On the Road Again - volume 3 - Page 2

Got to be from one big block that blew apart in what was once a thicker atmosphere and sploshed into either thick ice or quite a deep lake/sea of some sort.

Or have we found a new type of meteorite that parachutes in to avoid making a big crater in the sculpted drifting dunes.

Well spotted hort.

Burt and Knauth have suggested that a single giant iron impactor might be the origin of the unusually high concentration of hematite (iron oxide) at Meridiani compared with the rest of Mars.

The many iron meteorites that Oppy has seen could be spalled fragments from a big impactor. Not all of the incoming object ends up melted and mixed with target material in the bottom of a crater. Here is a Scientific American article that investigates a nickel-iron impact in the Saudi desert:

http://volcanoes.usgs.gov/jwynn/1998SciAm-Wabar.pdf

Note the text on page 70 and the diagram on page 68 to see how pieces of intact nickel-iron metal ended up scattered on the desert surface near the impact site.

Got to be from one big block that blew apart
Could be - but BI doesn't show any sign of having been spalled from a parent body either on impact or airburst. I guess that all we can say with conviction is that meteorites this size landed when there was a much thicker atmosphere billions of years ago. To be on the surface like this the surrounding plain must have been worn down.

The number of small craters we have seen during Opportunities traverse show the result of impacts after the atmosphere thinned to the extent that erosion was minimal and dunes at Meridiani were frozen. The timescales that we need to consider in trying to make sense of Meridiani are almost beyond our comprehension. These meteorites woud have had a tens or hundreds of millions of years to accumulate.

sol 2020 ( Sep 29, 2009 ) R12 of possible meteorite "Shelter Island":

Today is a driving day with a full filter sequence of "Shelter Island".

serpens, re your 23: What would be a sign that Block Island spalled from a larger body? (Spalling happens only on impact. Airburst is something else.) The deep pits on Block Island very likely mean that there has been considerable erosion on Mars. The presence of a fusion crust was still an open issue for the rover team at last update. We don't know if any part of the current surface of Block Island records anything about atmospheric transit or impact.

I have a problem with the meteorites burying themselves in craters in the bright rock at impact and them being exposed by subsequent erosion. (A thicker layer of soil, maybe.) It is just too much of a coincidence that they have all been completely exhumed but none seems to have interacted with subsequent erosion of the bright rock. There are no moats, yardangs or pedestals. I'm going with a simple explanation: the meteorites have been placed on the plains after the plains had reached something very close to their present condition. There are billions of cobbles on the plains that could not be moved by wind, and while being clustered in various ways, show no apparent correlation with impact craters. I suspect that the cobbles and the meteorites have been rearranged by the same process.

As far as impactors go, to me Meridiani's scattering of meteorites and native rock chunks don't look that different from what you observe for larger meteorite impacts with Earth. (if you remove from consideration that Earth's meteorite fragments can hide in more places...). You have obvious craters from impacts on Mars, many of them little-changed since impact and you have variably-sized meteorite fragments (such as Block Island), as well as the expected chunks of native material cast out from the craters made in those impacts. In that low gravity, I can easily believe that an impact forceful enough to produce Victoria (or Endeavour) Crater would cast off many such fragments of rock - both meteoritic and Martian - during the colossal impact.

Block Island just doesn't look like it was an individual impactor. It looks (and lies there) just like spalled meteorite shrapnel does anywhere you find it, albeit much modified by the Martian climate and its time exposed on the surface. Heck, I've got chunks of Earth's Shikote-Alin airburst over Siberia that resemble BI, though they are significantly smaller of course.

If you look at known meteorite impacts on Earth, it should be no surprise that most meteoritic finds are from desert areas - like Meridiani, where they have little or nothing to hide under. I'd expect to continue to see many more BI-like fragments during Oppy's journey, however much longer that journey might last.

Frankly, if I was wandering around the Gobi, Kalahari, or Sahara and came across something resembling BI and its little buddies, I would (after jumping up and down of course) immediately wonder where the huge crater that the parent meteorite made lies - and which would contain most of the unspalled remnants of the parent meteorite. I'd be very surprised if BI has ANY fusion crust at all. Probably never did have.

Meteorite planetary impacts can be immensely powerful. The Martian meteorites we have recovered on Earth were themselves chunks of basalt blown off Mars long ago by powerful meteorite impacts. Any impact powerful enough to blast large enough rocks into space to make it through the Earth's atmosphere to its surface would certainly be powerful enough to spall fragments many, many miles on Mars.

The link I gave is to an interesting story about a small iron-nickel meteorite's impact with Earth. Kind of gives an idea of what such an impact could do - at least on Earth, though I would think that the process would be at least loosely similar for such an impact on Mars.

OOps... lost the link...

http://www.scribd.com/doc/19800339/The-Day-the-Sands-Caught-Fire-The-meteorite-hunters-Part-1

sol 2022 L27R1 of "Shelter Island" meteorite:

sol 2020 pan in direction of "Falcon" crater:

Perhaps this small 1.8 meter crater was created by the final landing "skip" of "Shelter Island" which is about 11 meters away???

Why so many berries around these meteorites?

Horton brought up that he thought there might be more berries around the fracture fill areas, are there more beries around the meteors, or the impact areas?

Are the cobbles that have been seen from the get go, all family, of these larger chunks?

sol 2011 ( Sep 20, 2009 ) L257 7x1 pan of Nautilus crater:

with link to 3D of interesting block on the rim. I thought the block might be meteoric - but the cracks in the block means it's likely just bedrock.

sol 2024 ( Oct 3, 2009 ) L257 Shelter Island:

Maybe I'm too simply-minded, but do you consider the possibility that these meteorites never made impact. I mean : is it possible, that thay fell in a ( i don't know how deep ) water and just fell gently on the bottom, not making impact. Later the water evaporates, as we consider the Mars was wet..and these meteorites just stay whey they are...
Please somenone explain me why it's impossible...I know it's a crazy idea, but I can't find anything against it...

Large eroded cavities at the base, not at the top. Sitting in an acidic water table that only came midway up the meteorite? Probably not but worth a comment.

I dont't say that my explanation is right, but your is questonable too : What if the acidic water evaporates slowly, very slowly ? Than at the end, it becomes even more acidic because of the higher concentraion of minerals. It that case, it is possible that larger cavities can form on the bottom of the meteorite, and smaller at he top, because the bottom is exposed to stronger acids, and for longer period ?

sol 2001 (Sep 10, 2009 ) southern 3D view of bedrock pedistal under Block Island:

What is that in the distance (center)?

pk. Good eyes. There are a lot of meteorites of a similar size in this area. Elsewhere it has been suggested that this is a strewn field - that is a larger meteorite fragmented in the atmosphere so that the pieces impact in an elipse, kinda like pellets from a shotgun blast. Makes sense, and there are terrestial examples. But the bits will still create impact craters and for this to happen would necessitate a much denser atmosphere.

Hortonheardawho. Nice processing result at reply 36 thank you and pretty convincing. I wish I had a tenth of your talent.

sol 2024 ( Oct 3, 2009 ) false color infrared view of Shelter Island:

On sol 2027 ( Oct 6, 2009 ) Oppy moved in front of Shelter Island:

Looks like a closer study is planned for the weekend.

Shelter Island looks "similar" to Block Island in the infrared - but inter-image comparisons are forbidden in horticolor - so it may in fact not be similar. But my bet is that the APXS will reveal an almost identical composition.

The block in reply 7 is about 75 meters distant and is about 50 cm wide and 25 cm high. Yep, looks likes another meteorite.

Thanks for the kudos serpens.

sol 2027 ( Oct 6, 2009 ) L256R1 of possible meteorite:

with link to "next drive direction" panorama.

An MI sequence of Shelter Island is planned for today, sol 2029.