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Three types of windblown piles of stuff

A Piece of Mars: The wind blows different sorts of sediment in different ways. Ultimately they pile up because some oddity in nature makes one spot accumulate more sediment than other spots, allowing that windblown pile of stuff to grow. Sometimes it’s because of the wind interacting with the shape of the pile, and sometimes it’s because of the trajectories of moving grains as the wind blows them along the ground. Here’s an example of three types adjacent to each other: 1) a big dune on the left (migrating towards the right), which is covered in 2) smaller ripples, and downwind of the big dune are 3) brighter intermediate-scale piles (that are surrounded by larger and, presumably, better-developed versions of the “smaller ripples”). (HiRISE ESP_044515_1620, NASA/JPL/Univ. of Arizona)

Dune shadows

A Piece of Mars: Normally I post in color, but sometimes you need to back out to the grayscale images to see the big awesome things. This scene is 4.6×2.6 km (2.8×1.6 mi); the conical hill is 1.4 km (0.89 mi) wide. Sand-laden wind from the right is blowing streamers of dunes around the hill, which leaves a wake that stretches downwind. Some of the luckier hills on Mars have lovely dunes scarves like this, slowly shifting over the centuries as the wind brings in more sand. (HiRISE ESP_044258_1715 NASA/JPL/Univ. of Arizona)

How far does the wind blow stuff?

A Piece of Mars: Hargrave crater has an amazing array of colorful surfaces, each of which reflects a different type of rock (this scene is 480×270 m or 0.3×0.17 mi). I like the ripples sitting on top of it all; I’ve long thought that much of the material in those ripples hasn’t moved very far from where it originated. Here’s a good example of why. The ripples on the greenish surface have incorporated some local greenish material. The same is true of the tan ripples in the lower left. I’d bet most of this stuff has only moved as far as it took to make the ripple it’s in. (HiRISE ESP_044161_2005 NASA/JPL/Univ. of Arizona)

Sizes of worlds

A Piece of the Solar System: This isn’t my usual sort of post. But lately my 6 year old kid has been into planets, and thanks to the many informative videos on YouTube, has been reciting various names and numbers about the many worlds in the Solar System. I decided to show him just how much smaller than the Earth some of those worlds are. Here’s what I made for him. It’s not exhaustive, but it gives a good idea of just how small Pluto is relative to, say, the Moon. The scale is ~2 km/pixel. (Images attributable to NASA or ESA).

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…)