Originally posted by elevenaugust An amateur astronomer in West Chester, Penn. took a picture of a curious Martian cloud several nights ago Also, it moves with the planet (ruling out dust motes on the sensor) and seems to rise over the limb.

I think you have made a few rather astute comments regarding the feature seen on the limb of Mars recently. My thinking coincides with comments made by Christpohe Pellier, Richard McKim and others who have mentioned high altitude clouds as an explanation.

The MARCI weather map for March 12-18 has been posted and while it does not include March 20th, the date of Wayne's image, the map does cover the date of Marc DELCROIX’s image of March 12. I don’t see any indication of unusual dust or cloud activity in the March 12-18 map so I think that would tend to rule out the possible but highly unlikely scenarios of major impacts with their attendant dust plumes, major volcanic eruptions or some of the other interesting but fanciful explanations.

We know that the Olympus Mons orographic cloud should be at an altitude of 20 km at this season (Ls 87 degrees) and this cloud is not seen to project beyond or above the limb/terminator as far as I am aware. The
clouds within the equatorial cloud belt (ECB) may start out at low elevation on the morning side of the planet but rise to higher altitudes during the afternoon. As sunlight warms the surface, the escaping IR radiation warms
the CO2 atmosphere which raises the altitude of the water ice condensation layer. However, I think the ECB usually rises up to an altitude of only 10-20 km. So, to go one step further than the high altitude clouds already
mentioned my guess would be that they are mesospheric clouds which are at an altitude of 40-80 km.

More specifically, considering the proximity of the clouds to the terminator I would suggest they are similar to noctilucent clouds in Earth's atmosphere. One of my meteorology text books defines noctilucent clouds as "luminous night clouds". Since as Roger points out below, if this feature is high altitude clouds, they are not projecting above the actual limb of the planet but are high enough to be illuminated by sunlight, are seen beyond the terminator but against the night side of the surface, so are viewed in maximum contrast. I have only read a half dozen papers on mesospheric clouds so my knowledge is decidedly weak in this area as opposed to the lower altitude cloud types we usually see.

One of these papers (McConnochie et al, 2010) mentions that they could only identify two examples where an overlay of THEMIS-VIS images of mesospheric clouds could be matched up with a similar pattern of clouds as seen in the MOC daily global maps by Wang & Ingersoll so it may not be surprising that I did not see anything unusual in the MARCI maps. Dust, water ice and CO2 ice aerosols have all been observed in the martian mesosphere.
There does not appear to be an excess of dust in the MARCI images and the McConnochie paper states, "the mass of condensate is at least four times greater than the maximum amount of water vapor that can be contained in an atmospheric column, the condensate is most likely CO2 ice". I always assumed (apparently wrongly so) that the optical depth of martian mesospheric clouds would be considerably less (more tenuous) than lower altitude water ice clouds and that we would in all likelihood not be able to see high altitude CO2 ice clouds in ground based imagery. At least in some cases "particle sizes and optical depths also happen to be comparable to those of lower atmosphere water ice clouds"

(McConnochie 2010). I assume this last statement is referring to the ECB clouds as the volcano induced orographic clouds often appear much thicker with higher optical depth as compared to the ECB. Two types of mesospheric cloud forms seem to dominate, cumulonimbus and cirrus. The cumulonimbus clouds may at times have a clumpy structure which may indicate convective action. The cirrus clouds are by far the more numerous and may appear as filamentary structures sometimes organized in consecutive waves.

Certainly there are a few questionable aspects to the concept of this feature being mesospheric clouds, my own bias in thinking we can't see these things for one. Also, I have no idea if CO2 ice clouds should be visible in
all three primary RGB filtered images.
Cloud formation high up in the atmosphere can be induced by gravity waves but I have not researched the idea enough to know if this would be a preferred location for a quasi-stationary cloud. Is this feature moving in longitude and/or latitude?

To MIR, can you provide a comment on this feature we have been seeing? I would love to hear a more informed opinion even if to find my speculation is oh so wrong. BTW, thanks so much for sending the link to the Ashima web site. Love those maps!

McConnochie et al, THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology, Icarus 210 (2010) 545-565

Thanks,
Gary Rosenbaum

Originally posted by FraternitasSaturni

I found a strange feature over Acidalia (top left of the animation below)

Acidalia Planitia is a plain on Mars. It is located between the Tharsis volcanic province and Arabia Terra to the north of Valles Marineris, centered at 46.7°N 338.0°E. The plain contains the famous Cydonia region at the contact with the heavily cratered highland terrain.

It has been hypothesized by J.E.Brandenburg from Orbital Technologies Corp. that a large natural nuclear reactor in the northern Mare Acidalium, near the large, shallow depression north of Acidalia Colles (similar to Oklo on Earth) underwent catastrophic meltdown.