[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_*]

The methuselah outcrop has some of the strangest looking rocks weve seen from either rover, take a look at the "sandwich" on the right:

http://qt.exploratorium.edu/mars/spirit/pa...B2P2417L2M1.JPG

[jvandriel]

Here is a panoramic view of Jibsheet taken with the Pan Cam and L2 filter on Sol 477.

jvandriel

Attached thumbnail(s)

 

[Tman]

Here is a panoramic view of Jibsheet taken with the Pan Cam and L2 filter on Sol 477.

jvandriel

But where is Jibsheet exactly?

[Nix]

A rough low-res comparison for sol 478 and 481 microscopic camera mosaic..
Nico

Attached thumbnail(s)

 

[djellison]

The T shaped outcrop RIGHT in the middle of that mosaic

There are four middle frames on that mosaic - we're talking about the right half of the second one along

Doug

[hendric]

..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

Breccia is a type of rock commonly formed due to an impact. The impact partially melts the rock, so other rocks ("clasts") are embedded in a "matrix" of melted rock. Lunar rocks are almost all breccias. Usually the matrix and clasts are different colors and kinda obvious.

Last page of

Lunar Breccias

has a great example, and also explains how they happen.

[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

[JES]

..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.
...
... but I think I've got the concepts down fairly well.

-the other Doug

THANK YOU for this explanation! Most helpful for a non-geologist.

[Nirgal]

A rough low-res comparison for sol 478 and 481 microscopic camera mosaic..

NIX, I like the color MI image you posted above.
Did you take the colors from a corresponding pancam-image ?
I've tried a similar technique on the MIs but found that the
resolution of the pancam was not quite high enough on the scale
of the MI pixels ...
wonder if JPL could produce some kind of pancam "super-resolution"
(similar to the MGS "PROTO" technique) to assist the MI
system in order to provide color information down to the sub-centimeter
scale...

[dot.dk]

wonder if JPL could produce some kind of pancam "super-resolution"
(similar to the MGS "PROTO" technique) to assist the MI
system in order to provide color information down to the sub-centimeter
scale...

Dont't think it would be easy to have the rover pitch and roll while taking pancam pictures to obtain higher resolution

[djellison]

wonder if JPL could produce some kind of pancam "super-resolution"
(similar to the MGS "PROTO" technique) to assist the MI
system in order to provide color information down to the sub-centimeter
scale...

As said - you cant do CProto with a 1024 x 1024 CCD (as MGS MOC is a push-broom line ccd ) - but you can do a sort of super resolution imagery by taking many many images by moving the pancam head very slightly. However - it's been done with the L7 filter to generate a high resolution greyscale image - and then an L456 over the top for colour. However - we have the high res greayscale in the form of MI imagery. It's a no go really.

doug

[dvandorn]

Doug,

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

Chris

No problem. I enjoyed it.

Looking at Jibsheet some more, though, I'm reminded that breccias aren't the only kinds of rocks that have clasts embedded in a matrix. Conglomerate rocks can be simple sandstones and mudstones that have embedded stones or pebbles, and a congolmerate will often have a softer and more easily eroded matrix than harder stones embedded within it. Which is what we see at Jibsheet.

Breccias and ashflow tuffs can also have soft (the geological term is "friable," meaning soft and crumbly) matrices compared to their clasts... it all depends on the minerals that make up the rocks and the speed with which melted rock cooled and solidified. But whichever kind of rock Jibsheet is made up of, it's definitely shedding clumps onto the ground. Which, if nothing else, is different from most of the rocks we've seen at Gusev. Looks a little like Pot of Gold and others like that, with harder material surviving the erosion of softer pieces of a rock, but we haven't seen the shedding of uniform little clumps like this before, really...

Now, at Meridiani, on the other hand, soft rocks shedding fairly uniform-sized harder bits are the most common... so, go fig!

-the other Doug

[Nix]

Here's an L257 view of Jibsheet. AwalkonMars Still working on the MI mosaic though...
Nirgal, the difference in resolution is indeed an issue but overlapping the images can be a real pain..
Nico

Attached thumbnail(s)

 

[Tman]

Here's an L257 view of Jibsheet. AwalkonMars Still working on the MI mosaic though...
Nirgal, the difference in resolution is indeed an issue but overlapping the images can be a real pain..
Nico

Hi Nico, I just enjoyed your full resolution PNG of east Methuselah (or Jibsheet ). Exquisite such full resolution!
In respect of the size of 25MB I wonder about you generated a PNG-file of it. The orginal pics were sure JPEG - they must be JPEG Are you sure there would be a visible difference in comparison to a high JPEG version?

Note: For the responses in this issue it was opened a new thread in "Imagery & Tech Issues": http://www.unmannedspaceflight.com/index.php?showtopic=1001

This post has been edited by Tman: May 16 2005, 07:55 PM