrogen).
In addition, there are some unanswered questions about when and where the planet formed. While it may have formed in its current orbit, some evidence suggests that it could have formed farther from the sun before migrating inward. All of these questions, it is hoped, are things the Juno mission will answer.
Having launched on August 5th, 2011, the Juno spacecraft spent the next five years in space, and will reach Jupiter on July 4th, 2018. Once in orbit, it will spend the next two years orbiting the planet a total of 37 times from pole to pole, using its scientific instruments to probe beneath the gas giant’s obscuring cloud cover.
Instrumentation:
The Juno spacecraft comes equipped with a scientific suite of 8 instruments that will allow it to study Jupiter’s atmosphere, magnetic and gravitational field, weather patterns, its internal structure, and its formational history. They include:
Gravity Science: Using radio waves and measuring them for Doppler effect, this instrument will measure the distribution of mass inside Jupiter to create a gravity map. Small variations in gravity along the orbital path of the probe will induce small changes in velocity. The principle investigators of this instrument are John Anderson of NASA’s Jet Propulsion Laboratory and Luciano Iess of the Sapienza University of Rome.
JunoCam: This visible light/telescope is the spacecraft’s only imaging device. Intended for public outreach and education, it will provide breathtaking pictures of Jupiter and the Solar System, but will operate for only seven orbits around Jupiter (due to the effect Jupiter’s radiation and magnetic field have on instruments). The PI for this instrument is Michael C. Malin, of Malin Space Science Systems
Jovian Auroral Distribution Experiment (JADE): Using three energetic particle detectors, the JADE instrument will measure the angular distribution, energy, and velocity vector of low energy ions and electrons in the auroras of Jupiter. The PI is David McComas of the Southwest Research Institute (SwRI).
Jovian Energetic Particle Detector Instrument (JEDI): Like JADE, JEDI will measure the angular distribution and the velocity vector of ions and electrons, but at high-energy and in the magnetosphere of Jupiter. The PI is Barry Mauk of NASA’s Applied Physics Laboratory.
Juno spacecraft and its science instruments. Credit: NASA/JPL
Jovian Infrared Aural Mapper (JIRAM): Operating in the near-infrared, this spectrometer will be responsible for mapping the upper layers of Jupiter’s atmosphere. By measuring the heat that is radiated outward, it will determine how water-rich clouds can float beneath the surface. It will also be able to assess the distribution of methane, water vapor, ammonia and phosphine in Jupiter’s atmosphere. Angioletta Coradini of the Italian National Institute for Astrophysics is the PI on this instrument.
Magnetometer: This instrument will be used to map Jupiter’s magnetic field, determine the dynamics of the planet’s interior and determine the three-dimensional structure of the polar magnetosphere. Jack Connemey of NASA’s Goddard Space Flight Center is the instrument’s PI.
Microwave Radiometer: The MR instrument will perform measurements of the electromagnetic waves that pass through the Jovian atmosphere, measuring the abundance of water and ammonia in its deep layers. In so doing, it will obtain a temperature profile at various levels and determine how deep the atmospheric circulation of Jupiter is. The PI for this instrument is Mike Janssen of the JPL.
Radio and Plasma Wave Sensor (RPWS): This RPWS will measure the radio and plasma spectra in Jupiter’s auroral region. In the process, it will identify the regions of auroral currents that define the planet’s radio emissions and accelerate its auroral particles. William Kurth of the University of Iowa is the PI.
Ultraviolet Imaging Spectrograph (UVS): The UVS will record the wavelength, position and arrival time of detected ultraviolet photons, providing spectral images of the UV auroral emissions in the polar magnetosphere. G. Randall Gladstone of the SwRI is the PI.
In addition to its scientific suite, the Juno spacecraft also carries a commemorative plaque dedicated to Galileo Galilei. The plaque was provided by the Italian Space Agency and depicts a portrait of Galileo, as well as script that had been composed by Galileo himself on the occasion that he observed Jupiter’s four largest moons (known today as the Galilean Moons).
The Galileo plague aboard the Juno spacecraft. Credit: NASA/JPL-Caltech/KSC
The text, written in Italian and transcribed from Galileo’s own handwriting, translates as:
“On the 11th it was in this formation, and the star closest to Jupiter was half the size than the other and very close to the other so that during the previous nights all of the three observed stars looked of the same dimension and among them equally afar; so that it is evident that around Jupiter there are three moving stars invisible till this time to everyone.”
The spacecraft also carries three Lego figurines representing Galileo, the Roman god Jupiter and his wife Juno. The figure of Juno holds a magnifying glass as a sign of her searching for the truth, Jupiter holds a lightning bolt, and the figure of Galileo Galilei holds his famous telescope. Lego made these figurines out of aluminum (instead of the usual plastic) to ensure they would survive the extreme conditions of space flight.
Launch:
The Juno mission launched from Cape Canaveral Air Force Station on August 5th, 2011, atop an Atlas V rocket. After approximately 1 minute and 33 seconds, the five Solid Rocket Boosters (SRBs) reached burnout and then fell away. After 4 minutes and 26 seconds after liftoff, the Atlas V main engine cut off, followed 16 seconds later by the separation of the Centaur upper stage rocket.
After a burn that lasted for 6 minutes, the Centaur was put into its initial parking orbit. It coasted for approximately 30 minutes before its engine conducted a second firing which lasted for 9 minutes, putting the spacecraft on an Earth escape trajectory. About 54 minutes after launch, the spacecraft separated from the Centaur and began to extend its solar panels.
A year after launch, between August and September 2012, the Juno spacecraft successfully conducted two Deep Space Maneuvers designed to correct its trajectory. The first maneuver (DSM-1) occurred on August 30th, 2012, with the main engine firing for approximately 30 minutes and altering its velocity by about 388 m/s (1396.8 km/h; 867 mph).
The second maneuver (DSM-2), which had a similar duration and resulted in a similar velocity change, took place on September 14th. The two firings occurred when the probe was about 480 million km (298 million miles) from Earth, and altered the spacecraft’s speed and its Jupiter-bound trajectory, setting the stage for a gravity assist from its flyby of Earth.
Earth Flyby:
Juno’s Earth flyby took place on October 9th, 2013, after the spacecraft completed one elliptical orbit around the Sun. During its closest approach, the probe was at an altitude of about 560 kilometers (348 miles). The Earth flyby boosted Juno’s velocity by 3,900 m/s (14162 km/h; 8,800 mph) and placed the spacecraft on its final flight path for Jupiter.
During the flyby, Juno’s Magnetic Field Investigation (MAG) instrument managed to capture some low-resolution images of the Earth and Moon. These images were taken while the Juno probe was about 966,000 km (600,000 mi) away from Earth – about three times the Earth-moon separation. They were later combined by technicians at NASA’s JPL to create the video shown above.
The Earth flyby was also used as a rehearsal by the Juno science team to test some of the spacecraft’s instruments and to practice certain procedures that will be used once the probe arrives at Jupiter.
Rendezvous With Jupiter:
The Juno spacecraft reached the Jupiter system and established polar orbit around the gas giant on July 4th, 2016. It’s orbit will be highly elliptical and will take it close to the poles – within 4,300 km (2,672 mi) – before reaching beyond the orbit of Callisto, the most distant of Jupiter’s large moons (at an average distance of 1,882,700 km or 1,169,855.5 mi).
This orbit will allow the spacecraft to avoid long-term contact with Jupiter’s radiation belts, while still allowing it to perform close-up surveys of Jupiter’s polar atmosphere, magnetosphere and gravitational field. The spacecraft will spend the next two years orbiting Jupiter a total of 37 times, with each orbit taking 14 days.
Already, the probe has performed measurements of Jupiter’s magnetic field. This began on June 24th when Juno crossed the bow shock just outside Jupiter’s magnetosphere, followed by it’s transit into the lower density of the Jovian magnetosphere on June 25. Having made the transition from an environment characterized by solar wind to one dominated by Jupiter’s magnetosphere, the ship’s instruments revealed some interesting information about the sudden change in particle density.
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.
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.
While the mission team had hoped to reduce Juno’s orbital period to 14 days, thus allowing for it to conduct a total of 37 perijoves before mission’s end. However, due to a malfunction with the probe’s helium valves, the firing was delayed. NASA has since announced that it will not conduct this engine firing, and that the probe will conduct a total perijoves in total before the end of its mission.
End of Mission:
The Juno mission is set to conclude in February of 2018, after completing 12 orbits of Jupiter. At this point, and barring any mission extensions, the probe will be de-orbited to burn up in Jupiter’s outer atmosphere. As with the Galileo spacecraft, this is meant be to avoid any possibility of impact and biological contamination with one of Jupiter’s moons.
The mission is managed by the JPL, and its principal investigator is Scott Bolton of the Southwest Research Institute. NASA’s Launch Services Program, located at the Kennedy Space Center in Florida, is responsible for managing launch services for the probe. The Juno mission is part of the New Frontiers Program managed by NASA’s Marshall Space Flight Center in Huntsville, Ala.
As of the writing of this article, the Juno mission is one day, four hours and fifty-five minutes away from its historic arrival with Jupiter. Check out NASA’s Juno mission page to get up-to-date information on the mission, and stay tuned to Universe Today for updates!
We’re always talking about Pluto, or Saturn or Mars. But nobody ever seems to talk about Jupiter any more. Why is that? I mean, it’s the largest planet in the Solar System. 318 times the mass of the Earth has got to count for something, right? Right?
Jupiter with Io and Ganymede taken by amateur astronomer Damian Peach. Credit: NASA / Damian Peach
Jupiter is one of the most important places in the Solar System. The planet itself is impressive; with ancient cyclonic storms larger than the Earth, or a magnetosphere so powerful it defies comprehension.
One of the most compelling reasons to visit Jupiter is because of its moons. Europa, Callisto and Ganymede might all contain vast oceans of liquid water underneath icy shells. And as you probably know, wherever we find liquid water on Earth, we find life.
And so, the icy moons of Jupiter are probably the best place to look for life in the entire Solar System.
And yet, as I record this video in early 2016, there are no spacecraft at Jupiter or its moons. In fact, there haven’t been any there for years. The last spacecraft to visit Jupiter was NASA’s New Horizons in 2007. Mars is buzzing with orbiters and rovers, we just got close up pictures of Pluto! and yet we haven’t seen Jupiter close up in almost 10 years. What’s going on?
Part of the problem is that Jupiter is really far away, and it takes a long time to get there.
How long? Let’s take a look at all the spacecraft that have ever made this journey.
The first spacecraft to ever cross the gulf from the Earth to Jupiter was NASA’s Pioneer 10. It launched on March 3, 1972 and reached on December 3, 1973. That’s a total of 640 days of flight time.
But Pioneer 10 was just flying by, on its way to explore the outer Solar System. It came within 130,000 km of the planet, took the first close up pictures ever taken of Jupiter, and then continued on into deep space for another 11 years before NASA lost contact.
Pioneer 11 took off a year later, and arrived a year later. It made the journey in 606 days, making a much closer flyby, getting within 21,000 kilometers of Jupiter, and visiting Saturn too.
Next came the Voyager spacecraft. Voyager 1 took only 546 days, arriving on March 5, 1979, and Voyager 2 took 688 days.
So, if you’re going to do a flyby, you’ll need about 550-650 days to make the journey.
But if you actually want to slow down and go into orbit around Jupiter, you’ll need to take a much slower journey. The only spacecraft to ever stick around Jupiter was NASA’s Galileo spacecraft, which launched on October 18, 1989.
Instead of taking the direct path to Jupiter, it made two gravitational assisting flybys of Earth and one of Venus to pick up speed, finally arriving at Jupiter on December 8, 1995. That’s a total of 2,242 days.
So why did Galileo take so much longer to get to Jupiter? It’s because you need to be going slow enough that when you reach Jupiter, you can actually enter orbit around the planet, and not just speed on past.
And now, after this long period of Jupiterlessness, we’re about to have another spacecraft arrive at the massive planet and go into orbit. NASA’s Juno spacecraft was launched back on August 5, 2011 and it’s been buzzing around the inner Solar System, building up the velocity to make the journey to Jupiter.
NASA’s Juno spacecraft launched on August 6, 2011 and should arrive at Jupiter on July 4, 2016. Credit: NASA / JPL
It did a flyby of Earth back in 2013, and if everything goes well, Juno will make its orbital insertion into the Jovian system on July 4, 2016. Total flight time: 1,795 days.
Once again, we’ll have a spacecraft observing Jupiter and its moon.s
This is just the beginning. There are several more missions to Jupiter in the works. The European Space Agency will be launching the Jupiter Icy Moons Mission in 2022, which will take nearly 8 years to reach Jupiter by 2030.
NASA’s Europa Multiple-Flyby Mission [Editor’s note: formerly known as the Europa Clipper] will probably launch in the same timeframe, and spend its time orbiting Europa, trying to get a better understand the environment on Europa. It probably won’t be able to detect any life down there, beneath the ice, but it’ll figure out exactly where the ocean starts.
So, how long does it take to get to Jupiter? Around 600 days if you want to just do a flyby and aren’t planning to stick around, or about 2,000 days if you want to actually get into orbit.
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.
With 67 confirmed satellites, Jupiter has the largest system of moons in the Solar System. The greatest of these are the four major moons of Io, Europa, Ganymede and Callisto – otherwise known as the Galilean Moons. Named in honor of their founder, these moons are not only comparable in size to some planets (such as Mercury), they are also some of the few places outside of Earth where liquid water exists, and perhaps even life.
But it is Callisto, the fourth and farthest moon of Jupiter, that may be the most rewarding when it comes to scientific research. In addition to the possibility of a subsurface ocean, this moon is the only Galilean far enough outside of Jupiter’s powerful magnetosphere that it does not experience harmful levels of radiation. This, and the prospect of finding life, make Callisto a prime candidate for future exploration.
Discovery and Naming:
Along with Io, Europa and Ganymede, Callisto was discovered in January of 1610 by Galileo Galilei using a telescope of his own design. Like all the Galilean Moons, it takes its name from one of Zeus’ lovers in classic Greek mythology. Callisto was a nymph (or the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis.
The name was suggested by German astronomer Simon Marius, apparently at the behest of Johannes Kepler. However, Galileo initially refused to use them, and the moons named in his honor were designed as Jupiter I through IV, based on their proximity to their parent planet. Being the farthest planet from Jupiter, Callisto was known as Jupiter IV until the 20th century, by which time, the names suggested by Marius were adopted.
The Galilean moons to scale, with Callisto in the bottom left corner. Credit: NASA/JPL
Size, Mass and Orbit:
With a mean radius of 2410.3 ± 1.5 km (0.378 Earths) and a mass of 1.0759 × 1023 kg (0.018 Earths), Callisto is the second largest Jupiter’s moons (after Ganymede) and the third largest satellite in the solar system. Much like Ganymede, it is comparable in size to Mercury – being 99% as large – but due to its mixed composition, it has less than one-third of Mercury mass.
Callisto orbits Jupiter at an average distance (semi-major axis) of 1,882,700 km. It has a very minor eccentricity (0.0074) and ranges in distance from 1,869,000 km at periapsis to 1,897,000 km at apoapsis. This distance, which is far greater than Ganymede’s, means that Callisto does not take part in the mean-motion resonance that Io, Europa and Ganymede do.
Much like the other Galileans, Callisto’s rotation is synchronous with its orbit. This means that it takes the same amount of time (16.689 days) for Callisto to complete a single orbit of Jupiter and a single rotation on its axis. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing over the course of centuries due to solar and planetary gravitational perturbations.
Size comparison of Earth, Moon and Callisto. Credit: NASA/JPL/DLR/Gregory H. Revera
Unlike the other Galileans, Callisto’s distant orbit means that it has never experienced much in the way of tidal-heating, which has had a profound impact on its internal structure and evolution. Its distance from Jupiter also means that the charged particles from Jupiter’s magnetosphere have had a very minor influence on its surface.
Composition and Surface Features:
The average density of Callisto, at 1.83 g/cm3, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. Ice is believed to constitute 49-55% of the moon, with the rock component likely made up of chondrites, silicates and iron oxide.
Callisto’s surface composition is thought to be similar to its composition as a whole, with water ice constituting 25-50% of its overall mass. High-resolution, near-infrared and UV spectra imaging have revealed the presence of various non-ice materials, such as magnesium and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds.
Model of Callisto’s internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior. Credit: NASA/JPL
Beneath the surface is an icy lithosphere that is between 80-150 m thick. A salty ocean 50–200 km deep is believed to exist beneath this, thanks to the presence of radioactive elements and the possible existence of ammonia. Evidence of this ocean include Jupiter’s magnetic field, which shows no signs of penetrating Callisto’s surface. This suggests a layer of highly conductive fluid that is at least 10 km in depth. However, if this water contains ammonia, which is more likely, than it could be up to 250-300 km.
Beneath this hypothetical ocean, Callisto’s interior appears to be composed of compressed rocks and ices, with the amount of rock increasing with depth. This means, in effect, that Callisto is only partially differentiated, with a small silicate core no larger than 600 km (and a density of 3.1-3.6 g/cm³) surrounded by a mix of ice and rock.
Spectral data has also indicated that Callisto’s surface is extremely heterogeneous at the small scale. Basically, the surface consists of small, bright patches of pure water ice, intermixed with patches of a rock–ice mixture, and extended dark areas made of a non-ice material.
Compared to the other Galilean Moons, Callisto’s surface is quite dark, with a surface albedo of about 20%. Another difference is the nature of its asymmetric appearance. Whereas with the other Galileans, the leading hemisphere is lighter than the trailing one, with Callisto the opposite is true.
Interior density structures created by an outer solar system late heavy bombardment onto Ganymede (top row) and Callisto (bottom row). Credit: SwRI
An immediately obvious feature about Callisto’s surface is the ancient and heavily cratered nature of it. In fact, the surface is the most cratered in the Solar System and is almost entirely saturated by craters, with newer ones having formed over older ones. What’s more, impact craters and their associated structures are the only large features on the surface. There are no mountains, volcanoes or other endogenic tectonic features.
Callisto’s impact craters range in size from 0.1 km to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes, whereas those that measure 5–40 km usually have a central peak.
Larger impact features, with diameters that range from 25–100 km have central pits instead of peaks. Those with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact.
The largest impact features on Callisto’s surface are multi-ring basins, which probably originated as a result of post-impact concentric fracturing which took place over a patch of lithosphere that overlay a section of soft or liquid material (possibly a patch of the interior ocean). The largest of these are Valhalla and Asgard, whose central, bright regions measure 600 and 1600 km in diameter (respectively) with rings extending farther outwards.
Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter. Credit: NASA/JPL
The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them – the older the surface, the denser the crater population. Based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System.
The ages of multi-ring structures and impact craters depend on chosen background cratering rates, and are estimated by different researchers to vary between 1 and 4 billion years of age.
Atmosphere:
Callisto has a very tenuous atmosphere composed of carbon dioxide which has an estimated surface pressure of 7.5 × 10-¹² bar (0.75 micro Pascals) and a particle density of 4 × 108 cm-3. Because such a thin atmosphere would be lost in only about 4 days, it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto’s icy crust.
While it has not been directly detected, it is believed that molecular oxygen exists in concentrations 10-100 times greater than CO². This is evidenced by the high electron density of the planet’s ionosphere, which cannot be explained by the photoionization of carbon dioxide alone. However, condensed oxygen has been detected on the surface of Callisto, trapped within its icy crust.
Habitability:
Much like Europa and Ganymede, and Saturn’s moons of Enceladus, Mimas, Dione, Titan, the possible existence of a subsurface ocean on Callisto has led many scientists to speculate about the possibility of life. This is particularly likely if the interior ocean is made up of salt-water, since halophiles (which thrive in high salt concentrations) could live there.
In addition, the possibility of extra-terrestrial microbial life has also been raised with respect to Callisto. However, the environmental conditions necessary for life to appear (which include the presence of sufficient heat due to tidal flexing) are more likely on Europa and Ganymede. The main difference is the lack of contact between the rocky material and the interior ocean, as well as the lower heat flux in Callisto’s interior.
In essence, while Callisto possesses the necessary pre-biotic chemistry to host life, it lacks the necessary energy. Because of this, the most likely candidate for the existence of extra-terrestrial life in Jupiter’s system of moons remains Europa.
Exploration:
The first exploration missions to Callisto were the Pioneer 10 and 11 spacecrafts, which conducted flybys of the Galilean moon in 1973 and 1974, respectively, But these missions provided little additional information beyond what had already learned through Earth-based observations. In contrast, the Voyager 1 and 2 spacecraft, which conducted flybys of the moon in 1979, managed to image more than half the surface and precisely measured Callisto’s temperature, mass and shape.
New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL
Further exploration took place between 1994 and 2003, when the Galileo spacecraft performed eight close flybys with Callisto. The orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters. In 2000, while en route to Saturn, the Cassini spacecraft acquired high-quality infrared spectra of the Galilean satellites, including Callisto.
In February–March 2007, while en route to Pluto, the New Horizons probe obtained new images and spectra of Callisto. Using its Linear Etalon Imaging Spectral Array (LEISA) instrument, the probe was able to reveal how lighting and viewing conditions affect infrared spectrum readings of its surface water ice.
The next planned mission to the Jovian system is the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. Ostensibly geared towards exploring Europa and Ganymede, the mission profile also includes several close flybys of Callisto.
Colonization:
Compared to the other Galileans, Callisto presents numerous advantages as far as colonization is concerned. Much like the others, the moon has an abundant supply of water in the form of surface ice (but also possibly liquid water beneath the surface). But unlike the others, Callisto’s distance from Jupiter means that colonists would have far less to worry about in terms of radiation.
In 2003, NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto, for the purposes of investigating the possible existence of life forms embedded in the ice crust on this moon and on Europa.
Artist’s impression of a base on the icy surface of Callisto. Credit: NASA
The study proposed a possible surface base on Callisto where a crew could “teleoperate a Europa submarine and excavate Callisto surface samples near the impact site”. In addition, this base could extract water from Callisto’s ample supply of water ices to produce rocket propellant for further exploration of the Solar System.
The advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate exploration on other Galilean Moons, and be an ideal location for a Jovian system way station, servicing spacecraft heading farther into the outer Solar System – which would likely take the form of craft using a gravity assist from a close flyby of Jupiter.
Reports filed by NASA’s Glenn Research Center and Langley Research Center – in December and February of 2003, respectively – both outlined possible manned missions to Callisto, as envisioned by HOPE. According to these reports, a mission that would likely involve a ship using a Mangetoplasmadynamic (MPD) or Nuclear-Electric Propulsion (NEP) drive system, and equipped to generate artificial gravity, could be mounted in the 2040s.
So while Callisto may not be the best target in the search for extra-terrestrial life, it may be the most hospitable of Jupiter’s moons for human life. In either case, any future missions to Jupiter will likely include a stopovers to Callisto, with the intent of investigating both of these possibilities.
