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My February birthday fund-raising campaign for Behind the Black it now over. I sincerely and with deep gratitude thank all those who donated. Without your support I could not keep doing this, not so much because of the need for income to pay the bills, but because it tells me that there are people out there who want me to do this work. For those who did not contribute during the campaign, please consider adding your vote of support to Behind the Black, by giving either a one-time contribution or a regular subscription, in any one of the following ways:


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The edge of Mars’ south polar layered cap

The edge of the Martian south pole layered deposits
Click for full image.

Cool image time! The photo to the right, rotated and cropped to post here, was taken by the high resolution camera on Mars Reconnaissance Orbiter (MRO) on April 10, 2020, and shows the edge of what scientists have dubbed Mars’s south polar layered deposits. The high point, towards the south, is at the bottom, and the terraced layers descend downward to the plains as you move up the image, to the north.

In essence, this spot is the edge of the southern ice cap, though unlike the north polar ice cap, this edge is not the edge of the visible ice cap, but the edge of a much larger field of layered deposits of mixed dust and ice. In the north the ice cap almost entirely covers these layered deposits. In the south the residual ice cap does not. Instead, the layered deposits extend out far beyond the smaller residual ice cap.

The map below provides the geography of the south pole, with the location of this image indicated by the blue cross.

Map of Martian south pole

It is also worthwhile comparing this map to the map of the north pole that I provided in an earlier post. On large scales the differences outlined above are obvious.

That post focused on the edge of the north polar ice cap, so opening it in a second tab and comparing its cool image with today’s cool image is also very worthwhile. On small scales the differences are just as obvious. While the edge in the north is very steep and sudden, in the south the edge is much more gradual. Rather than a cliff, we are presented by a gradual series of terraces, each representing an older layer as you travel downward.

In the north pole cool image there appear to be many more layers, but that is because in the north they are all visible in one cliff edge. In the south the layers are exposed over a much more extensive area, much larger than can be captured in a single MRO photo. Today’s cool image only shows one small section of terraces, with many more to the south, beyond the bottom edge of this image and closer to the pole.

In the north the edge also included the many white layers of the residual cap of relatively pure ice, sitting on top of the layered deposits. In the south, in today’s image, we are quite far away from that residual ice cap, so its white layers are not present.

Essentially, the Martian poles are fundamentally different As I noted on July 8, 2020:

The south cap is thought to be 7 to 15 million years older. While the permanent residual ice cap in the north is large and almost completely covers the layered deposits of ice/dust below it, in the south the permanent ice cap is much smaller and covers only a small portion of those layered deposits.

The south pole sits at a higher elevation, on the rougher cratered southern highlands. The north pole is in the middle of the smoother northern lowland plains, with many fewer craters. While the north pole is surrounded by a vast sea of dunes, the dunes in the south are generally confined to the interiors of craters.

Most importantly, while each visible layer in both the north and south pole images represents a unique past climate cycle on Mars, all probably caused by the swings in the planet’s obliquity (its rotational tilt relative to the Sun), scientists have not yet been able to match up the layers between the two caps. They therefore think that they could represent different timelines, a puzzling circumstance that remains unexplained.

In fact, to understand and align the histories of both caps, which will also help us lay out the entire history of Mars’ past climate cycles, we will simply need to drill cores in both places, in numerous spots.

Such work however is likely not to happen until humans are living on Mars and are walking its surface.

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.

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