APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

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APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by APOD Robot » Tue Sep 24, 2019 4:05 am

Image Sand Dunes Thawing on Mars

Explanation: What are these strange shapes on Mars? Defrosting sand dunes. As spring dawned on the Northern Hemisphere of Mars, dunes of sand near the pole, as pictured here in late May by ESA's ExoMars Trace Gas Orbiter, began to thaw. The carbon dioxide and water ice actually sublime in the thin atmosphere directly to gas. Thinner regions of ice typically defrost first revealing sand whose darkness soaks in sunlight and accelerates the thaw. The process might even involve sandy jets exploding through the thinning ice. By summer, spots will expand to encompass the entire dunes. The Martian North Pole is ringed by many similar fields of barchan sand dunes, whose strange, smooth arcs are shaped by persistent Martian winds.

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Yhaal House » Tue Sep 24, 2019 6:38 am

Does look like the top of a cake!

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by JohnD » Tue Sep 24, 2019 9:58 am

As these are "ringing the Martian North Pole" it's the tops of the dunes that will catch the sunlight first. Yet it's the valleys between the dunes where we see dark sand coming through. Shurley shome mishtake in the explanation?

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by orin stepanek » Tue Sep 24, 2019 11:26 am

Nice white sand on Mars
DunesThawingOnMars_ExoMars_1080.jpg
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Ann » Tue Sep 24, 2019 12:09 pm

JohnD wrote:
Tue Sep 24, 2019 9:58 am
As these are "ringing the Martian North Pole" it's the tops of the dunes that will catch the sunlight first. Yet it's the valleys between the dunes where we see dark sand coming through. Shurley shome mishtake in the explanation?

John
I think the dark stuff is some Martian "gunk". Perhaps the infamous "tholins"?

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by neufer » Tue Sep 24, 2019 12:35 pm

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by JohnD » Tue Sep 24, 2019 12:51 pm

Ann,

No.
"Thinner regions of ice typically defrost first revealing sand whose darkness soaks in sunlight and accelerates the thaw. "
Dark sand.

Sure, the dark marks could be the hypothesized 'Martian geysers" - https://apod.nasa.gov/apod/ap060823.html - but I repeat : the ridges of the dunes are unsullied. The dark marks are all in the valleys between The description of today's APOD is nonsensical and should be rewritten, perhaps to reflect our ignorance about Martian high latitudes
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by De58te » Tue Sep 24, 2019 1:15 pm

Interesting observation. I can understand the frozen water subliming into gas in the thin atmosphere, but it also says the frozen carbon dioxide sublimes directly into a gas in Mars' thin atmosphere. Strange! Frozen Co2, also known as dry ice, also sublimes into a gas in Earth's much thicker atmosphere. I was wondering how thick and dense does an atmosphere have to get so as frozen Co2 turns into a liquid first upon thawing.

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by JohnD » Tue Sep 24, 2019 1:30 pm

De58te,
The answer is readily available at the Triple Point phase diagram for CO2.
Look it up, please, I can't post pictures easily here.

X-axis temperature, Y-axis Pressure, its divided into three areas for the three phases solid, liquid and gas, by two lines that join at the Triple Point (where it can exist in all three phases simultaneously). You'll see at once where any substance that can exist in these phases will sublime to gas from solid if the pressure is low enough.
The Triple Point for Co2 occurs at a temperature of −56.4°C and a pressure of 5.11 atm. So, on Mars or Earth, no chance of liquid CO2, outside a pressure vessel.
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by TheZuke! » Tue Sep 24, 2019 2:24 pm

The moon is made of green cheese,
and it looks like Mars is made of Bleu Cheese.

(I'll leave it to Neufer to post a video clip from Wallace and Grommit's Moon adventure B^)

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by MarkBour » Tue Sep 24, 2019 10:33 pm

JohnD wrote:
Tue Sep 24, 2019 9:58 am
As these are "ringing the Martian North Pole" it's the tops of the dunes that will catch the sunlight first. Yet it's the valleys between the dunes where we see dark sand coming through. Shurley shome mishtake in the explanation?

John
I see your reasoning, but I am still quite prepared to accept that these dunes are indeed near the Martian north pole; that they are thawing; and that the darker regions are exposed sand, moreso than the light, cream-colored regions. This leads me to wonder if I can debate your reasoning.

One thing to consider is that the region is very windy, which is what forms the dunes. So, what happens if some of the sand near the top of the dunes thaws? It will be the most mobile sand in the structure, and the wind will reallocate it to lower elevations. Also, as I look over the image carefully, I do see a lot of darkened regions that I think are high up and near the top of the dunes. I would therefore not describe this image as showing melting only in the valleys. It may appear more like this is the case, due to our mental biases when viewing images. We visually assume that the darker portions are lower, and are shadows. Then again, I have to admit, one sees an awful lot of dark patches that appear to be just next to the base of many of the dunes.

Just some food for thought. Wouldn't it be great to have an entire time-lapse video of this region as it thaws?
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Boomer12k » Tue Sep 24, 2019 11:02 pm

If the water ice sublimes into the atmosphere as a gas, without turning liquid first...then the dark areas are dark, DRY, sand... and not wet at all...and may not be indicative of the thawing process. We would then not SEE any water vapor, steam, or standing water, or damp sand at all in that case...

Great image...
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Chris Peterson » Fri Sep 27, 2019 11:31 pm

By curious coincidence, the same day this APOD ran I left for three days horse packing in the wilderness around Great Sand Dunes National Park. Saw the exact same kinds of structure as we see on Mars. Luckily, no sublimating dry ice, though! Or even water ice. Yet.
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by JohnD » Sat Sep 28, 2019 9:01 am

Looks great fun, Chris!
As our nearest to The Martian cowpoke, did you science the ehll out of it?

John

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Ann » Sat Sep 28, 2019 9:38 am

Chris Peterson wrote:
Fri Sep 27, 2019 11:31 pm
By curious coincidence, the same day this APOD ran I left for three days horse packing in the wilderness around Great Sand Dunes National Park. Saw the exact same kinds of structure as we see on Mars. Luckily, no sublimating dry ice, though! Or even water ice. Yet.
_
IMG_20190925_163423p.jpg
IMG_20190925_155437p.jpg
IMG_20190925_145345p.jpg
So you bright a three-day supply of water for you and your horses with you then, Chris?

How much water would we have to bring with us to Mars? Nothing? Because we could excavate underground water ice and thaw it and drink it and cook with it? And keep ourselves clean with it?

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by JohnD » Sat Sep 28, 2019 10:18 am

Or, catalyse the spare hydrazine to make hydrogen, then burn it to make water.
Simples!

Not
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Chris Peterson » Sat Sep 28, 2019 2:19 pm

JohnD wrote:
Sat Sep 28, 2019 9:01 am
Looks great fun, Chris!
As our nearest to The Martian cowpoke, did you science the ehll out of it?
Yeah, can't help myself there. All of the geology in Colorado is fascinating (and there's a lot of it). Where we made camp on the edge of the dunes, at the base of the Sangre de Cristo mountains, we were on dunes that had acquired a bit of organic material in the top meter or so, allowing Ponderosa pines and other vegetation to take hold. Looked like ordinary dirt except where water had cut gullies. Then you could see back through thousands of years of deposition, with dry periods where layers of sand were laid down, and wet periods where stones must have been washed over that. I wouldn't be surprised if some Martian dune fields had that kind of structure underneath, as well.
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Chris Peterson » Sat Sep 28, 2019 2:22 pm

Ann wrote:
Sat Sep 28, 2019 9:38 am
Chris Peterson wrote:
Fri Sep 27, 2019 11:31 pm
By curious coincidence, the same day this APOD ran I left for three days horse packing in the wilderness around Great Sand Dunes National Park. Saw the exact same kinds of structure as we see on Mars. Luckily, no sublimating dry ice, though! Or even water ice. Yet.
So you bright a three-day supply of water for you and your horses with you then, Chris?
Although this looks like desert, it isn't. The region gets a reasonable amount of rainfall. Just east of the dunes are the Sangre de Cristo mountains, with many 14,000 foot peaks and countless small streams coming out. To the west lies the San Luis Valley, where people ranch and farm. We made camp between the mountains and the dunes. Lots of ground water.
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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by Ann » Sat Sep 28, 2019 2:43 pm

Chris Peterson wrote:
Sat Sep 28, 2019 2:19 pm
JohnD wrote:
Sat Sep 28, 2019 9:01 am
Looks great fun, Chris!
As our nearest to The Martian cowpoke, did you science the ehll out of it?
Yeah, can't help myself there. All of the geology in Colorado is fascinating (and there's a lot of it). Where we made camp on the edge of the dunes, at the base of the Sangre de Cristo mountains, we were on dunes that had acquired a bit of organic material in the top meter or so, allowing Ponderosa pines and other vegetation to take hold. Looked like ordinary dirt except where water had cut gullies. Then you could see back through thousands of years of deposition, with dry periods where layers of sand were laid down, and wet periods where stones must have been washed over that. I wouldn't be surprised if some Martian dune fields had that kind of structure underneath, as well.
_
IMG_20190925_075623p.jpg
Like you said, finding such structures on Mars, at least reasonably deep down, wouldn't be surprising.

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Re: APOD: Sand Dunes Thawing on Mars (2019 Sep 24)

Post by neufer » Sat Sep 28, 2019 6:36 pm

Click to play embedded YouTube video.
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