Dear Masatsugu,

I sent Tom's images of Edom to Andy Young, a photometry expert at San Diego State U. His comments follow.

Kind regards,

Bill

****

Well! Isn't that something? Thanks for sending those pictures -- they certainly are remarkable.

To begin with, I note that the "flash" is clearly an unresolved instrumental point-spread function; you can see it's a little broader on the frame where the seeing is a little fuzzier. So it's clearly something on Mars and not some kind of instrumental artifact.

>From the contrasty images, I assume these were taken at some fairly long wavelength in the deep red, though the lack of limb-darkening is surprising -- can you supply information about the passband used? And what was the phase angle? What kind of processing has been applied to the image?

The bright spot seems brighter than anything else on Mars, but not by more than a factor of 2. So the average brightness of the PSF is about that of a light gray (say, "white") piece of paper the same size. As this is roughly 10^5 times dimmer than an image of the Sun, we can assume that only something like 10^(-5) of the area is filled by reflecting objects if they reflect like a perfect mirror. If they are ice crystals or mica flakes with a reflection on the order of 10%, then about 0.01% of the area of the PSF is filled with them. The PSF is about 1/20 of the diameter of the planet, so if all the reflectors are together in a coherent area (unlikely) this area is about 1/20 x 1/100 of the diameter of Mars across, or 1/2000 of 6800 km, or about 3 or 4 km across (assuming ice or mica). If the reflectors are more spread out, the peculiar area on Mars is somewhat larger, but of course no larger than the projected PSF of about 340 km diameter.

These sizes sound about like the diameter of a medium-sized impact crater; it may turn out to be a funny-looking crater floor?

In the old days we would gladly have read this as the leaves of some Martian heliotrope-field blowing in the wind; but such an interpretation today seems highly unlikely. The most pressing problem is to understand the short timescale -- much less than the time required for the solar image to pass across a stationary reflecting surface. So salt flats with well-developed crystal faces, and frost on the ground, are not possible interpretations. I would be inclined to favor some temporary alignment of crystals suspended in the air; but then the problem is to tilt them synchronously over such a large area, to make the "flash" wink on and off coherently.

Perhaps a Martian geyser is erupting here, and its plume is turning to ice crystals in a few seconds. But then it's hard to get the crystals to grow large enough to have well-developed faces, needed to give the desired directionality to the reflection. If it were simply a steam plume from geothermal -- er, areothermal -- activity, then how do we get it to cover such a large area (now we are talking about a temporary white spot some 100 km or more across) and yet to disappear again in a minute or so?

Might we be dealing with a dust cloud stirred by a major Marsquake? This might account for the timescale: seismic waves travel a few km/sec, so in 10 sec or se we can easily shake an area large enough to make the bright spot. If the dust is merely stirred a few meters into the air, it can fall out again on a similar time scale. Ask the geologists if the terrain suggest earthquake country.

My own inclination is to favor some such widespread brightening of the surface, rather than a localized glint of Sun on moving surfaces. But if the brightening is mild and spread over many tens or a few hundred km, a mechanism is needed to synchronize the display over that region, and I think the speed of sound on Mars (about 200 m/sec) is a bit slow to accomplish this with an atmospheric driver. If the phenomenon is basically atmospheric, then we need something that can appear and disappear in a few tens of seconds at most. A steam vent doesn't look promising at first glance, but maybe there is a way to make it serve.

A very remote possibility is a specular reflection from something in orbit around Mars. The problem then is that it has to be over a kilometer across, and tumbling enough to account for the temporal modulation. But then why hadn't it been noticed before, and why hadn't the physical librations been damped out?

You certainly have a puzzler here!

-- Andy

The follwoing another intersting email is also from from Bill SHEEHAN:

Date: Wed, 27 Jun 2001 15:10:52 -0500
From: "Bill and Debb Sheehan"(sheehans@tds.net)
To: vzv03210@nifty.ne.jp
Subject: Re: Edom brightening

Dear Masatsugu --

......Meanwhile, thanks much for the news of the recent dust-storm. I'll begin to monitor.

  Regarding the flashes on Mars, you may find this of interest. A comment from Walter Haas about Percival Lowell logging thousands of hours and not seeing anything jogged my memory. Now I'm sure you will find this of compelling interest, and we ought to mention it in our paper. Percival Lowell, in *Mars*, pp. 86-87: "An interesting phenomenon occurred in the cap on June 7, 1894 {Note the date!!-W.S.). On that morning, at about a quarter of six (or, more precisely, on June 8, 1 h. 17m, G.M.T.), as I was watching the planet, I saw suddenly two points like stars flash out in the midst of the polar cap. Dazzlingly bright upon the duller white background of the snow, these stars shone for a few moments and then slowly disappeared. The seeing at the time was very good. It is at once evident what the other-world apparitions were,--not the fabled signal-lights of Martian folk, but the glint of ice-slopes flashing for a moment earthward as the rotation of the planet turned the slope to the proper angle; just as, in sailing by some glass-windowed house near set of sun, you shall for a moment or two catch a dazzling glint of glory from its panes, which then vanishes as it came. But though no intelligence lay behind the action of these lights, they were none the less startling for being Nature's own flash-lights across one hundred millions of miles of space. It had taken them nine minutes to make the journey; nine minutes before they reached Earth they had ceased to be on Mars, and, after tehir travel of one hundred millions of miles, found to note them but one watcher, alone on a hill-top with the dawn."

Bill