A Piece of Mars: Is this 480×270 m (0.3×0.17 mi) scene showing a 150 m (492 ft) wide yin-yang symbol on Mars? Sort of, maybe, if you blur your eyes and lend me artistic license, but it’s not doing so intentionally. One side of the crater is dark and the other is light. Both have their tone because of windblown material blown from the same direction, but the different materials collected where they did for different reasons. The dark material is probably mafic sand (iron and magnesium-rich, like what’s found near many volcanoes), which was bounced along the ground from the lower right, and collected in the lee of the crater rim. The bright material is much finer-grained, dust carried aloft, and it probably settled down on the far side of the crater, and outside as well, as the crater rim poked into the wind and provided enough shelter to let some of the bright material settle out as airfall. (HiRISE ESP_016496_2000, NASA/JPL/Univ. of Arizona)
A Piece of Mars: The focus of this 0.96×0.96 km (0.6×0.6 mi) scene is one of many two-faced dunes on Mars. The bright sunlit slope is one face, formed recently by wind blowing from the upper right. The dark shaded slope is the other face – it’s a little older, formed by wind blowing from the left. Together these two winds alternate, probably in different seasons, forcing the sand into a needle-shaped point that carries sand in a direction that is, give or take, the sum of those two winds. Two-faced dunes like this are rare on Earth, as winds here typically quickly erase older crestlines. (HiRISE ESP_021716_1685, NASA/JPL/Univ. of Arizona)
A Piece of Mars: How do you tell when a planetary landscape shows Mars, instead of Mercury or the Moon or Europa? The easiest way to tell is to look for both craters and dunes, like what’s shown here in this 640×360 m (0.4×0.22 mi) scene. Not all martian landscapes have either feature, and there are some other worlds that do have both (Earth, Titan, maybe Pluto, and probably Venus but we need better data…), but it’s a pretty good bet that if you see both features together, you’re looking at Mars. Anyway, in this lovely view, the dark gray terrain (you’ll see boulders if you look closely enough!) is being eroded away slowly, revealing a much older, brighter surface beneath it. Unfortunately for those who would study ancient terrains on Mars, much of that older, lower surface is covered in dunes. But I like the dunes – they give us information about surface erosion rates and wind patterns. One person’s signal is another person’s noise. (HiRISE ESP_047762_1585, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Sometimes in the floors of small craters, the wind blows in from several directions to produce odd polygon-shaped dunes that look like crochet (maybe Mars is making sweaters for its craters – it is, after all, a cold place). This “sweater” segment is 480×270 m (0.3×0.17 mi) in size (the “stitches” are ~20 m, or 66 ft, across). The smaller interior lines are younger windblown features, that are superposed on the larger structures – their alignment is strongly controlled by the topography of the larger polygonal “stitches”. (HiRISE ESP_017833_1975, NASA/JPL/Univ. of Arizona)
Using a combination of space telescope data, as well as recent data acquired with the SOFIA Airborne telescope and lab experiments, a team of astronomers including researchers from the SETI Institute and Jet Propulsion Laboratory have revealed the presence of dust of exogenic origin at the surface of dwarf planet Ceres. This contamination likely stems from a dust cloud formed in the outer part of the main belt of asteroids following a collision in recent times. That study challenges the relationship proposed between Ceres and asteroids in the C spectral class and instead suggests an origin of this dwarf planet in the transneptunian region. This study was published on January 19 2017 in Astronomical Journal.
Interplanetary dust particles (IDPs), which form meteors when they cross Earth’s atmosphere, represent the largest fraction of extraterrestrial material accreted on Earth. A team led by Pierre Vernazza, research scientist CNRS in the Laboratoire d’Astrophysique de Marseille (LAM – CNRS/AMU), have shown that IDPs are also an important and continuous source of material captured on the surface of asteroids.
Pierre Vernazza explains that « by analyzing the spectral properties of Ceres we have detected material made up of fine particles of dry silicate called pyroxene. However, thermal evolution models proposed for Ceres have predicted a surface composed of aqueously alterated (e.g., clays, carbonates) which was confirmed from recent observations collected by the NASA Dawn mission. Hence the researchers concluded that it is unlikely that those fine grains of dry material could still be preserved in Ceres’ interior.
The team then searched for the possible source of contamination. Recently, observations from a variety of spacecraft have shown that the zodiacal light has significant structure including dust bands which are associated with debris from particular asteroid families, resulting from the destruction of a large asteroid. One of these dust bands produced in the main belt is likely the culprit. In particular, the so-called alpha dust band, produced via grinding within the Beagle family (part of the extended Themis family) formed less than 10 Myrs ago and represents a major source of dust in the outer region of the Main Belt. Recent observations also showed that pyroxene dust is a primordial constituent of the Themis family. Hence the alpha dust band is a plausible source of contamination of Ceres and neighboring asteroids.
If the pyroxene observed on Ceres’ surface is of exogenic origin then this challenges the relationship between Ceres and other Main Belt asteroids which has been inferred for decades based on their similar colors in visible light. Astronomers have classified Ceres and 75% of the asteroids in the so-called spectral class C, suggesting a similar composition. This result shows that the reality is certainly more complex and the detection of ammoniated clays on Ceres suggest a trans-neptunian origin. Evidence for ammonia or ammonium on another dwarf planet, Orcus, strengthens that connection.
This study further suggests that the so far unexplained detection of pyroxenes on metallic asteroids* might also originate from a similar dust source. This process likely acts on a global scale at least in the direct neighborhood of the dust band complicating significantly the work of astronomers who want to understand the composition of asteroids from their color.
« This study resolves a long-time question about the nature of the surface materials inferred from spectroscopic observations in the visible and near infrared, whether they reflect the intrinsic composition of the asteroid or contamination by exogenic material. Our results show that by expanding the study in the mid-infrared the asteroid initial composition remains identifiable despite contamination at a level of ~20%. » added Pierre Vernazza
Franck Marchis, planetary astronomer at the SETI Institute also a co-author of this article, stressed out that “The future of asteroid research would greatly benefit from a systematic study of the largest 400 main-belt asteroids. Based on this result, it is clear that mid-infrared spectroscopic observations are key to understand the true nature of an asteroid. Less than 30 of them were observed by the NASA Spitzer and ESA ISO space-based telescopes, and none can be observed with JWST, the next NASA mid-infrared telescope because they are too bright for its sensitive instrument. A dedicated instrument on board SOFIA airborne telescope or a future dedicated space telescope will reveal the true nature of those asteroids even in the presence of contaminations.”
- SETI PR: http://www.seti.org/seti-institute/press-release/observations-ceres-indicate-asteroids-might-be-camouflaged
- CNRS/LAM PR (en Francais): https://www.lam.fr/les-actualites/article/mise-en-evidence-de-la-contamination-de-la-surface-des-asteroides-par-les?lang=fr
- NASA/SOFIA web article: https://www.nasa.gov/feature/don-t-judge-an-asteroid-by-its-cover-mid-infrared-data-from-sofia-shows-ceres-true
- An interview after my first SOFIA flight in the stratosphere
- Observing in the stratosphere with SOFIA
A Piece of Mars: Get out your 3D blue/red glasses (or look here for a 2D version if you can’t find them). This is a 3.2×1.8 km (2×1.13 mi) scene showing dark dunes carving lanes 50-70 m (165-230 ft) deep into a stack of brighter sedimentary layers. Over time, the sand wears down the rock into yardangs, the elongated remnants of rock the sand didn’t manage to reach. Here we see the process ongoing; perhaps in a few million years there will be nothing left but a few streamlined peaks. Those murdering basterds [sic]. (HiRISE ESP_034419_2015, NASA/JPL/Univ. of Arizona)
The tortoise: The rippled surface at the top is high ground: the top of a dune. Wind pushes the ripples toward a steep sunlit slope, creating long thin, dark avalanches that slowly inch the slipface forward. At the bottom of the slope, which is shielded from winds blowing from the top, ripples have been formed by wind blowing from the left.
The hare: Oblivious to both the slow progression of ripples and dunes, 5-25 m wide dust devils have blazed on by, leaving behind erratic trails.
(HiRISE ESP_048592_2070, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Mars rarely does anything without drama. Long ago in this 0.96×0.54 km (0.6×0.34 mi) scene, large ripples formed and then, presumably, lithified (turned into rock). Some time after that, an impact formed the crater in the center, throwing debris into an ejecta blanket that covered the lithified ripples. That ejecta blanket sat around long enough to acquire some smaller impact craters of its own. Since then, most of that ejecta blanket has eroded away, exposing the ripples to view once again. (HiRISE ESP_011699_1910, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Nicholas Steno was a 19th century geologist, who came up with some principles that are still used today to guide interpretation of exposed sedimentary rocks. The principles seem a bit obvious, but then some of the most profound principles can be like that. Emily Lakdawalla of the Planetary Society describes them in more detail here, with really good examples. You can use these principles to do forensics on a landscape, to see what happened and when.
You can see all three principles at work in this image.
#1: Stuff makes horizontal layers. (This isn’t always true, e.g., dunes and deltas make tilted layers, but most sediments pile up into flat, horizontal layers.) You can see that at work here: A thick layer of dark gray stuff once piled up on a flat surface of brighter stuff. Some of the dark gray stuff has since eroded away, but you can see that both the gray and the brighter stuff originally piled up in flat-lying layers.
#2: Older stuff is at the bottom. (Because newer stuff buries the older stuff, like the papers on my desk and the veggies in my fridge.) In this image, the brighter stuff must be older than the darker gray stuff, because the bright stuff is on the bottom.
#3: You can’t see the layers until they’re exposed by erosion or tectonics. (Because they’re buried. So if you see layers, you know something has happened so you can see them.) You can see the edges of the dark gray stuff, so you know it’s been partially eroded away – otherwise you’d never know the underlying bright stuff was ever there. Some of the material from the dark gray layer has been reformed into dark bedforms on the brighter layer, and those bedforms are probably the youngest features in this scene.
What I like most about this image has to do with yet another principle of layered stuff: Things that cut across other things are younger. Things that have been cut across are older (Like if you chop down a tree, then the axe cuts on the tree trunk must have been made after the tree itself grew. Duh, right?). You can see that in this image: on top of the dark gray layer are some old bedforms. They must be quite old, even cemented or lithified (turned into rock that the wind can’t easily move), because they’ve been cut by erosion at the edge of the gray layer. So not only was the gray layer once more extensive, but it had ripples on it, and those ripples formed and became immobile before that erosion ever happened.
(HiRISE ESP_030460_1525, NASA/JPL/Univ. of Arizona)
A Piece of Mars: A low, broad dune occupies the center of this 800×450 m (0.5×0.28 mi) scene, blown by a dominant wind towards the lower left. The slip face on the lee side has several small avalanches, formed as the slope oversteepens (this is how dunes crawl along the surface). Upwind, among other fainter lines, is a prominent bright line: it is a former slip face of this dune, possibly formed from a thick accumulation of bright dust (maybe there was a big dust storm that year). Farther upwind, another dune slowly approaches. (HiRISE ESP_033955_2065, NASA/JPL/Univ. of Arizona)
I co-organized a session for the AGU 2016 meeting entitled “P42A: Solar System Small Bodies: Asteroids, Satellites, Comets, Pluto, and Charon“. Below the info on the session and the schedule.
We have three invited talks that will describe the New Horizons data of Charon, color of Kuiper Belt Object from a ground-based survey and a theoretical study of the formation of the asteroid belt.
Abstract: The composition and physical properties of Small Solar System Bodies
(SSSBs), asteroids and dwarf planets, remnants of the formation of planets, are key to better understand our solar system. Increased knowledge of their surface properties and their potential as resources are also necessary to prepare for robotic and human
exploration. Hints about the internal structure and composition of SSSBs
have been acquired recently thanks to flyby/rendezvous data from space
missions, study of complex multiple asteroid systems, or close encounter
between asteroids. In this session we will discuss results bringing
information on the internal structure and composition of SSSBs based on
space and ground-based data, numerical models, as well as instrument/mission
concepts in the prospect of future exploration.
Chairs and Conveners
Amanda R Hendrix
Planetary Science Institute Tucson
Krishan K Khurana
University of California Los Angeles
Padma A Yanamandra-Fisher
Space Science Institute Rancho Cucamonga
Thursday, 15 December 2016 10:20 – 12:20 Moscone West – 2007
10:20 P42A-01 New Horizons Results at Charon (Invited)
Bonnie J Buratti et al.
10:32 P42A-02 Colours of the Outer Solar System Origins Survey (Col-OSSOS): New Insights into Kuiper belt Surfaces (Invited)
Megan Elizabeth Schwamb et al.
10:44 P42A-03 Pebble Accretion and the Formation of the Asteroid Belt (Invited)
Katherine Kretke et al.
10:56 P42A-04 Constraints for the subsurface structure at the Abydos site on 67P/Churyumov-Gerasimenko resulting from CASSE listening to the MUPUS insertion phase
Martin Knapmeyer et al.
Replaced by P43B-2104 Meteoroid Impact Hazard based on Atmospheric Trajectory Analysis
11:08 P42A-05 Psyche: The Science of a Metal World
Linda T Elkins-Tanton et al.
11:20 P42A-06 Shapes and Densities of the Small Satellites of Pluto
Simon Porter et al.
11:32 P42A-07 CO2 and 12C:13C Isotopic Ratios on Phoebe and Iapetus
Roger Nelson Clark et al.
11:44 P42A-08 Ice Electric: Electron Irradiation Experiments with Porous Water Ice Samples
Andre Galli et al.
11:56 P42A-09 Laboratory Simulations and Spectral Analyses of Space Weathering of Non-Ice Materials on Ocean Worlds
Benjamin R Wing et al.
Moscone South – Poster Hall
P43B-2102 Secular Orbit and Spin Variations of Asteroid (16) Psyche
Bruce G Bills
P43B-2103 Chemistry and Spectroscopy of Frozen Chloride Salts on Icy Bodies
Paul V Johnson
P43B-2104 Meteoroid Impact Hazard based on Atmospheric Trajectory Analysis
P43B-2105 Spectrophotometric Characterisation of the Trojan Asteroids (624) Hektor et (911) Agamemnon
P43B-2106 Shapes and rotational properties of the Select Hilda and Jovian Trojan Asteroids
P43B-2107 Far-UV Spectral and Spatial Analysis from HST Observations of Europa
Tracy M Becker
P43B-2108 CONCAVE SHAPE MODEL OF ASTEROID (130) ELEKTRA BASED ON DISK-RESOLVED IMAGES FROM VLT/SPHERE
P43B-2109 A DIRECT OBSERVATION OF THE ASTEROID’S STRUCTURE FROM DEEP INTERIOR TO REGOLITH: TWO RADARS ON THE AIM MISSION
P43B-2110 Geomorphological Mapping of Sputnik Planum on Pluto: Convection, Glacial Flow, Sublimation and Re-deposition of Nitrogen Ice
Oliver L White
P43B-2111 Constraining the Ice Viscosity and Heat Flux on Enceladus During the Formation of the Leading Hemisphere
Erin Janelle Leonard
P43B-2112 Constraints on the properties of Pluto’s nitrogen-ice rich layer from convection simulations
P43B-2113 Elpasolite Planetary Ice and Composition Spectrometer (EPICS): A Low-Resource Combined Gamma-Ray and Neutron Spectrometer for Planetary Science
Laura C. Stonehill
P43B-2114 Dynamics of HVECs emitted from comet C/2011 L4 as observed by STEREO
Nour E. Raouafi
See you there!
PS: Thanks those who attended the session, as well as the speakers, and my co-chairs (Amanda and Padma).
Here a group picture taken after the session. From left to right: Padma A Yanamandra-Fisher, Maria Gritsevich, Roger Nelson Clark, Andre Galli, Katherine Kretke, Franck Marchis, Megan Elizabeth Schwamb, Benjamin R Wing, Simon Porter, Amanda R Hendrix. Missing on this picture: Bonnie J Buratti & Linda T Elkins-Tanton
A Piece of Mars: Higher ground is to the left. You’re seeing a tan layer sandwiched between two gray layers in this 0.96×0.54 km (0.6×0.34 mi) scene. Large ripples have accumulated in the lowest area to the right, which is the floor of an old river channel. Ripples have also formed on the gray upper layer. But not the middle tan layer – maybe it’s too fine-grained to erode into sand grains, or maybe it erodes too slowly to allow any eroded sand grains to pile into ripples before they’re blown away. (HiRISE ESP_048196_1995, NASA/JPL/Univ. of Arizona)
AGU Fall meeting is starting tomorrow. I co-organized a session entitled “Detection and Direct Imaging of Habitable Exoplanets: Progress and Future” to discuss 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 that are 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.
We have two invited talks, one by George Ricker on TESS and a second one by Shawn D Domagal-Goldman on HabEx, two NASA missions that could play a major role on identification and characterization of Earth-Like exoplanets.
Conveners & Chairs:
SETI Institute Mountain View
Ramses M Ramirez
Douglas A. Caldwell
SETI Institute Mountain View
Location: Room 2020 – Moscone West
Schedule of the Talks:
13:40 P13C-01 The Transiting Exoplanet Survey Satellite (TESS): Discovering Exoplanets in the Solar Neighborhood (Invited)
George R. Ricker and TESS Science Team
13:52 P13C-02 HabEx: Finding and characterizing Habitable Exoplanets with a potential future flagship astrophysics mission (Invited)
Shawn D Domagal-Goldman et al.
14:04 P13C-03 Next Generation Telescopes for Terrestrial Exoplanet Characterization
John M Grunsfeld et al.
14:16 P13C-04 Exoplanet detection and characterization with the WFIRST space coronagraph
Bruce Macintosh et al.
14:28 P13C-05 Enhancing Direct Imaging Exoplanet Detection and Characterization with Astrometry
Eduardo Bendek and Ruslan Belikov
14:40 P13C-06 Imaging and characterizing exo-Earths at 10 microns – The TIKI project
Franck Marchis et al.
14:52 P13C-07 Systematic Search of the Nearest Stars for Exoplanetary Radio Emission: VLA observations in L and S Bands
Daniel Winterhalter et al.
15:04 P11A-1845 Modeling Exoplanet Interiors From Host Star Elemental Abundances
Brandi Hamilton and Douglas Green
15:16 P13C-09 Long-Term Stability of Planets in the Alpha Centauri System
Jack J Lissauer and Billy Quarles
15:28 P13C-10 3D Modeling of the H2O Profile of Temperate Earth-Size Planets around Late-Type Stars, and the Signatures in Transit Spectra
Yuka Fujii et al.
Schedule of the Posters:
P11A-1842 Modeling Molecular Hydrogen Emission in M-Dwarf Exoplanetary Systems
William Ray Evonosky et al.
P11A-1843 Feasibility studies for the detection of atomic oxygen exospheres of terrestrial planets in the habitable zone of a low-temperature star with a UV space telescope
Hiroki Horikoshi et al.
P11A-1844 Cloud and Haze in the Atmospheres of Wide-Separation Exoplanets
P11A-1846 Multifractal Analysis of Expoplanetary Spectra
Sahil Agarwal et al.
P11A-1847 By Inferno’s Light: Characterizing TESS Object of Interest Host Stars for Prioritizing Our Search for Habitable Planets
Cayman T Unterborn et al.
P11A-1848 Dysonian SETI as a “Shortcut” to Detecting Habitable Planets
Jason Thomas Wright
See you there,
PS: Unfortunately I did not take a group picture with all our speakers after the session. However, I have a picture taken with our invited speakers. From left to right: Shawn D Domagal-Goldman, Franck Marchis, astronaut John Grunsfeld and George Ricker)
A Piece of Mars: Martian spiders, or araneiforms, are geological structures found at high latitudes on Mars. The dark splotch with branching arms in this 0.48×0.27 km (0.3×0.17 mi) scene is a good example. They form in the springtime, when bright frost still covers a darker sandy soil, but some sunlight filters through the frost to warm the underlying surface. Sublimation of gas (under the frost but just above the soil) creates enough pressure that little explosions occur like dry geysers, punching through the frost and blowing up sand that then falls back to the surface as a dark splotch. If the wind is blowing when this happens, then the dark splotch is carried a ways downwind, but that hasn’t happened in this case. (HiRISE ESP_048189_0985, NASA/JPL/Univ. of Arizona)
A Piece of Mars: The dunes climbing over a rocky surface in this 0.96×0.54 km (0.6×0.34 mi) scene are mostly yellow because they’re covered (and therefore kept immobile) by dust. The crest of one dune, though, shows recent activity: dark sand has been pushed by the wind up the lower right side, and then shot (cannonball-style) over the brink, where it slowly piles up on the upper left side. This pileup is called grainfall, because that’s what the sand grains have done here (rather than sliding downhill, avalanche-style, which is called grainflow). There’s a dune on the left side of the image that hasn’t experienced this activity, maybe because it’s a little more sheltered from the wind. (HiRISE ESP_047779_1655, NASA/JPL/Univ. of Arizona)
A Piece of Mars: Dunes and ripples most commonly form in topographic lows. But not in this 0.96×0.54 km (0.6×0.34 mi) scene. Here, and in other places on Mars, these bedforms (called TARs) form on plains, and sometimes appear to cling to the rims of craters – which are topographic highs, not lows. It’s not clear how this happens: Does the topography of the crater rim provide a wind shadow that allows windblown sediment to accumulate there? Or was there simply more loose material on the crater rims to begin with, allowing these things to form in place? I’m open to suggestions. (HiRISE ESP_047787_1910 NASA/JPL/Univ. of Arizona)
A Piece of Mars: Bright material (either dust or sand) has accumulated in the lee of wagon- to car-sized boulders in this 0.96×0.54 km (0.6×0.34 mi) scene. It’s perhaps something like the Rocknest sand shadow that Curiosity visited a few years back. The wind blows from lower right to upper left, carrying along sediment that occasionally gets trapped in the protected areas behind the boulders. These sand shadows aren’t very thick, as the underlying texture (polygonal terrain!) is visible through them. (HiRISE ESP_047798_1150, NASA/JPL/Univ. of Arizona)
It was a tough night … one in which any dreams we may have had of “American exceptionalism” were crushed.
Instead of serving, as it has so nobly, for more than two centuries as a beacon of hope and light to people everywhere, this nation will instead see our first African-American president hand the keys to the White House to a low-grade reality TV star who is endorsed by David Duke and the KKK, “alt-right” crazies, and American Nazis. To the horror of many of us, our next president will be someone who explicitly rejects science, reason, and the values we all hold dear—values that are our only hope for moving this country and the world forward.
Unfortunately, an America that many of us neither know nor understand decided yesterday that the American Dream is not big enough for everyone—especially not people of color, LGBTs, and struggling immigrants. It was painful to explain this to my children this morning, when they awoke and were stunned to discover that Donald Trump was their next president.
So, what happens now? We have to act.
We have four years to show Trump’s supporters, Americas who he has duped and deceived, that they were wrong. Four years to fight against the radical agenda of Vice President-elect Mike Pence. Four years to defend, at political rallies and protests, values we hold dear and have fought hard for. Four years to understand that posting comments to Facebook and Twitter is not taking action. Four years to reach the many Americans who will need our help when they realize that Trump’s promises were empty and will make their lives worse not better. In short, we have four years to create a movement that resonates with all Americans, even ones who feel abandoned by the system.
Right now, today, this very minute, it’s urgent that we stop the blame game and focus on what we value. We need to be ready to help the many, many people who are only months away from becoming victims of Donald Trump and his hideous agenda. That is why I urge you to act now to support groups that have been fighting for our rights and our values for years, but that we may have overlooked or even forgotten.
If you care about our civil rights, join ACLU: https://www.aclu.org/
If you care about the voice of science join UCS: http://www.ucsusa.org/
If you care about LGBT rights, join HRC: http://www.hrc.org/blog/hrc-statement-on-the-election-of-donald-trump-march-toward-equality-continu
If you care about our planet, join Greenpeace: http://www.greenpeace.org/usa/
If you care about our democracy http://front.moveon.org/
If you want a more progressive California and country, join Courage Campaign https://www.couragecampaign.org/
That was a terrible battle that we lost yesterday, but we can grow better by fighting together for what is right, true, and just. #AlwaysForward
A Piece of Mars: Wind from the upper left is blowing dark dunes toward the lower right in this 1.92×1.08 km (1.19×0.67 mi) anaglyph (if you don’t have your red/blue 3D glasses handy, you can also check out the black and white 2D version). The dunes are crossing through hurdles aligned to make their progress as difficult as possible, but the dunes nevertheless are slowly making their way through. Ironically, the bright “hurdles” are themselves lithified dunes that are perhaps billions of years old. (HiRISE ESP_020555_1755/ESP_047139_1755 NASA/JPL/Univ. of Arizona)
A Piece of Mars: This 1.6×2 km (1×1.24 mi) scene mostly shows what wind will do to fine-grained, weakly-consolidated surfaces. It has created topography that further strengthens wind scour in the hollows, which even leave kilometers-long grooves reminiscent of water-carved streams. If this were Earth I’d guess they had been carved by water first. But this is Mars, where the wind is in charge. (HiRISE ESP_046504_1785, NASA/JPL/Univ. of Arizona)