[X] My Assistant

[aldo12xu]

Hi folks I just summarized some of the abstracts I read on the alteration and chemistry of the rocks on the Gusev plains and Columbia Hills. The most interesting thing I found was that the rocks at West Spur seem to have gone through a different alteration process than those higher up in the Columbia Hills.

So check it out and feel free to correct me on any points.

Cheers,
Aldo.

[gpurcell]

Good write-up on the hills, Aldo. I think it explains why the Spirit team wants to examine the lower rocks in much more detail now that they have the power to do so...if they can find the transition rocks between the Olivine-rich upper strata and the Olivine-poor lower strata....

[dvandorn]

Good write-up on the hills, Aldo.  I think it explains why the Spirit team wants to examine the lower rocks in much more detail now that they have the power to do so...if they can find the transition rocks between the Olivine-rich upper strata and the Olivine-poor lower strata....

These newly-available analyses suggest to me a slightly modified sequence of events at Gusev:

1 - Creation of the ancient crust, actual minerology still not definitely determined.

2 - Creation by large impact of Gusev crater.

3 - Covering of the floor of Gusev (at least in some areas) by pyroclastic materials, something like ashflow tuff. Possibly over several episodes.

4 - Uplift of the Columbia Hills and other landforms, both within Gusev and extending on a fault line to the north of Gusev, exposing the layered beds of pyroclastic deposits along the fault line.

5 - Inundation of Gusev by very salty and probably acidic water, altering the ashflow tuff-like materials in the hills up to the level of standing water. This water inundation would have eroded and altered the olivine in the lower hills, breaking it into constituent elements and creating the hematite and other water-alteration byproducts we see in the West Spur rocks, but leaving the rocks farther up the hill relatively unaltered.

6 - Evaporation of standing water inside, and slow loss of some of the groundwater beneath, the Gusev floor.

7 - Flooding of the crater floor by basaltic lavas which embayed the uplifted hills, but to a lower level than the water had embayed those same hills.

8 - Migration of remaining groundwater up through the basalt "cap" flows, creating minor but detectable alteration of the basalt beds.

9 - Then, for the most recent several million/billion years, the whole thing gets slowly sandblasted by an ever-thinning atmosphere, creating ventifacts and other aeolian weathering features.

And there you have it -- a recipe for the creation of Gusev Crater as it exists today.

-the other Doug