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The Shallow Sky

Saltation Excitation

Akkana Peck

On the left is a picture of the sand dunes within Proctor Crater on Mars. This photo was taken using the HiRISE camera on the Mars Reconnaissance Orbiter. (Photo courtesy of NASA/JPL-Caltech/University of Arizona). On the right is a picture of sand dunes in the Mojave desert taken. (Photo courtesy of Akkana Peck). In the MRO image, “the larger, darker bedforms are dunes composed of sand, most likely of fine size. Ripples tend to move slower than dunes. Because of this, over time, ripples get covered with dust, possibly explaining the bright tone visible here. The dunes are dark probably because they are composed of basaltic sand (derived from dark, volcanic rock) that is blown by the wind enough that dust does not sufficiently accumulate to change their color.” (NASA)


Saturn has just passed opposition and is entering its prime viewing time in April. The ring tilt goes from just under 3 degrees at the beginning of the month to under 1 degree at month’s end – almost the edge-on view we got last year! It’ll stay that way through the end of May, so if you didn’t get enough of edge-on rings last year, you have lots more time to look.

Crescent Mercury and gibbous Venus share the evening sky during the first week or two of April – nice to have a pair of evening planets back.

Jupiter, Uranus and Neptune and Pluto are all visible in the predawn sky, for those hardy souls willing to brave the cold mornings.

But of course we’re still in thrall to Mars, a few months past opposition and still well placed for viewing. It’s high in the sky by nightfall and visible most of the evening. But it’s receding from us rapidly now, and has shrunk to 8”, less than 60% of its size at opposition a few months ago. It will be noticeably gibbous by now, its eastern limb darkened.

Syrtis Major is up against that dark eastern limb around 9pm as April opens. Hellas, nestled against the south end of Syrtis Major, is right on the limb, looking more than ever like a bright polar cap in the southeast, almost opposite the real polar cap in the north. Strung out along the rest of the southern temperate zone are Maria Tyrrhenum and Cimmerium. Along the equator toward the western limb, look for the subtle dark smudges of Cerberus and Trivium Charontis, and the nearby brighter Elysium. In the north, look for a dark fringe around the polar cap, but it’s tough to see detail there in the area stretching from Mare Boreum through Utopia.

By the weekend of the 10th, all the easy dark features have rotated away, and we see a bright, nearly featureless expanse. Cimmerium is barely visible along the south edge of the east limb, with Cerberus and Trivium Charontis closer to the equator and more challenging to see. In the southwest, Mare Sirenum is starting to be visible, while the middle-west and northwest shows Olympus Mons and a little of the Tharsis plateau. Not that you can see much there, besides a few clouds if you’re lucky. In the north, you can probably see the darkness of Mare Boreum contrasting with the polar cap. Can you make out the dark smudge of Propontis and Erebus, farther from the pole?

Fast-forward another week and Tharsis is dead center around 9pm. Still no detail there. But in the south, Solis Lacos and the Eye of Mars are starting to be visible. Since the southern hemisphere is tilted away from us, the Eye area will be foreshortened and it might be tough to see it as an eye. What shape do you see? Meanwhile, dark Acidalium is peeking out in the northwest.

By April 24th there’s plenty to see, with Erythraeum and the many finger-like bays of Margaritifer in the south, looming Acidalium in the north. Sinus Meridiani and Sabaeus peek over the western limb to round out the view.

Finally, by the very end of the month, Meridiani and Sabaeus are just south of center, Acidalium still visible on the northeast limb and Syrtis Major showing up in the southwest.

So, lots of detail at the beginning and end of April. But what do you do for those couple of weeks when we’re seeing mostly blank space?

Think about the dunes of Mars!

A new paper announced a few months ago, analyzes Martian sand dunes. They look remarkably like the ones here on Earth: for example, like the Kelso dunes in the Mojave desert a few hundred miles south of here. The ones at Kelso are formed by wind: it blows the sand grains along the surface of a dune, making them hop in a process that’s called “saltation”. If you’ve ever visited Mojave National Preserve and climbed up the big star dune at Kelso, or visited the similar dune fields in Death Valley or Imperial Dunes, you’ve experienced the blowing sand and seen the saltation that keeps those big dune fields alive.

But Mars has such a thin atmosphere that it’s hard to understand how Martian winds could excite enough saltation to create the elaborate dunes seen by the Mars orbiters. Of course, Mars has slightly lower gravity than Earth, but not enough to compensate for the extremely thin air. So it was thought that the Martian wind shouldn’t be able to blow sand grains off the ground very often.

But a new computer model shows that as one grain gets pushed a little by the wind, it can land and knock several other grains into motion. They call the process “splashing”. When a sand grain splashes, Mars’s low gravity means that the sand grains it hits can kick as much as a meter into the air, where they get caught in stronger wind, speed up and then splash down to knock more particles into the air.

That’s enough to build huge dune fields, the equal of anything on Earth. So think about that as you’re contemplating Mars in your telescope, or the next time you drive through the Mojave.


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