Posted on by Matt Williams

This was Exactly Where Cassini Crashed into Saturn

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.

Further Reading: NASA

Posted on by Susie Murph

Astronomy Cast Ep. 458: The Science of Cassini

And now Cassini’s gone. Smashed up in the atmosphere of Saturn. But planetary scientists are going to be picking through all those pictures and data for decades. Let’s look back at some of the science gathered up by Cassini so far, and we can still learn from this epic journey.

We usually record Astronomy Cast every Friday at 1:30 pm PDT / 4:30 pm EDT/ 20:30 PM UTC (8:30 GMT). You can watch us live on AstronomyCast.com, or the AstronomyCast YouTube page.

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Posted on by Susie Murph

Astronomy Cast Ep. 457: Why Did Cassini Have To Die? In Memoriam

It’s time to say goodbye to an old friend, NASA’s Cassini spacecraft, which has been orbiting within the Saturnian system since 2004. But why does a seemingly healthy spacecraft and mission need to come to an end? Today we look back at the mission, some of the amazing discoveries, and why its finale was necessary.

We usually record Astronomy Cast every Friday at 12:00 pm PDT / 3:00 pm EDT/ 19:00 PM UTC. You can watch us live on here on AstronomyCast.com, or the AstronomyCast YouTube page.

If you would like to support Astronomy Cast, please visit our page at Patreon here – https://www.patreon.com/astronomycast. We greatly appreciate your support!

If you would like to join the Weekly Space Hangout Crew, visit their site here and sign up. They’re a great team who can help you join our online discussions!

Posted on by Nancy Atkinson

Loss of Signal: Cassini Spacecraft Plunges Into Saturn

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

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.

Posted on by Nancy Atkinson

Cassini: The Mission That Will Live Forever

Artist rendition of the Cassini spacecraft over Saturn. Credit: NASA/JPL-Caltech/SSI/Kevin M. Gill.

“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

Posted on by Matt Williams

Saturn’s Hexagon Will be the Star of the Cassini Finale

This illustration shows Cassini above Saturn's northern hemisphere prior to one of its 22 Grand Finale dives. Credit: NASA/JPL-Caltech

The Cassini spacecraft is nearing the end of its lifespan. This September, after spending the past twenty years in space – twelve and a half of which were dedicated to studying Saturn and its system of moons – the probe will be crash into Saturn’s atmosphere. But between now and then, the probe will be making its “Grand Finale” – the final phase of its mission where it will dive between the planet and its rings 22 times.

In addition to exploring this region of Saturn (something no other mission has done), the probe will also be using this opportunity to study Saturn’s hexagonal polar jet stream in greater detail. This persistent storm, which rages around Saturn’s northern polar region, has been a subject of interest for decades. And now that it enjoys full sunlight, Cassini will be able to directly image it with every pass it makes over Saturn’s north pole.

This persistent storm was first noticed in images sent back by the Voyager 1 and 2 missions, which flew by Saturn in 1980 and 1981, respectively. As storms go, it is extremely massive, with each side measuring about 13,800 km (8,600 mi) in length – longer than the diameter of the Earth. It also rotates with a period of 10 hours 39 minutes and 24 seconds, which is assumed to be equal to the rotation of Saturn’s interior.

Image of Saturn’s hexagonal polar jet stream, taken by Cassini on Jan. 22nd, 2017. Credit: NASA/JPL-Caltech/Space Science Institute

When the Cassini spacecraft arrived around Saturn in 2004 to conduct the first part of its mission, this region was in shadow. This was due to the fact that the northern hemisphere was still coming out of winter, and was hence tilted  away from the Sun. However, since Saturn began its summer solstice in May of 2017, the northern polar region is now fully illuminated – at least by Saturn’s standards.

In truth, between its distance from the Sun (an average of 9.5549 AU) and its axial tilt (26.73°), the northern polar region only gets about 1% as much sunlight as Earth does. And from the perspective of the north pole, the Sun is very low in the sky. Nevertheless, the sunlight falling on the north pole at this point is enough to allow the Cassini mission to directly image the region by capturing its reflected light.

Images of the hexagonal jet stream (like the one above) will be taken by Cassini’s wide-angle camera, which uses special filters that admit wavelengths of near-infrared light. Already, Cassini has captured some impressive imagery during its first plunge between Saturn and its rings (which took place on April 26th, 2017). The rapid-fire images acquired by one of Cassini’s cameras were then stitched together to create a movie (posted below).

As you can see, the movie begins with a view of the vortex at the center of the hexagon, then heads past the outer boundary of the jet stream and continues further southward. Toward the end of the movie, the spacecraft reorients itself to direct its saucer-shaped antenna in the direction of the spacecraft’s motion, which is apparent from the way the camera frame rotates.

The images that make up this movie were captured as the Cassini spacecraft dropped in altitude from 72,400 to 6,700 km (45,000 to 4,200 miles) above Saturn’s cloud tops. As this happened, the features which the camera could resolve changed drastically – going from 8.7 km (5.4 mi) per pixel to 810 meters (0.5 mi) per pixel.

The movie was produced by Kunio Sayanagi and John Blalock – an associate of the Cassini imaging team and a  graduate research assistant (respectively) at Hampton University in Virginia – who collaborated with the Cassini imaging team. And thanks to this video, new insights are already being made into the hexagonal jet stream and the mechanisms that power it.

For example, as Sayanagi indicated in a NASA press release, the video captured the boundary regions of the jet stream rather nicely, which allowed him to note an interesting fact about them. “I was surprised to see so many sharp edges along the hexagon’s outer boundary,” he said. “Something must be keeping different latitudes from mixing to maintain those edges.”

Andrew Ingersoll, a member of the Cassini imaging team based at Caltech, expressed how similar movies will result from future plunges taken as part of the Grand Finale. “The images from the first pass were great, but we were conservative with the camera settings,” he said. “We plan to make updates to our observations for a similar opportunity on June 29th that we think will result in even better views.”

Between now and the end of the mission, who knows what we might learn about this mysterious storm? The next plunge – aka. Grand Finale Dive No. 4 – will take place on Sunday, May 15th at 4:42 p.m. UTC (12:42 p.m EDT; 9:42 a.m. PDT). A total of 22 dives will be made on a weekly basis before the probe takes the final plunge – the one that will cause it to breakup in Saturn’s atmosphere – on Friday, September 15th, 2017.

For more information, consult Cassini’s Grand Finale Orbit Guide. And be sure to enjoy this video of the final phase of the probe’s mission, courtesy of NASA:

Further Reading: NASA, NASA/JPL

Posted on by Nancy Atkinson

A Survivor’s Tale: Cassini Lives Through First Ring Dive

Artist's concept of Cassini diving between Saturn and its innermost ring. Credit: NASA/JPL-Caltech

One down, twenty-one to go! The Cassini spacecraft survived the first dive through the narrow gap between Saturn and its rings, and is now back communicating with Earth.

“No spacecraft has ever been this close to Saturn before. We could only rely on predictions, based on our experience with Saturn’s other rings, of what we thought this gap between the rings and Saturn would be like,” said Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California. “I am delighted to report that Cassini shot through the gap just as we planned and has come out the other side in excellent shape.”

It was a long day for Cassini scientists and engineers at the Jet Propulsion Laboratory while the spacecraft was out of contact for 20 hours during this first dive, signaling the beginning of the end for the mission.

Cassini, running out of fuel, is heading toward its ultimate death by crashing into Saturn on September 15, 2017. But during the next few months, Cassini will make twenty-one more passes through the gap, and in doing so, further our understanding of how giant planets, and planetary systems everywhere, form and evolve.

A raw image of Saturn’s polar vortex, taken on April 26, 2017 by the Cassini spacecraft during the first close pass between Saturn and its rings. Credit: NASA/JPL-Caltech.

Project Scientist Linda Spilker said Cassini will be able to make close up measurements of Saturn and its rings to finally help us understand the mass and internal structure of Saturn. And the images should be absolutely stunning.

Contact was lost as the ring-plane crossing started at 2 a.m. PDT (5 a.m. EDT) on April 26. NASA’s Deep Space Network Goldstone Complex in California’s Mojave Desert acquired Cassini’s signal at 11:56 p.m. PDT on April 26, 2017 (2:56 a.m. EDT on April 27) and data began flowing at 12:01 a.m. PDT (3:01 a.m. EDT) on April 27.

Cassini was programmed to collect science data while close to the planet. As a protective measure, the spacecraft used its large, dish-shaped high-gain antenna (13 feet or 4 meters across) as a deflector shield, orienting it in the direction of oncoming ring particles. This orientation put the spacecraft out of contact with Earth.

“In the grandest tradition of exploration, NASA’s Cassini spacecraft has once again blazed a trail, showing us new wonders and demonstrating where our curiosity can take us if we dare,” said Jim Green, director of the Planetary Science Division at NASA Headquarters.

The gap between the rings and the top of Saturn’s atmosphere is about 1,500 miles (2,000 kilometers) wide, and Cassini came within about 1,000 miles (1,600 kilometers) of Saturn’s cloud tops.

The best models for the region suggested that if there were ring particles in the area where Cassini crossed the ring plane, they would be tiny, on the scale of smoke particles. However, the spacecraft was traveling at speeds of about 77,000 mph (124,000 kph) relative to the planet, so small particles hitting a sensitive area could potentially have disabled the spacecraft.

The spacecraft is being destroyed after a successful 13 year mission at Saturn, as NASA needs to follow the protocol of planetary protection, and not allow a spacecraft with possible microbes from Earth to crash into a potentially habitable moon such as Enceladus or Titan.

Cassini’s next dive through the gap is scheduled for May 2.

You can see the Cassini raw images page here, but look for more processed images to be available soon.

Jason Major put together this animation of some of the first images from Cassini’s close pass:

Source: JPL

Posted on by Matt Williams

Adieu Titan: So Long & Thanks For All The Hydrocarbons

Artist's conception of Cassini winging by Saturn's moon Titan (right) with the planet in the background. Credit: NASA/JPL-Caltech

The Cassini spacecraft has done some amazing things since it arrived in the Saturn system in 2004. In addition to providing valuable information on the gas giant and its system of rings, it has also provided us with extensive data and photographs of Saturn’s many moons. Nowhere has this been more apparent than with Saturn’s largest moon, the hydrocarbon-rich satellite known as Titan.

And with just a few hours left before Cassini makes its final plunge between Saturn and its innermost ring (something that no other spacecraft has ever done), we should all take this opportunity to say goodbye to Titan. In the past few years, it has dazzled us with its methane lakes, dense atmosphere, and potential for hosting life. And it shall be sorely missed!

Cassini’s last encounter with Titan – where it passed within 979 km (608 mi) of the moon’s surface – took place on April 21st, at 11:08 p.m. PDT (April 22nd, 2:08 a.m. EDT). The probe also used this opportunity to take some radar images of the moon’s northern polar region. While this area has been photographed before, this was the first time that radar images were acquired.

Unprocessed image of Saturn’s moon Titan, captured by NASA’s Cassini spacecraft during its final close flyby on April 21st, 2017. Credit: NASA/JPL-Caltech/Space Science Institute

Over the course of the next week, Cassini’s radar team hopes to pour over theses images, which provide a detailed look at the methane seas and lakes in the northern polar region. It is hoped that this data will allow scientists to shed more light on the depths and compositions of some of the small lakes in the area, as well as provide more information on the evolving surface feature known as “magic island“.

With this last pass complete (its 127th in total), Cassini is now beginning the final phase of its mission – known as the Grand Finale. This will consist of the spacecraft making a final set of 22 orbits around the ringed planet between April 26th and September 15th. The maneuver will allow Cassini to go where no other probe has gone before and get the closest look ever at Saturn’s outer rings.

The final pass over Titan was part of this maneuver, using the moon’s gravity to bend and reshape the probe’s orbit so that it would be able to pass through Saturn’s ring system – instead of passing just beyond the main rings. As Earl Maize, Cassini project manager at JPL, said in a NASA press release:

“With this flyby we’re committed to the Grand Finale. The spacecraft is now on a ballistic path, so that even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn’s atmosphere on Sept. 15 no matter what.”

Some key numbers for Cassini’s Grand Finale and final plunge into Saturn. Credit: NASA/JPL-Caltech

Cassini’s final pass with Titan allowed it to acquire a boost in velocity, increasing its speed by 860.5 meters per second (3098 km/h; 1,925 mph). It then reached its farthest point in its orbit around Saturn (apoapse) on April 22nd, :46 p.m. PDT (11:46 p.m. EDT). This effectively began the Grand Finale orbits, with the first dive coming on April 26th, at 02:00 a.m. PDT (05:00 a.m. EDT).

This orbit will provide Cassini with its best look to date at Saturn’s north pole, which it will be studying with both its  Visible and Infrared Mapping Spectrometer (VIMS) and Composite Infrared Spectrometer (CIRS). These studies will lead to the creation of the sharpest movies to date in the near-infrared band, which will also allow the science team to study the motions of the hexagon pattern around Saturn’s north pole in more detail.

Between now and September, when the mission will end, the probe will provide information that is expected to improve our understanding of how giant planets form and evolve. Things will finally wrap on  September 15th, 2017, when the probe will plunge into Saturn’s atmosphere. But even then, the probe will be sending back information until its very last seconds of operation.

Safe journeys Cassini! And so long Titan! We hope to be exploring you again someday soon, preferably with something that can float or fly around inside your dense atmosphere, or perhaps investigate your methane seas in serious depth!

In the meantime, be sure to check out this narrated, 360-degree animated video from NASA. As you can see, it simulates what a ride on the Cassini spacecraft might look like as it makes its Grand Finale:

Further Reading: NASA, Cassini – The Grand Finale

Posted on by Bob King

Earth Beams From Between Saturn’s Rings in New Cassini Image

Credit: NASA/JPL-Caltech
NASA’s Cassini spacecraft captured the view on April 13, 2017 at 12:41 a.m. CDT. The probe was 870 million miles (1.4 billion km) away from Earth when the image was taken. The part of Earth facing toward Cassini at the time was the southern Atlantic Ocean. Look closely to the left of Earth; that pinprick of light is the Moon. Credit: NASA/JPL Caltech

Look at us. Packed into a gleaming dot. The entire planet nothing more than a point of light between the icy rings of Saturn. The rings visible here are the A ring (top), followed by the Keeler and Encke gaps, and finally the F ring at bottom. During this observation, Cassini was looking toward the backlit rings with the sun blocked by the disk of Saturn.

Cassini first photographed Earth from Saturn in July 2013. Credit: NASA/JPL-Caltech

Seen from Saturn, Earth and the other inner solar system planets always appear close to the sun much like Venus and Mercury do from Earth. All orbit interior to Saturn; even at maximum elongation, they never get far from the Sun. Early this month, as viewed from Saturn, Earth was near maximum elongation east of the sun, thus an “evening star,” making it an ideal time to take a picture.

In this cropped view of the April 13 image, you can better see the Moon, located a short distance to the left of the Earth. Credit: NASA/JPL-Caltech

Opportunities to capture Earth from Saturn have been rare in the 13 years Cassini has spent orbiting the ringed planet. The only other photo I’m aware of was snapped on July 19, 2013. Each is a precious document with a clear message: we are all tiny, please let’s be kind to one another.

This graphic shows Cassini’s flight path during the final two phases of its mission. The 20 Ring-Grazing Orbits are in gray (completed) and the 22 Grand Finale Orbits are in blue. The final partial orbit is colored orange. The first of the Grand Finale orbits begins on April 22 at 10:46 p.m. CDT. Credit: NASA/JPL-Caltech/Space Science Institute

We’ll soon miss the steady stream of artistic images of Saturn, its rings and moons by the Cassini team. The probe will make its final close flyby of the planet’s largest moon, fog-enshrouded Titan, at 1:08 a.m. April 22, at a distance of just 608 miles (979 km). That night at 10:46 p.m. CDT, Cassini will enter the first of its Grand Finale orbits, a series of 22 weekly dives between the planet and the rings. The first ring plane crossing is slated for midnight CDT April 25-26.


Cassini at Saturn and the Grand Finale

The coming week will be a busy one for Cassini. On each orbit, the probe will draw closer and closer to the butterscotch ball of Saturn until it finally tears across the cloud tops and burns up as a spectacular fireball on September 15. Scientists would rather see the craft burn up in Saturn’s atmosphere instead crash into a moon and possibly contaminate it.

Cassini will become a brilliant fireball streaking over Saturn’s cloud tops on the last day of its operation on September 15. Credit: NASA/JPL-Caltech

After nearly 20 years in space, seven of them spent traveling to the ringed planet, Cassini feels like family. It won’t be easy to say goodbye, but thanks to the probe, Saturn’s family album is bursting with remarkable images that will forever remind us the tenacity of this amazing machine and the vision and work of those who kept it operating for so many years.

Posted on by Evan Gough

The Bubbly Streams Of Titan

The appearing and disappearing feature observed in Titan's Lakes was dubbed "Magic Island". Image: NASA/JPL-Caltech/ASI/Cornell
The appearing and disappearing feature observed in Titan's Lakes was dubbed "Magic Island". Image: NASA/JPL-Caltech/ASI/Cornell

Saturn’s largest Moon, Titan, is the only other world in our Solar System that has stable liquid on its surface. That alone, and the fact that the liquid is composed of methane, ethane, and nitrogen, makes it an object of fascination. The bright spot features that Cassini observed in the methane seas that dot the polar regions only deepen the fascination.

A new paper published in Nature Astronomy digs deeper into a phenomenon in Titan’s seas that has been puzzling scientists. In 2013, Cassini noticed a feature that wasn’t there on previous fly-bys of the same region. In subsequent images, the feature had disappeared again. What could it be?

One explanation is that the feature could be a disappearing island, rising and falling in the liquid. This idea took hold, but was only an initial guess. Adding to the mystery was the doubling in size of these potential islands. Others speculated that they could be waves, the first waves observed anywhere other than on Earth. Binding all of these together was the idea that the appearance and disappearance could be caused by seasonal changes on the moon.

Titan's dense, hydrocarbon rich atmosphere remains a focal point of scientific research. Credit: NASA
Titan’s dense, hydrocarbon rich atmosphere remains a focal point of scientific research. Credit: NASA

Now, scientists at NASA’s Jet Propulsion Laboratory (JPL) think they know what’s behind these so-called ‘disappearing islands,’ and it seems like they are related to seasonal changes.

The study was led by Michael Malaska of JPL. The researchers simulated the frigid conditions on Titan, where the temperature is -179.2 Celsius. At that temperature, some interesting things happen to the nitrogen in Titan’s atmosphere.

On Titan, it rains. But the rain is composed of extremely cold methane. As that methane falls to the surface, it absorbs significant amounts of nitrogen from the atmosphere. The rain hits Titan’s surface and collects in the lakes on the moon’s polar regions.

The researchers manipulated the conditions in their experiments to mirror the changes that occur on Titan. They changed the temperature, the pressure, and the methane/ethane composition. As they did so, they found that nitrogen bubbled out of solution.

“Our experiments showed that when methane-rich liquids mix with ethane-rich ones — for example from a heavy rain, or when runoff from a methane river mixes into an ethane-rich lake — the nitrogen is less able to stay in solution,” said Michael Malaska of JPL. This release of nitrogen is called exsolution. It can occur when the seasons change on Titan, and the seas of methane and ethane experience a slight warming.

“Thanks to this work on nitrogen’s solubility, we’re now confident that bubbles could indeed form in the seas, and in fact may be more abundant than we’d expected,” said Jason Hofgartner of JPL, a co-author of the study who also works on Cassini’s radar team. These nitrogen bubbles would be very reflective, which explains why Cassini was able to see them.

The first-ever images of the surface of a new moon or planet are always exciting. The Huygens probe was launched from Cassini to the surface of Titan, but was not able investigate the lakes and seas on the surface. Image Credit: ESA/NASA/JPL/University of Arizona
The first-ever images of the surface of a new moon or planet are always exciting. The Huygens probe was launched from Cassini to the surface of Titan, but was not able investigate the lakes and seas on the surface. Image Credit: ESA/NASA/JPL/University of Arizona

The seas on Titan may be what’s called a prebiotic environment, where chemical conditions are hospitable to the appearance of life. Some think that the seas may already be home to life, though there’s no evidence of this, and Cassini wasn’t equipped to investigate that premise. Some experiments have shown that an atmosphere like Titan’s could generate complex molecules, and even the building blocks of life.

NASA and others have talked about different ways to explore Titan, including balloons, a drone, splashdown landers, and even a submarine. The submarine idea even received a NASA grant in 2015, to develop the idea further.

So, mystery solved, probably. Titan’s bright spots are neither islands nor waves, but bubbles.

Cassini’s mission will end soon, and it’ll be quite some time before Titan can be investigated further. The question of whether Titan’s seas are hospitable to the formation of life, or whether there may already be life there, will have to wait. What role the nitrogen bubbles play in Titan’s life question will also have to wait.