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I am now running my annual July fund-raising campaign to celebrate the twelfth anniversary of the establishment of Behind the Black. For many reasons, mostly political but partly ethical, I do not use Google, Facebook, Twitter. These companies practice corrupt business policies, while targeting conservative websites for censoring, facts repeatedly confirmed by news stories and by my sense that Facebook has taken action to prevent my readers from recommending Behind the Black to their friends.


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Two pits at opposite ends of Mars’ big volcanoes

Overview map

Regular readers of Behind the Black know that since 2018 I have regularly documented all the images of pits taken by the high resolution camera on Mars Reconnaissance Orbiter (MRO). (See my last pit post in January for a full list of these previous articles.) The black dots on the map to the right shows the location of all the pits near the volcanoes Arsia and Pavonis Mons that have so far been highlighted here.

The two white dots are the two most recent MRO pits, and are the subject of today’s cool image. They also happen to be the farthest north and south pits so far documented. The southernmost pit, which I am saving till last, is the most interesting.

PIt north of Pavonis Mons
Click for full image.

Pit south of Arsia Mons
Click for full image.

The first pit to the right, the image cropped to post here, was taken on January 5, 2021. It is located to the northeast of Pavonis Mons, and is similar to most of these likely skylights into an underground lava tube. When compared to the nearby similarly-sized impact crater to the east, the differences are obvious. The pit’s edges are sharp, with no rim as you’d see with a crater. It is also much deeper than a crater, with the strong suggestion that the pit’s edges are overhung. While we cannot see for sure whether there are any underlying passages leading off from this pit, there is a faint hint on the surface of a northwest-to-southwest depression under this pit, suggesting the lava tube itself.

The second pit to the right, also cropped to post here, was taken on January 7, 2021, and is the farthest south pit yet documented in this volcanic region. Unlike the first pit above, it appears not as deep, and with less apparent overhang at its edges.

What makes this pit really interesting however are two features. First, there is the bluish color at the pit’s bottom. A bright blue, according to guidance [pdf] from the camera’s science team suggest either coarse rocks or frost or ice. As the pit is in the dry equatorial regions at 20 degrees south latitude, with the picture taken during summer, it is unlikely this is ice. More likely the blue color indicates the breakdown material that fell when the roof collapsed.

The possibility of ice, however is very tantalizing. Other research has found evidence of past glaciers on the western slopes of Arsia Mons, suggesting that ice might still exist in these lava tubes.

Second, there is the surrounding terrain, which is an endless scattering of small pointed knobs and ridges surrounded by dust-covered smooth hollows. The lack of craters suggests it is a relatively young lava plain. The knobs and ridges suggest to me that for some reason it has been eroded over time. A quick review of all the other high resolution images of nearby pits suggests that as you descend the southern flank of Arsia Mons, the terrain gets rougher and rougher, suggesting the lower flanks are older. This is a pure guess on my part, but does make some sense, as the flows from newer eruptions should be higher up the mountain.

Both of these pits were documented in a 2015 study that used the context camera on MRO and found more than a hundred in this region. Since then the scientists have been methodically snapping high resolution pictures of these pits. As Chris Okubo of the U.S. Geological Survey explained to me in 2019,

We are just cataloging them in the hopes that someday one of us, or someone else will have the time to investigate these further.

I will in turn continue to catalog them here, with the hope that this might encourage someone to do exactly that.

Conscious Choice cover

From the press release: In this ground-breaking new history of early America, historian Robert Zimmerman not only exposes the lie behind The New York Times 1619 Project that falsely claims slavery is central to the history of the United States, he also provides profound lessons about the nature of human societies, lessons important for Americans today as well as for all future settlers on Mars and elsewhere in space.

Conscious Choice: The origins of slavery in America and why it matters today and for our future in outer space, is a riveting page-turning story that documents how slavery slowly became pervasive in the southern British colonies of North America, colonies founded by a people and culture that not only did not allow slavery but in every way were hostile to the practice.  
Conscious Choice does more however. In telling the tragic history of the Virginia colony and the rise of slavery there, Zimmerman lays out the proper path for creating healthy societies in places like the Moon and Mars.


“Zimmerman’s ground-breaking history provides every future generation the basic framework for establishing new societies on other worlds. We would be wise to heed what he says.” —Robert Zubrin, founder of founder of the Mars Society.


Available everywhere for $3.99 (before discount) at Amazon, Barnes & Noble, and all ebook vendors, or direct from the ebook publisher, ebookit. And if you buy it from ebookit you don't support the big tech companies and I get a bigger cut much sooner.


  • Jeff hiland

    Love your Mars posts

  • While the northernmost feature looks very much like a lava pit, I’m wondering whether the southern structure is. There are a number of similar, crater-like structures in the area, and they all appear ‘oriented’ in roughly the same direction. Curious if a nearly-cooled, and still somewhat viscous, lava flow could have been impacted by a flight of rocks. Perhaps a carbonaceous asteroid got a bit too close.

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