This is VERY interesting. The pre-grind surface "looked" pretty clean, but if there is a significant difference in APXS readings after the grind, then there may be a patina of "desert varnish" or the like on this rock, maybe a very thin hydration layer. The pre-to-post grind APXS difference will be very enlightening. I hope someone (pretty please...) drops a hint about what the difference is....

MI MI, oh my!

The new brushed-off/reground Marquette Island pictures are fascinating! Veins, large crystals, inclusions, there is a story to tell here from this piece of deep Mars, conveniently blasted here for our viewing pleasure.

I'm not enough of an igneous petrologist to wax eloquent about the specifics of much of what is seen here, but there seems to be some indication of a little disturbance during its partial-melt (veins), but not much settling-type of layering/sorting. This rock had long-ago hardened in place when it was presumably excavated by an impact.

So, compared to an Earth mantle/lower crust rock, we are seeing less differentiation (lower gravitational gradient and less/absent plastic convection--tectonics) for this Mars rock. Of course, the details of MI's unknown source and depth can have a huge effect, so one can't take this one "data point" too far, but it is information.

In the happy tradition of good science, this MI of MI raises more questions than it answers...

I can't wait to hear some composition results...

Quiet lately, Oppy must be amidst a spectrometer integration. Did a RAT grind actually happen (I think only a brushing took place)?

With regards to lack of both frost and dust devils, this is some indication of "geopraphical" climatic differences, and reinforces the impression of extreme current dryness of the Meridiani site.

Speaking of weather, does anyone recall any frost or dust-devil pictures from Meridiani? Is frost more rare and are dust devils less common than elsewhere (i.e. the other landing/rover sites)? It seems so...
The unaltered (chemically) face of MI implies that it has not seen very much moisture since it was emplaced.

Look at...
1M311952764EFFA9R5P2996M2M1.JPG
from 20-Dec-2009 plus or minus a couple files is a good example. The filled-in crack (maybe I should have used the term "vein"...) wanders horizontally just above the center of the brushing, narrowing going left-to-right.

I'll post a link...

http://qt.exploratorium.edu/mars/opportuni...R5P2996M2M1.JPG

The recent brushings have exposed the details of a filled crack in Marquette Island. Fascinating.

(Just a short note, the aforementioned Microscopic Imager pictures from the past were not soil brushings, they were imprints of the spectrometer.)

..."Grind?"...

Well, that is a picture of a soil brushing. On the other hand, both Odyssey and MRO are back on line, and maybe they are downloading a bunch of old stuff -- I think the just-downloaded Microscopic Imager files are older.

Yes, actually the whole rock is full of tiny crystals -- mostly of olivine. Gemstone-quality olivine is called peridot.
When an igneous rock is termed "coarse-grained", that means that the size of its component crystalline minerals is macroscopic -- sand-sized or better. That certainly is the case for Marquette Island.
There are also crystals of pyroxene and plagioclase feldspar (according to JPL), plus likely traces of other minerals in MI. On this side of the rock there is also some places where there is some sort of "stuff" in some of indents between grains. This might be pounded-in fine dust, or perhaps deposits of altered olivine (possible "serpentization" hydration of olivine). I did speculate about chemical spalling, perhaps this is evidence. We'll see what this stuff looks like after brushing. I'll bet my rock hammer that they brush this face.

Olivine (and some of the other components of peridotite) can react with any water to produce a hydrate called Serpentine, which also increases its volume. Even a small amount of moisture adsorbed into MI could cause chemical spalling, which there might be some evidence of in the recent photos. Also note that even coarse grained peridotites and gabbros can exhibit some layering. (The apparent layering could also be just the expression of impact fracturing in the parent body.)

Also, if the local impact of MI's parent was at Fresh Crater, it is less likely that a large fragment of the local soft Sulphate-salt sediment would survive the 350 meter toss from the fresh crater than would the gnarly olivine chunk. We'll obviously see a lot more of the local stuff churned up as we get closer.

(For a definition of Gnarly, see the April 1 edition of Geophysical Letters.)

That was a great reply and discussion about capture at perihelion. After thinking about it a bit, an easier-to-understand visualization came to mind...

When Mercury is at perihelion, the sun's gravity is stronger relative to Mercury's at a given distance from Mercury (compared to Aphelion), thus reducing the size of Mercury's "gravity well", and thus requiring less delta-v to stay in it.

So Messenger can get closer to Mercury before the dynamics become Mercury-dominated, rather than Solar-dominated. (Remember, this is a visualization, not a precision calculation...)

The goal is to learn as much as possible about Mars, and a likely piece of the planet from great depth has presented a great learning opportunity. I also suspect we will be here a while longer. A closer look (and maybe a brushing) at some of the other chunks may also be in the offing.

"Fresh Crater" has also become of more interest. It is reasonable to speculate the Fresh Crater is the impact site of the parent body of MI. If so, one would expect to continue to see an ultramafic component of debris as we approach the crater, and less-approaching-zero when we depart from the vicinity on the other side. If a case can thus be made for Fresh Crater being the local impact site, then the crater size and condition, plus an estimate of the ultramafic debris mass, can give good clues as to the origin of MI. But this is speculation, mind you.

Also, it is unclear whether MI has (or can yet be) labeled as a peridotite or rather a gabbro...

With both MRO and Odyssey contending with safe mode resets, etc., we must be sipping data from both MER's through a very long and thin straw -- direct-to-Earth transmission.
What effect is this having on the missions and the day-to-day choices? How is DSN time/schedule affected?

I think the Oberth effect is only an oblique influence in the positioning of the capture point of Messenger in Mercury's orbit. There may be some advantages from the positioning of the 3-body problem as far as Messenger's entry into Mercury's gravity well, but there were not rocket firings in any of the fly-by's, and the capture rocket firing will be for losing energy relative to Mercury (admittedly centered at Mercury closest approach, so that aspect of the capture rocket firing is Oberth-enhanced). What is not obvious is why this near-Mercury capture is more efficient at Mercury's perihelion, rather than elsewhere in its orbit.

Perhaps the net trajectory deflection by Mercury's gravitational field can be viewed in some sort of approximation as a delta-v impulse, and that is why the fly-by's and capture are grouped near Mercury's perihelion, and that is how the Oberth effect is realized.

Also from the release...

"A 3.3-minute firing of its bi-propellant engine provided nearly all of the probe’s 177 meter per second (396 mile per hour) increase in its speed relative to the Sun."

Quite a little kick -- it moves the periapsis of Messenger's orbit very close to Mercury's. Somehow, capture at periapsis is most energy-favorable, but it is not casually obvious why. (Maybe someone will be kind enough to post an explanation...)

Olivine: The brushed areas of M.I. just had the "look" of coarse-grained olivine, the way the crystal surfaces look and the non-red semi-translucent look. The sharper edged grains are likely pyroxene and feldspar.

I hope more areas of the rock get brushed before (and if) a grind site is chosen. I see veins and inclusions that merit a clear look.

The coarseness of grain and high olivine content definitely imply that this rock came from significant depth -- the coarse grain size implies slow crystallization (and slow cooling) far from the surface, and the veins imply shear and rework at depth (the filled in veins are as hard as the surrounding rock -- it did not fragment along the veins).

It took a pretty good impact to excavate this rock, perhaps a 100-km or much more sized crater would be left behind. Which crater would it be? The recent-ness (relative to the Meridiani surface) of the debris of this rock and it shards might cut the potential source list down to a short list. However, it is also possible that a smaller impact "recently" excavated this rock from the rim debris of the original big crater. Even that impact had to be big enough to "gently" throw this rock's parent debris clump on a sub-orbital trajectory. By gently, I mean an non-shock acceleration, since the rock is still intact.

Well, maybe this helps folks understand why this rock is so interesting.

Something about the look of the brushed rock says "Olivine" to me. No real factual basis...

Oh, what the heck, let's get wildly speculative...

1)
I would assert that today's movement was intentionally slight, a deliberate perturbation to confirm data on Spirit's mechanical and frictional state. This includes noting if the rock underneath is in significant contact (is it a pivot?). The response from this perturbational movement will help determine the next movement set. Mechanical engineers love perturbation theory. ...And the principal of virtual work...

2)
Digging myself deeper (not Spirit, hopefully), the JPL guys in the sand pit found a "stick-slip" type of behavior, so that a slight movement below a certain threshold (.5 mm?) would not cause the sandy dust under the wheels to break free and slip. Furthermore, the diurnal thermal cycle causes a "reset" of the grain inter-particle friction, so the slight stick-before-slip can be repeated the next Sol. So, we can expect Spirit to explode out of its hole at 1.5 cm/month.

3)
The right front wheel no longer turns, but it can be rotated left and right. In fact, if this is done while the other wheels are pushing, it reduces somewhat the effective drag that dead wheel provides. Helps Spirit shimmy out of there...

--There, now I feel better.

MI's of Marquette (just in) sure look like a crystalline basement rock with inhomogenuities and mineral-filled veins. The debris all about the site makes me want to put my money on it being part of a clump of secondary impact debris.

Re RAT:

As I recall, Opportunity's RAT had its brush bunged up, and there is at least one sensor problem. I think there are work-arounds in place, but it has been a while since I read about them. Perhaps one of the exalted "official ones" can enlighten us...

In regards to the Enceladus picture...

A detail I find interesting here is that the night limb of Enceladus is DARKER than the background sky. I am surprised that there is that much scattered light from the tenuous ring (or extended exosphere) derived from the plume.

--MarkG

Thanks for the tip on Celestia -- I'm working with it now.

At 8 million km distance today, the crescent of Mercury would be barely resolved by a human eye traveling with Messenger (~2 arc minutes).

As far as the shape and surface textures of Block Island goes, it is quite possible that the meteorite has tumbled one or more times over the aeons in erosion/burial cycles (remember the pedestal discussion). There is no particular reason (besides trusting in a couple billion years of inertia) that the orientation of BI has been unchanged.

Does anyone know of a detailed diagram of the flyby (#3). I actually want not just the inner stuff, but the stuff from a couple million KM out, to show the interactive dance of the 3-body problem to my son.

Both rear and forward hazcam pictures are showing up -- movement must be near. Hoping.....