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A dry lakebed on Mars?

Evidence of a past lake in a crater on Mars
Click for original image.

Today’s cool image illustrates in some ways the uncertainty of science. The photo to the right, rotated, cropped, reduced, and sharpened to post here, was taken on December 1, 2022 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). The science team intriguingly labeled it “Small Candidate Lake Deposit Downstream of Alluvial Fan.” I am not sure what they consider that lake deposit in the full image, so I have focused on the area of stucco-like ground, which resembles bedrock that has been corroded by some water process.

This area is just to the east of the central peaks of an unnamed 25-mile-wide crater in the southern cratered highlands. Many of the craters in this region are believed by scientists to have once harbored lakes formed by run-off from the glaciers that once existed on the craters’ inner rim. In this case it appears this stucco area is the head of an alluvial fan, coming down from the crater’s central peaks. You can see its beginning in this MRO high resolution image of the central peaks, taken in November 2016. As defined geologically,

An aluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation.

In this case the terrain is now arid, but shows evidence it once was icy wet.

Overview map

The black dot in the center-left of the overview map to the right marks the location of this 25-mile-wide crater, deep inside the dry equatorial regions of Mars.

Yet, it appears scientists suspect a lake once filled this crater. Other images of the crater flow also show a knobby terrain on a generally smooth flat surface, and on the western interior rim the scientists have also identified another alluvial fan.

In general, this crater fits the description of most of the suspected formerly lake-filled craters that scientists have identified in the southern cratered highlands. Its central floor is very flat and smooth, suggesting it was once at the bottom of a lake. Its rim is also heavily eroded, with much evidence that its material had been washed inward when that lake existed.

The problem with these conclusions is central to the uncertainty of science. The evidence of a past lake is purely circumstantial, from evidence seen only from a distance. The geology is also of an alien planet with a very alien climate with one third the gravity of Earth. Any conclusions we draw from this data must be treated with great skepticism. We can’t form any trustworthy theories until we have geologists on the ground, tapping the rocks with a hammer.

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.

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"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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