Posted on by David Dickinson

Images from Enceladus ‘Plume Dive’ Courtesy of Cassini

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Oh, to hitch a ride aboard NASA’s Cassini spacecraft this week. The Saturn orbiting sentinel recently completed an amazing series of passes near the enigmatic ice-covered moon Enceladus, including a daredevil dive only 49 km (31 miles) above the southern pole of the moon and through an ice geyser. Images of the dramatic flyby were released by the Cassini team earlier this morning, revealing the moon in stunning detail. 

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Enceladus vs the rings of Saturn. Image credit: NASA/JPL Caltech/Space Science Institute

“Cassini’s stunning images are providing us a quick look at Enceladus from this ultra-close flyby, but some of the most exciting science is yet to come,” says NASA mission project scientist Linda Spilker in today’s NASA/JPL press release.

Launched in 1997 from Cape Canaveral Florida in a dramatic night shot, Cassini arrived at the Saturnian system in 2004, and has delivered on some amazing planetary science ever since.

Discovered in 1789 by William Herschel, we got our very first views of Enceladus via the Voyager 1 spacecraft at 202,000 kilometers distant in 1980. Cassini has flown by the moon 21 times over the past decade, and ice geysers were seen sprouting from the surface of the moon by Cassini on subsequent flybys. one final flyby of Enceladus is planned for this coming December.

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Ice geysers ahead, in this Oct 28th view from Cassini. Image credit: NASA/JPL Caltech/Space Science Institute

 

Mission planners are getting more daring with the spacecraft as its mission nears completion in 2017. The idea of reaching out and ‘tasting’ an icy plume emanating from Enceladus has been an enticing one,  though a fast-moving good-sized ice pellet could spell disaster for the spacecraft.

NASA successfully established contact with the spacecraft on Wednesday night October 28th after the closest approach for the flyby at 11:22 AM EDT/ 15:22 UT (Universal Time) earlier in the day. Cassini is reported to be in good health, and we should see further images along with science data returns in the weeks to come.

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A closeup view of the icy terrain of the southern polar region of Enceladus from this weeks’ flyby. Image credit: NASA/JPL Caltech/Space Science Institute

A second, more distant flyby of Enceladus was completed by Cassini earlier this month as it passed 1,142 miles (1,839 kilometers) from the northern pole of Enceladus on October 14th, 2015 on its E-20 flyby.

But beyond just pretty post-cards from the outer solar system, Cassini’s successive passes by the mysterious moon will characterize just what might be occurring far down below.

Why Enceladus? Well, ever since ice geysers were spotted gushing from the fractured surface of the moon, it’s been on NASA’s short list of possible abodes for life in the solar system. Other contenders include Mars, Jupiter’s moon Europa, and Saturn’s giant moon, Titan. If the story of life on Earth is any indication, you need a place where an abundant level of chemical processes are occurring, and a subsurface ocean under the crust of Enceladus heated by tidal flexing may just fit the bill.

We’ll be adding further images and info to this post as more data comes in over the weekend, plus Cassini mission highlights, a look at the mission and final objectives and the last days of Cassini and more…

Stay tuned!

The end of Cassini in 2017 as it burns up in the atmosphere of Saturn will be a bittersweet affair, as our outer solar system eyes around the ringed planet fall silent. Cassini represents the most distant spacecraft inserted into orbit around a planet, and ESA’s Huygens lander on Titan marked the most remote landing on another world as well. Will we one day see a Titan Blimp or Ocean Explorer, or perhaps a dedicated life-finding mission to Enceladus?  Final mission objectives for NASA’s Cassini spacecraft include a final flyby of Saturn’s large moon Titan, which will set the course for its final death plunge into the atmosphere of Saturn on September 15th, 2017.

A high-resolution capture of Enceladus released this weekend by the Cassini team. The spacecraft was about 60,000 miles (96,000 kilometers) out when this image was taken. You can see the stark contract of the moon's fractured cantlope terrain, versus craters in the opposite hemisphere imaged criedt: NASA/JPL-CalTech/Space Science Institute
A high-resolution capture of Enceladus released this weekend by the Cassini team. The spacecraft was about 60,000 miles (96,000 kilometers) out when this image was taken. You can see the stark contract of the moon’s fractured cantaloupe terrain, versus craters in the opposite hemisphere imaged. Credit: NASA/JPL-CalTech/Space Science Institute

Want to see Enceladus for yourself? The moon orbits Saturn once every 1.4 days, reaching a maximum elongation of 13″ from the ring tips of Saturn and a maximum brightness of magnitude +11.7. Enceladus is one of six major moons of Saturn visible in a backyard telescope, and one of 62 moons of the ring planet known overall. The other five moons within reach of an amateur telescope are: Titan, Mimas, Dione, Rhea, and Tethys, and the fainter moon Hyperion shining at magnitude +15 might just be within reach of skill observers with large light bucket instruments.

Enjoy the amazing views of Enceladus, courtesy of Cassini!

Posted on by Nancy Atkinson

Are We Just ‘Lucky’ to See Activity on Enceladus?

Caption: Geysers on Enceladus. Credit: NASA, JPL, Space Science Institute

One of the most exciting but unexpected discoveries of the Cassini mission is seeing the activity taking place on Saturn’s small moon Enceladus. Between the active geysers, the unusual “tiger stripes” and the surprisingly young surface near the moon’s south pole, Enceladus has surprised scientists with almost all the images and data the gathered by the spacecraft. But is the moon always active, or are we just in the right place at the right time, lucky to be catching it during an active phase? A recent paper outlines a model in which the kind of geologic eruptions now visible on Enceladus only occur every billion years or so.

“Cassini appears to have caught Enceladus in the middle of a burp,” said Francis Nimmo, a planetary scientist at the University of California Santa Cruz. “These tumultuous periods are rare and Cassini happens to have been watching the moon during one of these special epochs.”

Nimmo and co-author Craig O’Neill of Macquarie University in Sydney, Australia propose that blobs of warm ice that periodically rise to the surface and churn the icy crust on Saturn’s moon Enceladus explain the quirky heat behavior and intriguing surface of the moon’s south polar region.

The most interesting features by far in the south polar region of Enceladus are the fissures known as “tiger stripes” that spray water vapor and other particles out from the moon. While Nimmo and O’Neill’s model doesn’t link the churning and resurfacing directly to the formation of fissures and jets, it does fill in some of the blanks in the region’s history.

Enceladus. Credit: NASA/JPL/Space Science Institute

“This episodic model helps to solve one of the most perplexing mysteries of Enceladus,” said Bob Pappalardo, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., of the research done by his colleagues. “Why is the south polar surface so young? How could this amount of heat be pumped out at the moon’s south pole? This idea assembles the pieces of the puzzle.”

But not everyone is convinced this model answers all the questions about Enceladus. Carolyn Porco, who leads the imaging team for Cassini said via Twitter regarding this paper, “Beware! Several different models out there say different things.”

About four years ago, Cassini’s composite infrared spectrometer instrument detected a heat flow in the south polar region of at least 6 gigawatts, the equivalent of at least a dozen electric power plants. This is at least three times as much heat as an average region of Earth of similar area would produce, despite Enceladus’ small size. The region was also later found by Cassini’s ion and neutral mass spectrometer instrument to be swiftly expelling argon, which comes from rocks decaying radioactively and has a well-known rate of decay.

Calculations told scientists it would be impossible for Enceladus to have continually produced heat and gas at this rate. Tidal movement – the pull and push from Saturn as Enceladus moves around the planet – cannot explain the release of so much energy.

The surface ages of different regions of Enceladus also show great diversity. Heavily cratered plains in the northern part of the moon appear to be as old as 4.2 billion years, while a region near the equator known as Sarandib Planitia is between 170 million and 3.7 billion years old. The south polar area, however, appears to be less than 100 million years old, possibly as young as 500,000 years.

O’Neill had originally developed the model for the convection of Earth’s crust. For the model of Enceladus, which has a surface completely covered in cold ice that is fractured by the tug of Saturn’s gravitational pull, the scientists stiffened up the crust. They picked a strength somewhere between that of the malleable tectonic plates on Earth and the rigid plates of Venus, which are so strong, it appears they never get sucked down into the interior.

These drawings depict explanations for the source of intense heat that has been measured coming from Enceladus' south polar region. Credit: NASA/JPL

Their model showed that heat building up from the interior of Enceladus could be released in episodic bubbles of warm, light ice rising to the surface, akin to the rising blobs of heated wax in a lava lamp. The rise of the warm bubbles would send cold, heavier ice down into the interior. (Warm is, of course, relative. Nimmo said the bubbles are probably just below freezing, which is 273 degrees Kelvin or 32 degrees Fahrenheit, whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Fahrenheit.)

The model fits the activity on Enceladus when the churning and resurfacing periods are assumed to last about 10 million years, and the quiet periods, when the surface ice is undisturbed, last about 100 million to two billion years. Their model suggests the active periods have occurred only 1 to 10 percent of the time that Enceladus has existed and have recycled 10 to 40 percent of the surface. The active area around Enceladus’s south pole is about 10 percent of its surface.

Source: JPL