Thanks Robert -- this Ls bidness is something I've never entirely understood. Even though it makes perfect sense~!

And Dan, thanks for the Solar System Live reference. I was not able to make seasonal sense of it though! (Blame my thick head.) Volatile escape of some sort would produce a white cloud such as we see; rising dust from a collapse event would probably not appear white, but dust mixed with volatiles could certainly do so.

So. The image is centered just about on the equator, at 1.70°N. Seems to me then, that the putative vent is some 25°N (m/l). The Ls 180.70° = Northern Autumn/Southern Spring start at 180°. Local time is just about 2:00pm at the image center -- perhaps 3:00pm at the vent location(?)

Ergo, at the location of the putative vent, the season would be the beginning of Autumn. However, it seems to me that if we are witnessing a venting incident, the season really wouldn't matter all that much. What would matter is the internal presence of volatiles (water would suffice) under pressure and a way to the surface. Other volatiles, such as Hoffman's beloved CO2, would work as well, provided sub-surface pressures could be maintained high enough and temperatures low enough.

We see in Tractus Fossae numerous pit/graben associations which we do not see among other graben sets nearby. Checking MOLA data, we also find that the Tractus assemblage is only a few kilometers higher than the reference datum ("sea level") for Mars, and considerably lower than <a
href="http://marsoweb.nas.nasa.gov/MOLA/regions/07_15/">nearby graben/horst terrain in Ceraunius Fossae (where there appear to be no pits).

We see extensive drainage of volatiles from Tharsis generally, though most signs indicate this drainage was relatively early in Mars' history, at least several hundred million years ago, by most assessments . Venting signs are relatively rare. There have been some disputed signs of recent/current venting in Tharsis, including geysers on the plains from the Viking era, and
interpolated from cloud formations over and around the major volcanoes. There is in Woodlock's image what appears to be a small spiral cloud in the middle of the Arsia caldera and what appears to be a relatively short linear cloud moving north from Pavonis (though this may be an image artifact). The pits in graben in Tractus, which I propose as possible sources for the
apparent plume in this image processed by Woodlock are reasonable sources for outgassing events. There is no way to tell how recent the pits are.

I see nothing in the region which suggests to me that there is any high-albedo outcrop anywhere in Tharsis (such as we find in Meridiani and elsewhere, for example), and certainly none anywhere that would mimic a plume.

But we do well to keep in mind that Mars is a great mimic no matter, and that very often, what we think we see is not at all what is there.

The Tractus Fossae eventually fade away in Tempe. Tempe, interestingly, shows ambiguous signs of drainage. This suggests that if there are volatiles in Tharsis, they mostly escape or drain before reaching the northern plains at Tempe and Arcadia. To the west of Tractus and Ceraunius Fossae, however, is the very evocative <a
href="http://barsoom.msss.com/moc_gallery/e01_e06/images/E03/E0302370.html">Olympica Fossae, </>which appears to be a "recent" drainage system. What may have been drained here is unknown.

Of course there are those who will insist that Mars is dead, dead, dead, no matter what, and all signs of present activity are illusional.

Cheers, B R Felix