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A change of fluids

A Piece of Mars: Water carved this ~800 m (0.5 mi) wide channel billions of years ago. The water dried up, and since then it’s been sand that flows through here (from the right), building up lovely dunes. A single crater on one of the dunes indicates that they’re not very active (dunes of this type on Mars all seem to be inactive, unlike their bigger, darker cousins). Look closely between the dunes and you might see a few little dots – these are boulders that have fallen, weathered out from the channel walls. (HiRISE ESP_022693_1530, NASA/JPL/Univ. of Arizona)

Another smoking gun in the search for life in Enceladus’ ocean

This illustration shows Cassini diving through the Enceladus plume in 2015.  Credits: NASA/JPL-Caltech

This illustration shows Cassini diving through the Enceladus plume in 2015.
Credits: NASA/JPL-Caltech

Today, NASA-funded scientists announced a major new step in the search for life on Enceladus, Saturn’s sixth-largest moon, thanks to new data collected by the NASA/ESA Cassini mission.

Enceladus has attracted a lot of interest because it has an active pole that spews jets of material into outer space. During its last flyby over that pole, an instrument on board the Cassini spacecraft detected the presence of a biomarker—molecular hydrogen. This suggests that the ocean we know lies beneath the moon’s surface could indeed contain an ecosystem similar to the ones we find in deep-sea hydrothermal vents on Earth. (more…)

Two directions

A Piece of Mars: Sometimes I just want to show the interior of a dune field, because it’s full of waves: ripples and dune crests, slip faces, all of which signs of movement. The dunes in this 0.67×0.47 km (0.41×0.29 mi) view have been made by two winds: one blowing from the top of the frame, and a more-recently-active one blowing from the right. Together, these two winds (and gravity) push this sand between a series of hills and down into Coprates Chasma, one of the longest canyons on Mars. (HiRISE ESP_035278_1655, NASA/JPL/Univ. of Arizona)

Where on Mars is this dune?

A Piece of Mars: This 0.48×0.27 km (0.3×0.17 mi) scene shows a rotund barchan dune. Can you tell from looking at it where on Mars it might be? To me the most obvious feature are the bumpy piles at the bottom of the slip face (at the foot of the dune on the right). They’re probably the remains of avalanches that occurred when there was still winter frost on the dunes. This is a summertime image, so the frost is long gone and the wind is reworking the dune, trying to erase signs of the cold season avalanches. This sort of pattern is best seen in dunes near the north pole. (HiRISE ESP_027674_2650, NASA/JPL/Univ. of Arizona)

A big rock in a big air stream

A Piece of Mars: Sand pours in from the top of this 1.95×1.95 km (1.21×1.21 mi) scene. The sand piles up and up (here ~115 m or 377 ft high), but ahead (at the bottom) is a mountain poking up. Like water diverting around a rock in a stream, the mountain affects the air flow just upwind of it, causing the sand to move around it. The steep dune slope is a slip face, caused by oversteepened sand avalanching. If you look closely, you’ll see some of those narrow avalanches near the bottom of the slip face (those at the top have been covered by ripples and falling sand). (HiRISE ESP_049045_1760, NASA/JPL/Univ. of Arizona)

More Earth-like views of Mars

A Piece of Mars: In a recent post (Dunes in a Colorful Hole), I showed some dunes crawling over layered terrain, with a view that looked a lot like some desert regions of Earth. Here’s another spot on Mars (0.95×1.1 km, 0.59×0.68 mi) showing yet more beautiful layers with dunes filling up the valleys. Part of what makes it seem Earth-like is the lack of craters, although if you go looking you’ll see there are some there. It’s hard to tell from here, but this whole scene is inside an old fluvial channel. The layers are thought to be lake deposits from when the river dammed up, ages ago. Since then the wind has taken over, taking apart the layers one grain at a time, and then building up dunes with some of those grains. (HiRISE PSP_010329_1525, NASA/JPL/Univ. of Arizona)

Windblown or not? Probably…

A Piece of Mars: This 0.95×0.95 km (0.59×0.59 mi) scene shows an eroding surface punctured by some old craters. Long, thin lines seem to form in the wake of many brighter knobs. Are those thin lines windblown in origin? They look like erosional features – things that are left behind when other stuff erodes away around it (not like sand dunes, which are things that pile up over time). If so, they don’t look like typical yardangs, which are streamlined bedrock, formed as sand wears down the rock. But this isn’t typical bedrock – it is easily erodible material. The bright knobs and crater rims are what’s left of a once-higher surface. The darker material may be a lag deposit that has built up as that brighter layer eroded down, leaving behind coarser grains that the wind has a harder time transporting (a similar process has occurred in Meridiani Planum, where the Opportunity rover drove through many kilometers of ripples, which now help protect the surface from erosion). If so, these long thin lines are a very unusual sort of yardang. (HiRISE ESP_016843_1590, NASA/JPL/Univ. of Arizona)

The Three Discoveries of Pan


Recent closeup of Pan, imaged by the Cassini spacecraft

This morning, NASA’s Cassini spacecraft obtained the first closeup images of Saturn’s innermost moon, Pan. The images show a peculiar body shaped like a “flying saucer”. Pan occupies a unique position in the rings, at the center of the 300-km wide Encke Gap. As best we can tell, Pan probably started its life as a more spherical moon, but it subsequently swept up a thick equatorial belt of ring-dust. A smattering of crevasses and craters across the surface add to our view of a moon that has endured a long and dynamic history. Yesterday, Pan was just a “tiny ring-moon”; today, it has been revealed as a world in its own right.

Seeing these images has brought back vivid memories of the day way back in June, 1990 when I became the first person to see Pan. That story is worth retelling, because Pan’s discovery story has almost as many twists and turns as Pan’s battered and bruised geology. For example, most planetary bodies have just a single discovery paper; Pan has three.

The story starts when the twin Voyager spacecraft flew by Saturn in 1980 and 1981. My colleague and mentor Jeff Cuzzi, of NASA’s Ames Research Center, was a member of the Imaging Team at the time, studying some of the newly revealed properties of the ring system. In those days, “image analysis” most often meant staring at photographic prints. Jeff had a stack of prints with him one day while waiting at the Albuquerque airport. By bending one particular print and looking along it lengthwise, he noticed something very strange–the Encke Gap had a wavy edge.


A sideways look at a Voyager image of the Encke Gap. By taking a print and sighting along the gap as shown, the wavy pattern at the inner (right) edge becomes visible.

Jeff realized that a small moon must be orbiting within this gap. As each particle at the gap edge passes the moon, it receives a gravitational tug that sets up this pattern. Once Jeff got home, he and his colleague Jeff Scargle set about examining all the other Voyager images of the Encke Gap. By assembling all of that data, they pinned down the moon–for now, let’s just call it “TBD”–into a 30-degree sector of longitude where no high-resolution images were available. They published their work in 1985 under the title, Wavy Edges Suggest Moonlet in Encke’s Gap. This is “discovery” paper #1.

Meanwhile, scientists were examining other ring data from the Voyager flyby. Two instruments had obtained “occultation profiles”–measurements of ring opacity at fine resolution across the rings. Both of the occultations showed periodic bright-dark variations near the Encke Gap, but at different locations and with different wavelengths. Jeff and I realized that these were slices through spiral patterns, which could be understood as another aspect of TBD’s influence on the ring. We called the phenomenon a “moonlet wake”. These patterns provided enough information that we were then able to pin down the precise orbit of still-unseen TBD. We published that paper in 1986, Satellite “wakes” and the orbit of the Encke Gap moonlet. “Discovery” paper #2.

After that, our interest in the topic subsided because it seemed, due to bad luck, that the Voyagers simply had not imaged the moon. (Also, meanwhile, we had the Voyager flybys of Uranus and Neptune to deal with.)

This brings us to June 1990, and one fundamental change. The Voyager images were finally available on that brand new, high-capacity storage medium, the CD-ROM. One morning it dawned on me that I had everything I needed to perform a truly comprehensive search for TBD. I had all 30,000 Voyager images of Saturn at my fingertips. I knew when they were taken and where they were pointed. Also, from paper #2, I knew exactly where TBD ought to be along its orbital path at each image time. I left for work that morning telling my husband that my plans for the day were to discover a moon of Saturn.


The discovery image of Pan. It occupies just a single bright pixel in the middle of the dark gap.

By early afternoon my program had printed out a table of every Voyager image that ought to contain TBD, along with where in the image to look. Scrolling down that list, I quickly identified the most promising image, loaded up the CDROM, and checked out the predicted location. Just there, I saw a single bright pixel in the middle of the Encke Gap! A single pixel could be a glitch, but I quickly examined the half-dozen next-best images, and they all showed a bright pixel at the predicted location. Nailed it!

“Eureka” moments don’t come along very often in a research career. That was mine.

Shortly thereafter, I published Visual Detection of 1981S13, Saturn’s Eighteenth Satellite, and its Role in the Encke Gap, discovery paper #3. However, in astronomy as in life, it’s seeing the thing that makes it real.

The Author

The Author, a very long time ago.

Two postscripts. First I am often asked how I chose the name. It was a no-brainer. The name of the process whereby a moon can open up a gap in rings is known as “shepherding.” Moons of Saturn are named after Greek gods. The god of shepherds in Greek mythology was that flute-playing satyr, Pan. Happily, the International Astronomical Union liked my reasoning and approved the name. Conveniently, the name is also short enough to fit on my license plate.

Second postscript. I proudly submitted my manuscript to the journal Nature. It was nearly rejected, however because the reviewer said, “we already knew the moon was there, so seeing it for the first time is no big deal.” Luckily for me, the editor was of the “seeing is believing” school of thought, and Nature published the paper anyway.



Hills made by wind and ice

A Piece of Mars: A fluid is something that fills a container it’s put into, and it includes both gas and liquids. This 0.7×0.5 km (0.43×0.31 mi) scene shows hills of sediment left behind by two different fluids (wind and ice). The hill on the left is a rippled sand dune, which has been piled up by the wind as it drops its sandy load. On the right is a layered sinuous hill, leftover from when ice flowed down a slope offscreen to the right. The dune is slowly encroaching on the hill, and will eventually be disrupted by it. (HiRISE ESP_048913_1330, NASA/JPL/Univ. of Arizona)

Dunes in a colorful hole

A Piece of Mars: Gray dunes have migrated over reddish rock, moving toward a narrowing cleft surrounded by tall tan cliffs. Bright lines on the dunes are exposed internal layers (bones of the dunes, really) that show you where the lee-side slopes once were (so you can tell they’ve moved to the left). The cliffs are made of layered rocks (extra points if you can find the fault), suggesting these are sedimentary layers, laid down long ago in Mars’ geologic past. The whole HiRISE image is worth a long look, it’s really amazing. (HiRISE ESP_049009_1520, NASA/JPL/Univ. of Arizona)