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http://marsrovers.jpl.nasa.gov/spotlight/o...y/20050427.html

Opportunity Discovers Tiny Craters on Mars

[CosmicRocker]

There is a crater diameter calculator here that might be useful. I've played with it, using various plausible scenarios, and managed to "create" craters with similar diameters.
http://www.lpl.arizona.edu/tekton/crater_c.html

I think the collapse feature idea would also have a problem explaining the small diameters of these two craters, considering the depth of the sediment below them. I too, am thinking along the lines that these are related to Opportunity's EDL sequence. These are very recent impacts. I can't imagine that they are more than a few years old, at the very most.

The fact that we have observed this pair of tiny, fresh craters not all that far from the descent and landing zones seems significant. I couldn't find an EDL sequence timeline with enough information to be useful, but I know there are quite a number of pyrotechnic devices that explode to release various parts along the way. It doesn't seem too far-fetched to imagine some miscellaneous fragments being accelerated in this direction.

This might be a long shot, but the heat shield impacted, what, a couple of kilometers north of here? Could that have launched secondaries this far in Mars' reduced gravity and thin atmosphere?

But, could they also be the result of natural meteorite impacts, or secondaries. I don't see why not.

[helvick]

This might be a long shot, but the heat shield impacted, what, a couple of kilometers north of here?  Could that have launched secondaries this far in Mars' reduced gravity and thin atmosphere?

But, could they also be the result of natural meteorite impacts, or secondaries.  I don't see why not.

I think the numbers I posted in my earlier reply show that they are very unlikely to be direct meteorite impacts but could very easily be secondaries. The big question for either case is age - I agree with the Edward Schmitz 100 year limit on the age and it may well be a lot less. It would take a very big impact somewhere reasonably close to do it though - does anyone know of any likely candidates from any of the orbiter imagery?

For debris from the heatshield - I seem to recall that we were told that it hit at around 200km/h (55 m/s). Assuming that the maximum velocity of any debris is 200km/h maximum range of the debris is given by v^2/g where g is 3.822 on mars. That gives absolute maximum range of around 800m. I think that might just be in range given the likely margin for error.

However even though the martian atmosphere is very thin it still has some effect. At ~50m/sec we need a piece of debris around 10cm diameter to make our tiny crater, that will weigh around 1kg. I have to recheck my calculations but it seems that the drag deceleration will amount to about 0.1m/s/s and that is proportional to v^2 so it will decrease as the object slows. The piece of debris reaching ~800m is in flight for around 10 seconds so I'd guess that we'd see a maximum range reduction of around 20-30m at most.

[Bob Shaw]

If the tiny craters are impact-related, where is the impacting object?

"Object not found", eh?

[dvandorn]

The one big problem I have with all the nice math above that calculates the minimum speed of a primary impactor is that, according to those numbers, nothing of *any* size that reaches the surface will be going slowly enough to avoid being vaporized upon impact.

And yet, Oppy found an iron meteorite. Just sitting on the ground. Didn't even dig a hole.

Which means that the numbers above *cannot* describe every possible primary impactor. Those equations fail the test of explaining observed phenomenah.

As I have said on a number of occasions, many objects break up explosively under entry heating conditions. Some of those objects are ejected from the "parent" body in such a way as to reduce their velocity relative to the ground to *much* slower speeds than are calculated above.

For example, on Earth, there is absolutely no trace of the main impactor that creted Meteor Crater in Arizona. However, there were literally hundreds of small (fist-sized, usually) chunks of unvaporized meteorite found *outside* the crater. Some of them don't apear to have been shocked or anything. So, while the main impactor that hit Arizona was vaporized, smaller pieces of it were shed during its descent in such a way as to reduce their impact speed and allow them to strike the ground and remain intact.

In other words, if a medium-sized body enters Mars' atmosphere at a shallow angle, it *can* break up explosively and some of the fragments can be slowed by the energy of the breakup (relative to the main body's motion) such that small pieces can drop to the ground at *much* sower speeds than that of the main body.

It just seems to me that if we observe a meteorite just sitting out on the ground, we have to include in our discussion of impactor sizes and terminal velocities events that will allow what we observe to actually happen.

-the other Doug

[helvick]

Doug,

I agree - the simple calculations do not characterise all possible events but they do give an idea of the probabilities of finding recent intact small objects or craters from them. Even on Mars they should be very rare compared with the Moon for example as its very thin atmosphere is still plenty thick enough to act as a substantial shield.

The Heat shield meteorite could be a fragment as you describe or the small remnant of a much bigger original. One thing to remember about it is that it could have been around for a very, very long time as it would be extremely resistant ot weathering. It could also have been the source of something like Fram but ended up bouncing out and rolling along before ending up where it is now. I also think that there may be a process similar to boulder heave that could operate on Mars which would tend to push buried objects up to the surface over time. I don't think it's outrageous to suppose that the Heat Shield meteorite could have been in more or less this spot for millions of years, maybe even tens or hundreds. Once timescales of that length are considered then even improbable events can happen often enough that evidence of them will be likely to be found.

The fragments from the Meteor Crater meteor that are found outside it are secondary debris and that is what I think the two tiny craters are most likely to have been caused by. The only problem with that is since they are so small and are just little holes in sand dunes they really have to be quite young. I'm curious about whether anyone knows of anything that looks like a very new crater nearby. It is possible that it wouldn't have to be near at all too - a big impactor hitting at 2500 m/s would eject material that could travel ballistacally a very long way (>1000 km).

Your point about meteors breaking up/exploding in such a way that some of the fragments will reach the surface intact seems plausible but I don't think it will be a common occurrance. All parts of the inbound object have tremendously high kinetic energy, as I pointed out in the earlier message for small objects it is greater than the atomisation energy required to fully reduce the object to its component atoms and it has to be dissipated one way or another. Yes the objects explode and break up but I don't think it _usually_ makes any difference, the resulting debris items still have to disippate their kinetic energy and one of the basic rules are that smaller objects will burn up faster than single large ones. Also if an object fragments its effective surface area rises significantly which causes the decelerating forces to rise making survival less likely. That doesn't mean it can't happen, just that it wouldn't happen very often.

I'm off to see if I can find more data on this. I'm very intrigued.

[dvandorn]

I'm curious about whether anyone knows of anything that looks like a very new crater nearby. It is possible that it wouldn't have to be near at all too - a big impactor hitting at 2500 m/s would eject material that could travel ballistacally a very long way (>1000 km).

There is a suspicious, very dark splotch to the east and somewhat to the south of Oppy's original landing spot. I don't have the images in front of me right now, but my (admittedly non-perfect) memory says that it's something like one-third of the distance we've already traveled to the south, and something like one-quarter to one-half that same distance to the east. Again, I'm reaching with my memory, here, but I seem to recall it's just a little east of straight north of Victoria, and maybe 1 to 1.5 km north of it.

The only other really dark splotches we've seen are the marks made by heatshields hitting the ground at high speed. I know there was a discussion in this forum, back before the trek to Victoria won out as the extended mission objective, that this dark splotch might well be the impact point of the cruise stage, or at least of the largest piece of the cruise stage that survived to the ground. There was a certain amount of sentiment for visiting that feature before going anyywhere else.

Depending on the orientation of the tiny dune craters in relation to this dark splotch, it night be a possibility that whatever made the dark splotch could have ejected a pebble that traveled as far as Oppy's current position...

-the other Doug