[X] My Assistant

[djellison]

Looks quite Fram-ish

Doug

[Gray]

Thanks, dot.dk.
Yes I was referring to the meteorite found next to the heat shield. Reading dvandorn's post about crater size vs meteor size made me wonder about the meteorite calmly sitting on the flat plain with no obvious crater or ejecta nearby. Perhaps it is a fragment of a larger meteorite that skidded to a stop there without much of an impact.
Gray

[dvandorn]

Thanks, dot.dk.
Yes I was referring to the meteorite found next to the heat shield.  Reading dvandorn's post about crater size vs meteor size made me wonder about the meteorite calmly sitting on the flat plain with no obvious crater or ejecta nearby.  Perhaps it is a fragment of a larger meteorite that skidded to a stop there without much of an impact. 
Gray

There are a lot of rocks in the solar system in orbits that intersect in all sorts of different ways. Some meteors approach Mars relatively slowly and are slowed to a relatively slow speed (I don't have an exact figure, but possibly as slow as a few hundred miles per hour) by the atmosphere.

Now, I doubt that this rock just entered and plopped down gently on the ground. I'd bet that a rather bigger rock landed as slowly as any meteor every lands on Mars somewhere within a km or two and this fragment was ejected relatively whole and undamaged. It probably bounced a time or two before landing in its present position, looking for all the world like it was just set down there by a giant's hand...

"Slow" impacts are relatively rare, I'm sure, but they do happen. I remember seeing an MGS high-res image of a Martian crater that literally looked like a long trench. You could see the impactor lying in a heap at the end of the trench, with dirt just splayed out from it like a bulldozer had shoved it out. The thing had obviously hit at a very low angle to the ground. The impactor must have been a good 5-6 feet across (it was two pixels or so wide, IIRC, on an image that was rated something like 1-2m per pixel resolution) , and the crater was only about three or four times wider than the rock that made it. Had that same rock impacted at the average velocity of impactors on Mars, the crater would probably have been a LOT larger.

(Yes, it might have been a secondary, but the dirt it excavated was a *lot* darker than the dust-covered soil around it, making it look very, very fresh. I guess it's possible there was a large enough impact relatively recently and nearby enough to have made a secondary like this, but I think you really could see this kind of crater morphology in a slowest-possible-speed primary impact.)

-the other Doug

[helvick]

Thanks, dot.dk.
Yes I was referring to the meteorite found next to the heat shield.  Reading dvandorn's post about crater size vs meteor size made me wonder about the meteorite calmly sitting on the flat plain with no obvious crater or ejecta nearby.  Perhaps it is a fragment of a larger meteorite that skidded to a stop there without much of an impact. 
Gray

There are a lot of rocks in the solar system in orbits that intersect in all sorts of different ways. Some meteors approach Mars relatively slowly and are slowed to a relatively slow speed (I don't have an exact figure, but possibly as slow as a few hundred miles per hour) by the atmosphere.

Cross posting from an earlier post of mine.

You can work these things out and the numbers are interesting.

vt=sqrt((2*W)/Cd*Rho*Area))

W=weight, mass*g. For a basketball sized lump of Nickel Iron mass will be ~ 60kg.
g on mars ~ 3.8m/s^2 I think
Cd - Coeff of drag, 0.7 seems acceptable for a lump like this.
Rho - Atmospheric density, around 12g/m3 at Meridiani give or take a few grams.
Area - 0.046 m^2 ssuming it is basketball sized
So vt ~ 1000m/s or 2500mph for those who haven't gone metric.

Then punching this into http://www.lpl.arizona.edu/tekton/crater_c.html
gives a 7-10m crater.

Assuming it is a meteorite that impact velocity will be more or less independant of the initial velocity of the object relative to mars - atmospheric drag will reduce anything faster than 1000m/sec and Martian gravity would accelerate anything that was slower in the very rare situation that the initial approach of the object to mars was somehow slower than 1000m/sec. Typical Martian asteroidal impacts are reckoned to be about 7km/sec and even a retrograde object of that size directly smacking into mars head on at 50-70km/sec would be slowed to the 1000m/sec terminal velocity. Gravity alone would accerate an object to around 1000m/sec if it could simply be dropped from a stationary point 150km above the martian surface. All this is assuming we're talking about objects of the same shape, size and density.

The numbers change significantly as the objects get bigger but it is entirely reasonable for the heatshield meteorite to have hit somewhere nearby, made a smallish hole we couldn't see and bounced\rolled to where it was found. Fram could even be a candidate in terms of size.

An interesting question which I don't know the answer to is what the initial size of such an object is likely to have been - assuming nothing ablated\melted away and assuming it did start off at the typical 7km/sec mars asteroidal impact rate it would have lost enough energy to raise its temperature to around 1340C which is fairly hot but not above its melting point, provided it was averaged all of which are fairly unreasonable assumptions.

As far as scaling with size is concerned the effects change dramatically so a 3m diameter Nickel Iron rock would weigh about 113 tons and hit at 3.8km/sec giving a 200-250m crater, and a 30m diameter version weighs in at 113million tons, hits terminal velocity at around 12km/sec yielding a 20 to 25km crater.

Helvick