The crater no longer exists. If it did, the meteorite creating it would be buried beneath it.

Remember that a significant amount of material has been worn away from the surface that Opportunity is roving over. The layer of "blueberries" lying on the surface were eroded out of a layer of rock that is long gone. The crater was in that rock, or some other long gone layer.

The possibility exists of a history of ice coverage at Meridiani, where it is possible that any impact crater in that ice would have since sublimed away, leaving the meteorite intact.

I don't doubt that at all. I think meteorites are way cool. But I didn't see that we could learn much from a meteorite on Mars that we couldn't have learned from much-easier-to-study meteorites on Earth. There have been a couple interesting suggestions since I posted my question:

• They sample metoroids from a different part of the Solar System.
• They may preserve material from other bodies, such as the early Earth, the Moon (and presumably Io, etc).

Thanks for the responses. The photos I see of BI today really do look like an iron or stony-iron meteorite to my untrained eye.

CE

How big would be the crater caused by a pretty thing like Block Island?
Looking at the image at a first glance I had the impression of seing a circular pattern with what? help me here...70, 100mts? BI and bigger dunes stay in the center, I'm probably seing too much and this doesn't make any sense at all...
Here's what I mean, sorry but the only available tool is Paint...

EDITED: I can distinguish it better in Tesheiner's map.

On this sandblasted plane Block Island is large enough to have caused local wind variations and isn't it possible that the larger ripples in the immediate area could be attributable to this rather than crater remnants? If BI is ejecta rather than an asteroidal meteorite then the impact could well have been at terminal velocity for Mars or less. No more than 1 km/s? That wouldn't punch a particularly large hole. We could even be looking at ejecta from Victoria. So even if it does have the composition of an iron meteorite, if it is a fragment spalled from the Victoria meteorite on impact it would again have hit at less than terminal velocity.

If it is a Victoria impactor fragment spalled on impact and ejected does that make it ejecta or meteorite?

My impression is that Victoria is too old for anything spalled from it's impactor to be lying on the current surface. Weren't we speculating a while back that Victoria* was formed before the hundreds of meters of sediment that Opportunity is traveling over were laid down?

*edit: Oops, getting my craters mixed up.. That was Endeavor. But the speculation on Victoria was that it was so old that it has been covered over and uncovered by deposition and erosion, still making it impossible that the current surface could be the surface on which part of it's impactor fell.

Actually, I think that's really the key concept to understanding the Meridiani meteorites. It doesn't seem like we see much exposed bedrock aside from regions near craters or between dunes. I get the impression that the environment is similar in one respect to Antarctica or glacier tops: the surface gets buried & exhumed periodically, and quite often what isn't sand or snow turns out to be meteoritic.

If this model's true, then these meteorites are probably all one-shot unrelated objects & not secondary artifacts of larger impacts.

And in line with the drift (pun intended) of Nprev and others, would it be correct to think that an iron meteorite is less likely to shatter on impact than a stony meteorite, and prove more resistant to a billion years or so of subsequent sandblasting?

The line of demarkation between impact ejecta and the pre existing sandstone is pretty clear at Victoria. In places the breccia is almost eroded away. So wouldn't the surface away fropm the annulus be pretty much at the level it was at the time of impact?

Of course. It seems likely that there would be more impact ejectites of Martian origin on earth than those of an earthly origin on Mars, but it is nice to know that some may be out there. For me, it is more exciting to think that our moon may be littered with early earthly ejecta, since that external body is so much more accessible to sample discovery and collection. On Mars, existing resources able to identify provenance are limited, but future instrumentation is in the pipeline.

Now, there's an interesting comparison, Tom.
How does the discovery rate of meteorites by the MERs compare with that of the Apollo moonwalkers?
Are there lessons here for learning, or is it "apples vs. oranges"?

Strictly speaking, our moon IS early earthly ejecta.

So far as I know none of the rocks collected by the Apollo astronauts have been identified as meteorites from elsewhere. However, this does not mean that every sample of lunar regolith has been examined for small fragments of exotic rocks.:

http://meteorites.wustl.edu/lunar/regolith_breccia.htm

I believe that many of the contributors to UMSF are geologists working for universities. Presumably NASA would respond favourably to those geologists if they requested a few ccs of lunar regolith to look for exotic rock fragments. Perhaps in the Apollo samples somewhere there is a tiny fragment of an Earth meteorite.

Perhaps a "moondust@home" web site could be set up along the lines of the "stardust@home" web site where members of the public could scrutinise a million fragments from the lunar regolith looking for something interesting.

EDIT: One implication of looking for fragments of Earth meteorites in samples of the lunar regolith is to prove that the Earth rock actually came from the Moon and did not represent contamination back on Earth. To overcome this problem the investigation would have to be done in clean room conditions.

(Admin edit. It's best to not cite this place as umsf dot com, as that's actually a functioning, different website that google will pick up on. Just UMSF - or the full URL, but don't turn the acronym into a URL)

Mars does of course have the advantage of an atmosphere, so hand-specimen sized meteorites can survive their arrival intact. Meteorites of any size hitting the moon are going to be mostly vaporized on impact.

John

Quick recap, comparison and celebration :-)