One possibility is that it's the last layer to form. It speaks to yet another water episode.
More Quotes from Steve Squyres:
For Spirit, the priority has been to reach a safe winter haven.Steve Squyres
I actually see the Mars year anniversary as being more significant, ... We will have seen Mars over an entire seasonal cycle.
Steve Squyres
Before launch it just seemed impossibly far away, it seemed unreachable. Tonight, I go out knowing our rovers have been there a whole Martian year...it's a much more familiar planet. We know the place.
Steve Squyres
It was a hot, violent place with volcanic explosions and impacts. Water was around, perhaps localized hot springs in some cases and trace amounts of water in other cases.
Steve Squyres
The hill is very steep in places ... and the orbital images show a lot of tasty geology, some of it in pretty nasty-looking places. All in all, the next several weeks are going to present us with some of the most interesting route-finding decisions that we've faced in a long time, on both sides of the planet.
Steve Squyres
One mystery weve been dealing with for a long time is the origin of the little dark cobbles that we occasionally see out on the plains,
Steve Squyres
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These are estimates that are done by the experts as to how much they expect we could get from the first lease sale that would take place in ANWR, and the estimate is about $2.5 billion.
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