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)
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.
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.
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.
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.
Join us tomorrow at the AGU Fall Meeting for a session on direct imaging of habitable exoplanets that I organized with my colleagues Ramses Ramirez from Cornell University and David Black.
This session consists in a discussion on the potential of new and future facilities and modeling efforts designed to detect, image and characterize habitable exoplanets, studying their formation, evolution and also the existence of possible biospheres. Topics to be covered in this session include signs of exoplanet habitability and global biosignatures that can be sought with upcoming instrumentation; instrument requirements and technologies to detect these markers; strategies for target selection and prioritization; and impacts of planetary system properties, ground-based and space telescope architectures, and impacts of instrument capabilities on the yield of potentially inhabited exoplanets.
Sorry for the pun in the title there, but NASA asked for it by naming their rover like that. And you’ve seen it done a hundred times, so let’s grit our teeth, smile, and carry on.
So I’m more excited now about a space mission than I have been in a long time. A Mars rover is finally visiting sand dunes, after so many years of peering at them from orbit and seeing them in rover images in the far distance. They took their time getting there, but now it’s there. Taking images of the dunes, and presumably other data as well. Here’s what it looks like from above:
Today, December 14, 2015, is Sol 1193 for Curiosity. On Sol 1176, Curiosity took a bunch of color images looking up at “High Dune”. Here’s what they look like all mosaicked together:
Looking from the bottom up, you are panning up the windward side of the dune, looking at the lovely ripples that cover the entire dune. You can see the biggest ripples in the first image from space. Using those images, my colleague Simone Silvestro and others (including me, yay) measured that they move downwind (towards the left in the above image) at a rate of 0.66 meters (26 inches) every Earth year. Here’s a 3-frame movie from that paper, showing how the dunes and ripples move over the course of 5 years (which is ~2.5 Mars years) in 2006, 2008, and again in 2011. Note that this isn’t the same dune that Curiosity imaged above.
Image credit: Silvestro, S., D. A. Vaz, R. C. Ewing, A. P. Rossi, L. K. Fenton, T. I. Michaels, J. Flahaut, and P. E. Geissler (2013), Pervasive aeolian activity along rover Curiosity’s traverse in Gale Crater, Mars, Geology, 41(4), 483–486, doi:10.1130/G34162.1.
So yes, these dunes are moving! We might even see changes as Curiosity takes more images. We might see sand grains that have bounced onto the deck of the rover. We might even be able to learn something about how the wind moves sand on Mars, how that process differs from that on Earth, and how strong the wind blows. Cool, eh? Check back next week for more pictures of these dunes. I’ll be showing some closeups of the sand and speculating on what it’s made of and why it’s different from windblown sand on Earth.
Announcement from the AAS
The American Astronomical Society (AAS) will convene an online press conference on Tuesday, 1 December, featuring exciting new results on exoplanets from Extreme Solar Systems III, a conference taking place from 29 November through 4 December 2015 at the Waikoloa Beach Marriott Resort & Spa on Hawaii Island.
ExSS III is the third in a series of conferences that began with Extreme Solar Systems in 2007 in Santorini, Greece, and was followed by Extreme Solar Systems II in 2011 in Jackson Hole, Wyoming. Next week’s conference, like the previous two, will cover all aspects of research on exoplanets. Some 350 researchers from all over the world are registered for the meeting. (more…)
A piece of Mars: This 480×270 m (0.3×0.17 mi) area is a steep slope that plunges down to the upper left. A pile of dark sand, covered by brighter tan dust, clings to the hillside. Usually the martian wind blows sand into ripples, and you can see where it’s tried to do that here. But the steep slope triggers thin dark avalanches of dark sand that compete with the wind in shaping the sandy surface. (HiRISE ESP_043085_1670 NASA/JPL/Univ. of Arizona)
A Piece of Mars: This 0.96×0.54 km (0.6×0.33 mi) area shows ripples forming on a layer of dark gray material. In a few spots, the gray layer has been eroded away (probably by wind scour), revealing the lighter, tan-colored terrain below. Geologists call these exposures windows, because you can see through one layer to another that’s underneath. (HiRISE ESP_043086_1715 JPL/NASA/Univ. of Arizona)
Thursday, November 12 2015 – 9:00 am, PST
AAS/SETI Institute press release presented at the DPS 2015 at National Harbor, MD, USA
The Gemini Planet Imager Exoplanet Survey (GPIES) is an ambitious three-year study dedicated to imaging young Jupiters and debris disks around nearby stars using the GPI instrument installed on the Gemini South telescope in Chile. On November 12, at the 47th annual meeting of the AAS’s Division for Planetary Sciences in Washington DC, Franck Marchis, Chair of the Exoplanet Research Thrust of the SETI Institute and a scientist involved in the project since 2004, will report on the status of the survey, emphasizing some discoveries made in its first year. (more…)