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On February 5, 2023 I will celebrate my 70th birthday. Yay! As I do every year during this birthday month, I run a campaign to raise money to support my work here at Behind The Black. I do not run ads. My only support comes from my readers, which leaves me utterly free to speak my mind openly about space, culture, and politics. Please consider supporting me in this work by giving either a one-time contribution or a regular subscription, in any one of the following ways:


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Skiing dry ice boulders on Mars

Dune slope, with grooves, in Russell Crater
Click for full image.

Cool image and video time! The image to the right, cropped and reduced to post here, shows something that when I spotted it in reviewing the newest image download from the high resolution camera on Mars Reconnaissance Orbiter (MRO), I found it very baffling. The photo was taken on March 3, 2020, and shows an incredible number of linear groves on the slope of a large dune inside Russell Crater, located in the Martian southern highlands at about 54 degrees south latitude.

If these were created by boulders we should see them at the bottom of each groove. Instead, the grooves generally seem to peter out as if the boulder rolling down the slope had vanished. Making this even more unlikely is that the top of the slope simply does not have sufficient boulders to make all these groves.

The image was requested by Dr. Candice Hansen of the Planetary Science Institute in Tucson, Arizona, who when I emailed her in bafflement she responded like so:

You will love the explanation for this one! (-:

We think that the grooves are formed from blocks of dry ice breaking off at the top and sliding down the sand dune.

…What is really fun (at least if you are a nerd like me) is to go to the grocery store and buy a block of dry ice, then head to any nearby sand dunes. The block will levitate and slide down if the sand is compact enough.

Because the block is sublimating away, the gas acts as a lubricant so that it can slide down the hill. If large enough, the dry ice block will stop at the base of the hill to disappear in a small pit. If small enough, it actually might completely vaporize as it slides, explaining the grooves that appear to gradually fade away.

Hansen and her colleagues actually did this, and posted the video of it on youtube.

Though the dune on Mars is in the high mid-latitudes, Hansen explained that it is possible for carbon dioxide to condense into a solid on Mars at latitudes as low as 20 degrees. “We think carbon dioxide gets cold-trapped up in the alcoves at the crest of the dunes.”

This theory is actually not new. I have posted about it previously — on July 10, 2014, on December 21, 2015, and on October 25, 2017 — with the last post also including the video above.

The idea is so strange however that it apparently never sunk in. Maybe this image will help me remember it the next time I see boulder tracks on Mars, made by boulders that have vanished.

Conscious Choice cover

Now available in hardback and paperback as well as ebook!


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.


All editions are available at Amazon, Barnes & Noble, and all book vendors, with the ebook priced at $5.99 before discount. The ebook can also be purchased direct from my ebook publisher, ebookit, in which case you don't support the big tech companies and I get a bigger cut much sooner.


Autographed printed copies are also available at discount directly from me (hardback $24.95; paperback $14.95; Shipping cost for either: $5.00). Just email me at zimmerman @ nasw dot org.


  • Phill O

    Cool! Literally.

  • APL

    Gullies on Mars are fairly deep until pretty much the end of the track. This would imply a relatively large starting block. No blocks are ever seen at the bottom or partway down the tracks. Lots of potential blocks at the top. Earth simulations seem to show very different types of tracks – they seem much shallower, almost invisible.

    I’m inclined to think something else is going on, although I admit I don’t another theory – not enough data.

  • Alex Andrite

    Ice Blocking on Earth. 50lb blocks of ice, terry cloth towels, well manicured golf course, at night, teenagers.
    Find well groomed golf course grass slope, place ice block near slope edge, place towel on top of block, sit on block, towel becomes obvious, link legs around buddy / buddyess in front of you to form a chain, no more than six is best.
    Nudge, scoot to edge of slope, until …
    Warp speed down the slope.
    Careful of flying ice blocks.
    Leave ice blocks at bottom of slide to mysteriously disappear by morning.
    Ice Blocking on Earth.

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