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Changing winds

PSP_002387_1985_1.0xA piece of Mars: There are two sets of ripples here: tan ones and gray ones, each oriented to a different wind (scene is 300×225 m, or 984×738 ft). The gray ones sit on top of the tan ones, so the gray ones are younger. Now come the fun questions: why the different colors? Are they made out of different material (and if so, why), or are the older tan ones different because the gray sediment has weathered to tan over time? (HiRISE PSP_002387_1985, NASA/JPL/Univ. of Arizona)

Missing bedrock

ESP_017173_1715_32768ppdA piece of Mars: Wind flow on Mars can be quite dramatic. Here, a single wind-sculpted hill stands 1.5 km (0.93 mi) wide and 600 m (1970 ft) high (color shows elevation). That sounds big, but vastly larger is the volume of material that has been removed to form it. A sandy ridge forming a “bow shock” indicates present-day winds still blow in the same direction. (HiRISE ESP_017173_1715, NASA/JPL/Univ. of Arizona)

Martian waves

ESP_037200_1765_0.501xA piece of Mars: The swirly candy stripes in these big dark dunes are layers inside that have been made visible by wind erosion (the scene is 1.5×0.9 km, or 0.93×0.56 mi). It’s rare to see the inside structure of dunes like this, but these are being eroded by wind blowing from the upper right. For similar examples on Earth, check out The Wave. (HiRISE ESP_037200_1765, NASA/JPL/Univ. of Arizona)

Holes around rocks

ESP_037201_2450_0.589xA piece of Mars: This scene (509×382 m, or 1670×1253 ft), aside from showing some lovely rippled dunes, has many car-sized boulders in it. Some are surrounded by ditches in the sand, like little moats. Why? The sand is blown away from the ground as wind impacts the rocks. My colleague Mark Bishop has studied these in more detail (read more here) (HiRISE ESP_037201_2450, NASA/JPL/Univ. of Arizona)

Which wind came first?

ESP_037156_1800_0.178xA piece of Mars: This scene (3.9×2.5 km or 2.4×1.6 mi) shows a surface carved by two different winds: one blowing from the right and one blowing from the bottom right. They’ve formed overlapping sets of streamlined rocks called yardangs. Can you tell which set of yardangs was formed first? It’s a little more complicated than it may first appear. (HiRISE ESP_037156_1800 NASA/JPL/Univ. of Arizona)

How the wind turns

ESP_037188_1835_0.603xA piece of Mars: The two shadowed hills in the upper part of this frame (497×373 m or 1631×1224 ft across) rest on a flat plain covered in large ripples. On the plain the ripples are aligned north-south, formed perpendicular to a wind blowing from the east (right). But those hills block the wind and turn it, so that the ripples between the hills change direction. This is how windblown landforms can be used as wind vanes in remote places (like on Mars). (HiRISE ESP_037188_1835, NASA/JPL/Univ. of Arizona)

Wind within vs. wind without

ESP_037195_1625_0257xA piece of Mars: Dunes outside the crater are straight but the ones inside the crater look like a spiderweb. Why? This image shows just how much the topography of a crater wall can affect the wind, which produces a much more complex set of dunes inside than out on the plains. (HiRISE ESP_037195_1625 NASA/JPL/Univ. of Arizona)

What on Mars?

ESP_037161_1785_1.0xA piece of Mars: What on Mars is this (the scene is 600×450 m, or 0.37×0.28 mi)? It can be hard to tell. The lines are ridges of windblown dunes or ripples, the dark gray stuff is active sand blowing between the dunes, and the underlying bedrock is pale tan. But if your eyes can’t make sense of it all, just sit back and enjoy the pretty patterns of Mars. (HiRISE ESP_037161_1785, NASA/JPL/Univ. of Arizona)

Small dunes up high, big dunes down low

ESP_036795_1760_0.331xA piece of Mars: This 1018×1352 m (0.63×0.84 mi) dune-covered scene has split topography: the the bottom part is up on a plateau, and the upper part is in a broad valley. The dunes up on the plateau are smaller than the ones in the valley. Why? Probably because there was more mobile dune-building sediment in the valley to begin with: the dunes up high ran out of material and stopped growing, but the ones in the valley kept getting bigger. (HiRISE ESP_036795_1760, NASA/JPL/Univ. of Arizona)

Summery dune

ESP_035997_2565_0.38xA piece of Mars: Last December I blogged about a picture of a sand dune taken in early northern spring. This is the same dune, without frost, now that summer has come to the northern hemisphere and all the frost is gone. It’s quite a difference. Apparently the dunes are controlled by ice in the winter and by the wind in the summer. (HiRISE ESP_035997_2565, NASA/JPL/Univ. of Arizona)