[X] My Assistant

[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]

I would like to know if anyone can calculate the terminal velocity for a rock or metallic object on mars.  An object as small as the one in question would have to have slowed to survive to surface impact.  We would also need a calc on crater sizes for those objects at terminal velocity in sand and dust.

I've come up with these:
Roughly spherical body with Cd of 0.7, Iron (8000 kg/m3)
20cm diameter - 985m/sec (3546 km/h), 33kg, Terminal KE 16MJ, 4m crater
10cm diameter - 696m/sec (2507 km/h), 4kg, Terminal KE 1MJ, 2 m crater
2cm diameter - 311m/sec (1121km/h), 33g, Terminal KE 1.6kJ, 40 cm crater

Roughly spherical body with Cd of 0.7, low density rock (2000 kg/m3)
20cm diameter - 492m/sec (1773 km/h), 8kg, Terminal KE 1MJ, 1.8m crater
10cm diameter - 348m/sec (1253 km/h), 1kg, Terminal KE 1MJ, 90cm crater
2cm diameter - 155m/sec (560km/h), 8g, Terminal KE 102J, 18cm crater

Using Martian atmospheric density of 12g/m3 and assuming it's uniform, Martian g at 3.822m/s^2.
Crater sizes from http://www.lpl.arizona.edu/tekton/crater_c.html with loose sand as the target.

So the ~2cm diameter low density rock looks like a candidate.

However the minimum size of a meteroric object that might survive to hit the ground is fairly complicated to calculate accurately and beyond me but if you take the Atomization energy of a given body and compare it to the energy that has to be disippated as it decelerates from its inital velocity (7-11km/sec) to its terminal velocity as calculated above you can see that it takes a fairly sizeable object to survive entry. The requirements for survival are that an object is strong, big (initially at any rate), slow and comes in at a steep angle so it doesn't bounce.

For an Iron object with an initial speed of 7km/sec (which is very, very slow) the object needs to be > 10m diameter in order to reach the ground using the above assumptions. If that speed rises to 20km/sec then the diameter needs to be around 82m and would weigh around 2.4million tons. At 50km/sec which is getting pretty fast it needs to be slightly over half a kilometer in diameter.

Approximately. :-)

Looking at that I'm pretty certain there is no chance that the tiny craters are the result of any sort of primary impact, even from something that exploded or fragmented in the uppder atmosphere, even the fragments would burn up. Parts of the lander's various stages would do it but it seems unlikely to me that two would be found right beside each other. Secondary debris from a large impact elsewhere is definitely plausible but the timing seems to make that very unlikely too.

I'm really intrigued too and would love to see Oppy go back to them and poke around.

[Bob Shaw]

Perhaps the 'Lion Ant' notion isn't so far off the mark - at least in the way that such pits seem to work? We're seeing evidence of Opportunity having almost fallen through a duricrust and into a soft and powdery subsurface. What happens if there is a natural flaw in the more mechanically strong layer - could we see a pit gradually grow, with loosely cemented particles reaching disequilibrium on the slope and tumbling out of the upper layer and into the powdery froth below? We know that the surface can flow (Endurance Crater, current Spirit images) so can it also gradually drain into holes? I'm wondering in particular how hoar frost might work on the edges of a small flaw, gradually causing it to loosen and decay still further...

[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.

[Guest_Richard Trigaux_*]

Helvick and all,

there is an instance of a 20km crater 30 M years old in southern Germany, which formed tektites found in Moldavia, more than 1000 kms away. This is very sure, as those tektites, called moldavites, have the same composition than the rock in Germany. Such tektites, being some centimetres large, and falling on the ground at sub-orbital velocities, could be good candidates for the tiny craters.

Do not forget too that in Meridiany planum there are large craters with rays of tens of kilometres long. We still no not know how this looks like on the ground, perhaps it is a field of tektites with millions of tiny craters.

Anyway on Earth there are many curious things about meteorite falls. Large meteorites can vaporise in the air without traces (like the famous Tunguska impacter) while we can found (In China if I remember well) a hundreds tons iron meteorite intact. Most often, fragments are in the range of 1cm-1m. So calculations do not really account for all this variety of behaviours. Initial velocity, density and angle may be not the main parameter if the meteorite explodes.

I sincerey regret that the Opportunity team never tried to dig the sand and have some view through the tiny craters.

[Guest_Edward Schmitz_*]

...

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.

...

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

As near as I can tell, nobody was segesting that a small primary impactor couldn't survive to the surface. The farthest I will go is to say that a small impactor can't make it to the surface without being slowed to terminal velocity. I think that bounce rock definitely didn't hit the surface anywhere near orbital velocity. Even if it survived the atmosphere at those speeds, it would disintigrate on impact with the ground.

The fact that bounce is just sitting on the surface, today, doesn't mean that it didn't dig a hole. It must have dug a hole! It is sitting on a plain that is in the process of eroding away. As the rock around it erodes, it settles down. The hole is gone because the rock that the hole was in is gone.

The impactor at meteor crater was not vaporised. The vast majority of it is sitting hundreds of meters under the crater in small fragments. It is unclear how much of it was intact when it hit. And I have no doubt that some of it broke apart and hit at terminal velocity. Which all seems perfectly consistent with what is currently being discussed.

I like the discussion that's happening. We can raise ideas, make mistakes, and have some fun.

Thanks for the calcs, helvick. That's cool stuff. Haven't had time to digest it all. I will, though.

Ed

[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

[Guest_Edward Schmitz_*]

On the heat sheild impact kicking something up...

CosmicRocker was asking if the heat shield impact could launch something that far. Helvick made some calcs based on the impact velocity of the heat shield and put it at the limit of possibility.

It actually could have launched a small object much farther. The heat shield had a lot of energy. There are a lot of ways that that energy can be transfered to another object. If it has less mass, it can get a lot of speed. One way to do this is on impact, the shield deforms under it's own weight, like a spring. As it rebounds back into shape, it launchs a small particle. The particle will have the same speed as the rebounding heat shield. Most of the heat shield was not very elastic but there were at least three springs sitting on the ground.

Long story short... A small particle can achive a higher velocity than the impactor had.

I still don't think the small crater could have come from that. The rover travelled away from the site for a few kilometers. One would expect to find a distribution of debris with an inverse square relation to the heat shield. Finding the two peices that flew a few kilometers away and landed together seems too unlikely.

But then landing in eagle crater seemed unlikely, too. And landing on bounce rock just seems astronomically unlikely. So... Who knows. Opportunity has been a lucky rover.

[Bill Harris]

I remember the "mysterious dark spot" to the SE of the heat shield, and wish that Oppy had visited it instead of making speed records. For that matter, I wish we'd looked in the heatshield "crater" in more detail.

I doubt that the TinyCraters are related to Oppy's reentry debris. The immediate subsurface soil tends to be a lighter orangeish color and the small craters do not have that color. They are newer than the surrounding plain surface, but not new enough.

--Bill

[edstrick]

I have a suspicion that some of those dark spots are scattered debris from lower mechanical strength meteorites, like carbonaceous chondrites, which "detonated" into a shotgun blast of debris on the way in, leaving gravel banks on the landscape.

[Jeff7]

I remember the "mysterious dark spot" to the SE of the heat shield, and wish that Oppy had visited it instead of making speed records.  For that matter, I wish we'd looked in the heatshield "crater" in more detail.

I doubt that the TinyCraters are related to Oppy's reentry debris.  The immediate subsurface soil tends to be a lighter orangeish color and the small craters do not have that color.  They are newer than  the surrounding plain surface, but not new enough.

--Bill

Yeah, they talked like they were all interested in looking at what may be the freshest crater on Mars, but then turned back, because they thought that the crater was kicking up dust that got onto the one hazcam shield.

[Bob Shaw]

Attached are a series of images of impact craters on weakly-cemented dunes at my local Mars analogue site (raindrop craters on the beach at Ardrossan!). Unlike the Opportunity craterlets, they certainly *do* follow a normal distribution!

Which leads me on to the final nail in the coffin for the impact hypothesis on Mars: OK, small bits of debris *could* make tiny craters; they *might* be from the Cruise Stage; it's *possible* they are evidence of secondary impacts, or even of rays - BUT, three or four in a row, landing close together by chance after long and involved trajectories, in just one spot on a huge plain? Nope! If they were impacts, we'd have seen them elsewhere...

Occam's Razor suggests to me that there's something odd about the spot on the plain. Let's turn that MER and go a-trenching (the JPL chappies (and chappettes) made a pretty darn good one over the last few weeks, so they got the practice in!)

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