Martian sand traps and elongated dunes
Click for full image.
Cool image time! The photo to the left, rotated, cropped, and reduced to post here, was taken on June 23, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). I was attracted by the uncaptioned image’s title, “Elongating Linear Dunes at Meroe Patera.” What are elongating linear dunes?
The photo shows two such dunes, stretching out to the southwest away from the pile of sand that abuts the cliff to the northwest. Unlike most dunes, which usually form and travel in groups, these for some reason form single straight lines extending for some distance.
I contacted the scientist who requested this image, Joel Davis of the Natural History Museum in London, hoping he could answer some questions about these strangely shaped dunes, and discovered that he was studying this exact subject for a paper since published. As noted in the paper’s introduction,
Linear dunes develop in multidirectional wind regimes and may migrate laterally (i.e., crest normal) and by elongation (i.e., crest parallel). Recent laboratory and numerical experiments demonstrate that in sediment‐starved regions (e.g., with nonerodible bedrock), linear dunes develop by elongation at the dune tips, aligned with the mean sediment transport direction (“finger‐mode”.
While on Earth such dunes form “extensive sand seas, linear dunes in nonpolar regions of Mars are rare.” The paper took a close look at multiple images of some these rare elongated dunes on Mars, taken over time, and found that they generally grew along their length, outward, parallel to the prevailing wind. Some were actively growing, some were not. Estimated ages ranged between 1,300 and 5,000 Earth years.
In the image above, as well as the wider shot of the same place to the right, the prevailing winds blow over that ridge, about 100 feet high, flow downward to hit the sand field nestled up against it, catch some of it and blow it outward to form the elongated dunes. As Davis explained, “The southwest side of the ridge is likely acting as a sand trap, where it is protected from the wind. The dunes then extent outwards from this field of sand.”
Imagine thick honey dripping down a shallow slope. Most of it might not move very much, but often a single drip might flow down, stopping when the supply of honey above peters out. In the case of these dunes, it isn’t gravity that is causing the flow, but the prevailing winds.
The wider shot however reveals some mysteries. For some reason the elongated dunes only occur in two specific spots. To the north we have the same ridgeline, the same field of sand sitting up against the ridge, but no elongated dunes. I asked Dr. Davis if there was an explanation for this. His wonderful answer, spoken like a true scientist:
I’m not sure, to be honest. It could be a number of factors due to winds, the availability of sand, or other conditions that make these particular locations ideal for dunes to nucleate.
One explanation that did occur to me (unproven of course) is that there might be a hollow in the ridge aligned with elongated dune, acting as funnel for wind and thus encouraging dune growth at just that point.
The location of these particular dunes on Mars, as shown by the blue cross in the overview map to the right, is interesting in itself. The dunes and ridge sit inside the caldera of one of Mars least dramatic looking shield volcanoes, Syrtis Major Planum, which ironically was the first documented surface feature identified on another planet, discovered by Christiaan Huygens during his observations of Mars in 1659.
This location, near to Jezero Crater where Perseverance will land in February 2021, is nestled between Isidis Basin, formed by a giant impact, and the dry and barren cratered highlands to the south. And like those highlands, I suspect that Syrtis Major is as dry and as barren. Unless someone discovers mineral resources here, the first colonizers of Mars will not come to this place first.
The support of my readers through the years has given me the freedom and ability to analyze objectively the ongoing renaissance in space, as well as the cultural changes -- for good or ill -- that are happening across America. Four years ago, just before the 2020 election I wrote that Joe Biden's mental health was suspect. Only in this year has the propaganda mainstream media decided to recognize that basic fact.
Fourteen years ago I wrote that SLS and Orion were a bad ideas, a waste of money, would be years behind schedule, and better replaced by commercial private enterprise. Even today NASA and Congress refuse to recognize this reality.
In 2020 when the world panicked over COVID I wrote that the panic was unnecessary, that the virus was apparently simply a variation of the flu, that masks were not simply pointless but if worn incorrectly were a health threat, that the lockdowns were a disaster and did nothing to stop the spread of COVID. Only in the past year have some of our so-called experts in the health field have begun to recognize these facts.
Your help allows me to do this kind of intelligent analysis. I take no advertising or sponsors, so my reporting isn't influenced by donations by established space or drug companies. Instead, I rely entirely on donations and subscriptions from my readers, which gives me the freedom to write what I think, unencumbered by outside influences.
Please consider supporting my work here at Behind the Black.
You can support me either by giving a one-time contribution or a regular subscription. There are five ways of doing so:
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Click for full image.
Cool image time! The photo to the left, rotated, cropped, and reduced to post here, was taken on June 23, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). I was attracted by the uncaptioned image’s title, “Elongating Linear Dunes at Meroe Patera.” What are elongating linear dunes?
The photo shows two such dunes, stretching out to the southwest away from the pile of sand that abuts the cliff to the northwest. Unlike most dunes, which usually form and travel in groups, these for some reason form single straight lines extending for some distance.
I contacted the scientist who requested this image, Joel Davis of the Natural History Museum in London, hoping he could answer some questions about these strangely shaped dunes, and discovered that he was studying this exact subject for a paper since published. As noted in the paper’s introduction,
Linear dunes develop in multidirectional wind regimes and may migrate laterally (i.e., crest normal) and by elongation (i.e., crest parallel). Recent laboratory and numerical experiments demonstrate that in sediment‐starved regions (e.g., with nonerodible bedrock), linear dunes develop by elongation at the dune tips, aligned with the mean sediment transport direction (“finger‐mode”.
While on Earth such dunes form “extensive sand seas, linear dunes in nonpolar regions of Mars are rare.” The paper took a close look at multiple images of some these rare elongated dunes on Mars, taken over time, and found that they generally grew along their length, outward, parallel to the prevailing wind. Some were actively growing, some were not. Estimated ages ranged between 1,300 and 5,000 Earth years.
In the image above, as well as the wider shot of the same place to the right, the prevailing winds blow over that ridge, about 100 feet high, flow downward to hit the sand field nestled up against it, catch some of it and blow it outward to form the elongated dunes. As Davis explained, “The southwest side of the ridge is likely acting as a sand trap, where it is protected from the wind. The dunes then extent outwards from this field of sand.”
Imagine thick honey dripping down a shallow slope. Most of it might not move very much, but often a single drip might flow down, stopping when the supply of honey above peters out. In the case of these dunes, it isn’t gravity that is causing the flow, but the prevailing winds.
The wider shot however reveals some mysteries. For some reason the elongated dunes only occur in two specific spots. To the north we have the same ridgeline, the same field of sand sitting up against the ridge, but no elongated dunes. I asked Dr. Davis if there was an explanation for this. His wonderful answer, spoken like a true scientist:
I’m not sure, to be honest. It could be a number of factors due to winds, the availability of sand, or other conditions that make these particular locations ideal for dunes to nucleate.
One explanation that did occur to me (unproven of course) is that there might be a hollow in the ridge aligned with elongated dune, acting as funnel for wind and thus encouraging dune growth at just that point.
The location of these particular dunes on Mars, as shown by the blue cross in the overview map to the right, is interesting in itself. The dunes and ridge sit inside the caldera of one of Mars least dramatic looking shield volcanoes, Syrtis Major Planum, which ironically was the first documented surface feature identified on another planet, discovered by Christiaan Huygens during his observations of Mars in 1659.
This location, near to Jezero Crater where Perseverance will land in February 2021, is nestled between Isidis Basin, formed by a giant impact, and the dry and barren cratered highlands to the south. And like those highlands, I suspect that Syrtis Major is as dry and as barren. Unless someone discovers mineral resources here, the first colonizers of Mars will not come to this place first.
The support of my readers through the years has given me the freedom and ability to analyze objectively the ongoing renaissance in space, as well as the cultural changes -- for good or ill -- that are happening across America. Four years ago, just before the 2020 election I wrote that Joe Biden's mental health was suspect. Only in this year has the propaganda mainstream media decided to recognize that basic fact.
Fourteen years ago I wrote that SLS and Orion were a bad ideas, a waste of money, would be years behind schedule, and better replaced by commercial private enterprise. Even today NASA and Congress refuse to recognize this reality.
In 2020 when the world panicked over COVID I wrote that the panic was unnecessary, that the virus was apparently simply a variation of the flu, that masks were not simply pointless but if worn incorrectly were a health threat, that the lockdowns were a disaster and did nothing to stop the spread of COVID. Only in the past year have some of our so-called experts in the health field have begun to recognize these facts.
Your help allows me to do this kind of intelligent analysis. I take no advertising or sponsors, so my reporting isn't influenced by donations by established space or drug companies. Instead, I rely entirely on donations and subscriptions from my readers, which gives me the freedom to write what I think, unencumbered by outside influences.
Please consider supporting my work here at Behind the Black.
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:
5. Donate by check, payable to Robert Zimmerman and mailed to
Behind The Black
c/o Robert Zimmerman
P.O.Box 1262
Cortaro, AZ 85652
You can also support me by buying one of my books, as noted in the boxes interspersed throughout the webpage or shown in the menu above. And if you buy the books through the ebookit links, I get a larger cut and I get it sooner.
Mars is way more interesting than the Moon.
My favorite Mars picture is of Dust Devils recorded by rovers.
https://earthsky.org/upl/2016/04/mars-opportunity-3-31-2016-NASA-e1460025108939.jpg
Hi Bob,
An orientation question. You note:
“The photo shows two such dunes, stretching out to the southwest away from the pile of sand that abuts the cliff to the northwest.”
Are the photos always oriented such that north is always at the top of the photo?
Thanks,
CMcL
Chris: Depends on where on Mars the picture was taken. Images from the equator to the high mid-latitudes will be oriented with north more-or-less up, but at an angle because of the orbit of MRO. At the poles that angle becomes more pronounced.
When I rotate I usually do it to make the area I am cropping vertical, to capture more of it. This generally makes north up.