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Posted by: jmknapp Jul 8 2008, 01:38 AM

Was looking at some MGS imagery:



Kind of a low-contrast scene with some darker splotches here and there. Running that through an equalizing filter gives this image:



Some surprising detail brought out there--it looks to me like there are small clouds casting shadows on the surface. Can that be right? They could be little hills I suppose.

Posted by: siravan Jul 8 2008, 01:49 AM

Likely JPEG lossy-compression artifacts.

Posted by: jmknapp Jul 8 2008, 01:55 AM

The original images are from IMG files (256 pixels per degree MGS images http://www.msss.com/mgcwg/mgm/), only converted to JPG in the final step. I believe IMG files are lossless.

Posted by: Del Palmer Jul 8 2008, 01:56 AM

They look like puffy water-ice clouds; here's a similar image from http://images.spaceref.com/news/2003/2003.07.23.M0105394.jpg.

Posted by: jmknapp Jul 8 2008, 02:08 AM

Interesting comparison. Equalizing that photo shows some similar shadows under those clouds in the expected direction too:

Posted by: dvandorn Jul 8 2008, 04:16 AM

I dunno -- look at the semi-regular spacing. Personally, I think you've discovered the dust clouds kicked up by the annual thoat drives into the great abattoires of Helium...

-the other Doug

Posted by: jmknapp Jul 8 2008, 11:33 AM

Looks somewhat less than semi-regular to me, if that isn't an oxymoron! Anyway, what do you think might cause semi-regular artifacts (other than said helium)?

BTW, here's the location of the image (arrow) on a map of water-ice cloud opacity predicted for early northern summer:



*from Malin et al, "Climate, weather, and north polar observations from the Mars Reconnaissance Orbiter Mars Color Imager," Icarus 194 (2008)

So it seems to be in the area of likely suspects at least, on the north slope of the plateau of the Tharsis volcanoes. The striations near the top of the image are part of Ceraunius Fossae. A paper, http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3147.pdf noted clouds in this area:

Posted by: siravan Jul 8 2008, 11:42 AM

I'm not sure. I agree, IMG per se is lossless, but MOC had the ability to compress images on board using Walsh-Hadamard or Discrete Cosine techniques, which are lossy. I wonder whether you can see the same low-contrast, semi-regular patterning in other MOC wide-angle images.

Posted by: ugordan Jul 8 2008, 11:59 AM

I don't see these as compression artifacts, the features very much look like real fuzzy bright objects with corresponding darker shadows in the right places. DCT doesn't do that, it just produces blocky artifacts more or less.

Posted by: ElkGroveDan Jul 8 2008, 01:55 PM

I don't think there is any question that we are looking at clouds in both images.

Posted by: jmknapp Jul 8 2008, 02:02 PM

Here's one of interest, from a simulated view from MRO as it was approaching the Phoenix site the day after the landing:



So there's a bit of the "cloud" effect on the left but not so much elsewhere. Could that be an area where ground ice is subliming and condensing again at height?

Unfortunately there's a bit of a dataset problem around the Phoenix site, approximately at the center of this MGS mosaic:



Looks like some inconsistent lighting on the stitched-together sections. Also, maybe clouds were there for one pass and absent the next? Here is what MRO was looking at just a minute or so later:



Some pretty large clouds? Anyway, there are plenty of different kinds of artifacts (stitching, posterization, dropouts, etc.) but not compression per se, IMO.

Note that the HIRISE field of view is much less than this image (like 1/50th the height).

Posted by: dvandorn Jul 8 2008, 04:09 PM

I wouldn't be at all surprised to see clouds around the Phoenix site. In fact, I'm pretty certain Phoenix herself captured some images of clouds from the ground. Here's a highly downsampled image from Sol 5:

This is just one of several sky images from that sol. They all show patterning in the sky that seems to suggest clouds, to me.

Besides, we're in the Land of the Polar Hood Clouds. This latitude on Mars has been observed from Earth as developing cloud decks which regularly obscure the scene from telescopic observers.

-the other Doug

Posted by: edstrick Jul 9 2008, 09:15 AM

.....the annual thoat drives into the great abattoires of Helium.......

I can imagine Tars Tarkas having to file an environmental impact statement on that, and verifying to PETA that no thoats were abused during the drive.

Posted by: jmknapp Jul 9 2008, 05:54 PM

Interesting... makes sense, as the subliming H2O has to go somewhere!