Another example of a meteor exploding and raining debris
Tagish Lake

Yes, if you look at the collection of meteor bits that nasa has collected, it will be apparent that small chunks can make it to the surface. If anyone has any doubt about that, start looking for the bits that have been found in antartica. They can be quite small. And I'm sure smaller peices make it. They are just really hard to spot from a moving snow mobile. That is the method used to search for them.

The more important part is the velocity that they strike with. I beleive that on both earth and mars that they must slow to terminal velocity to survive the lower atmosphere. That will make the calcs that helvick made particularly relavent. The crater size can be used to back calculate the impactor size of a given matterial type.

Thinking about this a little further...

The antartica model works pretty well for meridiani. Things that are out of place really have very few possible origins. Falling from the sky is the prime choice. The difference is that they don't find craters in antartica, only meteors. The meteors they find in antartica are usually very old compared to the craters that opportunity found.

So what theories are still on the tabel.

1) meteor.
2) lightning.
3) edl / cruse stage debris.

Can't think of anything else. Numbers 2 & 3 are seeming increasingly unlikely to me. Number one is very unlikely, too. The antartic model would suggest that the amount of debris showering down on meridiani should be quite small. The famous mars meteor with "fosil life" sat on the ice for 13,000 years. Without another choice, it would seem that opportunity must have been extremely lucky to have come across this find. Seems like really long odds.

This is a really good one - not only is some of the debris more or less the right size but lots of it got to the ground, and many of the parts are very close together, multiple examples in this map are <100m apart on a debris field 16 x 5km.

Tagish Lake Debris field

And, contrary to my assertion that the debris objects need to be strong to survive, many of the meteorites collected are soft enough to crumble.

Now it still seems unlikely that Oppy would stumble across something like this because it seems that the little craters must be recent.

However are they really that recent, if they could be many thousands of years old then it might not be that unlikely that Oppy would come across them occassionally.

Does anyone have any good estimates of deposition\weathering rates at Meridiani?

OK, let's say that they're the result of a friable meteorite impact, probably a carbonaceous chondrite - where's the evidence on/near the surface, and what should we be looking for?

I still think we could be looking at something coming up from below the ground, but in the absence of a closer view we could speculate a long time - can we hope that Oppy will go back for another look?

There is a rim on the crater. Unless something is digging it's way out, I can't figure what could cause it from below.

I think, if the sediment is falling into some void below, one could calculate the expected diameter and depth of the resulting pit. The angle of repose of typical sediments under earth's gravity is about 35 degrees. If that angle is independent of gravitational acceleration, one could simply sweep a 35 degree cone from a depth equal to the estimated depth of the sediment, to approximate the expected diameter of the crater.

I have found some research suggesting that the angle should become steeper under lower gravity conditions, but empirical observations of these small craters in the MER images suggests angles approximately similar to terrestrial ant lion pits in sand. Regardless of such arcane musings, wouldn't one expect the bottom of such pits to be near/at the base of the overlying sediment? Since these small craters only penetrate to a fraction of the overburden depth, can they seriously be attributed to subterranian sapping?

The "fractures" that Opportunity visited after leaving Eagle crater show geometric properties suggesting drainage of loose sediments into subsurface fractures, but they are clearly highly modified by surface transport of the sediments, and really obvious features suggesting drainage were not clearly visible to the rover.

Certainly, if the mini-craters are the result of impact, we'd expect to see the impacting body nearby or debris from it, but they're not evident.

I think that we'll end up with several published papers modelling and trying to explain them, without conclusive results, but with a "preferred" origin.

CosmicRocker's observations lead me, once more, to wonder whether it's not sapping undermining the surface but something from below gently blowing the fine silt away...

The tiny craters are not that far away. I say after we get unstuck, we go back and rototill one and see what we find. Otherwise it is going to just be speculation on how they formed.

Mob rule! We'll storm the control center at JPL and take control of the rovers. We're going BACK!!! HA, HA, HAAAA!!!

On the Deep Impact website there is an amazing video about a "High-speed digital sequence of a vertical impact by a copper sphere traveling at 4.5 km/sec into porous pumice (density of about 1g/cc)". The caused hole looks similar this tiny crater in the dune.

(below on the site) http://deepimpact.jpl.nasa.gov/science/cratering.html

(Tiny crater on Mars) http://marsrovers.jpl.nasa.gov/spotlight/o...y/20050427.html

Sol 503 navcam released a round panoramic view of the landscape (still unchanged since the beginning of this thread).


Two tiny craters are visible:

here (right bottom) (top centre).


here top left.

This discution is somewhat mixed with the one on Round Stain Near Purgatory Dune although there are no evidences that the two objects are linked.

Obviously, the tiny craters are formed by martian ant-lions, also known as doodlebugs.
Since we haven't seen martian ants, the ant-lions can only eat in the season when the ants come out, so they must be hungry....
The full sized ones can only live in deep, thick sand deposits. If Opportunity crosses a thicker deposit, it might fall into a trap-crater and be EATEN!.....

<and if you believe this, you've been reading too many articles at www.enterprisemission.com>

Perhaps these are the Martian equivalent of the Sarlac Pit from "Return of the Jedi." ON this scale, Jabba the Hutt will be about the size of a caterpillar.

<TOOTHY GRIN> mm.... Snack sized Jabba worms!

I went to check on that www.enterprisemission.com website estrick mentioned, and even though I noted the Star Trek reference early on, I dindt get it until I reached the page where Iapetus were shown together with the Darth Vaders evil round spaceship and started laughing until I had tears in my eyes.
Wonderful satire site bookmarked, thank you for the link edstrick.

Here is an image returned by Opportunity on July 20:

Navcam image

That *has* to be an impact crater in the center of the image. It even has ejecta aligned circumferentially in the crater walls.

I'm no expert as to the focal length of the navcams, but this thing looks like it might be three or four feet across -- while not "tiny," it's also a rather inconvenient size for a primary impact crater on Mars

Even disregarding size, this thing looks very, very much like a secondary crater to me -- the impact wasn't energetic enough to push the largest pieces of ejecta out of the crater. Although I admit that the rest of the ejecta may be covered up by the drifting.

Isn't it interesting, too, that the ejecta arranged circumferentially around the inside of the crater looks a lot like the small, dark rocks we see littering the ground in between drifts?

I'm starting to think that this whole area has been showered with relatively low-velocity debris. A number of times. And if the freshness of some of the smaller craters is any clue, it seems to happen regularly, the latest perhaps quite recently.

So, the next question is: Where is this debris coming from? Impact? Volcanic expulsion? Other? And how can *any* of these processes account for the relatively large number of small craters seen, in varying stages of degradation (and, therefore, of varying ages)?

-the other Doug

Tman, thanks for reference to Deep Impact cratering videos. Not only do those craters look like the small feature we see on Mars, but the dynamic ejecta cone is more or less identical to what we saw on Temple 1. Glenn

I think that the rest of debris are caused by the meteorite impact or a big stone expulsed by a nerby volcan are sepulted by the wind sands. Other possibility is that these artificial mini craters are caused by some kind of underground water filtration.

Rodolfo

Looks like Oppy's found another micro crater:

1N182876287EFF62OUP1834L0M1.JPG

Seems like it's been a while since we've seen one of those (or maybe I haven't been paying close enough attention)...

That's a really good one, wonder if they'll try to pke around in it with the MI?

I noticed that one, too. There have also been two craters seen recently on the dunes between Oppy and Erebus.

--Bill

since its so small, the impactor is probably buried shallowly under the sand below the divot, if they could use part of the arm's instrument assembly to drag the sand away and exhume the impactor, we could probably be able to get an MI of it.

So would that create a bunch of dust, too dangerous to risk?
but wouldnt RATing do that anyway?

The new micro-crater is certainly much more like an impact crater than the other ones. It's still a very odd structure - for example, how can a dune made of fluffy dust have withstood that much of an impact with no other effects? No cracks, no slumping, no damage at all...

And the sphericity of the crater bowl is really remarkable - it's not just a hole gouged out of the surface (like the mess the heat shield made). Somehow, rocks from the sky seem so unlikely (yes, I know, I know).

I'm trying to think what might have made the effect we're seeing, and I'm beginning to wonder if we might be seeing the effect of (wait for it) hailstones. It was previously suggested that the atmosphere of Mars would actually permit rain - so, why not hail? If a hailstone, or a fluffy lump of snow landed, then sublimated away, would it not leave a sort of blast pit?


Bob Shaw

I cant imagine enough convection on mars to keep a raindrop elevated long enough for a good sized hailstone to form.

It beats me though - one would have thought that the dunes are active and that something like this wouldnt last long.

Are we seing the results of the cruise-stage entering behind the spacecraft? Small components making into the surface and bouncing out, and the resultant crater being 'softened' over 600 sols by wind?

Doug

These damn things are really curious. When we first came across them I was convinced they were created by impacts, and possibly impacts from lander entry debris that Opportunity happened to pass by. But the rover couldn't possibly be so lucky or so clever to find its way to so many recent and tiny impact craters, unless tiny impact craters are very common on Mars.

But Spirit has not observed these things, as I recall. I really have a hard time imagining them as being caused by underground sapping, but I think some images by Opportunity from not so long ago displayed some tiny craters that were aligned with bedrock fractures seen nearby. I'll try to find those observations, but it may take a while to sift through this darn image archive.

It's just hard to imagine subsurface sapping that would extend up into a fluffy drift and create a dimple in its side or crest.

If they're from impacts, do they have to be from *meteor* impacts? What is the largest grain size particle that can be lifted to any significant height by Martian winds? Especially by mega-dust devils, which might pull small stones kilometers in height and toss them out more kilometers from the top or side of the funnel?

Perhaps there are localized extremely high winds or pressure gradients within Martian dust storms, not easy to observe from above, which would tend to pick up stones large enough to create these dimples?

I'm just trying to think of *anything* that could toss a small stone into a fluffy drift and make a nice little dimple "crater"...

-the other Doug

What is the terminal velocity on mars of a quarter-inch diameter drop of rain? I'll bet it is pretty high.

Scott

Assuming it's ice and not rain - 123m/sec given the martian atmospheric density (12g/m^3), g (3.822m/s^2) and assuming a drag coefficient of 0.7. Mass would be about 8.2g , Terminal KE 62J. This Hail analysis paper states that on earth you would need a 2.5" diameter hailstone to get the same amount of energy.

I can't find any data on what that would do to loosely packed sand\dust drift but I'd be very surprised if it couldn't puch a hole at least as big as this one.

There is one thing that makes it highly unlikely, if not impossible, that rain drops or even hail stones form in the martian atmosphere: the incredibly low water vapor content. The amount of precipitable water in Mars' atmosphere can reach up to 40 mikrometers (if you condense all atmospheric water vapor on the surface, eg.: http://www.physicstoday.org/vol-57/iss-4/c...s/p71cap1.html). That is about a million times less than on Earth. Before a hail stone forms you need a lot of supercooled liquid water in the atmosphere, too. That water would be visible as huge convective clouds, and these have not been observed on Mars to my knowledge (which is understandably due to the low water vapor content). Cirrus clouds which are frequently found, are not able to produce hail stones, only snow. And that snow would quickly sublime away on its way to the surface under current consitions.

Michael

Must be something between 600 and 800 km/hour. But i do not believe it rains on Mars. There's no evidence for that whatsoever. The change that a liquid drop of water appears in the atmosphere of Mars is very close to zero. And IF it can form, it immediately would freeze to form hail. But if that does, it would not be the only hailstone would it ? If hail could form and could make craters like this.....the place should be littered with it i'd say. So that one isn't plausible.

Debris of the cruisestage isn't either for me: the size distribution of them (the couple we saw) is too uniform.

Impacts from outer space can't convince me too: The pace at which things tend to flatten out (dune formation, infilling of even oppy's tracks within months) at Meridiani are much to fast to let impact scars of this size existent for longer than, let's say, years. If one would have been spotted, OK. But we saw several within miles, which makes it not very valid to think meteorite impacts made these small holes: Meteorite impacts are VERY rare nowadays. Even on Mars.

Matter of fact is: these holes are misterious !

My gut feeling says it must have something to do with a process within the surface. Resettling of particles ? Sinkholes (into voids in the evaporite layer after a sufficient amount of loose material accumulated there) ? I hope they'll find out.

Good points. Something has been nagging me a lot though about the assertion that ice would always sublime rapidly on mars. I've seen anecdotal articles (like this one and slightly more official ones like this article from NASA that touch on the issue but I can't find any clear explanation of why we need to assume that all ice will sublime "rapidly".

My understanding [ from this Phase Diagram of Water for example is that at typical Martian atmospheric pressures (~600Pa\6mBar) the sublimation temperature is around 240-250K. That pretty much rules out any (pure) ice lasting for long on the surface as the daily temperature rises well above 250K everywhere but the arctic regions. However it seems to me that the atmospheric conditions should be pretty conducive to keeping ice\snow frozen (ignoring for a moment the (im)possibility of it occurring in the first place).

This atmospheric temperature\pressure profilechart for Mars from Stanford shows that the temperature is (well) below the relevant sublimation point as you rise up through the atmosphere.

Thoughts?

Well, even though temperature may be below the relevant thresholds, there always is an exchange of water molecules between the surface (water surface/ice crystal) and the atmosphere. If you leave a glass of water standing outside and the vapor pressure of the water vapor above the water is below the saturation vapor pressure (ie. relative humidity is below 100%), more water molecules escape to the gas phase than return to the liquid phase. Thus, the water would slowly evaporate even though temperature is well below boiling point.
As there are no "low level" clouds at the equator on Mars, it is quite safe to assume that realtive humidity is below 100% below approximately 10-15 km. Ice crystals (depending on their shape) usually have a very large surface compared to their mass. Therefore, they have a much higher loss of water molecules than more compact objects. They would have a lot of time to completely sublime on the 15 km to the surface. But the main reasons are the low realtive humidity and their large surface.


The atmosphere would be extremely dry at 250 K. Under such conditions the saturation vapor pressure is 1 hPa - 15% of the surface pressure. But, there is only approximately 0.002 hPa of water vapor present, a relative humidity of 0.002 hPa / 1 hPa = 0.2%. Therefore, snow would not last long under these conditions, as described above. At lower temperatures, eg. 200 K, the saturation pressure would be a lot lower, ~ 0.001 hPa (saturation vapor pressure decreases exponentially with decreasing temperature). 100% relative humidity is quickly reached and fog would form (as is often observed in winter even at the equator). Then snow/water ice would survive on the surface, as is the case at the poles.

Michael

I realize that you've probably looked at this microcrater closely, and I don't want to do the Bugs and Bunnies Contingent trick of a 700x enlargement so we can look at the Martian Crinoids, but...

Take a look at this enlargement of the microcrater. In the bottom, there is a flat (horizontal) deposit of light, fine material (probably the eroded evaporite dust blowing by and deposited by still air). And on the lower edge, there is what looks like an incipient slump feature-- you can see how the base is being pushed out.

We need some MIs of this feature...

--Bill

I recently read in a Nature article that the plains' ripples have been inactive for quite a while (http://www.nature.com/nature/journal/v436/n7047/full/nature03641.html). According to that article they were active in the "more distant past". And "recent" (referring to this article: http://marsrovers.jpl.nasa.gov/spotlight/o.../20050427.html) could mean anytime between now and 100 million years ago. That is a fairly long time span and it would not be surprising to find some very small impact craters in this area which is so smooth and without rocks strewn allover the place like at Gusev. Also, these little things are quite small and would not provide much shelter for loose dust. Certainly, some of them may be covered by dust (only visible as a dark patch or not at all), but others might have gotten exhumed during a recent dust storm. Small meteorites are decelarated quickly until they simply drop out of the sky in free fall. To me this still is the most likely explanation.

Michael

The mini-holes that I have seen are usually on the top or close to the rift. It strikes to my mind that these holes are made by the under sand process to fill any internal not well compacted.

I don't feel comfortable that these holes are made of:
- hailstones : If not, Mars would have life!
- MER landing debris: Ask NASA. It seems strange if some loose piece had lost to that?

The other possibility that sounds logical to me is:
- meteorite : hole has no rims. But...if after the impact, this hole would be bigger and with rims but after many soles, perhaps, millions, the hole might have shrunk to that...

According to my Earthly off road experience over dunes (kilometers and many days of surfing), I have not seen a such thing like the Mars hole....Maybe, I haven't pay much attention on that but the next time, I will be looking for these...

Rodolfo

Thing about that though, meteors are fairly common - there's a constant flow of interplanetary debris, most of it just tiny pebbles or dust. With Mars' very thin atmosphere, there's a better chance of tiny pieces reaching the surface. Once they get very small from the little bit of atmosphere that there is, wind resistance might slow them, so they wouldn't outright plow into the ground at hypersonic speeds, but just more or less fall quickly. That would kick up something like what we see.
Or at least, in my mind that's how it plays out.

Well, I have kept quiet the last few days just to see if you guys could figure it out yourself... But now it is time for me to reveal the truth about these micro craters!


They are secondary impact craters.
The primary impact very likely to be the MER heatshield or other hardware.

In the Mars atmosphere and gravity tiny particles will travel long distances and will impact with serious velocity. The low angle of attack of these craters proofs that the impactor didn't originate from space (or high altitude).

Cheers!

There was the slightly heretical theory about mini-comets being observed hitting Earth's upper atmosphere... ...I wonder whether we might be seeing such a process on Mars?

I'm firmly of the conservative school in these matters (100MY old dust drifts don't come as a surprise to me, and I laugh in the face of the noodle-spotters), but these micro craters are *so* strange that they almost insist on a strange cause!

Oh, for a back-hoe!

Bob Shaw

Yes, and also the fact that this and the last set of craters were aklso:
- very close to the top of the dune
- about the same exact size
- had the same sort of 'pushed-out slump' at the lower part.
...hmmmm....
lets dig it out from below and MI the cross-sections as we go along

It's true that the martian atmosphere should allow more interplanetary debris to get through but it's not as ineffective as you might think. The atmosphere is actually a bit higher than the earths (~100-110km vs ~90km on earth) with a larger scale height (11km vs 6km) and so is not really any thinner than the earth's at the altitude where most of the meteor interaction effects take place (specifically the initial shock and deceleration), especially the faster ones. See this report which gives some charts detailing the height measured for the Leonids in 1998. The majority of the action for the higher speed events happens above 75km where the earth's atmosphere is about as dense as Mars' is at 55km. For the slower post Leonid events a more significant number happens right down to 35km where earth's atmospheric pressure is higher than that on the surface of mars which would indicate that a high percentage of meteors like that would reach the surface although that number would be affected by the additional distance the meteor(ites) would have to travel through the deeper martian atmosphere.

So it's safe to assume that there will be more impacts on mars than on earth but it isn't easy to estimate just how many there will be. While it isn't easy there are certainly some bright sparks out there (probably even here ) who could make a good attempt.

The odds against finding craters in a random 6km x 10m box on the surface entirely depends on just how slow the erosion processes actually are. Figure out the first number and the fact that we've seen +- 10 should tell us quite a lot about erosion\deposition rates. I suspect they are very, very slow.

How about a volcano. Or a steam eruption.

ed

The distribution pattern is not consistant with any of the hardware impacts.

There appear to be more of these "mini-craters" in the sol 617 images.

This one:
http://qt.exploratorium.edu/mars/opportuni...N8P1600R0M1.JPG

This one could be the same as the one from sol 616 (I think, not sure), but seen from farther away (upper left of image):
http://qt.exploratorium.edu/mars/opportuni...N8P1600L0M1.JPG

And this one (also upper left of image):
http://qt.exploratorium.edu/mars/opportuni...00P2400L2M1.JPG

As RNeuhaus noted, these seem to usually be close to the top of the drifts, plus their generally similar size range, which also makes me a bit less than certain that these are simply from impacts (but if not, then from what)...?

So where did the cruise stage remains impact then

Doug

Thanks, Paul, I noticed those too and wondered if I was hallucinating.

Another one is on top of the big dune on the left:

http://qt.exploratorium.edu/mars/opportuni...00P2400L2M1.JPG

Lots of questions, lots of theories with frayed edges...

--Bill

Maybe there on the upper part of a dune they last a longer period than between or lower part in the dunes. Maybe the actual conditions fill only holes in the lower part. Therefore we see more of them close to the top of the drifts.

Perhaps these craters are aeolian features. The ripples appear to have a consistent "desert pavement" of blueberries and other fragments. However, if there is a statistical fluctuation in the distribution of the blueberries, this could result in the localized wind deflation of the fines, and a lowering of the ripple surface until the berry pavement re-establishes itself. I can visualize that this erosional point could be circular and bowl-shaped.

I'm not 100% on this idea, but it's a "bunny in the cloud" I'd like to toss out. We really need a close look at one of these critters....

--Bill

Probably up range. And probably not much of it. The entry vehicle is is much more massive and aerodynamic. Those small pieces would not make it this far. They are much more likely to melt and have a higher drag/mass ratio.

Nope, you're not hallucinating, unless I am too!

Thanks for pointing out the other one, I missed it before. Looks like one, but wish we were closer. And a bit larger too perhaps, given the distance to that dune.

I've just become rather curious now about these features (among so many interesting ones of course). I also would really like to see some MIs of these things.

Paul