APOD: Dark Sand Cascades on Mars (2015 Nov 29)

[APOD Robot]

Dark Sand Cascades on Mars

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 while the image was being taken.

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[Bric]

I saw this picture when it first came out and I'm still having trouble understanding it. Are we looking straight down or at an angle? Are the bases of the "trees" lined up along the top of the dunes? Then the slides are all flowing toward the top of the picture. Why are none coming down the near slopes?

[tetrodehead]

This must have caused excitement for the imaging team!
Flipping the image verticaly doesn't help. They just look like giant eyelids.
I'll stick with petrified trees on a cloudy day...

[heehaw]

Still hard to believe it is not asparagus.

[bystander]

http://asterisk.apod.com/viewtopic.php?t=30192
http://asterisk.apod.com/viewtopic.php?t=17988

http://www.uahirise.org/ESP_035033_2635

[Jouni]

Dear APOD team and the teams of photo producers, thanks a lot for sharing the astonishing image of the fringed hills of Mars.
Also I am still struggling to understand the structure and the mechanism of its formation. The image legend was unclear to me, in this respect.

It looks to me that the "trees" would really be standing up from the hills, i.e. they would not be merely optical illusions such as casted shadows or flat surface streaks. Right?

Am I gathering it right that the fringes, during the Martian season when they were born, were totally buried inside the pink hills? In this case they would be rigid remains of previous, nearly vertical, subterranean veins, enriched and encrusted by some darker minerals? And if harder, or with other sublimation temperature than, the pink ice, they would stay longer up when the ice evaporates?

And if the dark veins would be somewhat brittle, they could later tumble down when the supporting pink ice would get too scanty?

But it is also possible that the fringes would really grow (or at least add to their previous height) in the Martian athmosphere? Are there any sulphur gases or dust floating around which would selectively crystallize, or adhere, on these structures?

Any hints to understand it better would be welcome. Thanks!

Jouni, Finland

[Chris Peterson]

It looks to me that the "trees" would really be standing up from the hills, i.e. they would not be merely optical illusions such as casted shadows or flat surface streaks. Right?

These are purely surface markings. Nothing is standing up above the surface. That effect is an illusion.

[Dr Davies]

Same picture as 26/11/2012 - how sad am I that I know this!

[Ann]

It looks to me that the "trees" would really be standing up from the hills, i.e. they would not be merely optical illusions such as casted shadows or flat surface streaks. Right?

These are purely surface markings. Nothing is standing up above the surface. That effect is an illusion.

I certainly don't question you, Chris, but I agree with those who think these dark markings look strange. I think they look strange for the following reasons:

Click to view full size image

Oak trees buried in sand.
Photo: Insuremekevin.com

1) For every row of dark markings, there seems to be a "ridge" of some sort concealing the "roots" or beginnings of the dark markings. These "ridges" make the dark streaks look as if they appear very suddenly out of the ground. This certainly enhances the resemblance between the dark streaks and trees that just suddenly rise out of the ground.

2) Every row of dark markings seems to change from straight narrow "trunks" into wider and darker "crowns". Again these things look like trees.

Of course they are not trees. But I would like to know, Chris, where these things begin and where they end. Do they start at the "ridges", and do they end at the "crowns" of the "trees"?

Ann

[Chris Peterson]

Of course they are not trees. But I would like to know, Chris, where these things begin and where they end. Do they start at the "ridges", and do they end at the "crowns" of the "trees"?

I can't really say. They look as bizarre to me as they do to nearly everyone else. Making sense of them is very difficult. (BTW, if they were tree-like, given the camera resolution, they'd be at least 50 meters high- probably higher given that obliquity of the shot and the altitude of the spacecraft- similar to Giant Redwoods.)

[BMAONE23]

What I believe we are seeing is:
The ridge line, where the wind blown upslope meets the angle of repose, is where the dark streaks begin at the ridge line and flow down the repose side branching slightly at the bottom
The even side is the top of the slope and the uneven side is the bottom

[Jouni]

Thanks for explaining, Chris. I tried to study it in more detail, now, based on your comment. Could you please still confirm or dismiss the following:

Reasons to get the illusion of tree-like fringes seems to consist of at least two factors:

a. - from this photo's direction of sight, one is mislead to conceive that there would be a "bottom" side (like a straight line or a horizontal seam of soil, on the base of each group of trees), and a "top" side, where the variably long "tops" of trees transverse several different terrain zones of the backgound.

b. - From the curved contours, one is mislead to conceive that the terrain is seen in an oblique angle (e.g. 45 grades), therefore we conceive them as vertical objects.

As to (a), is it so that the "bottom" seams of each little "row of trees" is actually the sharp top of a long ridge, and the dark landslides run in this photo "upwards"?

As to (b), is it so that the photo is actually taken (nearly) directly from above? I.e. towards the center of the planet?

Am I on the right track of thought?

Anyway, a 3D model of the same terrain, some day/year in the future, might be interesting for the readers.

Exact anatomy of the starting points of the landslides might also be interesting.

Thanks, best wishes
Jouni

[MarkBour]

Thanks, bystander, for posting the viewtopic links. Lots of discussion and explanation there.

[workgazer]

the image works better upside down, using ifran view i turned it upside down and played with the contrast, i can see the light (near white vally's) with the sandy dune sides having this dark flow on them, soory dont know how to post the image

[Bric]

The image makes more sense rotated 180 degrees because it makes the downhill streaks run toward the bottom. But why are all the streaks on the same side of every dune? Did not one slip the other way? And why are the streaks black when all the other material on both sides of the dunes is light? I'm still leaning toward the trees hypothesis, but only because I so want it to be true.

[Chris Peterson]

The image makes more sense rotated 180 degrees because it makes the downhill streaks run toward the bottom. But why are all the streaks on the same side of every dune? Did not one slip the other way? And why are the streaks black when all the other material on both sides of the dunes is light? I'm still leaning toward the trees hypothesis, but only because I so want it to be true.

There is no "trees hypothesis".

[BMAONE23]

The image makes more sense rotated 180 degrees because it makes the downhill streaks run toward the bottom. But why are all the streaks on the same side of every dune? Did not one slip the other way? And why are the streaks black when all the other material on both sides of the dunes is light? I'm still leaning toward the trees hypothesis, but only because I so want it to be true.

The streaks are on the same sides of the dunes because of the physics behind the dune formation. The prevailing winds that created the dunes comes from the bottom of the image and blows toward the top of the image. this creates long slopes on the windswept sides and short slopes on the slump side. It is this Slump Side that Slumps to create the streaks.

The sand that creates the Dunes is Dark but is covered by both a Lighter Dust and a slight CO2 frost. The Frost creates the White patches and mixed with the Dust creates the Pink Cast.

Whatever process is causing the streaks, they are simply the underlying Dark Sand sliding down the Slump Side of the Dunes.

Since the process is Wind Driven, there should be No Reverse Slumping.
The streaks are Dark because the underlying Sand is Dark.

As Chris stated...No Trees on Mars

[Nitpicker]

I was thinking that the streaks were all on the south side, because that's where the Sun is. It is probably documented somewhere, but I haven't yet had time to "properly" study this mesmerizing image (in my amateurish way). I wonder what the obliquity of the image is? Initially, based on the high resolution and small scale of the image, I was assuming the view was more or less straight down, but Chris's comments suggest otherwise.

[Normus Svanstukker]

Though it is a bit confusing, we must keep in mind that the image was taken with the orbitng camera pointed straight down at the target area. The slipfaces of the dunes then would be nealy invisible, so the dust - sand trails would go at first in line of the camera's POV, then along the lesser curve at the dunes' bases. Thus it seems the trails point away from the surface though the trick of illusion.

[Nitpicker]

Upon reviewing some of the details in the "above image" link a little more closely, I've managed to determine that North is to the bottom-left(ish) of the APOD, and the sunlight is coming from the top-right(ish), from about 20 degrees West of South and about 27 degrees above the local horizon.

The following, much wider grayscale image of the same area, with the same orientation as the APOD, shows what must be the "mother dune" of all these little dunes, which much surely be a significant geographical factor in the orientation of the dark streaks:
http://hirise-pds.lpl.arizona.edu/PDS/E ... browse.jpg

[mvanneste]

This is not a scientific comment, but rather perhaps an amusing coincidence, Pantone has declare that the 2 colors of the year are rose quartz and blue "serenty". Did the fashion editors of Pantone have seen this picture to make their decision few days after it was published in APOD ?
http://www.pantone.com/color-of-the-yea ... ium=social