Tag Archives: Saturn

Methane detected on Enceladus could come from microbes

The uncertainty of science: New research has found that the methane that Cassini detected being released from Enceladus’s interior could conceivably come from at least one Earth-type microbe.

Using various mixtures of gases held at a range of temperatures and pressures in enclosed chambers called “bioreactors,” Rittmann and his co-authors cultivated three microorganisms belonging to the oldest branch of Earth’s tree of life, known as Archaea. In particular, they focused on Archaean microbes that are also methanogens, which are able to live without oxygen and produce methane from that anaerobic metabolism. The team examined the simplest types of microbes, which could be the primary producers of methane at the base of a possibly more complex ecological food chain within the moon.

They tried to simulate the conditions that could exist within and around Enceladus’s hydrothermal vents, which are thought to resemble those found at a few deep-sea sites on Earth, often near volcanically active mid-oceanic ridges. According to their tests, only one candidate, the deep-sea microbe Methanothermococcus okinawensis, could grow there—even in the presence of compounds such as ammonia and carbon monoxide, which hinder the growth of other similar organisms.

There are a lot of fake news stories today trumpeting this result as proof that alien microbes can exist on Enceladus. The data does no such thing. All it shows that one methane producing microbe could possibly live in an environment that researchers guess might somewhat resemble the situation on Enceladus. However, as the article admits,

Scientists do not really know the precise conditions on Enceladus yet, of course. And in any case it is possible any life there, if it exists, is nothing like any DNA-based organism on our planet, rendering our Earth-based extrapolations moot. What’s more, these findings only show microbial life might exist in one particular subset of possible environments within the moon’s dark ocean.

This result is interesting, but it really proves nothing about Enceladus itself.

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Titan’s liquid hydrocarbon seas have a global sea level

New research using Cassini data has revealed that the liquid hydrocarbon oceans of Saturn’s moon Titan have a global sea level, with some small lakes perched at higher elevations.

The new study suggests that elevation is important because Titan’s liquid bodies appear to be connected under the surface in something akin to an aquifer system at Earth. Hydrocarbons appear to be flowing underneath Titan’s surface similar to the way water flows through underground porous rock or gravel on Earth, so that nearby lakes communicate with each other and share a common liquid level.

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Saturn’s rings are very young

Data from Cassini’s last ring-diving orbits has now strengthened the hypothesis that Saturn’s rings formed very recently, just a few hundred million years ago.

Saturn acquired its jewels relatively late in life. If any astronomers had gazed at the sky in the time of the dinosaurs, they might have seen a bare and boring Saturn.

It was then that some sort of catastrophe struck the gas giant. Perhaps a stray comet or asteroid struck an icy moon, tossing its remnants into orbit. Or maybe the orbits of Saturn’s moons somehow shifted, and the resulting gravitational tug-of-war pulled a moon apart. However it happened, two new lines of evidence from Cassini make it clear that the rings were not around in the early days of the solar system 4.5 billion years ago, as scientists had long believed, says Jeff Cuzzi, a ring specialist at NASA’s Ames Research Center in Mountain View, California. “It rules out the primordial ring story,” Cuzzi says. “That’s what it looks like to me.”

At the moment there is no consensus on what might have caused the rings formation so recently.

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Cassini’s last full mosaic of Saturn and its moons

Cassini's last full mosaic of Saturn

Cool image time! The image above, cropped and reduced in resolution to show here, was created by Cassini from 42 images taken on September 13, 2017, two days before the spacecraft dived into Saturn to end its mission.

Be sure to take a look at the full image. It shows six of Saturn’s moons, as well as many stars.

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Billionaire Yuri Milner considering funding mission to Enceladus

Capitalism in space: Billionaire Yuri Milner, who already funds several astronomy projects aimed at interstellar travel, is now considering funding a planetary probe to the Saturn moon Enceladus.

At the moment all he is doing is holding workshops with scientists and engineers to see if such a mission can be done for an amount he can afford. Considering that Elon Musk’s first concept to send a private probe to Mars, before SpaceX existed, was stopped because of high launch costs, thus becoming the inspiration for SpaceX itself in order to lower those costs, Milner’s private effort might actually be affordable now.

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The glory of Cassini’s Saturn

The glory of Saturn

Cool image time! The image on the right, reduced in resolution to post here, was taken by Cassini on August 17, 2017, one month before the spacecraft dived into Saturn to end its mission.

This view looks toward the sunlit side of the rings from about 19 degrees above the ringplane. The image was taken in green light with the Cassini spacecraft wide-angle camera on Aug. 12, 2017. Pandora was brightened by a factor of 2 to increase its visibility.

The view was obtained at a distance to Saturn of approximately 581,000 miles (935,000 kilometers) from Saturn. Image scale is 35 miles (56 kilometers) per pixel. The distance to Pandora was 691,000 miles (1.1 million kilometers) for a scale of 41 miles (66 kilometers) per pixel.

The moon Pandora can be seen in the full resolution image just beyond the outermost ring near the top of the screen.

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New findings from Cassini’s final months

Link here. Among a bunch of other interesting results, this one I found the most tantalizing:

The spacecraft’s Ion and Neutral Mass Spectrometer (INMS) returned a host of first-ever direct measurements of the components in Saturn’s upper atmosphere, which stretches almost to the rings. From these observations, the team sees evidence that molecules from the rings are raining down onto the atmosphere. This influx of material from the rings was expected, but INMS data show hints of ingredients more complex than just water, which makes up the bulk of the rings’ composition. In particular, the instrument detected methane, a volatile molecule that scientists would not expect to be abundant in the rings or found so high in Saturn’s atmosphere.

The results generally leave behind more questions than answers, but that is exactly why exploring the universe is so much fun.

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Storms on Titan dump a lot of liquid methane, quickly

A new climate model for Titan that suggests the planet’s methane rainstorms occur about once a Saturn year (29.5 years) and at about 60 degree latitude correlates with Cassini surface data that found a high concentration of alluvial fans at that latitude.

“The most intense methane storms in our climate model dump at least a foot of rain a day, which comes close to what we saw in Houston from Hurricane Harvey this summer,” said Mitchell, the principal investigator of UCLA’s Titan climate modeling research group.

Sean Faulk, a UCLA graduate student and the study’s lead author said the study also found that the extreme methane rainstorms may imprint the moon’s icy surface in much the same way that extreme rainstorms shape Earth’s rocky surface. On Earth, intense storms can trigger large flows of sediment that spread into low lands and form cone-shaped features called alluvial fans. In the new study, the UCLA scientists found that regional patterns of extreme rainfall on Titan are correlated with recent detections of alluvial fans, suggesting that they were formed by intense rainstorms.

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The kittens of Saturn’s rings

The scientists who used Cassini to identify about 60 transient clumps in Saturn’s rings have dubbed them “kittens” and have been naming them appropriately.

Saturn’s kittens are a group of small clumps and baby moons, or moonlets, that occupy the planet’s F ring. Like the rest of Saturn’s rings, this thin outer ring is made up of countless particles that range in size. When enough of those particles bump into one another and stick together, they aggregate into larger clumps — and become eligible for a kitten name.

So far, the list of Saturn’s kitten names includes several classics, like Fluffy, Garfield, Socks and Whiskers. These are unofficial nicknames for more-complicated (and less adorable) official titles like “Alpha Leonis Rev 9” (aka, Mittens). The technical names for these features come from events called stellar occultations, during which Cassini was able to detect the little clumps. In a stellar occultation, a star passes behind Saturn’s rings from Cassini’s point of view.

Most of these clumps will likely never be found again, so their unofficial kitten names are essentially just for fun.

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Cassini’s last image of Iapetus

Iapetus

The image on the right is a cropped and scaled up version of one of Cassini’s last images of Saturn’s moon, Iapetus.

The moon is unique in that its east and west hemispheres have completely opposite albedos, with one being very dark and the other very bright. It also has a very distinctive large crater, seen in this image. Scientists do not quite understand what causes the dichotomy, though they have models that partly explain it, partly from material being deposited on the moon’s leading hemisphere combined with the temperature differences at different latitudes.

The cause of the extreme brightness dichotomy on Iapetus is likely to be thermal segregation of water ice on a global scale. Thermal effects are usually expected to act latitudinally. That is, polar areas are colder than equatorial terrain in most cases due to the more oblique angle of the solar irradiation. Therefore, an additional process is required to explain the longitudinal difference as well. In one model, dark, reddish dust coming in from space and preferentially deposited on the leading side forms a small, but crucial difference between the leading and trailing hemispheres, which is sufficient to allow the thermal effect to evaporate the water ice on the leading side completely, but only marginally on the trailing side.

It was this moon’s strange dichotomy that had Arthur Clarke use it in 2001: A Space Odyssey. While Cassini’s images clearly prove that the brightness difference was not created by an alien civilization, as imagined by Clarke, those images have not really provided us a full explanation for its cause. The uncertainty of science marches on!

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The Worst Part of Losing Cassini Is That It Has No Replacement

Link here. This article is an honest review of the current lack of concrete plans by anyone to send a new probe to Saturn. While there are some tentative missions in the works, nothing is certain.

[I]f looking back on Cassini’s major discoveries at Saturn, Titan, and Enceladus have left you thirsty for more, we have some bad news: That thirst is going to go unquenched for a while. Talks of Uranus and Neptune missions are tentative at best. The best hope for Saturn now comes from NASA’s New Frontiers program, which looks for excellent medium-cost missions has spawned spacecraft including Jupiter’s Juno and Pluto’s New Horizons. This round of New Frontiers missions must launch by 2024, and there are two Enceladus proposals, a Titan proposal, and a Saturn atmospheric probe under consideration. We may hear word about those proposals by the end of the year “Hang tight, we’re going through the evaluations now and we’ll be announcing at the end of the year what some of the finalists will be,” Jim Green, NASA Planetary Science Director, said at the Cassini press conference Friday morning.

This list of possible Saturn missions sounds great, but they are all competing against each other and a number of other equally (and possibly more) interesting missions to other places. And with the federal budget out of control and mired in debt, there isn’t really a lot of money to go around.

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Cassini’s mission ends

Enceladus as seen by Cassini two days before mission end

After a seven year journey and thirteen years in orbit around Saturn, Cassini’s mission ended early this morning with a dive into Saturn’s atmosphere.

The image above, cropped to show here, is actually one from a short movie, showing Enceladus setting behind the horizon of Saturn. The images were taking two days ago, but provide a fitting image for the mission’s end.

Some of the best images from the dive, including Cassini’s last image, can be found here.

While most of the press will rightly wax eloquent about the magnificence of this mission, my focus remains on what will come next. We no longer have any way to observe what is happening on Saturn. We are blind. We should not be.

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Cassini on final approach to Saturn

Engineers have confirmed that Cassini is now on its final approach to Saturn, with a scheduled dive into the gas giant set for Friday, September 15.

The mission’s final calculations predict loss of contact with the Cassini spacecraft will take place on Sept. 15 at 7:55 a.m. EDT (4:55 a.m. PDT). Cassini will enter Saturn’s atmosphere approximately one minute earlier, at an altitude of about 1,190 miles (1,915 kilometers) above the planet’s estimated cloud tops (the altitude where the air pressure is 1-bar, equivalent to sea level on Earth). During its dive into the atmosphere, the spacecraft’s speed will be approximately 70,000 miles (113,000 kilometers) per hour. The final plunge will take place on the day side of Saturn, near local noon, with the spacecraft entering the atmosphere around 10 degrees north latitude.

We will get Cassini’s last images about 90 minutes after its death.

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An oral history of the Cassini mission to Saturn

Link here. Those who have read my book on the building of the Hubble Space Telescope will recognize many of the same people and political maneuvers used to get the project off the ground and funded.

Note too that the idea of Cassini was first proposed in 1982, but it didn’t actually launch until 1997. Fifteen years. While today I think such a spacecraft could go from concept to launch much faster, this timeline gives us a guide on when the next Saturn orbiter might launch. At the earliest do not expect another mission to Saturn to launch before 2025.

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Cassini says goodbye to Saturn

Saturn, October 2016

Cool image time! The picture above, reduced in resolution to show here, was taken in October 2016 during one of Cassini’s last distant orbits that gave it a global view of Saturn and its rings. Since it began its dives close to the gas giant such views have not been possible.

The mission ends this coming Saturday with a dive into Saturn. It was launched in October 1997, and after a seven year journey has spent the last thirteen years in orbit around the planet, providing us the first long term glimpse of a gas giant as its seasons evolved.

Cassini has been orbiting Saturn for nearly a half of a Saturnian year but that journey is nearing its end. This extended stay has permitted observations of the long-term variability of the planet, moons, rings, and magnetosphere, observations not possible from short, fly-by style missions.

When the spacecraft arrived at Saturn in 2004, the planet’s northern hemisphere, seen here at top, was in darkness, just beginning to emerge from winter (see Cassini’s Holiday Greetings​). Now at journey’s end, the entire north pole is bathed in the continuous sunlight of summer.

The spacecraft was also able to observe the seasonal changes that occurred to Titan. It also studied the plumes coming from the tiger stripe cracks on Enceladus, shown below the fold in a movie created by Cassini over a 14 hour time period in August 2017.

I expect that scientists will be exploring Cassini’s data archive for decades, finding many things not noticed in their initial viewing. Unfortunately, we will not have another spacecraft taking new pictures in orbit around Saturn to compare with Cassini’s past images for many decades to come. On Saturday, we go blind.
» Read more

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Saturn’s magnificent rings

Saturn's rings

The Cassini science team released two sets of images taken by the spacecraft of Saturn’s rings.

The image above, reduced in resolution to show here, is from the second link. As they note,

The pale tan color is generally not perceptible with the naked eye in telescope views, especially given that Saturn has a similar hue.

The material responsible for bestowing this color on the rings—which are mostly water ice and would otherwise appear white—is a matter of intense debate among ring scientists that will hopefully be settled by new in-situ observations before the end of Cassini’s mission.

The different ringlets seen here are part of what is called the “irregular structure” of the B ring. Cassini radio occultations of the rings have shown that these features have extremely sharp boundaries on even smaller scales (radially, or along the direction outward from Saturn) than the camera can resolve here. Closer to Saturn, the irregular structures become fuzzier and more rounded, less opaque, and their color contrast diminishes.

Check out both. They reveal to me that our understanding of these rings remains essentially nil, even after more than a dozen years of study by Cassini.

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Cassini movie flying past Saturn’s rings

movie of Cassini flying past Saturn's rings

Cool image time! The Cassini science team has assembled a short movie from 21 images taken by the spacecraft during its August 20th dive between the rings and Saturn. That animation is to the right.

Only two weeks remain in the Cassini mission. The spacecraft dives into Saturn on September 15.

The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn’s upper atmosphere. But on the way down, before contact is lost, eight of Cassini’s 12 science instruments will be operating. In particular, the spacecraft‘s ion and neutral mass spectrometer (INMS), which will be directly sampling the atmosphere’s composition, potentially returning insights into the giant planet’s formation and evolution. On the day before the plunge, other Cassini instruments will make detailed, high-resolution observations of Saturn’s auroras, temperature, and the vortices at the planet’s poles. Cassini’s imaging camera will be off during this final descent, having taken a last look at the Saturn system the previous day (Sept. 14).

The second link above gives a detailed moment-by-moment breakdown of the final six days.

I would have posted this earlier this week, but my limited internet access made it impossible. Sorry about that.

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The gentle storms of Saturn

Saturn's gentle storms

Cool image time! The Cassini image above, cropped and reduced slightly, shows a close-up view of Saturn’s cloud tops, taken during Cassini’s May 18, 2017 fly-by.

Clouds on Saturn take on the appearance of strokes from a cosmic brush thanks to the wavy way that fluids interact in Saturn’s atmosphere. Neighboring bands of clouds move at different speeds and directions depending on their latitudes. This generates turbulence where bands meet and leads to the wavy structure along the interfaces.

What I see is a much less turbulent storm pattern, when compared with Jupiter. This is not to say that the weather of Saturn is quiet or peaceful, for it certainly cannot be, considering the gas giant’s size and the depth of the atmosphere. Still, this image suggests that the turbulence is less violent here, possibly because Saturn is farther from the Sun and is hit with less solar energy, and because Saturn is smaller and thus produces less of its own internal energy.

Either way, it is beautiful and mysterious, in the way are all such images of alien places.

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Cassini’s last close look at Titan

Titan's magic lake district

The Cassini science team has released the last radar swath that the spacecraft will take of Titan, imaged on April 22.

You can see the full swath up close here. The image above is my crop of the section on the swath’s right portion, showing the shoreline of the hydrocarbon lake Ligeia Mare, where periodically an island has been seen by radar to intermittently appear and disappear.

No “island” feature was observed during this pass. Scientists continue to work on what the transient feature might have been, with waves and bubbles being two possibilities.

The fly-by also took the first, and last, depth measurement of 8 other lakes, finding that they all had the same depth, suggesting they are connected by an underground “water” table. In this case, it ain’t water, but liquid hydrocarbons like methane.

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Cassini’s last five orbits of Saturn

Cassini is about to begin its last of five orbits of Saturn, before it is sent into the planet’s atmosphere to burn up.

Cassini will make the first of these five passes over Saturn at 12:22 a.m. EDT Monday, Aug. 14. The spacecraft’s point of closest approach to Saturn during these passes will be between about 1,010 and 1,060 miles (1,630 and 1,710 kilometers) above Saturn’s cloud tops.

The spacecraft is expected to encounter atmosphere dense enough to require the use of its small rocket thrusters to maintain stability – conditions similar to those encountered during many of Cassini’s close flybys of Saturn’s moon Titan, which has its own dense atmosphere. “Cassini’s Titan flybys prepared us for these rapid passes through Saturn’s upper atmosphere,” said Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory (JPL) in California. “Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict.”

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Looking down at Saturn’s rings

Looking down at Saturn's rings

Cool image time! The image on the right, reduced in resolution to show here, was taken by Cassini in May and looks down at the outer rings of Saturn. The moon Prometheus can also be seen in the large gap between the main rings and the outermost F ring.

Most of the small moon’s surface is in darkness due to the viewing geometry here. Cassini was positioned behind Saturn and Prometheus with respect to the sun, looking toward the moon’s dark side and just a bit of the moon’s sunlit northern hemisphere.

Also visible here is a distinct difference in brightness between the outermost section of Saturn’s A ring (left of center) and rest of the ring, interior to the Keeler Gap (lower left).

The image clearly shows the gravitational influence of the moon on the outer ring. As Prometheus orbits past the F ring its mass creates waves through the ring’s materials.

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The endless bands of Saturn

Cool image time. One of the images released by the Cassini science team this week when it announced the first results from the spacecraft’s weekly dives between Saturn and its innermost rings was a short video made from 137 images taken on its first dive on April 26, 2017.

It is absolutely worthwhile to view this video. It begins at Saturn’s north pole, looking down into what appears to be a bottomless vortex, and continues south to almost the equator. Along the way the movie captures what seems to be innumerable horizontal bands across the gas giant’s surface. Not only are do we see the major bands that have been observed from Earth for centuries, there are bands within bands, and bands within those bands. Like a fractal it appears that the deeper you go, the more horizontal jet streams you see.

Like Juno at Jupiter, the mysteries of a gas giant like Saturn is overwhelming. This is a big and very active planet. We understand almost nothing about its weather systems, its atmosphere, and its interior. And this glimpse by Cassini is only that, a mere glimpse. When Cassini’s mission ends in September, it will leave us a treasure trove of knowledge. It will also leave us a much larger library of unanswered questions, all of which will remain unanswered until we can return, decades hence, with new probes..

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First results from Cassini’s dives between Saturn and its rings

The first results from Cassini’s weekly dives between Saturn and its innermost rings have now been released.

The big surprise so far is the lack of a tilt to Saturn’s magnetic field.

Based on data collected by Cassini’s magnetometer instrument, Saturn’s magnetic field appears to be surprisingly well-aligned with the planet’s rotation axis. The tilt is much smaller than 0.06 degrees — which is the lower limit the spacecraft’s magnetometer data placed on the value prior to the start of the Grand Finale.

This observation is at odds with scientists’ theoretical understanding of how magnetic fields are generated. Planetary magnetic fields are understood to require some degree of tilt to sustain currents flowing through the liquid metal deep inside the planets (in Saturn’s case, thought to be liquid metallic hydrogen). With no tilt, the currents would eventually subside and the field would disappear.

Any tilt to the magnetic field would make the daily wobble of the planet’s deep interior observable, thus revealing the true length of Saturn’s day, which has so far proven elusive.

They also have gotten lots of much better images of the planet’s cloud tops.

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The soft craters of Epimetheus

The soft craters of Epimetheus

Cool image time! The image on the right, reduced in resolution to show here, is the highest resolution image that Cassini has taken of the Saturn moon Epimetheus, taken from only 9,300 miles away on February 21, 2017.

Epimetheus (70 miles or 113 kilometers across) is too small for its gravity to hold onto an atmosphere. It is also too small to be geologically active. There is therefore no way to erase the scars from meteor impacts, except for the generation of new impact craters on top of old ones.

Below is the inset at full resolution, showing several craters, with ponds of dust on their floor. Overall, the surface of this tiny moon looks soft. The craters are all shallow, as if any impact merely plunged into a blob of ice cream. Any ejecta from those impacts eventually rained back down, and then settled slowly in the moon’s low points, forming those ponds of dust.

close-up of soft craters

In many ways this image is very revealing, as it shows what the early accretion process of any planetary body will look like. Nor is this unique. Earlier images taken of the asteroid Eros by the NEAR probe saw many of these same features, as have images of Saturn’s other small moons. In the early stages, new material gets absorbed easily because it finds it easy to bore into the body of the newly formed and not very dense planetary body. There isn’t much ejecta, and what there is doesn’t fly that far away so that it can settle back down on the surface and add to the new body’s total mass.

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Titan’s clearing northern skies

Titan's clearing northern skies

The image of Titan on the right, reduced in resolution to show here, was taken by Cassini less than a week ago, on June 9, as it continues its last orbits of Saturn prior to crashing into the gas giant’s atmosphere in September.

NASA’s Cassini spacecraft sees bright methane clouds drifting in the summer skies of Saturn’s moon Titan, along with dark hydrocarbon lakes and seas clustered around the north pole. Compared to earlier in Cassini’s mission, most of the surface in the moon’s northern high latitudes is now illuminated by the sun. …Summer solstice in the Saturn system occurred on May 24, 2017.

When Cassini arrived more than a decade ago, it was winter on Titan’s northern hemisphere, and the atmosphere was thick with haze. Now the sky is clearing as the stormy weather shifts to the winter in the southern hemisphere.

As with yesterday’s global map of Mimas, this image is in many ways a farewell look at Titan. While Cassini will likely get a few more global images of the Saturn moon before the mission ends in September, this image essentially marks the end of our ability to observe this strange planet closely, for decades to come. When Cassini crashes into Saturn, our vision at Saturn will go blind. And no one knows when our sight there will return, as no mission is presently in the works, or is even being considered, to return to Saturn.

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A new map of Mimas

Global map of Mimas

The Cassini science team has released what will be the best map of Saturn’s moon Mimas for many decades to come. A reduced resolution version is above, and was updated after the most recent fly-bys in November 2016 and February 2017.

The moon’s large, distinguishing crater, Herschel, is seen on the map at left. The map is an equidistant (simple cylindrical) projection and has a scale of 710 feet (216 meters) per pixel at the equator. The mean radius of Mimas used for projection of this map is 123.2 miles (198.2 kilometers). The resolution of the map is 16 pixels per degree.

Since Cassini is in its final orbits and will crash into Saturn in September, and no other Saturn mission is even being planned, do not expect a better map for a very long time.

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Pole of Saturn’s moon Enceladus shifted 55 degrees in the past

Data from Cassini now suggests that the moon Enceladus shifted its pole as much as 55 degrees sometime in the past, possibly due to an impact.

Whether it was caused by an impact or some other process, Tajeddine and colleagues think the disruption and creation of the tiger-stripe terrain [at south pole] caused some of Enceladus’ mass to be redistributed, making the moon’s rotation unsteady and wobbly. The rotation would have eventually stabilized, likely taking more than a million years. By the time the rotation settled down, the north-south axis would have reoriented to pass through different points on the surface — a mechanism researchers call “true polar wander.”

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After 7 years Cassini reaches Saturn’s solstice

In 2010 NASA extended the Cassini mission orbiting Saturn for seven years to the planet’s next solstice, so that the spacecraft could observe Saturn, its moons, and its rings, for one full season of its 28 year orbit.

Today, Cassini reached that target.

NASA’s Cassini spacecraft still has a few months to go before it completes its mission in September, but the veteran Saturn explorer reaches a new milestone today. Saturn’s solstice — that is, the longest day of summer in the northern hemisphere and the shortest day of winter in the southern hemisphere — arrives today for the planet and its moons. The Saturnian solstice occurs about every 15 Earth years as the planet and its entourage slowly orbit the sun, with the north and south hemispheres alternating their roles as the summer and winter poles.

The article provides a detailed review of all the changes that have occurred during this long time period.

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Propellers by the dozens

Propellers

Cool image time! The latest image releases from Cassini, taken during its recent close fly-by campaign of the rings of Saturn, focus on the propeller features produced in the rings by larger ring objects. The image on the right, reduced to show here, reveals dozens of propellers of all shapes and sizes.

The original discovery of propellers in this region in Saturn’s rings … was made using several images taken from very close to the rings during Cassini’s 2004 arrival at Saturn. Those discovery images were of low resolution and were difficult to interpret, and there were few clues as to how the small propellers seen in those images were related to the larger propellers Cassini observed later in the mission….

This image, for the first time, shows swarms of propellers of a wide range of sizes, putting the ones Cassini observed in its Saturn arrival images in context. Scientists will use this information to derive a “particle size distribution” for propeller moons, which is an important clue to their origins.

The parallel pattern of rings in the center of the image is a series of density waves in the ring structure, caused by an interaction with one of Saturn’s larger nearby moons.

They have also released the best view we can now expect of a propeller by Cassini.

This is the third and final propeller to be targeted for a close flyby observation during Cassini’s ring-grazing orbits (the period from Nov. 2016 to April 2017 when Cassini’s orbit passed just outside the main rings). …Because propellers are seen in the outermost parts of the main rings, the ring-grazing orbits provided Cassini’s best opportunity to see them up close.

Cassini is now diving between the rings and the planet, so the propellers are farther away.

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