[X] My Assistant

[Bob Shaw]

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!

[Jeff7]

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?

[Guest_Edward Schmitz_*]

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

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.

[Guest_Richard Trigaux_*]

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.

[John M. Dollan]

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. ...

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...

[Guest_Richard Trigaux_*]

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

[Marcel]

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.....

[dvandorn]

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

[John M. Dollan]

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...

[Bob Shaw]

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!

[Guest_Richard Trigaux_*]

sorta crop circles (assuming that any are genuine at all, one suggestion is that they get formed by (ahem) whirlwinds!).

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...

[dvandorn]

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?

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.

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?

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.

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

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

[Bob Shaw]

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

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

[Jeff7]

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.

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

[Guest_Edward Schmitz_*]

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.