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

Opportunity Discovers Tiny Craters on Mars

Looks to me like there are a few more around as well:

http://nasa.exploratorium.edu/mars/opportu...DIP1992L0M1.JPG

We may even have driven right over one:

http://nasa.exploratorium.edu/mars/opportu...DIP2531L2M1.JPG

I can't find a hazcam image that shows it, though.

Chris

Well found ! I'm starting to think that it might not have anything to do with impacts. I don't see any elevated rims on these ones. Maybe the evaporite has voids where the topsoil settles in. Ancient karst process ?

I think most of the dimples we see are, in fact, meteor craters. The ones without raised rims are just older and more eroded.

However, I also think there is plenty of evidence for sinkhole or karst-like structures out on the plains -- especially the cracks like Anatolia. But while it seems obvious that there are voids down in the evaporite, I think we'll find that these voids are arrayed beneath the weathered concretion layer (the dark soils out on the plains) as a series of cracks defining polygonal crack patterns caused by the drying-out of the ancient seabeds. Such crack patterns are visible in a lot of places on Mars -- they're just filled and covered over here at Meridiani (for the most part) by the concretion erosion residue.

-the other Doug

I know that there is no chance in heck for this to be true, but these tiny craters made me do a double take when I first saw them. They look exactly like the point of contact between some sand dunes and lighting strikes from when I lived in New Mexico. While we didn't see the strikes that caused the little pits, we did dig one of them up and discovered a foot and a half long fulgerite.

Again, I'm sure we would more than likely spontaneously hear from the Beagle 2 than discover active lightning on Mars, but the resemblances between the pits struck me. I only wish I had pictures of those New Mexico pits to share.

...John...

I like that. Could they be lightning strikes. There are so many problems with all the other explainations that I've heard.

Does anybody know of hard data regarding lightning on Mars? I have some recollection about dust devils and lightning... Happens with tornados.

I have been racking my brain trying to come up with another explaination. I think this is worth considering.

Not sure - is an electrical discharge even possible in the martian atmosphere?

Doug

Didn't see any hard evidence of lighting of some sort on Mars. Anyone else did ?Earth lightning happens because frictional energy between large bodies of water vapour converts into electrical potential differences, which level off on contact (or near contact). I suppose the amount of water vapour (or any other electrical conducting constituent of the Martian atmosphere) is far too low to cause ionic channels for discharging energies, high enough to cause any noticable material displacement.

Didn't see any hard evidence of lighting of some sort on Mars. Anyone else did ?Earth lightning happens because frictional energy between large bodies of water vapour converts into electrical potential differences, which level off on contact (or near contact). I suppose the amount of water vapour (or any other electrical conducting constituent of the Martian atmosphere) is far too low to cause ionic channels for discharging energies, high enough to cause any noticable material displacement.

I imagine that dust devils might be a source for lightning. As a youngster I was a mad scientist and one of my creations was a series of Tesla coils, up to 1000KV. Discharges in the air were snappy and lightning-like, but discharges through a clear incandescent lamp (which do not contain a vacuum, but are filled with an inert gas such as nitrogen or argon at reduced pressure) are diffuse and aurora-like. Possible, IMO.

--Bill

Electrical discharges *are* possible in the martian atmosphere, but lightning is probably *not* possible.

A paper in Science in 1970 reported taking a simulated martian soil <dessicated basalt sand and limonite dust?>, putting some of it in a transparent jar, backfilling the jar with CO2 <dunno if they added any other gasses.. minor gas composition of the martian atmosphere was unknown then>, and sucking it down to martian surface pressure.

When they shook the jar of simulated Mars in the dark, IT GLOWED.

Charges were being separated mechanically, but the 1/2 of 1% Earth pressure gas couldn't support high voltages and the dust discharged with corona discharges.

Mars atmosphere is fairly near the pressures inside neon tubes and fluroescent bulbs, and can easily support diffuse discharges. This is actually a problem for exploration.. high voltage cannot be exposed to the martian atmosphere as it promotes arcing and discharges. Zap. Sput. Oops.

I suspect that the situation during global dust storms may be *interesting* in terms of electrostatic effects, too - in fact, anything with violent motion may have peculiar sparks and pops associated with it!

Anyone here ever heard of 'singing sand'?

Oh, and there's triboluminescence to contend with when you stress materials, too...

...maybe the hills really *are* alive with the sound of music (and lights! and action!)...

I know that dry sand and dust can produce electricity, but I'm not sure of any of the other requirements.

And that is an interesting thought... Is it beyond the realm of possibility (if one were to be writing something of a hard science fiction book) to have singing sand on Mars, especially with some of the massive temperatre changes?

If so, I wonder what it would sound like.

...John...

I forgot to add to my last post here: much like the "plasma globes" available nowadays. Filled with N, Ar, He, etc gas at reduced pressure with a high frequency, high voltage discharge applied, it has a very beautiful diffuse diacharge. I have one. I also have a Wimshurst static electricity machine. The plasma sphere glows a nicely with static electricty as with RF.

So electrical potential under Martian conditions may get "shorted out" before it can build up. The universe is an odd place, no?

--Bill

Have a look at this:

http://www.cosis.net/abstracts/EGU05/02197/EGU05-J-02197.pdf

sounds like at least small-scale lightning strikes might be possible

Now there you are, marooned on the top of a dune in an almost entirely flat sea of sand, and an electric dust devil comes your way...

...I hope Opportunity's PanCam Mast has a lightning-rod attached!

That'd just be weird. Struck by lightning on Mars while stuck in a sand dune. Time to break for the cover of Victoria Crater as quickly as possible?

Wow! Good work. I still wouldn't bet the farm on it. But I think it sounds more plausible than a rock that just barily made it to the surface from space.

If it were a meteor, there should be a normal distribution of craters. Few large ones, more mid-sized, and lots of small ones. But the distibution stops abruptly after a certain point.

The other thing is that after it left endurance, they inspected the tracks they made six months earlier. It showed significant modification. The landscape is active. That crater probably happened in the last few years. Centuries at the most.

Interesting discution about those strange little pits.

They look like the pits formed by certain insects to catch ants. Those insect pits are of course funnel-shaped, but I think that it those martian pits were made from sand flowing into a cavity, they would be larger, or much more blunt, from the thickness of the sand in this place. Such were the pits near Anatolia: much larger.

About electric discharges, on Earth they can occur at very high altitude, between the highest cumulonimbus and the ionosphere. But these discharges do not look like classical lighning: rather sorts of flames, and not very bright, this explaining they were discovered only recently. So there is an abrupt transition in air property, at a given pressure, far above Mars pressure: under this pressure, electric discharges are corona like, above they are lighning-like. So I think it is difficult to have large lighning on Mars, powerfull enough to make such pits.

About lighning pits observed by John M Dollan, I would be curious to see their shape: funnel without rim, or crater with rim? The explosion formed by a lighning would be very similar to the one formed by a meteorite, so the comparison is interesting. Anyway the smallest craters observed on the Moon had no rim, as there is a strong scale effect in crater shape: small craters above 10m are just holes, without rims, as the material would be ejected at distances far superior to the diametre.

So I think the pits are meteorites impacts, but as noticed by Edward Schmitz, their distribution is not what we could expect. But we have a very small sample, and also meteorites often travel in groups, so the pits could have formed together.

Pity they did not made close images, after certain images I had an impression that the largest pit contained a stone, which would had be thrown here by a larger impact, as the "bounce " stone. Simplest explanation.

From what I recall, there was no rim. It looked very similar to an ant lion's pit, albeit a bit larger (maybe 6 inches? It's been a LONG time...). There was a fulgerite directly beneath it, however, so I'm certain that it was formed by lightning.

...John...

Thanks John for your precisions. As I expected, your lightning impacts had no rim, as the tiny martian pits. So it strengthen the idea of small explosion pits, althought it does not disprove that those martian pits were lightning impact pits.

By the way it is about ant lions I was speaking.

As far as I know fulgerites are alway formed by lighning, they are tubes (often hollow) of glassy molten rock that we can find on the lightning path into the soil, so it is not astonishing to find them directly under an inpact, right on the countrary it makes surer that the pits you found were lightning impacts.

Richard

Wowowow, don't forget that there are more ways to make a hole in the ground. I don't believe discharges in the Martian atmosphere can be powerfull enough to displace material in this manner. Simply, because there's not enough molecules to make the amps (and thus watts, and thus Joules per second) go through ionized channels.

Since there's no rim around this features, my vote would be that it is material that somehow collapsed into voids underneath.

But i'm not sure.....

The freshest of the tiny craters we've seen in these dunes *does* have a raised rim, though.

I just have to figure these are little secondary impact craters, caused by small stones ejected from a larger impact somewhere nearby. Either that, or the freshest ones *may* be from stray bolts, nuts or flakes of paint that came off the lander's cruise stage and survived to hit the ground. But that last is *awfully* hard to prove.

-the other Doug

Do we have any idea about the movement rate of material in this region of Mars? How long would it take for such a small pit, for instance, to be filled in by wind-blown material?

If the time frame is not that long, and the pits *are* formed by material falling into a void beneath the surface, would that imply that there is some sort of active geology going on to open these voids enough to cause the material to fall in? Or could the pits be very, very old?

Conversely, what is the liklihood of very small meteorites making it through the thin atmosphere and to the surface on a regular basis?

...John...

I think that karst landforms were already suggested for the snaking depressions we saw previously, and that was because of the feeling that we were seeing something being undermined, and dropping. Could the tiny craters be (a) blow-holes from ice deposits which are boiling off or ( sorta crop circles (assuming that any are genuine at all, one suggestion is that they get formed by (ahem) whirlwinds!).

Oh, how the conspiracy buffs will like this one!

Bob, I lived in Sahara for three years, and saw many dust devils, and even entered in some (small). But I have never seem them forming holes like that. Dust devil are alway moving at slow but constant speed, in fact, and when they left traces it is rather "lanes" as the ones we see in Gusev. I can imagine that in a corn field they could form lanes too, but not circles!

But I never saw crop circles actually forming, so I cannot say something about this. So conspiracy buffs will think that I am part of the conspiracy... But i am not. But saying I am not is evidence that I am, etc. etc...

I don't have the images right at hand, but I have seen a few nearly-erased craters in the dunes, too. If I recall correctly, there is one very good example of a fresh crater and one very good example of a nearly-obliterated circular depression of similar size. Also if I'm not mistaken, they were taken from the same location, so they were located on fairly near-to-each-other dunes.

Unless we sit for months at a time at one of these highly drifted/duned locations and take a bunch of images over a fairly long baseline, I don't think we can state with any kind of certainty the time needed to fill in such a little dimple crater.


Some of the dunes near Oppy's current position show layering, so we know the dunes have built up over some span of time. And, of course, dune/drift formation is by definition a constructive process whereby new material is slowly built up onto the duneforms. So, depending on the speed of dune/drift formation (and dune/drift erosion -- it goes both ways, after all), these tiny craters could be years old or millions of years old. But since we see degradation of tiny craters, I'd have to think that really fresh-appearing tiny craters are probably not ancient.


You can tell from the lower limit of the size of what appear to be primary impact craters the size of impactors that get through the air. On Mars, these *seem* to get as small as a few meters, which can be made by marble-sized to fist-sized chunks traveling at cosmic velocities. The tiny craters we're seeing in the dunes are more likely caused by something smaller than a peanut, probably less than half the size of a shelled peanut, or smaller -- if they are primary impacts.

I'm really not convinced that these are primary craters, though. They look exactly like what I would expect from a pebble or stone ejected from a nearby impact, traveling at relatively slow speed, hitting the side of a powdery-sand dune. I would say the most likely thing to have caused the really fresh one, if it *is* a primary impact, would be a small survivng piece from the cruise stage that broke off and traveled downrange a bit. But again, that would be very, very difficult to prove.

-the other Doug

I don't suppose it wos the Mysterons whot done the blag, Cap'n?

(Shameless attempt to do Parker, m'Lady!)

Not too tough to prove - drive up to one, and use the RAT to brush at the crater. Hopefully something odd would turn up.

There is a rim on this hole. It's not big but it is there. It is not a collapse feature.

The cruse stage fragment is also extremely unlikely. Not impossible - but not likely. As the poorly aerodynamic cruse stage enters the atmosphere, it heats dramatically. This causes fragmentation which increases the surface area to mass ratio quickly. Only a very large, aerodynamic, heat resistant piece could ride as far down range as the lander did.

Remember that the space craft was travelling 12000 kph on atmospheric interface. It decelerated to 180 kph and was travelling nearly straight down when it deployed the chute about 5.5 minutes later. A quick calculation says that it travelled around 550 kilometers in that time. The vast likelyhood is that the debris field is 100's of kilometers uprange and if we saw anything from that, it would be the largest peices, such as a fuel tank.

One more possibility, could it be a fragment from the chute deployment or heat sheild separation? The ground track for entry was from the west but a tiny peice, like the one in question, could be carried by the wind while the lander had enough momentum to cut cross wind. Does anyone know what the wind profile was that they came up with. I'm pretty sure that was done.

Also, we do have evidence for how fast the landscape is changing. When opportunity left Endurance, they imaged the six month old and new tracks side by side. And there was a dramatic difference. That crater could not be more than 100 years old, in my opinion. millions is straight out of the question.

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. This will give a good aproximation of the size. The hole is most of the way up the dune. What's the grain size - we must know that by now. If no one can do these calcs easily, I can get them done, eventually. We might get through the calcs and find out that the object should be visible in the bottom of the hole. In which case we need to look for another explanation.

I'm getting the feeling that the electrical discharge theory is not holding up. I've heard pro's and con's. But I haven't heard anyone say anything with conviction. I don't know much about how lightning forms. Does it need a thick atmosphere to support it?

I really like this little hole in the ground. It's a mystery... I'll bet we can get to the bottom of it. Figuratively not litterally.

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.

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.

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...

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.

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

"Object not found", eh?

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

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.

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.

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

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

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.

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

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.

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.

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!)

And some more - bigger versions are available, if any crater-counters want to try counting, er, craters.

Nice summary Bob, I have to admit that none of the impact explanations really seem very likely, wherever the source of the theorised impactor.

I also would love for Oppy to go back and have a poke around - it might even turn up something that would explain why it trenched into the current dune rather than rolling over the top.

The only problem with the explosion hypothesis leading to small(ish) meteorites is that if it happens in the atmosphere the end result is a bunch or particles with at least the same total kinetic energy while the effective surface area increases significantly. This causes increased drag ==> more burning up\disintegration, etc. Certainly in some cases some significantly sized particles will make it through but the probability off survival to the ground decreases significantly if the object explodes or disintegrates.

Don't get me wrong, I know that despite what I've said above this must still happen, after all many meteorites found on earth have come from larger bodies that disintegrated in the atmosphere, but the likelihood of it happening must be very slim and that makes it unlikely that what we're seeing here can be explained in this way.

Bob Shaw's comments regarding the clustering seems to be pretty conclusive to me, if small craters can be explained by impact debris, and we're looking at a cluster in one spot then they should be very common. The NASA\JPL folks seem happy with that conclusion so I wonder if there is something else out there that we've missed.

Hi helvick,

I somewhat agre with the general conclusions about the previous posts, but:

1) meteorites are often not homogenous. For instance a carbonated chondrite may contain sand or small stone chondres. If such a meteorite disintegrates, most of its mass can never reach the ground (On Mars it would majke a dark splotch) while still sending a shower of grains or small stones which could reach the ground at significant terminal velocities.

2) a shower of meteorite debris often does not follow a gaussian statistical repartition, it can be on the countrary very inhomogenous. In space meteorites of any size often go by little groups. And when a meteorite disintegrates, it often does it by stages, breaking first in several large blocks, and each block disintegrating in turn in smaller blocks. This can make a very inhomogenous distribution of impacts, for instance a large range with relatively few impacts by kilometres, but with some clusters of closely linked impact. An exemple was (alas) the disintegration of the space shuttle in the atmosphere, where debris where often found by groups in some hundred metres, or scatered over hundreds of kilometres. So I think the argument of two crater close by does not forbid an impact origin (primary or secondary meteorite, or from the MER landing).

This example being a case in point:
chicago meteorite
Most disintegrated in the fireball but some fairly big pieces hit the ground. I don't fully follow the argument that you'd get a "dark splotch" surely most of the mass is completely disintegrated into microscopic dust that is to small to simply fall out of the atmosphere. It must be said, I have nothing to base that assumption on, I'm not finding much of relevance in my searches on the topic.



As above this seems to be borne out and proves that it is possible but does it seem likely that we'd find a small cluster of impacts in one spot and (almost) no others?

I've doubts about the shuttle wreckage as an example of a "typical" debris field, maybe it just feels wrong but I suppose the cruise\descent stage bits might behave like bits of shuttle.