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Is it windblown or not?

A Piece of Mars: This 480×270 m (0.30×0.17 mi) scene shows what are being called “ridges”. Were these ridges once dunes that have now been stabilized and eroded? They have some dune-like characteristics: nearly parallel crests, one slope is steeper than the other, that steep slope seems to have exposed layers, and sometimes the crests meet in what is called a “Y-junction” (based on the letter’s shape). But although they’re common in some areas on Mars, they’re not like any dunes or ripples I’m familiar with. I’m inclined to think they’re not ancient dunes, but it’s likely that the wind had a hand in their formation. I’m open to suggestions… (HiRISE, ESP_046998_1365 NASA/JPL/Univ. of Arizona)

Light and dark

A Piece of Mars: This 0.96×0.54 km (0.6×0.34 mi) late winter scene is a study in contrast. The dark top half is uniformly rippled. This is the shady surface of the main windward side of one of Mars’ biggest dunes, in Kaiser crater. On the bottom is the sunlit side of the dune, strewn with gullies colored by CO2 frost (white), dark basaltic sand (black), and what may be oxidized fines (orange). (HiRISE ESP_045614_1330, NASA/JPL/Univ. of Arizona)

Shades and textures

A Piece of Mars: This 480×270 m (0.3×0.17 mi) scene shows the contact between two very different terrains. On the left is a bright surface with polygonal cracks (characteristic of periglacial terrain – this is at a high latitude). On the right is a dark rippled sand sheet that superposes the polygonally-cracked surface. The long meandering furrows might be the beginnings of polygonal cracking in the sand, which might expand if wind doesn’t erase them. (HiRISE PSP_006473_1125, NASA/JPL/Univ. of Arizona)

Eroded dune

A Piece of Mars: Barchan dunes on Mars have a characteristic crescent shape, with a steep slope (“slip face”) on the inside of the sharpest curve (see examples like this, this, these, or this). This image (873×491 m, or 0.54×0.31 mi) shows an example of a dune that probably looked a bit like those other dunes did once, but it’s been highly eroded so that the characteristic curved slip face is no longer the steepest slope. This dune is located pretty far north, so I’m betting it’s been stabilized by ice, so that the wind can no longer easily reshape it into a typical barchan. (HiRISE ESP_036404_2590, NASA/JPL/Univ. of Arizona)

Where does the windblown stuff come from?

A Piece of Mars: How far do windblown materials move on Mars? This scene (0.9×1.2 km, 0.56×0.75 mi) shows a bright layer of bedrock (top right) that is eroding, exposing a darker, bluish rock (bottom left). Ripples 5-20 m wide have slowly moved towards the lower right, with some migrating into the darker terrain. Those near the interface show that they’re made of stuff from the brighter terrain, as they are still brighter than the dark, bluish bedrock. But those at the bottom are much more blue. This means that this type of ripple incorporates material from nearby rocks: unlike other kinds of windblown material, they don’t travel far from their source. (HiRISE, ESP_017262_1560, NASA/JPL/Univ. of Arizona)

Erosional remnants

A Piece of Mars: The erosionally-streamlined bright areas are on high ground. They are remnants of a vast dusty mantle that once covered this whole area – the rest of it has been blown away. The surrounding regions (check out the whole image) are still covered by that mantle, but here you can see through to the underlying, dark surface made of dark, cratered lava flows. (HiRISE ESP_017914_1685, NASA/JPL/Univ. of Arizona)

Ancient ripples?

A Piece of Mars: Potential signs of wind activity are everywhere on Mars. Take this 0.96×0.54 km (0.6×0.34 mi) scene, which is on bedrock dated to be several billion years old. There’s a fabric of ridges trending from the upper right to lower left. The smaller and smoother ones are clearly windblown bedforms. The larger, bright ones are shedding boulders, so if they’re old bedforms then they’ve been lithified. How old are they? Billions of years old? Or did they form sometime in the intervening years? (HiRISE ESP_046389_1695, NASA/JPL/Univ. of Arizona)

Fossil dunes

A Piece of Mars: This 1.92×1.08 km (1.19x 0.67 mi) scene shows eroded ridges that are, in fact, lithified dunes. They are so old that you might not recognize them as dunes without more context. This doesn’t happen much on Earth, where inactive dunes are quickly eroded, buried, and/or destroyed by other geologic processes, so enjoy this uniquely martian wonder! (HiRISE ESP_046597_1670, NASA/JPL/Univ. of Arizona)

Dunes not in the global dune database

A Piece of Mars: Ten years ago I participated in a global survey of martian dunes. But we missed a few dune fields, like these beauties. They’re small, low, and in rugged terrain, which made them difficult to spot in the lower resolution data set we used. I keep a list of dune fields we’ll have to add if we get a chance to update the database. This scene is km (0.6×0.34 mi) wide. (HiRISE ESP_043582_1555, NASA/JPL/Univ. of Arizona)

When the martian surface is eroded, pretty things emerge

A Piece of Mars: Just like at Earth’s Grand Canyon, erosion on Mars has created some really beautiful landscapes. This 480×270 m (0.3×0.17 mi) scene shows rugged terrain that was once buried in sediment. Does the texture here represent the landscape before it was buried, or was it created in the process of scouring off all that overlying sediment? Probably a mixture of both. And we get a pretty view because of it! (HiRISE ESP_046198_1750, NASA/JPL/Univ. of Arizona)