Dr. Hoffman:

Thanks for the informed response. I don't mean to put you or anyone on the spot about the issue, but it seems to me that a little more thorough thinking about the problem of the Hellas Basin is in order. Not to claim I am the person to do that, but I'm trying!

Arguably, Hellas is the largest structure of its kind in the Solar System, some 2000 km in diameter and at its deepest, some 9 km below the average reference level. By comparison, similar structures on Mars, such as Argyre and Isidis, are only 2-4 km below the local surface level at their deepest.

Similar structures on other planetary bodies, such as the Caloris Basin on Mercury and Mare Orientale on the Moon are only 2-3 km below the local reference surface, despite their enormous diameters.

It is argued that the impacts which created these basins occurred very early in the history of the bodies impacted. These bodies were still in the process of formation and were still relatively hot; their crusts were relatively thin, and their interiors were relatively fluid.

The double ring and surprisingly flat floors of Caloris and Orientale contrast starkly with the distinctive, partially sediment filled, deep pit found at Hellas. Essentially, there is no "pit" at Caloris or Orientale, and Argyre and Isidis are almost entirely "filled." Caloris and Orientale show no sedimentary deposits at all, while both Argyre and Isidis show substantial deposits.

There is evidence of heavy erosion around the rim of Hellas -- indeed, part of the rim is absent. About half the rim of Isidis is absent. While most of the rim of Argyre is present, both the rim and the floor of Argyre appear significantly eroded, and significant parts of the floor of Argyre appear to have once hosted ice or glacial covers.

The floor of the Hellas basin was once quite a mystery as the basin was almost always shrouded in clouds and fog during the Mariner and Viking missions; sometimes it was the source of dust storms. The appearance of the Hellas floor as revealed by the Mars Global Surveyor is surprising. The material that is found on the floor of Hellas is almost all sediment -- of some kind -- and is of a substantial depth, at least several kilometers. Some of this material may be icy. Some may be dessicated mud flows. Some may be lava. Some of this material likely originated at volcanoes found around the rim of Hellas. Some may have originated in the planetary interior beneath the Hellas basin itself. Some no doubt is the result of channel flows. Some may be the result of windblown dust deposits. Whatever the compostion of the floor material in Hellas, it has not even come close to filling the basin. And, whatever its composition, the material is at least partially of recent origin -- as there appear to be remarkably few impact craters on the surface.

Of course, if -- as has been suggested -- the Hellas Basin was once an open sea, fairly recently dried up, some of what we see now would be explained.

The problem of accounting for the depth of the formation would remain. Now earlier, Dr. Hoffman argued that at the time of Hellas' formation (~4 Ga +/-) the Martian lithosphere was only 10km thick. A large body impacting such a lithosphere would easily puncture it, destroying the surface layer (pulverizing and vaporizing a good deal of it) and explosively contacting the essentially fluid mantle below. A transient cavity in this mantle would be formed that might initially be several tens of kilometers deep, but almost immediately -- it seems to me -- the nearly fluid mantle material would flow into this cavity, and unless prevented somehow, would completely fill this cavity to the overall mantle level and be forced into the lithosperic hole leaving an impact structure that is very nearly at the level of the surrounding surface, as we see at Caloris, Orientale, Argyre, and Isidis, (indeed at every other basin on Mars) but not at Hellas.

Lately Dr. Hoffman argues that istosasy may have something to do with the appearance of Hellas -- though I confess I'm not sure what. If we assume that the crust at the Hellas site was much thicker than the 10km earlier argued, then perhaps the failure of Hellas to "heal" could be related to a failure of the impacting body to actually breech the lithosphere. If there was no mantle contact, then it would be more difficult (though hardly impossible) for mantle material to reach and thus fill the cavity. The presence of a number of large vocanic structures around the Basin indicates that at least some flow from the interior occurred related somehow to the formation of the basin.

But we are still left with the question of why, after nearly the whole history of the Solar System, erosion and dust transport and lava flow and so on has not even come close to filling the Basin. After so many billions of years, it seems to me, the Basin should be nearly full of sediment. It is not.

I don't know what the history of Hellas is, but it is easy enough to see that it is a unique formation, unlike any other basin on Mars or any other comparable basin on any other planetary body. Uniqueness inspires questions, such as "Why?" "What could have happened here to cause this unique appearance?" "Were there and are there unique conditions here and nowhere else that cause Hellas to be the way it is?" "Are there alternative explanations?"

Dr. Hoffman seems to be suggesting something remarkable: at the time of Hellas's formation (c. 4+ Ga) the planet was already quite "cold." Not only was the surface crust solidified, but the mantle, too, had a "thick skin" -- presumably solid or nearly solid as well -- to a considerable depth, unlike the case of, say, the Moon, which remained hot enough and fluid enough beneath the solid surface to form extensive maria until 3 Ga ago or even less.

Then I'd want to know why the Moon was still hot while Mars was already quite cold... and how could such a cold planet form such monstrous volcanoes???

Well, one question leads to another, doesn't it?

Still wondering!

Cheers, BFelix