Basically KK asked if SS could give a complete, drive by drive rundown of Oppy's plans from now until January, and SS rightly said "no", cos, I mean, how the (insert own choice of word here) could he *know* that?

After nearly eight years exploring the surface of Mars with the robot rover Opportunity, scientists announced Wednesday they have found "the single most powerful piece of evidence" yet that water once flowed abundantly on the Martian surface.

It must have gushed through underground fractures on the planet billions of years ago, they said, and as the water flowed, it formed a broad range of minerals that the rover has recently discovered near the edge of a crater that Opportunity has just begun exploring.

Led by Steven Squyres of Cornell University, the Mars mission scientists reported their latest findings from Opportunity's 20-mile journey at a meeting of the American Geophysical Union at San Francisco's Moscone Center.

In the course of its travels across the crater-pocked terrain of Mars' southern hemisphere - with many stops to investigate promising sites - Opportunity has already found iron and magnesium minerals that on Earth are known to form in water. But to Squyres and his colleagues, discovering the mineral calcium sulfate - gypsum, as it's most commonly known on Earth - makes it a "slam dunk case" for water.

"...hydrated calcium sulfate that on Earth is used to make drywall and plaster of Paris..."

Crystaline gypsum is a lot differnt from plasterboard. Think of the crystals in the Mexican cave...

Nonetheless, gypsum is relatively soft relative to other minerals.

Two frame MI mosaic...

Nonetheless, gypsum is relatively soft relative to other minerals. It's "2" on Moh's Scale of Mineral Hardness, roughly in between the hardness of a pencil "lead" (graphite) and a fingernail.

Here you are : gypsum ...and coming from our Museum of Plaster close to Paris.

Hmm, that arrowhead is made of flint if I'm not mistaken. Or did I misunderstand something?

Gypsum arrowheads are very common here....

Gypsum is too soft, surely. Hardness 2. How would it stand up to erosion in this way?

Gypsum is so soft, how can it make upstanding veins?

Nonetheless, gypsum is relatively soft relative to other minerals. It's "2" on Moh's Scale of Mineral Hardness, roughly in between the hardness of a pencil "lead" (graphite) and a fingernail.

It's clearly harder than the rock surrounding it, as evidenced by the drive-over. Perhaps the vein has only been exhumed relatively recently in geological terms.

With loss of water, gypsum can change to anhydrite, which rates 3.5 on Mohs. That's still pretty soft, especially compared to minerals in basalt (5ish-7ish). Moreover, anhydrite is about 25% denser than gypsum, so the transformation of gypsum to anhydrite would be accompanied by shrinking rather than expansion, which would have been another potential way to explain the vein's positive relief.

"Since Mini-TES [miniature thermal emission spectrometer] is not working, we really couldn't pin down the mineral phase based on the infrared. So the next best thing was to look at the vein with Pancam. It shows a dip at 1 micrometer, which is probably related to the presence of molecular water in the mineral, so it's calcium, and sulfur, it's water bearing. The best bet is it's gypsum, which is calcium sulfate with 2 waters in the unit cell," he explained.

Mineral (hardness) and weatherability don't always corrrelate. Remember, in some climes limestone can be a resistant ridge-former and in others a weak valley-former. ...

I found this explanation of the formation of gypsum "spearheads" ...

Fascinating...I wonder if ancient peoples actually used these twinned crystals as effective weapon points.

I couldn't resist...

If Homestake is embedded within soft sulfate sediments similar to those we have become familiar with on the Meridiani plains, perhaps this occurance resembles a gypsum crystal embedded within wallboard

An extreme range of depth in this anaglyph from 2800:

Here's mhoward's beautiful new pan in polar format.

Phil

... 1/ Homestake doesn't look like a feature produced by crystal growth into a loose matrix, like desert sand roses. It looks like it formed in a pre-existing crack in rock that was harder when it cracked than it is now in its weathered state.
2/ Erosion-resistent veins have been seen at Victoria and elsewhere, but the team are saying this is the first vein of gypsum found so far.
One lesson for me from all this: I didn't realise how mechanically weak the Meridiani stuff becomes when freeze-dried on the surface and that it could be significantly harder when not so exposed.

The rock wasn't necessarily harder when it cracked. You can form cracks in wet sand.

The first paper below relates them to rinds "actually developed only at the margins of fractures."

Because it's a nice round number, or maybe because we have so many overlapping navcam pans in this location, I thought I'd output a 1cm/pixel map of our current (and recent) locations.

Because it's a nice round number, or maybe because we have so many overlapping navcam pans in this location, I thought I'd output a 1cm/pixel map of our current (and recent) locations.