[X] My Assistant

[dvandorn]

This is more of a general question/comment about Martian geology, but since Oppy is in such a unique environment (unlike any of the other landing sites we've visited), I figure it belongs here more than anywhere else.

As far as I've been able to tell in my reading of the results of the various Mars probes, there doesn't seem to be much in the way of granite on the surface of Mars. There's a lot of basaltic and andesitic lava-rock (with a lot of olivine, pyroxene and even ilmenite), some ancient feldspathic/anorthositic rock, a lot of sulfates and other salts, a lot of rusted iron -- but little to no granite.

There's almost no granite in any of the lunar samples, either. Quartz also seems to be very rare, both in lunar samples and in what we see on Mars.

Has anyone here heard any good theories on why Earth produced so much granite and neither of the other rocky bodies we've looked at in detail seem to have much, if any?

-the other Doug

[edstrick]

I don't know what those magnetic stripes are. They're poorly resolved... we can't see substructure in them from orbit worth a damn. I have major doubts they resulted from plate tectonics "as we know it". It's a good arm-waving guess, but it's trying to assume the role of "preferred hypothesis" instead of "none of the above".
A magnetic/gravity anomaly mapping scout class mission that would be in a "powered" ultra-low orbit, possibly using 2 spacecraft like the current Earth-orbiting GRACE mission, would be enormously valuable for martian geophysical studies, probing the earliest history of the planet.

By powered orbit, I mean a vehicle with either ion propulsion or some few hundred kilograms of chemical propulsion. Once aerobraked into low circular orbit, such a mission would descend to an orbit that requires periodic, say weekly or daily orbit reboosts to keep from doing an atmosphere entry. On mission end, as propellant runs low, the spacecraft would need redundant systems to reboost for planetary quarentine reasons.

What really gets me about the stripes and other magnetic anomalies is that there's no correlation with relict ancient topography, other than avoiding the impact basins, and being weak in the northern plains. Look at the MOLA altimetry maps, and the southern highlands are full of scarps and ridges and mountain massifs and other features that don't seem to be the remains of craters or nearly obliterated impact basins. I'd *** LOVE *** to see some researcher use AI aided image processing to progressively intelligently airbrush away craters, starting with small fresh ones and moving to larger and less fresh ones, removing the visual distraction of younger impact dominated geology, and giving a good low-distraction look at the topographic remnant of the oldest topographic landforms on Mars.

[dvandorn]

I wholeheartedly agree -- there may well be other explanations for the magnetic features that we see (even though they are so very poorly resolved) in the southern highlands. It's interesting, though, that there are *no* anomalies (at the current resolution limits) around the easily identifiable impact basins, such as Hellas and Argyre. There is very little remanent magnetism at all in the basin floors and ringwalls. Which argues that Mars' magnetic field died before they were formed.

I'm just afraid that trying to reconstruct ancient landforms in the southern highlands is akin to observing an omelet and trying to reconstruct the chicken that laid the eggs. Impact modification has been so extensive and so drastic that it's nearly impossible to identify pre-impact landforms in the underlying crust. Add to the impact modification the stretching and cracking of much of the surviving ancient southern highlands crust due to tectonic response to the immense, and far younger, lava structures of the Tharsis Bulge, and it becomes nearly impossible to identify with any certainty *any* topographic features in the highlands that might pre-date both the high-impact-flux era and the Tharsis construction era.

I like your idea of powered orbiters. I despair that we'll never see the funding required to fly them, but they would be invaluable in answering some basic questions about Mars' early history. I think it's important to find out whether or not any other planet in the solar system has ever displayed plate tectonic processes, since those processes are so important to the history and ongoing evolution of our own planet.

-the other Doug