Dear Masatsugo:

Regarding the message from Andrew Young (a specialist in stellar photometry at San Diego State University) that was forwarded by Bill Sheehan and is currently posted on the CMO web site, I would like to relate the following comments...

Frankly, I think Young fails to grasp a vital point essential to understanding the flare phenomena. The rotation of Mars will displace the angle of any reflecting surface by about 1.2 degrees of longitude every five minutes, so discrete patches of frost don't have to "disappear" for pulsating flares to be seen! Moreover, Young's notion that just before local noon (i.e. when the flashes in Edom were seen) frost would not be present due to temperatures being too high is not consistent with MOC images taken at approximately 2 PM local time that do appear to show frost!

Rather than the exotic (and in my opinion highly improbable) mechanisms proposed by Young, I believe that Martin Gaskell has provided a far more plausible explanation.

Gaskell writes:

(1) The reflector is inclined to the horizontal at a fair bit - this strongly rules out clouds. It's got to be on the surface.

(2) The range of inclinations is readily explained by a range of slopes on the surface.

(3) The rapidity of the fluctuations tells us that there are regions of the reflector with slightly different slopes (by a fraction of a degree).

(4) The size of region needed to explain the flashes of a few seconds is only a few times bigger than a football field. There are plenty of flat regions on this scale. I think the faces of sand dunes are an interesting possibility, although by no means the only one.

(5) These flashes are only seen when the weather is right (not every day), so they are fog/frost induced. It's not shiny rocks.

Here's my scenario for what happens:

In the morning the sun heats the ground and makes water evaporate. The Martian air is always close to saturation and, unlike the earth's, is significantly colder than the ground. Ice crystal therefore condense in the air above the ground forming a fog (as see on the earth when the sun shines on wet ground and as imaged on Mars). The ice crystals fall on the ground creating a frost. Fog and frost must go hand in hand.

Ice crystals have a very high albedo so they inhibit any more heating of the ground where they fall and they can stay there for quite a while. On Mars, unlike on the earth, the surface temperature is ruled almost entirely by the amount of sunlight absorbed and by the emissivity of the surface, not by the atmosphere (on the earth, with a much denser atmosphere, heating by the air dominates instead).

Why does fog/frost form in valleys? Answer: because the wind is calm there and the water vapor content of the air is not reduced by turbulent mixing with drier air (the same holds for the earth).

What governs when frost is seen in Schiaparelli? Answer: How windy it is. Fog and frost will only be seen on the calmest days (as on Earth).

I am convinced that these ideas of Gaskell's contain the essential truth of the matter.

Warmest regards,

Tom