Explanation: They might look like trees on Mars, but they're not. Groups of dark brown streaks have been photographed by the Mars Reconnaissance Orbiter on melting pinkish sand dunes covered with light frost. The above image was taken in 2008 April near the North Pole of Mars. At that time, dark sand on the interior of Martian sand dunes became more and more visible as the spring Sun melted the lighter carbon dioxide ice. When occurring near the top of a dune, dark sand may cascade down the dune leaving dark surface streaks -- streaks that might appear at first to be trees standing in front of the lighter regions, but cast no shadows. Objects about 25 centimeters across are resolved on this image spanning about one kilometer. Close ups of some parts of this image show billowing plumes indicating that the sand slides were occurring even when the image was being taken.
“When occurring near the top of a dune, dark sand may cascade down the dune leaving dark surface streaks . . .”
I still don’t get it. I didn’t four years ago, and I still don’t. I’ve read all 120 responses to the Bad Astronomy article, and can’t “see” sand cascading down the dunes. The dark brush-bristle objects look too straight and parallel to be anything rolling down a slope. Maybe my eyes are playing tricks, but they appear more like something sticking up from the top of the dunes. I think it takes a real stretch of the imagination to see sand cascading down a slope. Anyone else have a problem with the explanation?
I loved this picture when it appeared on APOD on 19 January 2010, and I still love it. I think word for word the same commentary is a bit poor, though.
I have no problem with seeing dark sand cascading down a slope. I don't think the dark marks look particularly straight or particularly parallel, especially when I view the hi-res version. @Mactavish, it might help if you think of sand cascades as flowing like a liquid, rather than "rolling" down the slope.
Last edited by bystander on Sun Nov 25, 2012 5:12 pm, edited 1 time in total.
Reason:added link to previous discussion
As to what one is seeing here ... It is difficult to even tell up from down, but moving sand/mud/whatever from a higher to a lower elevation directly down the fall-line does seem a plausible explanation. When flying over mountain ranges I have seen somewhat similar patterns in the snow cover caused by small avalanches.
I found the links at http://apod.nasa.gov/apod/ap080311.html to be of help.
At first glance, the image still looks creepy, some kind of skin disorder.
Each streak points downhill. Perhaps a 3D image might make that more apparent? The light at the Martian north pole doesn't bring out the shadows on the leeward side of the dunes.
I don't know the triple point of carbon dioxide or at what temperature-pressure CO2 ice is stable. We know the air pressure makes water ice sublimate at the robot latitudes, but it is probably somewhat colder at the poles.
There is life, but we haven't figured out how to talk to paramoecia let alone space monkeys.
<<In geography, an oasis (plural: oases) or cienega (Southwestern United States) is an isolated area of vegetation in a desert, typically surrounding a spring or similar water source. The word oasis comes into English via Latin: oasis from Ancient Greek: ὄασις óasis, which in turn is a direct borrowing from Demotic Egyptian. The word for oasis in the later attested Coptic language (the descendant of Demotic Egyptian) is wahe or ouahe which means a 'dwelling place'. The location of oases has been of critical importance for trade and transportation routes in desert areas. Caravans must travel via oases so that supplies of water and food can be replenished. Thus, political or military control of an oasis has in many cases meant control of trade on a particular route.
Oases are formed from underground rivers or aquifers such as an artesian aquifer, where water can reach the surface naturally by pressure or by man made wells. Occasional brief thunderstorms provide subterranean water to sustain natural oases, such as the Tuat. Substrata of impermeable rock and stone can trap water and retain it in pockets, or on long faulting subsurface ridges or volcanic dikes water can collect and percolate to the surface. Any incidence of water is then used by migrating birds who also pass seeds with their droppings which will grow at the water's edge forming an oasis. People who live in an oasis must manage land and water use carefully; fields must be irrigated to grow plants like dates, figs, olives, and apricots. The most important plant in an oasis is the date palm, which forms the upper layer. These palm trees provide shade for smaller trees like peach trees, which form the middle layer. By growing plants in different layers, the farmers make best use of the soil and water. Many vegetables are also grown and some cereals, such as wheat, barley and millet, are grown where there is more moisture.>>
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
I have viewed this photo many times and, while I accept the explanation, I still cannot twist the image enough to get away from seeing "trees". I guess I'll go to the dementia site again and take the faultering brain test.
Seeker wrote:I could never see this until I flipped the image 180 degrees. It now makes sense though I don't know why this was not done to begin with years ago.
Thank you, Seeker. I don’t know why I didn’t think of the upside-down trick before. Now I see the downward flow. A 180 degree flip also makes a world of difference when trying to determine whether a crater is a crater or a mound. When nothing else works, stand on your head!
Seeker wrote:
I could never see this until I flipped the image 180 degrees. It now makes sense though I don't know why this was not done to begin with years ago.
Thank you, Seeker. I don’t know why I didn’t think of the upside-down trick before. Now I see the downward flow. A 180 degree flip also makes a world of difference when trying to determine whether a crater is a crater or a mound. When nothing else works, stand on your head!
i agree that the description doesn't really explain what we see in the picture.
What we need is a stereo picture, two close togethr or two pictures from different angles. These should be available, does anyone know where they might be sourced?
keirvt wrote:i agree that the description doesn't really explain what we see in the picture.
What we need is a stereo picture, two close togethr or two pictures from different angles. These should be available, does anyone know where they might be sourced?
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
Seeker wrote:I could never see this until I flipped the image 180 degrees. It now makes sense though I don't know why this was not done to begin with years ago.
This makes a world of difference. Our minds aren't used to digesting upside-down dunes.
Flipped 180 deg, it now looks like Mars is bleeding.
I find the last note in the caption inaccurate and disingenuous. Clearly the picture that comes up when one explores the "Close ups of some parts of this image show billowing plumes indicating that the sand slides were occurring..." are NOT a part of this image blown up, and are not even of similar terrain. The close up of the avalanche plume is taken at the edge of the polar ice cap, NOT even remotely a dune environment.
This is not just a typo. I leave it to others to determine whether or not this is a deliberate attempt to mislead. I don't (as however many others do) have trouble with the basic explanation about materials having slid down a dune face. But, clearly there is something else going on in this image. Places where there is no high slope angle show black materials erupting from 'beneath' the pink sand-like materials; consistent with other images of dunes in the defrost latitudes.
Mis-attributing an image that is both mysterious and controversial does not work for the credibility of this effort.
edibleplantguy wrote:I find the last note in the caption inaccurate and disingenuous. Clearly the picture that comes up when one explores the "Close ups of some parts of this image show billowing plumes indicating that the sand slides were occurring..." are NOT a part of this image blown up, and are not even of similar terrain. The close up of the avalanche plume is taken at the edge of the polar ice cap, NOT even remotely a dune environment.
I didn't interpret the caption as suggesting that the linked APOD was of the same area. The image was in reference to dust plumes, which are easier to see on the earlier APOD than the more subtle plumes seen in the current APOD. APOD captions link other images all the time to help clarify aspects of the specific image under discussion.
This is not just a typo. I leave it to others to determine whether or not this is a deliberate attempt to mislead. I don't (as however many others do) have trouble with the basic explanation about materials having slid down a dune face. But, clearly there is something else going on in this image. Places where there is no high slope angle show black materials erupting from 'beneath' the pink sand-like materials; consistent with other images of dunes in the defrost latitudes.
How are you determining whether a slope angle is high or low? I think that is almost impossible to do in a single image with this perspective.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Right in the middle of the image 1/2 way down and 1/3 from the left side there is a debris cloud that shows the direction of travel ot that most recent slide activity
Dark Sand Cascades on Mars
