If it's the latter, was it perhaps weathered by that ice sheet -- with a thin bottom layer of liquid water -- that Michael Carr and James Head think filled the lowlands during the Noachian Age? (See my previous chapters.) IR spectra show that the Type 2 material has little or no clay minerals in it -- but it's possible that it instead contains "zeolites", the claylike minerals made when the glass in volcanic rock weathers (which takes less water than clays do anyway).

Andesite -- which is also richer in potassium and thorium than basalt -- is formed when basalt magma (underground lava) is exposed to a lot of liquid water. On Earth, it's formed mostly because of crustal tectonics ("continental drift"), when the edge of one of Earth's great basalt crustal plates collides with another one on an ocean floor and is slowly shoved down into the hotter semi-molten crust underneath, carrying a lot of ocean water with it.

But Mars has had little or no crustal tectonics, so it's much harder to explain how andesite could form there. This would seem to be an argument for the theory that the Type 2 northern rock is indeed basalt exposed to liquid (if very cold) water on or near Noachian Mars' surface.

And -- while Mars Pathfinder's measurements of the element makeup of the rocks at its northern Ares Valley landing site also showed them to be rich in silicon and potassium, which at first was taken as a further sign that they were andesite -- the "APX" spectrometer that it used could only analyze the outermost few hundredths of a millimeter of a rock, which may mean that it too was just analyzing a thin water-weathered crust on regular Type 1 basalt. (Indeed, the recent final recalibration of Pathfinder's data suggests that it sensed about 2% water chemically incorporated in the samples, which also suggests that it was analyzing a water-weathered crust on the rocks.)

But the most recent data from Odyssey, unfortunately, is still ambiguous on this point.

Its THEMIS camera can provide far sharper maps of the distribution of Type 1 and 2 rocks than Mars Global Surveyor's TES spectrometer could. In some places in the northern Arabia lowlands, it's found craters which have more Type 1 basalt soil on their floors and more Type 2 soil on the downwind side of their inner crater walls, suggesting strongly that the Type 2 rock is in the form of finer particles that have been separated out of the mix by the wind and blown downwind -- which in turn would mean that the Type 2 material is made of smaller and softer particles separated out of the mixed soil by the wind, indicating that it is indeed chemically weathered rock.

But the THEMIS camera has also examined Nili Patera, the caldera at the top of the big shield volcano dominating the Type 1-rich Syrtis Major area. The caldera itself is filled with solid Type 1 basalt lava -- but there are two deposits of Type 2 material bordering on it, whose very sharp edges suggest that they may be more andesitic lava thrown out of the volcano at a different time, rather than water-weathered basalt whose distribution would be blurrier.

The Type 2 Nili material, however, is less dense and seems to be a now-hardened deposit of old volcanic ash -- which would have been more water-rich when erupted than the solid caldera basalt was, but would also have been weathered far more easily by the kind of obliquity-related soil weathering processes, involving repeated temporary traces of liquid water in the soil, that I described in my last chapter.

Complicating things further, Odyssey's GRS maps of the distribution of potassium and thorium show both elements to be dramatically more concentrated in the northern Type 1 rock deposits than in the Type 2 material or anywhere else on Mars -- and since this instrument analyzes the top 30 cm of Mars' surface, this suggests that it's looking at genuine andesite rather than just at hair-thin weathering crusts on the surfaces of pieces of type 1 rock. And the biggest such Type 2 region -- the Acidalia plain -- has what may be the ancient remains of some small volcanoes that erupted underneath a subsurface icy layer, suggesting that this might have helped produce andesitic rock.

However, different processes involving liquid water can also slowly leach both potassium and thorium out of rock and soil and carry them elsewhere to be redeposited, and in fact GRS has also found big enough differences in the distribution of the two elements all over Mars to suggest that such leaching processes have been occurring. It's possible that both the potassium-leaching process and the thorium-leaching one (which involves acidic water) may have operated to drain both elements out of neighboring terrain and redeposit them in the two big Type 2 regions, making them look like andesite to GRS' eyes when they really are water-weathered basalt. And the GRS's maps of Mars' surface silicon levels aren't sensitive enough to provide good evidence one way or the other.

In short, alas, Mars' geological processes are complex enough that we can't yet really interpret Odyssey's new data as good evidence for the theory that its northern lowlands were filled during the Noachian with a vast ice sheet with a thin melted base layer of liquid water -- or for the alternate view that the ice sheet didn't exist, and that the northern Type 2 material is instead andesite created when magma encountered water so deep down in Mars' fiery interior that it has no relevance to whether Noachian Mars might have been able to support life. The two upcoming MER rovers -- which carry both several different compositional instruments and a grinding wheel that can scour weathering rinds completely off rocks to analyze the surface underneath -- may enlighten us on this.

Click for Part Three

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