Until We Get Another Mission at Saturn, We’re Going to Have to Make Do with these Pictures Taken by Hubble

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.

Continue reading “Until We Get Another Mission at Saturn, We’re Going to Have to Make Do with these Pictures Taken by Hubble”

Cassini Data Has Revealed a Towering Hexagonal Storm at Saturn’s Northern Pole

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.

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Planets on Parade: Saturn at Opposition 2018

2018 planets
2018 planets
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.

vesta
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.

path of saturn
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
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.

iapetus
Two-faced Iapetus as imaged by Cassini. Image credit: NASA/JPL/Space Science Institute.

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.

Love to observe the planets? Check out our new forthcoming book, The Universe Today Ultimate Guide to Viewing the Cosmos – out on October 23rd, now up for pre-order.

New Saturn Storm Emerging?

Saturn Storm
Saturn Storm
The tell-tale white notch of a new storm system emerging on Saturn on April 1st. Image credit and copyright: Damian Peach.

Are you following the planets this season? The planetary action is about to heat up, as Jupiter, Saturn and Mars all head towards fine oppositions over the next few months.

Spying the Storms of Saturn

Astrophotographer Damian Peach raised the alarm on Twitter this past week of a possible bright storm emerging of the planet Saturn. The spot was noticeable even with the naked eye and in the raw video Peach captured, a sure sign that the storm was a biggie.

Though outbursts of clusters of white spots on the surface of Saturn aren’t uncommon, it’s rare to see one emerge at such a high latitude. The storm had faded considerably the next observing session Peach performed on April 5th, though observers should remain vigilant.

Saturn Storm 2
A storm subsiding? The followup view a few days later on April 5th. Image credit and copyright: Damian Peach.

It’s sad to think: Cassini and our eyes in the outer solar system are no more… and the situation will probably remain this way for some years to come. Juno also wraps up its mission at Jupiter (pending extension) this year, and New Horizons visits its final destination Ultima Thule (neé 2014 MU69) on New Year’s Day 2019, though it’ll likely continue to chronicle its journey through the outer realms of the solar system, much like the Voyager 1, 2 and Pioneer 10, 11 missions, also bound to orbit the galaxy, mute testaments to human civilization. But even though proposals for Europa Clipper, a nuclear-powered quad-copter for Saturn’s moon Titan, and a Uranus and/or Neptune Orbiter are all on the drawing board, the “gap decade” of outer solar system exploration will indeed come to pass and soon.

saturn storm
Catching a storm on Saturn, Cassini style. Credit: NASA/JPL-Caltech/SSI

But dedicated amateur astronomers continue to monitor the outer solar system for changes. This month sees Saturn rising around 1:30 AM local and transiting highest to the south for northern hemisphere observers at 6:00 AM local, just before sunrise. Saturn crosses the constellation Sagittarius in 2018, bottoming out at its most southerly point this year for its 29 year path around the Sun. Saturn currently shines at +0.4 magnitude, extending 40” across (including rings) as it heads towards a fine opposition on June 27th. After opposition, Saturn formally crosses into the dusk sky. The amazing rings are an automatic draw, but last week’s storm admonishes us not to forget to check out the saffron-colored disk of Saturn itself as well. For example, I’ve always wondered: why didn’t we see the hexagon before? It’s right there festooning the northern hemisphere cap, plain as day in modern amateur images… to be sure, we’re in a modern renaissance of planetary astrophotography today, what with image stacking and processing, but surely eagle-eyed observers of yore could’ve easily picked this feature out.

And the view is changing as well, as Saturn’s rings reached a maximum tilt in respect to our line of sight of 27 degrees in 2017, and now head back towards edge-on again in 2025. And be sure to check out Saturn’s retinue of moons, half a dozen of which are easily visible in a telescope at even low power.

Finally, here’s another elemental mystery poised by Saturn related to the current storm, one that Cassini sought to solve in its final days: how fast does Saturn rotate, exactly? The usual rough guesstimate quoted is usually around 10.5 hours, but we’ve yet to pin down this fundamental value with any degree of precession.

One thing’s definitely for sure: we need to go back. In the meantime, we can enjoy the early morning views of the most glorious of the planets in our Solar System.

Tiangong 1 Falls, Blue Moon Rises and Mars Takes Aim At Saturn

A couple watches the Moon rise from the icy shore of Lake Superior in Duluth earlier this month on March 1. Credit: Bob King

I apologize for the end-of-the-world title, but everything in it is true. And the world will still be here after it’s all done. On Friday (March 31) at 7:36 a.m. Central Time, the Moon will be full for the second time this month, which makes it a Blue Moon according to popular usage. Enjoy it. What with January’s Blue Moon and now this, we’ve chewed through all our Blue Moons till Halloween 2020.

I look forward to every full moon. Watching a moonrise, we get to see all manner of amazing atmospheric distortions play across the squat, orange disk. Once the sky’s dark, its outpouring of light makes walking at night a pleasure.

When a full moon occurs in spring, it hurries south down the ecliptic, the imaginary circle in the sky defining Earth’s orbit around the Sun. For northern hemisphere skywatchers, this southward sprint delays its rising by more an hour each night, forcing a quick departure from the evening sky. And that means blessed darkness for hunting down favorite galaxies and star clusters.


Tiangong 1 and a reentry simulation

As the Moon rolls along, the hapless Chinese space station Tiangong 1 hurtles toward Earth. Drag caused by friction with the upper atmosphere continues to shrink the spacecraft’s orbit, bringing it closer and closer to inevitable breakup and incineration. Since the Chinese National Space Administration (CNSA) lost touch with Tiangong 1 in March 2016, mission control can no longer power thrusters to de-orbit it at chosen time over a safe location like the ocean. The 9.3-ton (8,500 kg) station will burn up somewhere anywhere over a vast swath of the planet between latitudes 43°N and 43°S. Included within this zone are the southern half of Europe, the southern two-thirds of the U.S., India, Australia and much of Africa and South America.

Not until the day of or even hours before will have a clear idea of when and where the station will meet its fate. According to the latest update from the Aerospace Corp., which monitors falling spacecraft, reentry is expected on Easter Sunday (April 1) at 10:30 UT / 5:30 a.m. Central Time plus or minus 16 hours. This morning (March 29), the space station is circling Earth at about 118 miles (190 km) altitude. The lowest a satelllite can still make a complete orbit of the planet is about 62 miles (100 km). Below that, break-up begins.

A high definition TV camera on an aircraft took this photo of the cargo ship ATV-1 reentering the atmosphere in September 2008. Tiangong 1 is about the same size and will likely shatter and burn in similar fashion. Credit: ESA/NASA

For up-to-the-minute updates on when to expect Tiangong 1’s orbit to decay and the machine to plunge to Earth, check out Joseph Remis’ Twitter page. Most of the space station is expected to burn up on reentry, but larger chunks might survive all the way to the ground. Since much more of the Earth’s surface is water these remnants will likely end up in the drink … but you never know. If Tiangong-1 does come down over a populated area, observers on the ground will witness a spectacular, manmade fireball day or night.

Mars (right) and Saturn pair up in Sagittarius this morning, March 29 at dawn seen from Duluth, Minn. The two planets were 2.2° apart. Details: 35mm lens, f/2.8, 13 second exposure at ISO 800. Credit: Bob King

On the quieter side but nearly as eye-catching, Mars will overtake Saturn in the coming week, passing just 1° south of the ringed planet in a thrilling dawn conjunction on April 2. If the weather forecast doesn’t look promising that morning, the two planets will remain within 2° of each other now through April 6th, providing plenty of opportunities for a look.

You can easily tell them apart by color: Mars is distinctly red-orange and Saturn looks creamy white. Both are bright at around magnitude 0 though Mars is now a hair brighter by two-tenths of a magnitude. Will you be able to see the difference?

Mars passes close to Saturn on Monday, April 2. Look low in the southeastern sky shortly before and at dawn. Try getting a picture of the lovely couple by setting up your camera on a tripod and doing a series of time exposures from 5-30 seconds at f/3.5 and ISO 800. No fancy telephoto equipment is needed: a 35-55mm lens is perfect. Created with Stellarium

In most telescopes at low magnification both planets will comfortably fit in the same field of view. Saturn’s rings are tilted nearly wide open and quite beautiful. Mars appears gibbous and though still rather small, it’s brightening rapidly and drawing closer in time for its closest approach to Earth since 2003. Wishing you clear skies!

New Study Says Enceladus has had an Internal Ocean for Billions of Years

When the Cassini mission arrived in the Saturn system in 2004, it discovered something rather unexpected in Enceladus’ southern hemisphere. From hundreds of fissures located in the polar region, plumes of water and organic molecules were spotted periodically spewing forth. This was the first indication that Saturn’s moon may have an interior ocean caused by hydrothermal activity near the core-mantle boundary.

According to a new study based on Cassini data, which it obtained before diving into Saturn’s atmosphere on September 15th, this activity may have been going on for some time. In fact, the study team concluded that if the moon’s core is porous enough, it could have generated enough heat to maintain an interior ocean for billions of years. This study is the most encouraging indication yet that the interior of Enceladus could support life.

The study, titled “Powering prolonged hydrothermal activity inside Enceladus“, recently appeared in the journal Nature Astronomy. The study was led by Gaël Choblet, a researcher with the Planetary and Geodynamic Laboratory at the University of Nantes, and included members from NASA’s Jet Propulsion Laboratory, Charles University, and the Institute of Earth Sciences and the Geo- and Cosmochemistry Laboratory at the University of Heidelberg.

Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL

Prior to the Cassini mission’s many flybys of Enceladus, scientists believed this moon’s surface was composed of solid ice. It was only after noticing the plume activity that they came to realize that it had water jets that extended all the way down to a warm-water ocean in its interior. From the data obtained by Cassini, scientists were even able to make educated guesses of where this internal ocean lay.

All told, Enceladus is a relatively small moon, measuring some 500 km (311 mi) in diameter. Based on gravity measurements performed by Cassini, its interior ocean is believed to lie beneath an icy outer surface at depths of 20 to 25 km (12.4 to 15.5 mi). However, this surface ice thins to about 1 to 5 km (0.6 to 3.1 mi) over the southern polar region, where the jets of water and icy particles jet through fissures.

Based on the way Enceladus orbits Saturn with a certain wobble (aka. libration), scientists have been able to make estimates of the ocean’s depth, which they place at 26 to 31 km (16 to 19 mi). All of this surrounds a core which is believed to be composed of silicate minerals and metal, but which is also porous. Despite all these findings, the source of the interior heat has remained something of an open question.

This mechanism would have to be active when the moon formed billions of years ago and is still active today (as evidenced by the current plume activity). As Dr. Choblet explained in an ESA press statement:

“Where Enceladus gets the sustained power to remain active has always been a bit of mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy.”

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. Credit: NASA/JPL-Caltech

For years, scientists have speculated that tidal forces caused by Saturn’s gravitational influence are responsible for Enceladus’ internal heating. The way Saturn pushes and pulls the moon as it follows an elliptical path around the planet is also believed to be what causes Enceladus’ icy shell to deform, causing the fissures around the southern polar region. These same mechanisms are believed to be what is responsible for Europa’s interior warm-water ocean.

However, the energy produced by tidal friction in the ice is too weak to counterbalance the heat loss seen from the ocean. At the rate Enceladus’ ocean is losing energy to space, the entire moon would freeze solid within 30 million years. Similarly, the natural decay of radioactive elements within the core (which has been suggested for other moons as well) is also about 100 times too weak to explain Enceladus interior and plume activity.

To address this, Dr. Choblet and his team conducted simulations of Enceladus’ core to determine what kind of conditions could allow for tidal heating over billions of years. As they state in their study:

“In absence of direct constraints on the mechanical properties of Enceladus’ core, we consider a wide range of parameters to characterize the rate of tidal friction and the efficiency of water transport by porous flow. The unconsolidated core of Enceladus can be viewed as a highly granular/fragmented material, in which tidal deformation is likely to be associated with intergranular friction during fragment rearrangements.”
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

What they found was that in order for the Cassini observations to be borne out, Enceladus’ core would need to be made of unconsolidated, easily deformable, porous rock. This core could be easily permeated by liquid water, which would seep into the core and gradually heated through tidal friction between sliding rock fragments. Once this water was sufficiently heated, it would rise upwards because of temperature differences with its surroundings.

This process ultimately transfers heat to the interior ocean in narrow plumes which rise to the meet Enceladus’ icy shell. Once there, it causes the surface ice to melt and forming fissures through which jets reach  into space, spewing water, ice particles and hydrated minerals that replenish Saturn’s E-Ring. All of this is consistent with the observations made by Cassini, and is sustainable from a geophysical point of view.

In other words, this study is able to show that action in Enceladus’ core could produce the necessary heating to maintain a global ocean and produce plume activity. Since this action is a result of the core’s structure and tidal interaction with Saturn, it is perfectly logical that it has been taking place for billions of years. So beyond providing the first coherent explanation for Enceladus’ plume activity, this study is also a strong indication of habitability.

As scientists have come to understand, life takes a long time to get going. On Earth, it is estimated that the first microorganisms arose after 500 million years, and hydrothermal vents are believed to have played a key role in that process. It took another 2.5 billion years for the first multi-cellular life to evolve, and land-based plants and animals have only been around for the past 500 million years.

Knowing that moons like Enceladus – which has the necessary chemistry to support for life – has also had the necessary energy for billions of years is therefore very encouraging. One can only imagine what we will find once future missions begin inspecting its plumes more closely!

Further Reading: ESA, Nature Astronomy

Forecast for Titan: Cold, with a Chance of Noxious Ice Clouds

During the 13 years and 76 days that the Cassini mission spent around Saturn, the orbiter and its lander (the Huygens probe) revealed a great deal about Saturn and its systems of moons. This is especially true of Titan, Saturn’s largest moon and one of the most mysterious objects in the Solar System. As a result of Cassini’s many flybys, scientists learned a great deal about Titan’s methane lakes, nitrogen-rich atmosphere, and surface features.

Even though Cassini plunged into Saturn’s atmosphere on September 15th, 2017, scientists are still pouring over the things it revealed. For instance, before it ended its mission, Cassini captured an image of a strange cloud floating high above Titan’s south pole, one which is composed of toxic, hybrid ice particles. This discovery is another indication of the complex organic chemistry occurring in Titan’s atmosphere and on it’s surface.

Since this cloud was invisible to the naked eye, it was only observable thanks to Cassini’s Composite Infrared Spectrometer (CIRS). This instrument spotted the cloud at an altitude of about 160 to 210 km (100 to 130 mi), far above the methane rain clouds of Titan’s troposphere. It also covered a large area near the south pole, between 75° and 85° south latitude.

Artist concept of Cassini’s last moments at Saturn. Credit: NASA/JPL.

Using the chemical fingerprint obtained by the CIRS instrument, NASA researchers also conducted laboratory experiments to reconstruct the chemical composition of the cloud. These experiments determined that the cloud was composed of the organic molecules hydrogen cyanide and benzene. These two chemicals appeared to have condensed together to form ice particles, rather than being layered on top of each other.

For those who have spent more than the past decade studying Titan’s atmosphere, this was a rather interesting and unexpected find. As Carrie Anderson, a CIRS co-investigator at NASA’s Goddard Space Flight Center, said in a recent NASA press statement:

“This cloud represents a new chemical formula of ice in Titan’s atmosphere. What’s interesting is that this noxious ice is made of two molecules that condensed together out of a rich mixture of gases at the south pole.”

The presence of this cloud around Titan’s southern pole is also another example of the moon’s global circulation patterns. This involves currents of warm gases being sent from the hemisphere that is experiencing summer to the hemisphere experience winter. This pattern reverse direction when the seasons change, which leads to a buildup of clouds around whichever pole is experiencing winter.

Artist’s impression of Saturn’s moon Titan shows the change in observed atmospheric effects before, during and after equinox in 2009. Credit: NASA

When the Cassini orbiter arrived at Saturn in 20o4, Titan’s northern hemisphere was experiencing winter – which began in 2004. This was evidenced by the buildup of clouds around its north pole, which Cassini spotted during its first encounter with the moon later than same year. Similarly, the same phenomena was taking place around the south pole near the end of Cassini’s mission.

This was consistent with seasonal changes on Titan, which take place roughly every seven Earth years – a year on Titan lasts about 29.5 Earth years. Typically, the clouds that form in Titan’s atmosphere are structured in layers, where different types of gas will condense into icy clouds at different altitudes. Which ones condense is dependent on how much vapor is present and temperatures – which become steadily colder closer to the surface.

However, at times, different types of clouds can form over a range of altitudes, or co-condense with other types of clouds. This certainly appeared to be the case when it came to the large cloud of hydrogen cyanide and benzene that was spotted above the south pole. Evidence of this cloud was derived from three sets of Titan observations made with the CIRS instrument, which took place between July and November of 2015.

The CIRS instrument works by separating infrared light into its constituent colors, and then measures the strengths of these signals at the different wavelengths to determine the presence of chemical signatures. Previously, it was used to identify the presence of hydrogen cyanide ice clouds over the south pole, as well as other toxic chemicals in the moon’s stratosphere.

Artist’s impression of the Cassini orbiter’s Composite Infrared Spectrometer (CIRS). Credit: NASA-JPL

As F. Michael Flasar, the CIRS principal investigator at Goddard, said:

“CIRS acts as a remote-sensing thermometer and as a chemical probe, picking out the heat radiation emitted by individual gases in an atmosphere. And the instrument does it all remotely, while passing by a planet or moon.”

However, when examining the observation data for chemical “fingerprints”, Anderson and her colleagues noticed that the spectral signatures of the icy cloud did not match those of any individual chemical. To address this, the team began conducting laboratory experiments where mixtures of gases were condensed in a chamber that simulated conditions in Titan’s stratosphere.

After testing different pairs of chemicals, they finally found one which matched the infrared signature observed by CIRS. At first, they tried letting one gas condense before the other, but found that the best results were obtained when both gases were introduced and allowed to condense at the same time. To be fair, this was not the first time that Anderson and her colleagues had discovered co-condensed ice in CIRS data.

For example, similar observations were made near the north pole in 2005, about two years after the northern hemisphere experienced its winter solstice. At that time, the icy clouds were detected at a much lower altitude (below 150 km, or 93 mi) and showed chemical fingerprints of hydrogen cyanicide and caynoacetylene – one of the more complex organic molecules in Titan’s atmosphere.

Artist’s impression of the Cassini orbiter entering Saturn’s atmosphere. Credit: NASA/JPL

This difference between this and the latest detection of a hybrid cloud, according to Anderson, comes down to differences in seasonal variations between the north and south poles. Whereas the northern polar cloud observed in 2005 was spotted about two years after the northern winter solstice, the southern cloud Anderson and her team recently examined was spotted two years before the southern winter solstice.

In short, it is possible that the mixture of the gases was slightly different in the two case, and/or that the northern cloud had a chance to warm slightly, thus altering its composition somewhat.  As Anderson explained, these observations were made possible thanks to the many years that the Cassini mission spent around Saturn:

“One of the advantages of Cassini was that we were able to flyby Titan again and again over the course of the thirteen-year mission to see changes over time. This is a big part of the value of a long-term mission.”

Additional studies will certainly be needed to determine the structure of these icy clouds of mixed composition, and Anderson and her team already have some ideas on how they would look. For their money, the researchers expect these clouds to be lumpy and disorderly, rather than well-defined crystals like the single-chemical clouds.

In the coming years, NASA scientists are sure to be spending a great deal of time and energy sorting through all the data obtained by the Cassini mission over the course of its 13-year mission. Who knows what else they will detect before they have exhausted the orbiter’s vast collections of data?

Future Reading: NASA

Loss of Signal: Cassini Spacecraft Plunges Into Saturn

Until the very end, Cassini displayed just how robust and enduring this spacecraft has been throughout its entire 20 years in space and its 13-year mission at Saturn. As Cassini plummeted through the ringed-planet’s atmosphere, its thrusters fought the good fight to keep the antenna pointed at Earth for as long as possible, sending as much of the last drops of science data as it could.

Cassini endured about 40 seconds longer than expected before loss of signal was called at 11:55:46 UTC

“I hope you’re all deeply proud of this accomplishment,” said Cassini Project Manager Earl Maize in JPL’s Mission Control Center after Cassini’s signal was lost. “This has been an incredible mission, and incredible spacecraft and an incredible team. I’m going to call this the end of mission. Project Manager off the net.”

Of course, the actual demise of Cassini took place about an hour and 23 minutes before, as it took that long for the signal to travel the 1.5 billion km distance from Saturn to Earth.

“This is a bittersweet moment for all of us,” said JPL Director Mike Watkins, “but I think it is more sweet than bitter because Cassini has been such an incredible mission. This is a great time to celebrate the hard work and dedication of those who have worked on this mission.”

Watkins added that almost everything we know about Saturn comes from the Cassini mission. “It made discoveries so compelling that we have to back,” he said. “We will go back and fly through the geysers of Encleadus and we’ll go back to explore Titan… These are incredibly compelling targets.”

Cassini program manager at JPL, Earl Maize, left, and spacecraft operations team manager for the Cassini mission at Saturn, Julie Webster, right, embrace after the Cassini spacecraft plunged into Saturn, Friday, Sept. 15, 2017 at NASA’s Jet Propulsion Laboratory in Pasadena, California. Photo Credit: (NASA/Joel Kowsky)

Cassini launched on Oct. 15, 1997, and arrived at Saturn’s in 2004. It studied Saturn’s rings and sent back postcards almost every day of its journeys around the Saturn system, pictures of complex moons, the intriguing rings and the giant gas planet.

It revealed the moon Enceladus as one of the most geothermally active places in our solar system, showing it to be one of the prime targets in the search for life beyond Earth.

Saturn’s active, ocean-bearing moon Enceladus sinks behind the giant planet in a farewell portrait from NASA’s Cassini spacecraft.
Credits: NASA/JPL-Caltech/Space Science Institute

Also, piggybacking along was the Huygens probe to study Saturn’s largest moon, Titan. This landing in 2005 was the first spacecraft to land in the outer solar system.

During its final plunge, Cassini’s instruments captured data on Saturn’s atmosphere, sending a strong signal throughout. As planned, data from eight of Cassini’s science instruments will be providing new insights about Saturn, including hints about the planet’s formation and evolution, and processes occurring in its atmosphere.

This death plunge ensures Saturn’s moons will remain pristine for future exploration.

Over 260 scientists from 17 countries and hundreds of engineers worked with Cassini throughout the entire mission. During Cassini’s final days, mission team members from all around the world gathered at JPL to celebrate the achievements of this historic mission.

Here is the last picture taken by Cassini’s cameras, showing the place where Cassini likely met its demise:

This monochrome view is the last image taken by the imaging cameras on NASA’s Cassini spacecraft. It looks toward the planet’s night side, lit by reflected light from the rings, and shows the location at which the spacecraft would enter the planet’s atmosphere hours later. Credit: NASA/JPL-Caltech/Space Science Institute

If you can’t get enough of Cassini, there will be more information coming about this final data, and of course, you can go look at all the images it has sent back here. Also, NASA has provided an ebook for download that includes information and images from the mission.

Cassini: The Mission That Will Live Forever

“With Cassini, we had a rare opportunity and we seized it,” said Linda Spilker, Cassini Mission Scientist.

And on Friday, September 15, we say goodbye to this incredible spacecraft.

Since 2004, Cassini has been orbiting Saturn, exploring the magnificent gas giant planet while weaving through an incredibly diverse assortment of 60-plus icy moons, and skimming along the edges of the complex but iconic icy rings.

Cassini’s findings have revolutionized our understanding of the entire Saturn system, providing intriguing insights on Saturn itself as well as revealing secrets held by moons such as Enceladus, which should be a big iceball but instead is one of the most geothermally active places in our solar system. And thanks to the Huygens lander, we now know Saturn’s largest moon, Titan is eerily Earthlike, but yet totally alien.

“The lasting story of Cassini will likely be its longevity and the monumental amount of scientific discovery,” Cassini Project Manager Earl Maize told me last year. “It was absolutely the right spacecraft in the right place at the right time to capture a huge array of phenomena at Saturn.”

But after 20 years in space, the Cassini spacecraft is running out of fuel, and so Cassini will conduct a sacred act known as ‘planetary protection.’ This self-sacrifice will ensure any potentially habitable moons of Saturn won’t be contaminated sometime in the future if the drifting, unpowered spacecraft were to accidentally crash land there. Microbes from Earth might still be adhering to Cassini, and its RTG power source still generates warmth. It could melt through the icy crust of one of Saturn’s moons, possibly, and reach a subsurface ocean.

Diagram of Cassini’s final week, showing some of the milestones as the spacecraft heads for its plunge into Saturn. Credit: NASA/JPL-Caltech

For a mission this big, this long and this unprecedented, it will end in spectacular fashion. Called the Grand Finale — which actually began last spring — Cassini has made 22 close passes through the small gap between Saturn’s cloud tops and the innermost ring. This series of orbits has sent the spacecraft on an inevitable path towards destruction.

And tomorrow, on its final orbit, Cassini will plunge into Saturn’s atmosphere at tens of thousands of kilometers per hour. Like the science-churning machine it has been throughout its mission, Cassini will continue to conduct science observations until the very end, sending back long-sought after data about Saturn’s atmosphere. But eventually, the spacecraft will be utterly destroyed by the gas planet’s heat and pressure. It will burn up like a meteor, and become part of the planet itself.

There’s no real way to sum up this amazing mission in one article, and so I’ll leave some links and information below for you to peruse.

But I’ll also leave you with this: Instead of feeling like the mission is over, I prefer to think of Cassini as living forever, because of all the data it provided that has yet to be studied. Linda Spilker told me this last year:

“In one way,” Spilker said, “the mission will end. But we have collected this treasure trove of data, so we have decades of additional work ahead of us. With this firehose of data coming back basically every day, we have only been able to skim the cream off the top of the best images and data. But imagine how many new discoveries we haven’t made yet! The search for a more complete understanding of the Saturn system continues, and we leave that legacy to those who come after, as we dream of future missions to continue the exploration we began.”

But if you want to say goodbye to Cassini, scientist Sarah Hörst might have suggested the best way to do it:

You can watch the live video coverage of Cassini’s end of mission on Friday starting at 7 a.m. EDT. on NASA TV

NASA has a great “Grand Finale” feature on its website, which is well worth the visit.

NASA also has all sorts of “Grand Finale” images, graphics and videos available here.

Follow the @CassiniSaturn twitter account for the latest info.

Imaging wizard Kevin Gill has put together a “Visions of Cassini” video with a great compilation of images from the mission. Here’s a shorter two minute version:

Or a longer, two and a half hour version!

These are one of the highest-resolution color images of any part of Saturn’s rings, taken on taken on July 6, 2017, with the Cassini spacecraft narrow-angle camera. This image shows a portion of the inner-central part of the planet’s B Ring. Credit: NASA/JPL-Caltech/Space Science Institute