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Mars’ giant sinkholes

The floor of one of Mars' giant sinkholes

Cool image time! The picture to the right, rotated, cropped, reduced, and sharpened to post here, was taken on June 27, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows a small section of the floor and northern slope of Hebes Chasma, one of the many very large enclosed pits that can be found to the north of Valles Marineris, the largest canyon in the solar system. Though Hebes seems small next to the 1,500 mile long Valles Marineris, it still is 200 miles long by 80 miles wide, and could easily fit a half dozen Grand Canyons within it.

For example, the Grand Canyon is from 4,420 to 5,400 feet deep, hiking down from the south and north rim lodges respectively, which sit about ten miles apart. On this picture, the peak on the right sits about 5,300 feet above and only about 3.8 miles from the low spot on the bottom left, which means this one small picture encapsulates the Grand Canyon. And yet, the northern rim of Hebes sits another 21,000 feet higher and twelve miles away. And the entire chasma itself extends 50 miles to the west, 150 miles to the east, and 50 miles to the south.

Overview map

The white dot on the overview map and inset to the right marks the location of the picture above, inside Hebes Chasma.

Note the white line I have added to the map, cutting through Hebes and beyond and aligned with the fissure to the east. This line likely marks the location of a long fault line.

Hebes likely formed when underground ice and dry ice settled into this faultline, slowly widening it during the annual seasonal freeze-sublimation cycle. Eventually, that ice and dry ice began to sublimate away, as this sits in Mars’ dry equatorial region where it is difficult for near surface ice to survive.

As that material sublimated into gas, it left behind a void. Eventually the gas pressure would have pushed through cracks to the surface. At that point, the cracked and weakened ceiling, no longer supported from below, would have collapsed, leaving behind what on Earth we call a sinkhole.

On Earth such sinks are generally rarely larger than a few hundred feet across. On Mars, we instead get collapse pits like Hebes, hundreds of miles across. This formation process also likely applies as well to all the other chasma and Valles Marineris itself. While the underground ice and dry ice might have flowed, either as liquid or glaciers, these pits and canyons probably formed when that material disappeared and the roof crashed in. In Valles Marineris subsequent ice or liquid flows produced further shaping, but in an enclosed chasma like Hebes, there were no major flows. This gigantic pit was formed almost entirely by collapse.

The Grand Canyon was likely formed partly by the same process, but the conditions on Earth made liquid erosion play a much greater part. The Colorado River has flowed through the canyon for eons, and essentially carved it as the land around slowly rose. On Mars it is more likely that any liquid flows were of very short duration, because the liquid would have sublimated away in its thin and cold atmosphere.

Ice flows however might have lasted for many thousands of centuries, and created features we associate with flowing rivers.

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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