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[Guest_AlexBlackwell_*]

A Primordial and Complicated Ocean of Magma on Mars

--- Geophysical and geochemical calculations indicate that total melting of Mars during its formation could have led to large-scale heterogeneities in its mantle.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
posted March 31, 2006

[Guest_Richard Trigaux_*]

A Primordial and Complicated Ocean of Magma on Mars

--- Geophysical and geochemical calculations indicate that total melting of Mars during its formation could have led to large-scale heterogeneities in its mantle.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
posted March 31, 2006

Very interesting article, thanks for it. In a short, marsian mantle may have solidified from an early magma. From their geochemical properties, different minerals may have solidified at different depths. But it happens that lighter minerals solidified at greater depths, resulting in an overturning of the whole martian mantle. This well explains the different rocks found on mars and the fact that may exist different magma reservoirs.

I would add that, if the bulk of these events took place during the first 150 million years of Mars, the movements seem not completelly settled today, explaining the Tharsis dome, localized volcanism and lowlands.

[nprev]

Mmm...sure, maybe. However, pretty much every landing site we've done to date is situated on some sort of floodplain....do these results jibe with the 'ground truth' of Viking, at least?

[Guest_Richard Trigaux_*]

Hmmm... This confort my intuition about what the Mars mantle is largely heterogenous.


But if there is an overal overturning of the mantle, we should see surface features about the marks left by it. Furthermore, even if most of the overturning took place in the very early Mars history, some residual (smaller dimention - slower) movements could have taken place more lately, and even recently.

The Tharsis dome would be a consequence of the overturning, and relatively recent faults around it too.

But we should also see subsidence zones: hollows.

And we can find such hollows around hellas Planitia (which stands out in dark blue in Google Mars). We find two "umbilics" hollows at the south west of Hellas, and a large flat volcano. There are two volcanoes at east (or at least features which look like volcanoes) and one more spectacular "umbilic" with a circular network of faults at the north of Hellas. (Sorry, Google Mars don't give latitudes and longitudes, which makes of it rather a toy than a real tool). At a pinch, I even wonder if Hellas itself would be really an impact bassin. Theres is another such large bassin south east of Tharsis dome (in green), and it is much shallower. Large bassins tend to be larger and larger compared to depth, and Hellas is simply much too deep, and in more it is oval, and even a bit tear-drop shaped.


It is difficult to tell what exactly are the "umbilics" as they also ressemble some caldeiras, like the one of t Arsia Mons. But there overal shape strongly suggest some subsidence process.

[dvandorn]

Actually, Richard, I would be more prepared to believe that Hellas *is* an impact basin that has been highly modified by, among other things, crustal subduction along fault lines created by the impact.

A thorough analysis of the height of the ridges around Hellas actually shows that there is more material in what appear to be the basin rings than would fit back into the basin itself. This could happen if the crust were being pushed up around Hellas by subduction. Or had been in the past.

All in all, the boundaries of Hellas are a pretty good place to look for ancient subduction zones in the Martian crust.

-the other Doug

[Guest_Richard Trigaux_*]

Yes we all "KNOW" that Hellas IS an impact bassin. There are many other examples of such large bassins on many other bodies. And it really looks like a crater. But:
-it is oval
-it is very deep. There is a law which sets rigorously the ratio between diametre and depth of a crater, as a function of the diametre. Such large craters rather look like "flat" circles showing circular and radial faults and cliffs. And a rim, of course.
-Hellas has no rim, unless we count all the surrounding highlands as a rim.


As for the smaller "umbilics", it is rather difficult to guess what they are exactly, as they are all surrounded by large zones with less craters. It looks as if they were former volcanoes which later subsided into the ground, a rather unique feature.

another strange thing is that the volcanoes (if they are volcanoes) show V shaped radial valleys, as Earth volcanoes which were eroded by rain. Are they so ancient that they bear testimony of a time where rain was occuring on Mars? Arsia Mons also exhibits V shaped valleys.


What I point out is that Hellas and its surroundings show interesting large scale features that we don't yet understand, but which could be subduction zones, like the fossae on Earth. Simply martian subduction zones, being not constrained by horizontal movements, appear more or less circular, while Earth subduction zones, being constrained by plates movement, appear very narrow and elongated.

There was a discution in another thread as what there are magnetic traces of a plate tectonics in the southern highlands. If there was an overturning of the mantle, it could have appeared on the surface like a plate tectonics. But the overturning is a one-time event, while plate tectonics lasts long.


On Earth, the plate tectonics creates magnetic traces which are parallel bands along an extention zone (ocean ridge) where new crust is created. On Mars there are similar features in some parts of the southern highlands. So we may similarly find a ridge where extention occured. There are many elongated ridges in the southern highlands, but we cannot use Google Mars to find which would be a former extention ridge, as it don't give magnetic anomalies.