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Ancient flood lava in the Martian cratered highlands

Ancient flood lava on the cratered highlands of Mars
Click for original image.

Cool image time! The picture to the right, cropped, reduced, and sharpened to post here, was taken on February 4, 2024 by the high resolution camera on Mars Reconnaissance Orbiter (MRO).

The ridges were the primary reason this photo was taken, as they cover a 50-mile-square region of relatively flat terrain that also appears to be a series of steps downward to the west. The dotted line on the picture indicates one of those steps downward, with the plain to the west of that line about 100 to 200 feet lower that the plain to the east.

My first guess was that these ridges might be inverted channels, but that really didn’t make sense considering their random nature completely divorced from the downward grade. Then I took a wider view, and came up with a better guess.

Overview map

The white dot on the overview map to the right marks the location, in Arabia Terra, the largest transistion zone between the northern lowland plains and the southern cratered highlands of Mars. Arabia however has more in common with the cratered highlands, as it is cratered in much the same way, suggesting it is very ancient terrain left over from the heavy bombardment during the formation of the planets, four billion years ago.

That ancient terrain however has been significantly reshaped since that heavy bombardment, most often by volcanic activity. It appears that the dotted line actually denotes the edge of one flood of ancient lava, of which there appear to be several. The oldest is to the west. As you go east and north and climb those steps, you are traversing up onto later floods, one by one. The different floods show up as plains of different brightnesses, with the most recent the brightest.

But what formed the ridges? I suspect these ridges are wrinkle ridges, formed when that lava solidified and shrank. That shrinkage formed cracks, similar to what you see in dried mud, that were later filled with more resistent material. When later erosion stripped away the top of the lava, the ridges remained.

This is still a mere guess. What is more certain is the volcanic nature of these plains. Much of this part of Arabia Terra was formed from volcanic events in the far past, lava spewing out of vents to flood large areas, but forming no volcanoes of note.

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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