[X] My Assistant

[dot.dk]

Looks like Spirit is ready to attack Jibsheet



When I saw this picture I thought I had seen it somewhere else before. Some place at Meridiani

http://marsrover.nasa.gov/gallery/all/1/f/...54P1214L0M1.JPG

[Guest_Sunspot_*]

great mosaic there........parts of this rock remind me of popcorn lol

[dvandorn]

great mosaic there........parts of this rock remind me of popcorn lol 

Looks like the "popcorn" assembled at the base of the rock are pieces that dropped out of, or at places peeled off of, the rock face. To me, it looks like a breccia with dense clasts and a softer matrix has been eroding there, and the clasts are lying on the ground.

This is the first thing I've seen that looks like it might be impact melt. That's just a WAG, but the appearance just reminds me of something like that. I think it's interesting that a breccia with a melt matrix would have harder clasts than the matrix, though...

-the other Doug

[chris]

..snip..

This is the first thing I've seen that looks like it might be impact melt.  That's just a WAG, but the appearance just reminds me of something like that.  I think it's interesting that a breccia with a melt matrix would have harder clasts than the matrix, though...

-the other Doug

Doug,

I don't wish to lower the level of discussion, but, um, could you provide a translation for us non-geologists, please?

Impact melt I get, but the rest....

Chris

Edit: Remove nested quote

[dvandorn]

..snip..

This is the first thing I've seen that looks like it might be impact melt.  That's just a WAG, but the appearance just reminds me of something like that.  I think it's interesting that a breccia with a melt matrix would have harder clasts than the matrix, though...

-the other Doug

Doug,

I don't wish to lower the level of discussion, but, um, could you provide a translation for us non-geologists, please?

Impact melt I get, but the rest....

Chris

Well, I'm nothing more than an enthusiastic amateur, but sure, I can give a description of breccia (pronounced BRETCH-yah), here.

Breccia is a multi-rock kind of rock. It has what are called clasts within a matrix. It's sort of like concrete, in which you have gravel "suspended" in a matrix of cement, which is itself composed of crushed rock grains and other chemicals that make it harden.

A clast in a breccia is a small rock or stone that's been embedded in another kind of rock. In concrete, people make up that "other kind of rock" as a slushy cement mix, and it hardens because of chemical changes (polymerization) that occur within the cement. In a breccia, the matrix is often rock that is molten or semi-molten that has surged across the landscape, picking up stones as it goes -- it hardens as it cools. The outsides of the stones are heated, often shocked, and sometimes partially melted, but they retain their original minerology and gross structure. As the molten or semi-molten rock that "picked up" the clasts cools, it forms rock with these inclusions, or clasts, within it. That rock is called breccia. A landscape that has been altered by multiple impacts that have created thick layers of such breccia is referred to as "brecciated."

On the Moon and on Mars (and on any other rocky body where impact has had a large hand in shaping the landscape), an impact event melts the target rocks, creating what's called an impact melt sheet. This sheet ends up (roughly) lining the crater and gets splashed outside of the crater.

In the center of the melt sheet (thickest in the bottom of the crater itself), it's pretty pure melt -- it gets melted thoroughly and cools into what looks exactly like igneous rock (because, except for being melted by an impact instead of being melted deep within the planet's crust, it formed the same way, as molten rock that cooled). At the edges of the melt sheet, the molten rock begins to cool even before it hits the ground, and as it strikes other rocks it embeds them rather than simply melting or vaporizing them.

Sometimes, the impact melt picks up rocks from the surface, but the molten rock can also accrete around cooler pieces of non-molten rock within the ejecta cloud as it flies away from the impact crater. Sometimes the clasts are partically molten themselves and recrystallize within the melt matrix.

So, every impact creates a certain amount of impact melt, and some of that melt will pile into unmelted or partially melted rock and incorporate it into the new rock. This new rock is breccia.

On Earth (and I suppose, on Mars as well), breccias can also be formed during pyroclastic flows from volcanic eruptions, when extremely hot ash created by the eruption flows away from the vent and incorporates cool surface rocks as it goes. These breccias are called ashflow tuff, since the ash is usually not hot enough to melt completely and the resulting rock looks less crystalline than does rock that cooled from a pure melt. From orbit, there are a lot of places on Mars that look like they have ashflow tuff deposits, so finding ashflow tuff at some places on Mars is quite likely.

But impact has carved Mars' landscape at least as pervasively as has volcanics. Especially in the ancient cratered southern highlands. So the vast majority of breccia found on Mars is probably going to be impact-created.

Now, the really interesting thing about impact melt is that when the impact melts the target rocks (i.e., the rocks that make up the ground where a meteor or asteroid strikes), the minerals that make up the target rocks get homogenized and the new impact melt is made up of the minerals that had been in all of the rocks. So, for example, if a meteor hits a layered rock bed that includes layers of granite and layers of iron-rich basalt and even layers of sandstone, the resulting melt will include the *average* chemical and mineral composition of *all* of the rock layers that were melted. So impact melt from a large crater (like Gusev, for example) hints at the composition of not just the rocks that had been on the surface when the impact happened, but also the rocks excavated from the deepest part of the crater.

So, for example, if the surface rocks are mostly low-magnesium, high-titanium basalts but the impact melt from a nearby deep crater includes a noticeably higher proportion of magnesium and a lower proportion of titanium than the surface rocks, you can tell that there are layers of rock underlying the current surface that are higher in magnesium and lower in titanium than what you're standing on. Even if there aren't any pristine examples of that high-magnesium, low-titanium rock lying around anywhere on the surface.

Finally, impact melt is valuable when you can use it for radio-isotopic dating, since the melting and recrystallization of the rock resets its "radiological clock" and you can tell from it when the impact event occurred.

Which is why it's interesting to see something that looks like it might be impact melt in Gusev. Even though we don't have the instruments to perform radiological dating on the MERs, it's important to know if we can find and recognize impact melt on Mars, for when we actually have the capability of selecting samples for such dating.

I hope the explanation wasn't worse than the stuff you wanted explained... And I'm sure that there are *real* professional geologists in our little forum who could correct me on terminology here and there... but I think I've got the concepts down fairly well.

-the other Doug

[chris]

I hope the explanation wasn't worse than the stuff you wanted explained...    And I'm sure that there are *real* professional geologists in our little forum who could correct me on terminology here and there... but I think I've got the concepts down fairly well.

-the other Doug

Doug,

That was about as clear an explanation as I could possibly have wished for. Fantastic. Thanks a lot.

Chris