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Phobos
hendric
post Jul 30 2008, 10:34 PM
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I just had a really dumb idea, looking at that moon again. I was noticing how much it resembled the rocks around Opportunity at Victoria, and I started thinking...Hmm, what if those grooves were caused by differential micrometeoroid weathering due to layers? They do resemble at a macro scale the aeolian cross bedding we've seen before. That would add weight to the "Phobos is a chunk of Mars" idea!


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jasedm
post Jul 31 2008, 12:31 PM
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The best (for me) explanation I've heard for the grooves so far was highlighted by Emily over at TPS as being the result of Phobos sweeping up ejecta in chains from Martian impacts. Scroll down on the linked article to see a computer simulation which is a good match to the observed grooves.
But looking at the latest pictures, they seem to be far too straight and of regular width to be secondary crater chains - looking much more like collapse features to my untrained eye.
The mystery remains....
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Paolo
post Aug 21 2008, 07:42 AM
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In Astronomy & Astrophysics: New astrometric observations of Phobos with the SRC on Mars Express
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Harkeppler
post Aug 21 2008, 09:52 AM
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Mayby grooves on phobos seem not to be due to secondary impacts from asteroid impacts on Mars; they are too defined and regularly for that. These computer simulations seems to give only the trajectory of the ejected masses but the the width of the shower. And there are too much of these grooves and they are too regularly placed. This counts for some effects connected with material resistance against stress forces per length unit.

Especially, one set of grooves seems to be more or less concentric or better: coaxial along the Mars-anti-Mars-axis and can be explaned easily with tidal effects. The "craters" along the grooves may be simply sinkholes, especially because there are no ejecta. The other set of grooves (here highlighted in red) is nearly perpendicular to the other one and can be formed earlier by the same way; a large impact may have altered phobos orientation by chance and after that tidal fractures occure in a new set. So, that older bunch of grooves is orientated around the old Mars-anti-Mars axis of Phobos. Here:

http://www.planetary.org/blog/article/00001391/

or here

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2195.pdf

You can see the both sets, but you need another cartographic projection to see the old set as a concentric/coaxial one as well as the other.

Both groove sets are not (the old one) or not well (the new one) converging to two points but slicing phobos more like a cut potato. Here, the non-sperical shape of phobos and cartographic problems with this fact should be considered; the pseudo-convergence is an articfact of the cylindrical projection!

I would also consider that Phobos had been in an elliptic orbit first, so diffential stresses du to the tidal force change between apoareum and periareum and libration (nodding) effects due to the constant rotation period and the inconstant orbital angular velocity would have forced a result. A large impact, mayby the Stickney forming impact, would have altered the meanwhile circular orbit to an elliptic one again and it altered Phobos spin and orientation. So a second set of grooves came to existence.

You may recognize that the younger grooves, which are mostly sharper defined and smoother than the older ones (red) cross through them (and not opposite) which will give a hint which groove set was the former one.

The Stickney-impact itsself may be "buffered" by the weak material itsself, like the craters on asteroid Mathilda, and does not harm the whole structure. It is like shooting a projectile in styrofam...

It would be interesting to look for some grooves on several other moons like Amalthea; in the Saturnian system so much dust maybe hide features like that on the innermost moons.


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SteveM
post Aug 21 2008, 09:48 PM
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QUOTE (Paolo @ Aug 21 2008, 02:42 AM) *
The "new obervations" reported don't include the recent batch of observations discussed here. The most recent observation in the article is dated 2007-02-25T02:44:11.994

The main concern is with the fine details of Phobos' orbit.

Steve M
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charborob
post Oct 16 2008, 07:23 PM
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Interesting article about Phobos on the ESA website, based on the recent flyby:

ESA closes in on the origin of Mars’ larger moon
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tanjent
post Oct 18 2008, 03:35 AM
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In the article just quoted by charborob, there is an animation of Mars Express being diverted from its orbit during a close pass by Phobos. The interesting thing about this simulation is the way MEX returns to its original orbit after the deviation. It looks kind of like the maker of the simulation has added the symmetric plot of a normal distribution to the original unperturbed track of the spacecraft. Is this really how it would happen, or would the spacecraft experience a permanent change in its orbit due to the small gravity assist derived from Phobos? Observatoire Royal de Belgique must know a lot more than most of us about orbital mechanics, but maybe in this case they are pulling our legs and expecting us not to notice?
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peter59
post Nov 6 2009, 02:47 PM
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I checked "forgotten" Mars Express archive and I stated that this year Phobos has been photographed several times. Unfortunately, the quality is not stunning.
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and Phobos over Mars craters.

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Phil Stooke
post Nov 6 2009, 02:55 PM
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I like the last two transit images! More close passes would certainly be nice, especially if they showed the Phobos-Grunt landing region better.

Phil


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charborob
post Nov 6 2009, 04:17 PM
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Transit image modified to show the Martian surface better.
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elakdawalla
post Nov 6 2009, 04:57 PM
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Why is the Martian surface so dark in those transit images? Is it because we're on the nightside of Mars' terminator?


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Phil Stooke
post Nov 6 2009, 05:06 PM
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The shading suggests that's not right.. not near-terminator either. Maybe it's the filter?

Phil


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ugordan
post Nov 6 2009, 05:39 PM
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Couldn't the skylight provide some illumination, mainly from the west, even past the terminator? Didn't one of Dawn's Mars flyby images capture a similar scenario?


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vikingmars
post Nov 12 2009, 02:48 PM
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QUOTE (peter59 @ Nov 6 2009, 03:47 PM) *
I checked "forgotten" Mars Express archive and I stated that this year Phobos has been photographed several times. Unfortunately, the quality is not stunning.

Peter 59, please, could you tell us the orbit # of those pics, so that we can retrieve the original pics ? Thanks in advance smile.gif
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peter59
post Nov 12 2009, 06:16 PM
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QUOTE (vikingmars @ Nov 12 2009, 03:48 PM) *
Peter 59, please, could you tell us the orbit # of those pics, so that we can retrieve the original pics ? Thanks in advance smile.gif


Orbit number is contained in the names of the attached images:
H5984_0004_SR2, H6042_0005_SR2, H6217_0005_SR2, H6128_0003_SR2

This is a great honor for me to talk with someone who has played a significant role in the Viking mission. Words of respect.


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