Raised Features

• by wassock moderator

  I'm having trouble seeing the features in this image as depressions. There is a lot of dark material which appears to be trapped between the folds of the apparently raised features which makes sense if they are raised but not it these are channels. I've marked three points of interest. in the top most plume the plme blows across one of the channel features which appears to remain clear of the black stuff - makes sense if its a hill, none if it's a gully. the two plumes win the middle (ish) which run bottom to top pass over a "hill" ridge, the source side of the "ridge" has a thick deposit and down wind of the ridge its more diffuse.

  Posted January 10, 2013 6:11 PM

• by Wounded_Knee

  A trick ive learnt is to identify the position of the sun using the channels, in this case its to the right, then look at the centre of a clear patch near shallow channels, then keep thinking that this area is raised an follow the channel back to its origin, the thing that gives the channels a raised look is the brightness, once you convince the eye that there sunken not raised it starts to look right.....allegedly 😃

  Posted January 10, 2013 6:58 PM

• by wassock moderator

  So why do the 2 sse - nnw plumes appear to have had trouble carrying over gullies? Also if the sun is right then it would appear to be bottom right, which, assuming that the pic is presented with North at the top, doesn't work for a pic of the south pole.

  Posted January 10, 2013 7:26 PM

• by Portyankina scientist

  Hi!

  The image is not projected to the map, so the orientation should be figured out for each image depending on the movement of the spacecraft when it took the image. I am sorry not to have more info, but these things are depressions, we know that for sure.
  The "trouble with carrying dust over gullies" is an interesting one! The solution is actually not that there is no dust in that gully, but that there is still a lot of CO2 ice. The layer of dust in not dense, this allows dust particles heat up from the sun and sink through the ice layer below them. On Earth boulders do similar thing in glaciers. So, top of the ice layer becomes cleaner and looks brighter. The process is most efficient in the inclinations that catch more solar light - like on sides of these gullies. This is why gullies look bright, while flat surfaces - not.

  Anya

  Posted January 10, 2013 8:22 PM

• by wassock moderator

  Hi Anya - I can see that as an explanation for why the gullies remain 'clean' but the 2 features I'm referring too have a fan leaving dark deposits upwind of the 'gully' but abruptly less dense downwind of it. It looks like something is preventing the dust carrying over the feature.

  Posted January 10, 2013 8:32 PM

• by wassock moderator

  Also from the FAQ's and elsewhere it seems that 1 if there are fans then the surface is covered in ice and that the obsrerved channels are caused by gas movement BENEATH the ice if both these hold why can we see the channels here?

  Posted January 10, 2013 11:09 PM

• by chrisbayes

  i am having a similar problem seeing some of these as channels too
  this one and this one which has a plateau and a hole up to the top left. if you try to look at it so that the ridges are channels and illuminated from the other side then it just doesn't work 😦

  mschwab pointed me at this link which had me convinced for a while but it isn't easy 😄

  Posted January 11, 2013 1:13 AM

• by chrisbayes

  Anya, if

  The layer of dust in not dense, this allows dust particles heat up from the sun and sink through the ice layer below them.

  then it is more likely to happen on the lips of the channel than in the bottom of the channel where there is less sun

  unless you look at them as ridges and then the extra sun they get might cause the black stuff to sink through the ice layer and hence show ridges as white

  Posted January 11, 2013 1:20 AM

• by Wounded_Knee

  The first one is a tricky one as it has a even contrast I think, the second one is a mix of channels, ridges and plateau`s so your will get conflicting shadows, again only a guess 😃

  Posted January 11, 2013 1:21 AM

• by chrisbayes

  maybe they are quantum gullies 😄 they are actually both ridges and channels depending on the season

  in the summer they are channels which is why they look like they do in this link with the stereo image (which i can't see because i dont' have specs). then as the CO2 starts to freeze it does it in the channels (less sun) and they become colder than the area around. as the season goes on more CO2 condenses onto colder spiders and they build up more and more until they become ridges. then in the early spring when the fans start to erupt and are blown by the wind they hit the ridges like in this image. then in the summer all the CO2 has evaporated and they are channels again.

  Posted January 11, 2013 1:38 AM

• by chrisbayes

  look at this one

  surely the illumination is from the left ?

  there is a depression in the top right quadrant which is in shade and the top left is in sunlight?

  even the kinks in the spiders (just left of centre) can't be illuminated from the right?

  Posted January 11, 2013 2:15 AM

• by Portyankina scientist

  Hi again,

  1. Sure, this cleaning is more likely to happen on the sides of channels than in the middle, and this is what we observe in many places when we have highest resolution images - there is often dark line of dust runs alond the centre of the channel. Here channels are very wide and most probably U-shaped. They get light all the time, but they are actually not that much brighter than the flats.
  2. Any change of topography would disturb the dust carried over the feature: the hole would catch the dust inside, the ridge would stop its movement.
  3. to chrisbayes: I was also thinking about this possibility some time ago, because they do look like popping up. But I have cross-checked couple of similar images with the spacecraft data to get the orientation of the sun right and channels still happen to show shadows where depressions would. If you want to check it yourself, go to HiRISE page (http://hirise.lpl.arizona.edu/) and search for images with spiders (search box top right corner) and with solar longitude between 180 and 220 deg. Solar longitude is the measure of season on Mars, this range is when CO2 is present on the surface in southern polar areas. Then look for Sub-solar azimuth - it will tell you where the sunlight is coming from. This angle is counted clockwise from the center-to-right line. Be careful of differences between map-projected and not-map-projected images, they have different azimuths, but both are listed in the gray box with all additional information below each image - pick the one that fits to the image you download. And there you go, you'll know the sun direction and you can figure out which are the shadows and if these channels are depressions. Let me know about your finds! 😃

  Anya

  Anya

  Posted January 11, 2013 2:29 AM

• by wassock moderator

  2 so dust blown over a hole is some how subject to some sort of mystery force which makes it settle faster than it would over flat terrain?

  Posted January 11, 2013 2:19 PM