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I am now in the third week of my annual February birthday fund-raising drive. The first two weeks were good, but not record-setting.


There are still two weeks left in this campaign however. If you have been a regular reader and a fan of my work and have not yet donated or subscribed, please consider doing so. I take no ads, I keep the website clean from pop-ups and annoying demands (most of the time). Thus, I depend entirely on my readers to support me. Though this means I am sacrificing some income, it also means that I remain entirely independent from outside pressure. By depending solely on donations and subscriptions from my readers, no one can threaten me with censorship. You don't like what I write, you can simply go elsewhere.


You can support me either by giving a one-time contribution or a regular subscription. There are five ways of doing so:


1. Zelle: This is the only internet method that charges no fees. All you have to do is use the Zelle link at your internet bank and give my name and email address (zimmerman at nasw dot org). What you donate is what I get.


2. Patreon: Go to my website there and pick one of five monthly subscription amounts, or by making a one-time donation.

3. A Paypal Donation:

4. A Paypal subscription:

5. Donate by check, payable to Robert Zimmerman and mailed to
Behind The Black
c/o Robert Zimmerman
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Cortaro, AZ 85652

Martian dunes with strange splotches

Martian dunes with splotches

Cool image time! The picture to the right, cropped to post here, was taken on December 20, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows what the science team labels as “Dunes with Blotches.”

The blotches, or as I call them splotches, are the round dark patches on dunes themselves. Though their darkness is reminiscent of the dark patches that appear as spider features in the south polar regions of Mars, there are problems linking the two. The spiders form when the winter mantle of dry ice that falls as snow begins to weaken when the Sun reappears in the spring. Sunlight travels through the clear dry ice to warm the base of the mantle, causing it to sublimate into carbon dioxide gas. That gas however is trapped at the base, and only escapes when the thin mantle cracks at weak points. As the gas puffs out it carries with it dust, which leaves dark patches on the surface that disappear when the mantle disappears entirely by summer.

In the southern hemisphere at the poles the ground is somewhat stable, so the trapped gas appears to travel along the same paths each year to the same weak spots. This in turn causes it to carve spidery patterns in the ground, like river tributaries, except here the tributaries of gas flow uphill to their escape point. At the north pole the ground is not as stable. Instead we have many dunes, so that the dry ice mantle sublimates away at different places each year. There is no chance to form such spider patterns over time.

Making these splotches more puzzling is the season. This picture was taken in the winter, at a time one would think no dry ice is sublimating away.

Overview map

The white dot on the overview map to the right marks the location, inside a half-mile-deep and 12-mile-wide unnamed crater in the high southern latitude of Mars. At 63 degrees south latitude this location is just outside the arctic circle of Mars, which means there will be a day/night cycle year round, even if the days are extremely short in the winter.

The splotches therefore could be caused by sunlight during the short winter days, shining through the dry ice to create CO2 gas which then bursts through the mantle at dune weak points. In the full image it appears these weak points are more often found at the low points between dunes. In the cropped section above however the weak points are found both high and low on dunes.

This variety of weak point locations, combined with the dunes here, suggest that at this southern location the formation process is more like that seen in the north. The dunes are less stable, so weak points appear at different spots each season in a more random manner, and no spiders form.

One last thought: Note the colors. The orange near the bottom of the picture, on a part of the crater floor where there are no dunes, suggests Martian dust and debris. The blue color on the dunes suggest that water ice is being ejected from below by the CO2 gas when it escapes. These dunes therefore might also be impregnated with glacial material.

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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