...a mini-volcano!

http://tinyurl.com/z4qx4

http://www.sciencemag.org/cgi/content/full/311/5769/1858b

"Mars rover scientists have concluded that the cryptic "Home Plate" that the Spirit rover spent 3 months reaching is the remains of a little ash-spewing volcano"

Magazine subscription or article purchase necessary to read full article, but there should be other updates soon?

Well, I'll be a monkey's uncle! Assuming you've read the whole article, would you care to summarize the lines of reasoning/evidence in 100 words or less? Shoot, take a thousand if you need 'em, Paul!

Here's the whole Home Plate passage, which, as the article's title ("Snapshots From the Meeting") notes, was basically just a blurb from the conference:

"Martian ring of fire. Most Mars rover scientists have concluded that the cryptic 'Home Plate' that the Spirit rover spent 3 months reaching is the remains of a little ash-spewing volcano. The 90-meter-wide, 2-meter-high platform of layered ash has a distinctive chemical composition linking it to nearby lavas on the floor of Gusev impact crater and to rocks on the adjacent Columbia Hills, says team member Harry McSween of the University of Tennessee, Knoxville. He thinks volcanism driven from beneath Gusev blanketed any lake sediments mission planners expected to find on the floor of Gusev.

"Hobbled by a broken right-front drive wheel, Spirit is now on a 'drive or die' mission to a nearby hillside. It must tilt its dust-laden solar panels toward the sinking sun to boost power production and survive the coming winter, or sit the winter out in hibernation."

My first guess.

http://www.unmannedspaceflight.com/index.p...indpost&p=39980

I'm not sure this little blurb really tells us much that is new, but the way it was portrayed was a little misleading, I think. Titling a piece about Home Plate as the "Martian ring of fire" seems to imply that HP is the vent of the "little ash-spewing volcano." What we have observed is that the upper sediments of HP are rather spectacularly crossbedded (or cross-laminated, if you prefer) in a fashion known on earth to be formed by wind-blown ripples or dunes. We have also seen that the grains in the upper units are well rounded and well sorted, characteristics that are not typical of a primary ash. It seems as if this must be reworked ash, if it is ash. I don't have a problem with a model where the particles that comprise HP originated in a volcano. I'm not convinced HP is the volcano.

My notes about HP from Steve Squyres' presentation at LPSC are interesting (and in a few places, illegible, because I was writing so fast). I got the impression he was fascinated by HP, had some ideas about what it was, but didn't have the whole story sorted out yet. To put things into perspective, Spirit had just driven off of HP to head for the hills, and had looked back to take that detailed panorama of the newly visible edge.

He commented on the lovely cross-stratification visible in that panorama, and that the clasts seen in the MIs were "extremely well sorted and extremely well rounded." He then said that El Dorado was the closest thing they had seen up to this point, as a modern analog for HP. He went on to describe the minerological composition as 18% olivine, 23% pyroxene, 30% magnetite, and 28% something that is illegible in my notes. He said this was essentially an altered basalt. Steve described the APXS results as a "somewhat altered basalt" and mentined that they don't know how widespread the Home Plate formation is, and that there may be some on McCool Hill.

Then, he went on to speculate that the lower HP strata are volcaniclastic or impact related, and that the upper layers may be basal surge or aeolian in origin. (I'd love to see an example of a X-bedded basal surge deposit that displays such well-formed sets.) He mentioned the possibility that the upper strata might be the result of aeolian reworking of material from the lower units, since they have the same compositions.

Since we have left HP we haven't seen much other than somewhat variable dips and a bedrock that looks much like lower HP. Let's hope we soon see Spirit leave its salty quagmire.

"Ring of Fire" is doubly misleading in that it implies organized volcanism resulting from tectonic activity. We've seen nothing like that (discounting the Tharsis region) here.

My take, groomed and shaved with Occam's razor, is that is an erosional remnant of windblown ashfall. It likely extends over the Inner Basin and surrounding hills. The coarser "Spongebob Beds" may be related ro coincidental. But wait, there may be more-- however, we need to look further at this locale as we've barely scratched the surface.

I suspect that the sulfate zones we see are weathering-related and may or may not be related to the layered deposits. See "PSRD Antarctic Guide to Martian Weathering" at http://www.psrd.hawaii.edu/April05/DryValleysSoils.html for an overview of what _may_ be happening.

At least our intrepid explorer has become hobbled at a good location.


---Bill

I saw this on the MarkCarey.com/mars forum:

3D of brushed rock with yellow soil at base of homeplate, Gusev crater, Mars. Taken by Spirit February 14, 2006.
http://www.flickr.com/photos/hortonheardawho/105427976/

Click on the zoom button to see the most detailed view of the image.
The undulating quality of the lower layers, like a wave flowing, gives the impression of a material that flowed and then later solidified.



Bob Clark

This would certainly lend support to the thought that Pitcher's Mound looks like a volcanic cone, too.

Er, no, not *really*. Yes, it's sticking up, and looks a bit out of place. No debate there! But it looks more like a feature with Home Plate deposits draped over it, or perhaps a better way to describe it would be as a remnant of the prviously much more extensive draped units. I'm sure there *are* baby cinder cones on Mars, but Pitcher's Mound is just Baby Mound with a hat on.

Bob Shaw

Do I detect skepticism about the HP conclusions (in general here)...?

If the geologists on the MER team have reached this conclusion about HP, they must feel they have good evidence for it, which again, we have not seen most of yet. Could they be wrong? Sure, but I'd prefer to just wait for the more detailed reports to come out before assuming they are, that's all.

Re PM, how do we know it isn't a cinder cone? We haven't even looked at it close-up or done any analysis on it yet. We do know there are other small cinder cones on Mars already anyway though, because they were photographed in the northern regions by MEX and reported on by ESA.

Paul:

We can see the draped material both from above in the MGS images we know and love so much, and - as ground truth - edge-on from the side of Home Plate.

I don't think Home Plate itself is 'special', except inasmuch as it has not been brushed away over the last gazillion years, and I expect that we'll eventually consider it to be just one - good - example of a generalised formation. That's *not* to lay claim to it being something other than a volcanic feature, just that I can't see it as being a one-off orphan.

What was the old saw about observations being valueless, except in context?

Bob Shaw

I'll side with Bob on this: PM ain't a cindercone. It looks like Bob says it does.

I'll confess that when we had the initial look at the Inner Basin I was one of those who informally called it "the cinder cone" because it superficially looks like one.

We'll know more when we get a closer look in the Spring.

--Bill

MARSDAILY

- Home Plate Hints At Explosive Past

http://www.spacedaily.com/reports/Home_Pla...osive_Past.html

Pasadena CA (SPX) Apr 13, 2006 - New images from NASA's Spirit rover show
coarse-grained layers from around the edge of a low plateau called Home Plate
inside Gusev Crater on Mars. One possible origin for the material it fell to the
ground after being thrown aloft by an explosion such as a volcanic eruption or
meteorite impact.