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Patches of volcanic Martian ash covering patches of frozen volcanic dunes

Patches of volcanic Martian ash over frozen volcanic dunes

Cool image time! The picture to the right, cropped, reduced, and sharpened to post here, was taken on December 24, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO).

What makes this terrain intriguing are the series of parallel ridges that cover most of the picture, with smaller ridges at right angles filling the hollows between. It appears we are looking at two different sets of dunes, the larger ridges indicating the southeast-to-northwest direction of the prevailing winds, while the smaller ridges in the hollows suggest the wind patterns within the hollows, causing smaller ripple dunes to form at right angles.

Note however the flat patches in the lower left. The material there appears to fill the hollows, covering the dunes. We can tell this by the hollows to the east, which have an almost identical dune pattern. Those flat patches then are likely covering similar dunes, with the patched material either having been blown away to expose the lower dunes, or having been blown here to cover them in patches. That the dunes appear unchanged under this patched material when exposed also suggests strongly that these dunes are hardened into stone, no longer soft sand that can be blown by the wind.

Overview map

The white dot on the overview map to the right marks the location, on the edge of the Medusae Fossae Formation, the largest volcanic ash field on Mars. In the inset you can see that in this region the ash field is very patchy in general. The higher plateaus generally mark places where it is thick, with the lower areas in-between either exposed bedrock with little or no ash or ash in much thinner layers.

The picture above however tells us a lot about the creation of this vast volcanic ash field. It wasn’t placed here by one event, but by many individual events over long eons, with lots of time in-between to allow the formation and then hardening of those dunes into stone.

Genesis cover

On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.

The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.

The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.

"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News

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