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It’s a rock-eat-rock world

A Piece of Mars: This 738 x 415 m (0.46 x 0.26 mi) scene shows dark sand flowing down a channel bisected by a ~60 m (~200 ft) tall, thin “island”. That island, and many others around it (see the whole image), is what remains after windblown sand slowly carved away the rest of the rock, the same way rivers slowly cut through rock on Earth. The presence of the dark sand shows that the process is still active today. (HiRISE ESP_04400_1750, NASA/JPL/Univ. of Arizona)

The smallest dunes

A Piece of Mars: There are two small dome-shaped dunes in this frame (0.96×0.54 or 0.6×0.33 mi). If they got any larger, they’d form slip faces. Any smaller and they’d just be random drifts of windblown sand. Dunes form at a particular size (~125 m in this case) related to the distance it takes for sand grains to accelerate to the background wind speed. This distance is bigger on Mars than on Earth, where the smallest dunes are ~20 m across. (HiRISE ESP_044198_1480 NASA/JPL/Univ. of Arizona)

Gravitational wave detection rumors may end on Feb 11

It is official. NSF, together with scientists from Caltech, MIT and the LIGO collaboration will give an update on their effort to detect gravitational waves.

What is LIGO? Check out this article published in Arstechnica by Eric Berger.

I am not going to speculate on the announcement and will simply wait for it. Joe Giaime a California Institute of Technology physicist who manages the lab and also a professor at Louisiana State University was pretty clear in the Arstechnica interview about the way this group works: “We’re really kind of old school,” he said. “We analyze our data. If there’s anything interesting we write it up in papers. We send the papers to the journals. If and only if there’s an interesting discovery that passes muster, and it has been accepted for publication by a journal, then we blab about it. Anything before that, you’re not going to get anything out of me.”

So if they indeed have detected those gravitational waves, we will also get a paper.

Computer simulation of a black hole collision. When two black holes merge into one, enormous amounts of energy are released in the form of gravitational waves.

Computer simulation of a black hole collision. When two black holes merge into one, enormous amounts of energy are released in the form of gravitational waves.

Below the official announcement.


Remnants of erosion

A Piece of Mars: The gray area in the center of the 480×270 m (0.3×0.17 mi) area is an erosional remnant: once, more of this area was covered by the gray stuff, but some of it has eroded away (most likely by the wind) to reveal the underlying terrain below. The wind probably blew from upper left to lower right, lifting away the finer grains and leaving behind the larger, heavier ones. Some of the larger grains have formed into ripples, that in some places may be the only sign that the overlying layer was ever there. (HiRISE ESP_043136_2020, NASA/JPL/Univ. of Arizona)

Springtime for sand dunes and polar ice

A Piece of Mars: This 480×270 m (0.3×0.17 mi) scene shows a dark dune peeking out from under its cover of winter frost. In this picture it’s late spring, but still cold up at this latitude – it’s as far north here as the Greenland town of Qaanaaq (pop. 656 as of 2013). The CO2 frost here lingers on shady slopes until summer, preventing the dunes from migrating until it’s gone. (HiRISE ESP_043799_2570 NASA/JPL/Univ. of Arizona)

How to explain the inconceivable

I am often asked to comment on what happened in Paris last December since I have both French and American citizenships and I live in the US. Like a lot of my compatriotes, it has been difficult to watch those events unfold on Friday afternoon December 13 (I was working at George Mason University in DC ). Since then, he has been also impossible to rationalize what really happened and to give a sense on those horrific events. Today I listened to “Geopolitique”, a short program aired on France Inter which described events and their consequences in the geopolitical scale. Bernard Guetta summarized very well what are my thoughts on the Paris events and its consequences, so I decided to share with you  an English translation which has been freely adapted. The French version   “Comment expliquer l’inconcevable” is available on the France Inter web site.

PeaceForParis by the artist Jean Jullien

PeaceForParis by the artist Jean Jullien

The perpetrators of the most recent murders have no excuse whatsoever, especially not one that seeks to blame the societies they live in. Nor, for that matter, did Mohamed Merah before them, or the killers of November 13, or the killers of January 7 in Paris. They were certainly not mentally ill and they certainly were responsible for their actions — and cannot claim that the challenges of integrating into a new society make them the bloodthirsty monsters they became. (more…)

That which curves and that which is straight


A Piece of Mars: The long meandering lines snaking across the image (3.2×1.8 km or 2×1.1 mi across) are inverted channels. They are river deposits that once were the lowest part of the landscape (rivers always are), but then the water dried up and wind erosion took over. The river channels were more resistant to erosion and so now they stand above the rest of the terrain. The wind left behind straight, streamlined hills called yardangs. Given enough time, the wind will scrape at the surface until both the yardangs and the river channels are gone, but for now there’s a beautiful landscape. (HiRISE PSP_002424_1765 NASA/JPL/Univ. of Arizona)

Observing planet formation at close range: Gemini Planet Imager’s view of the TW Hya disk

Investigations of star and planet formation have long focused on the rich stellar nurseries of Taurus, Ophiuchus, Chamaeleon, and a handful of similarly nearby (but lower mass) molecular clouds. These regions, which lie just beyond 100 pc, are collectively host to hundreds of low-mass, pre-main sequence (T Tauri) stars with ages of a few million years and less. They hence provide large samples of stars with orbiting circumstellar disks that span a wide range of evolutionary stages.


Curiosity about sand dunes (part 2/2)

Today is December 21, 2015 (northern winter and southern summer solstice on Earth). On Mars it is Ls = 84º, Mars Year 33 (about 12 sols from northern summer and southern winter solstice on Mars). It is sol 1200 of Curiosity’s mission on Mars, and the rover is working its way around the southern side of Namib Dune. Part 1 of my previous post shows part of the windward (northeastern) side of High Dune. This time I’ll show pictures of the slip face of Namib Dune.

The dunes in this part of the Bagnold Dune Field are slowly marching towards the southwest. Wind blows from the north-northeast and the sand piles up, only to oversteepen on the lee side and form avalanches in what we call a slip face. This happens over and over as the dune moves: saltating sand flies over the dune crest and settles on the upper slip face (what we call grainfall). When enough sand piles up, it oversteepends, and eventually there’s a slope failure: stuff higher up moves down, like a little landslide (what we call grainflow). We see grainflows most commonly when the wind is blowing nearly directly across the crest of the dune.

But when is nature ever so uniform? Sometimes people ask me how strong the wind blows on Mars, as if I could just give them a single value that would apply to all of Mars (its mountains, polar caps, steep crater rims, and flat plains) at all times (its CO2-covered winters, convectively-turbulent summer days, regions prone to seasonal dust storms, and nighttime low-lying flows). Go outside on Earth for a moment and tell me if you can feel the wind moving from different directions, at different speeds, and do it again in 12 hours and again in 6 months. In most places you won’t get the same result, and it is the same on Mars.

You don’t get the same winds blowing here in the Bagnold Dune Field either, even though the dunes are telling us that the strong winds mostly blow from the north-northeast. What can the slip face tell us about that? Let’s have a look:


What an awesome view! Here’s another view of the whole slip face:


It’s totally different from the other side of the dune, which is covered in ~2 m sized ripples that are themselves covered in smaller ripples. Instead, this looks like a giant wall of sand, textured with features that all look like they’ve moved downhill. The smoother surfaces are probably the newest: these are fresh grainflows. But some of the slip face is covered in small ripples. Here’s what I mean:


What you’re seeing is the interplay of at least two different winds. There’s the main wind that blows over the crest, which forms grainflows, and then a secondary wind that blows in a different direction, forming ripples out of sand from the grainflows. Based on the orientations of other ripples, that secondary wind probably blows from the northwest, which is roughly along the slope of the slip face. That northwesterly wind is responsible for making the large ripples at the base of the slip face – when that wind blows, those large ripples would march towards the camera.

It’s pretty typical for two to three wind directions to dominate all other winds in a region, at least in terms of sand transport. On Earth this usually comes down to seasonal changes in weather patterns: winter storms vs. steady summer winds. Perhaps where you live, most of the weather arrives from one direction, but occasional storms may blow in from another direction. Those winds that blow strongest are most likely to move the most sand. This appears to be the case on Mars as well.

It looks to me like the most recent wind activity has formed grainflows, suggesting that the NNE wind has been more recently active than the ripple-forming wind. However, most of the slip face is covered in small ripples, suggesting that this NW wind was, until recently, the prevailing wind. The ripples weren’t able to fully rewrite the topography of the slip face, as you can see they cover slightly larger undulations that were probably older grain flows – this supports the idea that they are formed by a secondary wind that cannot move enough sand to rewrite the entire dune (if it did then the slipface would point towards the southeast, instead of towards the southwest as it does now). We’re probably seeing a seasonal tradeoff between the NNE and NW winds. I might even cautiously suggest that the grainflow-forming NNE wind is active in the current season (local autumn) and that the ripple-forming NW wind blew in a previous season (perhaps local spring or summer). I’d love to get a shout-out from the REMS folks, if they can pull out any new wind data from their partially-broken anemometer.

Happy solstice everybody, and I hope you have a good holiday.

AGU 2015 session: Solar system small bodies-relics of formation and new worlds to explore

Tomorrow is the last day of the Fall AGU Meeting and I am convening and chairing another session on ASTEROIDS!

This session entitled “Solar system small bodies: relics of formation and new worlds to explore” was organized with my colleagues Padma A Yanamandra-Fisher from Space Science Institute and Julie C. Castillo from Jet Propulsion Laboratory.

NASA's Dawn spacecraft found that bright spots on dwarf planet Ceres are most likely salt deposits. (Photo: Twitter/EdmundoCalero)

NASA’s Dawn spacecraft found that bright spots on dwarf planet Ceres are most likely salt deposits. (Photo: Twitter/EdmundoCalero)

We have a surprise for the oral session scheduled on Friday 18 December from 10:20 to 12:20, since we managed to replace two last minute cancelation by a talk of 30-min given by C. Russel to review the latest findings with Dawn at Ceres. See below.