Artist's concept of a dust storm on Titan.
Credits: IPGP/Labex UnivEarthS/University Paris Diderot – C. Epitalon & S. Rodriguez
Ever since the Cassini orbiter entered the Saturn system in July of 2004, scientists and the general public have been treated to a steady stream of data about this ringed giant and its many fascinating moons. In particular, a great deal of attention was focused on Saturn’s largest moon Titan, which has many surprising Earth-like characteristics.
These include its nitrogen-rich atmosphere, the presence of liquid bodies on its surface, a dynamic climate, organic molecules, and active prebiotic chemistry. And in the latest revelation to come from the Cassini orbiter, it appears that Titan also experiences periodic dust storms. This puts it in a class that has so far been reserved for only Earth and Mars.
This image of Saturn shows the planet and some of its moons in opposition. It's a composite image taken by the Hubble on June 6th, 2018. Image: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI); CC BY 4.0
We can’t seem to get enough of Saturn. It’s the most visually distinct object in our Solar System (other than the Sun, of course, but it’s kind of hard to gaze at). The Cassini mission to Saturn wrapped up about a year ago, and since then we’re relying on the venerable Hubble telescope to satisfy our appetite for images of the ringed planet.
A new study based on data from the Cassini mission is revealing something surprising in the atmosphere of Saturn. We’ve known about the storm at the gas giant’s north pole for decades, but now it appears that this massive hexagonal storm could be a towering behemoth hundreds of kilometers in height that has its base deep in Saturn’s atmosphere.
This grey-scale image of Saturn’s northern polar vortex was captured by the Cassini spacecraft. This image was captured from a distance of about 1.2 million km. A portion of Saturn’s rings are barely visible in the top right. Image: NASA/JPL-Caltech/Space Science Institute.
This image shows the recent observations of the planets Mars and Saturn made with the NASA/ESA Hubble Space Telescope. Credit: NASA, ESA, STScI, M. Mutchler (STScI), A. Simon (GSFC) and the OPAL Team, J. DePasquale (STScI)
During the summer of 2018, the planets of Mars and Saturn (one after the other) have been in opposition. In astronomical terms, opposition refers to when a planet is on the opposite side of the Earth relative to the Sun. This not only means that the planet is closer to Earth in its respective orbit, but that is also fully lit by the Sun (as seen from Earth) and much more visible.
As a result, astronomers are able to observe these planets in greater detail. The Hubble Space Telescope took advantage of this situation to do what it has done best for the past twenty-eight years – capture some breathtaking images of both planets! Hubble made its observations of Saturn in June and Mars in July, and showed both planets close to their opposition.
An artist's illustration of the Cassini probe's Grand Finale. Image: NASA/JPL/CalTech
In 1997, the NASA/ESA Cassini-Huygens mission launched from Earth and began its long journey towards the Saturn system. In 2004, the Cassini orbiter arrived around Saturn and would spend the next thirteen years studying the gas giant, its rings, and its system of Moons. On September 15th, 2017, the mission ended when the probe entered Saturn’s upper atmosphere and burned up.
This was known as Cassini’s “Grand Finale“, which began with the probe plunging into the unexplored region that lies between Saturn’s atmosphere and its rings and culminated with live coverage of it entering the atmosphere. In honor of the mission and NASA’s outstanding coverage of its final months, NASA was recently nominated for an Emmy Award by The Academy of Television Arts & Sciences.
The award is in the category of Outstanding Original Interactive Program, which recognizes the JPL’s multi-month digital campaign that celebrated the mission’s science and engineering accomplishments – which included news, web, education, television and social media efforts. It is also a nod to the agency’s success in communicating why the spacecraft concluded its mission in the skies of Saturn.
Essentially, the spacecraft was intentionally destroyed in Saturn’s atmosphere to prevent the possibility of it contaminating any of Saturn’s moons. Throughout the thirteen years it spent studying the Saturn system, Cassini found compelling evidence for the possible existence of life on Titan and in Enceladus’ interior ocean. In addition, scientists have speculated that there may be interior oceans within Rhea and Dione.
In this respect, Cassini ended its mission the same way the Galileo probe did in 2003. After spending 8 years studying Jupiter and its system the moons, the probe crashed into the gas giant’s upper atmosphere in order to prevent any possible contamination of Europa or Ganymede, which are also thought to have an interior oceans that could support life.
The “Grand Finale” campaign began on April 26th, 2017, and continued until the craft entered Saturn’s atmosphere on Sept. 15th, 2017, with the spacecraft sending back science to the very last second. The campaign utilized several different forms of media, was interactive, and was very comprehensive, providing regular updates and vital information about the mission.
“The multi-faceted campaign included regular updates on Twitter, Facebook, Snapchat, Instagram and the Cassini mission website; multiple live social, web and TV broadcasts during which reporter and public questions were answered; a dramatic short film to communicate the mission’s story and preview its endgame; multiple 360-degree videos, including NASA’s first 360-degree livestream of a mission event from inside JPL mission control; an interactive press kit; a steady drumbeat of articles to keep fans updated with news and features about the people behind the mission; state-standards aligned educational materials; a celebration of art by amateur space enthusiasts; and software to provide real-time tracking of the spacecraft, down to its final transmission to Earth.”
The short film, titled “For Your Consideration: The NASA Cassini Grand Finale“, showcases the missions many accomplishments, pays tribute to all those who made it happen and who helped inform the public and communicate the importance of the mission.
The Primetime Emmys will be awarded be on September 17th in Los Angeles. The Creative Arts Emmys, which includes interactive awards, will be presented during a separate ceremony on Saturday, Sept. 15th, at the Microsoft Theatre in Los Angeles. Other contenders include Back to the Moon, a Google Spotlight Stories App; Blade Runner 2049: Memory Lab, Coco VR, and Spiderman Homecoming, three Oculus VR experiences.
And be sure to check out the videos, FYC: NASA Cassini Grand Finale, below:
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Saturn, Mars and Jupiter all beckon this summer. Image credit and copyright: Sharin Ahmad (@shahgazer)
Saturn, Mars and Jupiter all beckon this summer. Image credit and copyright: Sharin Ahmad (@shahgazer).
We’re in the midst of a parade of planets crossing the evening sky. Jupiter reached opposition on May 9th, and sits high to the east at dusk. Mars heads towards a fine opposition on July 27th, nearly as favorable as the historic opposition of 2003. And Venus rules the dusk sky in the west after the setting Sun for most of 2018.
June is Saturn’s turn, as the planet reaches opposition this year on June 27th, rising opposite to the setting Sun at dusk.
In classical times, right up until just over two short centuries ago, Saturn represented the very outer limit of the solar system, the border lands where the realm of the planets came to an end. Sir William Herschel extended this view, when he spied Uranus—the first planet discovered in the telescopic era—slowly moving through the constellation Gemini just across the border of Taurus the Bull using a 7-foot reflector (in the olden days, telescopes specs were often quoted referring to their focal length versus aperture) while observing from his backyard garden in Bath, England on the night of March 13th, 1781.
Looking east tonight at sunset… note Vesta to the upper left. Credit: Stellarium.
Orbiting the Sun once every 29.5 years, Saturn is the slowest moving of the naked eye planets, fitting for a planet named after Father Time. Saturn slowly loops from one astronomical constellation along the zodiac to the next eastward, moving through one about every two years.
The path of Saturn through 2018. Image credit: Starry Night Education software.
2018 sees Saturn in the constellation Sagittarius the Archer, just above the ‘lid’ of the Teapot asterism, favoring the southern hemisphere for this apparition. Saturn won’t cross the celestial equator northward again until 2026. Not that that should discourage northern hemisphere viewers from going after this most glorious of planets. A low southerly declination also means that Saturn is also up in the evening in the summertime up north, a conducive time for observing. Taking 29-30 years to complete one lap around the ecliptic as seen from our Earthly vantage point, Saturn also makes a great timekeeper with respect to personal life milestones… where were you back in 1989, when Saturn occupied the same spot along the ecliptic?
Saturn also shows the least variation of all the planets in terms of brightness and size, owing to its immense distance 9.5 AU from the Sun, and consequently 8.5 to 10.5 AU from the Earth. Saturn actually just passed its most distant aphelion since 1959 on April 17th, 2018 at 10.066 AU from the Sun.
Saturn’s in 2018 Dates with Destiny
Saturn sits just 1.6 degrees south of the waning gibbous Moon tonight. The Moon will lap it again one lunation later on June 28th. Note that the brightest of the asteroids, +5.7 magnitude 4 Vesta is nearby in northern Sagittarius, also reaching opposition on June 19th. Can you spy Vesta with the naked eye from a dark sky site? 4 Vesta passes just 4 degrees from Saturn on September 23rd, and both flirt with the galactic plane and some famous deep sky targets, including the Trifid and Lagoon Nebulae.
Saturn reaches quadrature 90 degrees east of the Sun on September 25th, then ends its evening apparition when it reaches solar conjunction on New Year’s Day, 2019.
Saturn is well clear of the Moon’s path for most of this year, but stick around: starting on December 9th, 2018, the slow-moving planet will make a great target for the Moon, which will begin occulting it for every lunation through the end of 2019.
It’s ironic: Saturn mostly hides its beauty to unaided eye. Presenting a slight saffron color in appearance, it never strays much from magnitude -0.2 to +1.4 in brightness. One naked eye observation to watch for is a sudden spurt in brightness known as the opposition surge or Seeliger Effect. This is a retro reflector type effect, caused by all those tiny iceball moonlets in the rings reaching 100% illumination at once. Think of how the Full Moon is actually 3 to 4 times brighter than the 50% illuminated Quarter Moon… all those little peaks, ridges and crater rims no longer casting shadows do indeed add up.
Saturn in all its glory (note the moons Enceladus and Tethys, too!). Image credit and copyright: Efrain Morales.
And this effect is more prominent in recent years for another reason: Saturn’s rings passed maximum tilt (26.7 degrees) with respect to our line of sight just last year, and are still relatively wide open in 2018. They’ll start slimming down again over the next few oppositions, reaching edge-on again in 2028.
Even using a pair of 7×50 hunting binoculars on Saturn, you can tell that something is amiss. You’re getting the same view that Galileo had through his spyglass, the pinnacle of early 17th century technology. He could tell that something about the planet was awry, and drew sketches showing an oblong world with coffee cup handles on the side. Crank up the magnification using even a small 60 mm refractor, and the rings easily jump into view. This is what makes Saturn a star party staple, an eye candy feast capable of drawing the aim of all the telescopes down the row.
If seeing and atmospheric conditions allow, crank up the magnification up to 150x or higher, and the dark groove of the Cassini division snaps into view. Can you see the shadow of the disk of Saturn, cast back onto the plane of the rings? The shadow of the planet hides behind it near opposition, then becomes most prominent towards quadrature, when we get to peek around its edge. Can you spy the limb of the planet itself, through the Cassini Gap?
Though the disk of Saturn is often featureless, tiny swirls of white storms do occasionally pop up. Astrophotographer Damian Peach noted just one such short-lived storm on the ringed planet this past April 2018.
Saturn’s retinue of moons are also interesting to follow in there own right. The first one you’ll note is +8.5 magnitude smog-shrouded Titan. Larger in diameter than Mercury, Titan would easily be a planet in its own right, were it liberated from its primary’s domain.
Though Saturn has 62 known moons, only six in addition to Titan are in range of a modest backyard telescope: Enceladus, Rhea, Dione, Mimas, Tethys and Iapetus. Two-faced Iapetus is especially interesting to follow, as it varies two full magnitudes in brightness during its 79 day orbit. Arthur C. Clarke originally placed the final monolith in 2001: A Space Odyssey on this moon, its artificial coating a beacon to astronomers. Today, we know from flybys carried out by NASA’s Cassini spacecraft that the leading hemisphere of Iapetus is coated with dark in-falling material, originating from the dark Phoebe ring around Saturn.
Owners of large light bucket telescopes may also want to try from two fainter +15th magnitude moons: Hyperion and Phoebe.
Fun fact: Saturn’s moons can also cast shadows back on the planet itself, much like the Galilean moons do on Jupiter… the catch, however, is that these events only occur around equinox season in the years around when Saturn’s rings are edge-on. This next occurs starting in 2026.
Cassini finished up its thrilling 20 year mission just last year, with a dramatic plunge into Saturn itself. It will be a while before we return again, perhaps in the next decade if NASA selects a nuclear-powered helicopter to explore Titan. Until then, be sure to explore Saturn this summer, from your Earthbound backyard.
This image of Deimos and Saturn was taken by the Super Resolution Channel of Mars Express’ High Resolution Stereo Camera. Credit: ESA/DLR/FU Berlin
The ESA’s Mars Express probe has been studying Mars and its Moons for many years. While there are several missions currently in orbit around Mars, Mars Express‘s near-polar elliptical orbit gives it some advantages over the others. For one, its orbital path takes it closer to Phobos than any other spacecraft, which allows it to periodically observe the moon from distances of around 150 km (93 mi).
Because of this, the probe is in an ideal position to study Mars’ moons and capture images of them. On occasion, this allows for some interesting photo opportunities. For example, in November of 2017, while taking pictures of Phobos and Deimos, the probe spotted Saturn in the background. This fortuitous event led to the creation of some beautiful images, which were put together to produce a video.
Since 2003, Mars Express has been studying Phobos and Deimos in the hopes of learning more about these mysterious objects. While it has learned much about their size, appearance and position, much remains unknown about their composition, how and where they formed, and what their surface conditions are like. To answer these questions, the probe has been conducting regular flybys of these moons and taking pictures of them.
Phobos and background star (circled in red). Credit: ESA/DLR/FU Berlin
The video that was recently released by the ESA combines 30 such images which show Phobos passing through the frame. In the background, Saturn is visible as a small ringed dot, despite being roughly 1 billion km away. The images that were used to create this video were taken by the probes High Resolution Stereo Camera on November 26th, 2016, while the probe was traveling at a speed of about 3 km/s.
This photobomb was not unexpected, since the Mars Express repeatedly uses background reference stars and other bodies in the Solar System to confirm positions of the moons in the sky. In so doing, the probe is able to calculate the position of Phobos and Deimos with an accuracy of up to a few kilometers. The probes ideal position for capturing detailed images has also helped scientists to learn more about the surface features and structure of the two moons.
For instance, the pictures taken during the probe’s close flybys of Phobos showed its bumpy, irregular and dimpled surface in detail.The moon’s largest impact crater – the Stickney Crater – is also visible in one of the frames. Measuring 9 km ( mi) in diameter, this crater accounts for a third of the moon’s diameter, making it one the largest impact craters relative to body size in the Solar System.
In another image, taken on January 15th, 2018, Deimos is visible as an irregular and partially shadowed body in the foreground, while the delicate rings of Saturn are just visible encircling the small dot in the background (see below). In addition, Mars Express also obtained images of Phobos set against a reference star on January 8th, 2018 (see above) and close-up images of Phobos’ pockmarked surface on September 12th, 2017.
This image of Deimos and Saturn was taken by the Super Resolution Channel of Mars Express’ High Resolution Stereo Camera. Credit: ESA/DLR/FU Berlin
In the future, the Mars Express probe is expected to reveal a great deal more about Mars’ system of moons. In addition to the enduring questions of their origins, formation and composition, there are also questions about where future missions could land in order to study the surface directly. In particular, Phobos has been considered for a possible landing and sample-return mission.
Because of its nearness to Mars and the fact that one side is always facing towards the planet, the moon could make for an ideal location for a permanent observation post. This post would allow for the long-term study of the Martian surface and atmosphere, could act as a communications relay for other spacecraft, and could even serve as a base for future missions to the surface.
If and when such a mission to Phobos becomes a reality, it is the Mars Express probe that will determine where the ideal landing site would be. In essence, by studying the Martian moons to learning more about them, Mars Express is helping to prepare future missions to the Red Planet.
Be sure to check out the time-lapse video of Phobos and Saturn, courtesy of the ESA:
The site of the Cassini probe's crash in Saturn's atmosphere, circled in white. Credit: NASA/JPL-Caltech/Space Science Institute
On September 15th, 2017, after nearly 20 years in service, the Cassini spacecraft ended its mission by plunging into the atmosphere of Saturn. During the 13 years it spent in the Saturn system, this probe revealed a great deal about the gas giant, its rings, and its systems of moons. As such, it was a bittersweet moment for the mission team when the probe concluded its Grand Finale and began descending into Saturn’s atmosphere.
Even though the mission has concluded, scientists are still busy poring over the data sent back by the probe. These include a mosaic of the final images snapped by Cassini’s cameras, which show the location of where it would enter Saturn’s atmosphere just hours later. The exact spot (shown above) is indicated by a white oval, which was on Saturn’s night side at the time, but would later come around to be facing the Sun.
From the beginning, the Cassini mission was a game-changer. After reaching the Saturn system on July 1st, 2004, the probe began a series of orbits around Saturn that allowed it conduct close flybys of several of its moons. Foremost among these were Saturn’s largest moon Titan and its icy moon Enceladus, both of which proved to be a treasure trove of scientific data.
Artist’s impression of the Cassini spacecraft orbiting Saturn. Credit: NASA/JPL-Caltech/Space Science Institute
On Titan, Cassini revealed evidence of methane lakes and seas, the existence of a methanogenic cycle (similar to Earth’s hydrological cycle), and the presence of organic molecules and prebiotic chemistry. On Enceladus, Cassini examined the mysterious plumes emanating from its southern pole, revealing that they extended all the way to the moon’s interior ocean and contained organic molecules and hydrated minerals.
These findings have inspired a number of proposals for future robotic missions to explore Titan and Enceladus more closely. So far, proposals range from exploring Titan’s surface and atmosphere using lightweight aerial platforms, balloons and landers, or a dual quadcopter. Other proposals include exploring its seas using a paddleboat or a even a submarine. And alongside Europa, there are scientists clamoring for a mission to Enceladus and other “Ocean Worlds” to explore its plumes and maybe even its interior ocean.
Beyond that, Cassini also revealed a great deal about Saturn’s atmosphere, which included the persistent hexagonal storm that exists around the planet’s north pole. During its Grand Finale, where it made 22 orbits between Saturn and its rings, the probe also revealed a great deal about the three-dimensional structure and dynamic behavior of the planet’s famous system of rings.
This montage of images, made from data obtained by Cassini’s visual and infrared mapping spectrometer, shows the location on Saturn where the NASA spacecraft entered Saturn’s atmosphere on Sept. 15, 2017. Credits: NASA/JPL-Caltech/University of Arizona
It is only fitting then that the Cassini probe would also capture images of the very spot where its mission would end. The images were taken by Cassini’s wide-angle camera on Sept. 14th, 2017, when the probe was at a distance of about 634,000 km (394,000 mi) from Saturn. They were taken using red, green and blue spectral filters, which were then combined to show the scene in near-natural color.
The resulting image is not dissimilar from another mosaic that was released on September 15th, 2017, to mark the end of the Cassini mission. This mosaic was created using data obtained by Cassini’s visual and infrared mapping spectrometer, which also showed the exact location where the spacecraft would enter the atmosphere – 9.4 degrees north latitude by 53 degrees west longitude.
The main difference, of course, is that this latest mosaic benefits from the addition of color, which provides a better sense of orientation. And for those who are missing the Cassini mission and its regular flow of scientific discoveries, its much more emotionally fitting! While we may never be able to find the wreckage buried inside Saturn’s atmosphere, it is good to know where its last known location was.
Ligeia Mare, shown in here in data obtained by NASA's Cassini spacecraft, is the second largest known body of liquid on Saturn's moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan's north polar region. Credit: NASA/JPL-Caltech/ASI/Cornell
Thanks to the Cassini mission, we have learned some truly amazing things about Saturn and its largest moon, Titan. This includes information on its dense atmosphere, its geological features, its methane lakes, methane cycle, and organic chemistry. And even though Cassini recently ended its mission by crashing into Saturn’s atmosphere, scientists are still pouring over all of the data it obtained during its 13 years in the Saturn system.
And now, using Cassini data, two teams led by researchers from Cornell University have released two new studies that reveal even more interesting things about Titan. In one, the team created a complete topographic map of Titan using Cassini’s entire data set. In the second, the team revealed that Titan’s seas have a common elevation, much like how we have a “sea level” here on Earth.
This true-color image of Titan, taken by the Cassini spacecraft, shows the moon’s thick, hazy atmosphere. Credit: NASA
In the first paper, the authors described how topographic data from multiple sources was combined to create a global map of Titan. Since only about 9% of Titan was observed with high-resolution topography (and 25-30% in lower resolution) the remainder of the moon was mapped with an interpolation algorithm. Combined with a global minimization process, this reduced errors that would arise from such things as spacecraft location.
The map revealed new features on Titan, as well as a global view of the highs and lows of the moon’s topography. For instance, the maps showed several new mountains which reach a maximum elevation of 700 meters (about 3000 ft). Using the map, scientists were also able to confirm that two locations in the equatorial regions are depressions that could be the result of ancient seas that have since dried up or cryovolcanic flows.
The map also suggests that Titan may be more oblate than previously thought, which could mean that the crust varies in thickness. The data set is available online, and the map which the team created from it is already proving its worth to the scientific community. As Professor Corlies explained in a Cornell press release:
“The main point of the work was to create a map for use by the scientific community… We’re measuring the elevation of a liquid surface on another body 10 astronomical units away from the sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans.”
False-color mosaic of Titan’s northern lakes, made from infrared data collected by NASA’s Cassini spacecraft. Credit: NASA
Looking ahead, this map will play an important role when it comes tr scientists seeking to model Titan’s climate, study its shape and gravity, and its surface morphology. In addition, it will be especially helpful for those looking to test interior models of Titan, which is fundamental to determining if the moon could harbor life. Much like Europa and Enceladus, it is believed that Titan has a liquid water ocean and hydrothermal vents at its core-mantle boundary.
The second study, which also employed the new topographical map, was based on Cassini radar data that was obtained up to just a few months before the spacecraft burned up in Saturn’s atmosphere. Using this data, Assistant Professor Hayes and his team determined that Titan’s seas follow a constant elevation relative to Titan’s gravitational pull. Basically, they found that Titan has a sea level, much like Earth. As Hayes explained:
“We’re measuring the elevation of a liquid surface on another body 10 astronomical units away from the sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans.”
This common elevation is important because liquid bodies on Titan appear to be connected by something resembling an aquifer system. Much like how water flows underground through porous rock and gravel on Earth, hydrocarbons do the same thing under Titan’s icy surface. This ensures that there is transference between large bodies of water, and that they share a common sea level.
Artist concept of Cassini’s last moments at Saturn. Credit: NASA/JPL.
“We don’t see any empty lakes that are below the local filled lakes because, if they did go below that level, they would be filled themselves,” said Hayes. “This suggests that there’s flow in the subsurface and that they are communicating with each other. It’s also telling us that there is liquid hydrocarbon stored on the subsurface of Titan.”
Meanwhile, smaller lakes on Titan appear at elevations several hundred meters above Titan’s sea level. This is not dissimilar to what happens on Earth, where large lakes are often found at higher elevations. These are known as “Alpine Lakes”, and some well-known examples include Lake Titicaca in the Andes, Lakes Geneva in the Alps, and Paradise Lake in the Rockies.
Last, but not least, the study also revealed the vast majority of Titan’s lakes are found within sharp-edged depressions that are surrounded by high ridges, some of which are hundreds of meters high. Here too, there is a resemblance to features on Earth – such as the Florida Everglades – where underlying material dissolves and causes the surface to collapse, forming holes in the ground.
The shape of these lakes indicate that they may be expanding at a constant rate, a process known as uniform scarp retreat. In fact, the largest lake in the south – Ontario Lacus – resembles a series of smaller empty lakes that have coalesced to form a single feature. This process is apparently due to seasonal change, where autumn in the southern hemisphere leads to more evaporation.
While the Cassini mission is no longer exploring the Saturn system, the data it accumulated during its multi-year mission is still bearing fruit. Between these latest studies, and the many more that will follow, scientists are likely to reveal a great deal more about this mysterious moon and the forces that shape it!
