The baffling case of the tiny craters on the Meridiani dunes

[silylene old]

Jon, the great maps can be found here:<br />http://www.unmannedspaceflight.com/index.php?showtopic=681&st=855<br /><br />unmannedspaceflight.com is an excellent astronomy science forum. Hope to see you there, your geologic insight would be very well appreciated!<br /> <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[fossils]

Silylene, I put forth that these "micro-craters" are formed by a very tiny amount of liquid water wetting the surface/sub-surface for a very short period of time causing a compaction of grains. Repeated episodes of wetting create the "crater". I will try to explain.<br /><br />First we must understand the time scale. How old are these "micro-crater" features or how long does it take for the ripples to change significantly? 10years? 100 years? 10,000 year, 100,000 years? It is very hard to judge, but about 5,000 to 10,000 years seems right.<br /><br />5000 - 10,000 years - is a long time by our life experiences. Within this time period, the surface of Meridiani gets a frost now and again. The frost very quickly evaporates (sublimes) way since it is just a tiny layer of ice particles. But once in a while, the frost is heavy (by Mars standards) and likely it gets blown around where it collects on the leeward side of ripples and between the rocks (for example around the outside of Endurance). These tiny "drifts" of frost, are almost like dust and they usually evaporate way.<br /><br />Occasionally, during the 5,000 to 10,000-year period, the frost is extra heavy, almost like a snow. Maybe it is then covered by layer of dust, and/or is quickly followed by warm days. In any case, somehow, the wind blown pile of frost/snow melts a tiny bit, enough to wet the surface with liquid water, in a few isolated spots, under the cover of ice, under the cover of dust or on the surface of an upturned south facing rock, or at a spot here and there near the south facing tops of ripples.<br /><br />However the melting occurs, it occurs.<br /><br />Near the ripple crests, the liquid wets the surface only for a very short period of time, ONLY in isolated spots, but it is enough to cause the compaction of a small layer of grains. Over time if the process repeats in the same spot, the compaction creates the micro-crater. Once the micro-crater starts to form, the process may be accelerat

 

[telfrow]

Fossils! Good to see you posting again. <div class="Discussion_UserSignature"> <strong><font color="#3366ff">Made weak by time and fate, but strong in will to strive, to seek, to find and not to yeild.</font> - <font color="#3366ff"><em>Tennyson</em></font></strong> </div>

 

[mlorrey]

<blockquote><font class="small">In reply to:</font><hr /><p><br />- Why are most near the crests of dunes? <br />- Why are they generally similar in size? <br />- Why do they seem to be mostly in dunes near bedrock areas? <br />- Why have they been more frequently seen lately? <br /><br /><p><hr /></p></p></blockquote><br /><br />1) they happen typically near the crests of dunes for two reasons: a) the hammer always hits the nail that sticks out the most, b) you don't see craters in the valleys because the material there is rock/compacted, so craters there would either not occur (i.e. object hits and bounces) or would displace less material for a given impact energy than would occur when hitting the soft material in a dune peak.<br />2) similar size: this is a good question. It could be that these craters are produced by the hematite "blueberries" being picked up by major wind storms, which then drop when the winds die, and thus hit the newly created dunes. Since the blueberries are typically all of the same size range, they should also all produce craters of similar size.<br />3) why in dunes near bedrock: because that is where the blueberries originate: in the sedimentary bedrock.<br />4) why more seen more recently? We've also seen some dust storms and dust devils recently, which could be the mechanism that picks up the blueberries (as we've seen the dust devils have been cleaning the dust off the solar cells of the rovers, which is nice of them).

 

[fossils]

Hi Telfrow, I have been very busy lately, working to all hours of the morning, day after day. It would be nice to see someone discussing the amazing hematite on Mars and what it means. Just a few odd concretions in the rock, eh?

 

[telfrow]

<font color="yellow">Just a few odd concretions in the rock, eh?</font><br /><br />Maybe. Maybe not. <img src="/images/icons/wink.gif" /> <br /><br />I'm miss the old threads and the <i> good natured</i> debates that took place - something sorely missing around here right now. <div class="Discussion_UserSignature"> <strong><font color="#3366ff">Made weak by time and fate, but strong in will to strive, to seek, to find and not to yeild.</font> - <font color="#3366ff"><em>Tennyson</em></font></strong> </div>

 

[JonClarke]

Thanks!<br /><br />SDC is my home forum, and am trying to make it work. I also try to not post in too many fora. But if I get tired of here (or get kicked of for calling "ESASA_IS_A_LIE" a mythogical creature who lives under bridges crossed by domestic herbivores" I may well end up there on a regular basis.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

The problem with that is the extreme regularity of the craters. The only process I know that makes such extremely regular craters is high velocity impact. Or perhaps something that mimics radial excavation of material.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[silylene old]

Jon,<br /><br />About the impact hypothesis:<br /><br />I just don't think the microcraters would have a long lifetime, given the wind-driven erosional processes. Perhaps the lifetime would be in the range of decades to a hundred years (judging from observed erosional rates from oribiting photography of larger craters in other areas of Mars).<br /><br />Also some craters are not round. For example, the picture linked below.<br /><br />I also don't like coincidences. It just strikes me as unlikely that we would happen to stumble on so many fresh microcraters.<br /><br />Also, the microcraters are not evenly distributed. Only here. Not at other lander locations. Odd. Perhaps this could be explained if these were secondary impacts...but then where is the nearby big impactor?<br /><br />Here is an explanation I posted previously in the other forum, and am reposting here of my sapping hypothesis, from melting subsurface ices:<br /><br /><i>Summarizing all the proposed hypotheses from this thread, and my opinions (for whatever they are worth):<br />1. micrometeorites, perhaps from comet tails<br />- unlikely, these tiny meteors would burn up in the Martian atmosphere<br />2. micrometeorites, from larger meteors exploding in the upper atmosphere.<br />- possible, but I think we see too many microcraters to account for this origin<br />3. subsidence into holes in the substrate<br />- unlikely that pre-existing holes in the substrate would last 4B years without being already filled.<br />4. microcraters formed from secondary impacts 'tektites'<br />- most likely, of the impact hypotheses<br />5. lightning stikes<br />- very unlikely (?)<br />6. dust devils<br />- they wouldn't form pits<br />7. outgassing vents<br />- very unlikely<br />8. ant lions<br />- overactive imaginations<br /><br />I do believe that the aelian erosion with the winds and dust devils observed here is rapid enough that the microcraters are unlikely to remain more than a hundred years (and probably much less) without being re</i> <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[silylene old]

Welcome Fossils!<br /><br />Your hypothesis is interesting. But how wuold you account for pits on the <i>top</i> of dunes, near the crests? I think your hypotheses would be more likely if the pits were only observed in the lowest depressions. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[silylene old]

Jon et al.,<br /><br />I too am very happy to see scientific discussion again in this forum.<br /><br />I sure hope we members allow it to remain this way! <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[mlorrey]

Yes, quite so. I am now convinced that my idea that it is blueberries picked up and dropped by dust storms and/or dust devils are producing the microcraters. The blueberries are of very similar sizes, and so should produce very similar crater sizes. I'll email JPL's folks and suggest they excavate some of these craters looking for blueberries.

 

[silylene old]

Can't be the blueberries.<br /><br />Try this experiment at home:<br />Get a Bic pen tube, a BB gun, some BBs, a bowl filled with plaster powder, and a bowl filled with rough sand.<br /><br />Blueberries are the size of BBs.<br /><br />1) Try tossing the BBs into the bowls of sand and plaster powder.<br />2) Try throwing the BBs into the bowls.<br />3) Using the Bic pen tube as a blowgun, fire BBs into the bowls.<br />4) Fire the BBs into the bowls with the BB gun.<br /><br />Observe the craters.<br /><br />The craters you produce in this experiment simply aren't big enough to make craters ranging from 10 cm to 40 cm sizes (? estimated) observed in the Martian microcraters. (And don't forget Mars has only 1/3 the gravity of Earth too). <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[JonClarke]

We'll do our best! <img src="/images/icons/smile.gif" /><br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[centsworth_II]

<font color="yellow">"I also don't like coincidences. It just strikes me as unlikely that we would happen to stumble on so many fresh microcraters." -- silylene</font><br />Opportunity's "hole in one. The "water story" told by the outcrops visible upon the opening of Opportunity's eyes. Bounce rock, a sister of martian meteors found on Earth, found immediately upon leaving Eagle crater. The metalic meteor lying on the surface next to Opportunity's heat shield. What's one more coincidence?<br /><br />The freshness of the microcraters simply says that the event that caused them occured recently. Why is that not possible?<br /><br /><font color="yellow">2. micrometeorites, from larger meteors exploding in the upper atmosphere.</font><br />This is my favorite theory. But I would say the explosion, or breakup, took place lower. That would explain the number of microcraters. The debris did not spread much due to the low altitude of breakup.<br /><br /><font color="yellow">"...the microcraters are not evenly distributed. Only here. Not at other lander locations."</font><br />Each location is different. Some are more rocky with less sand for microcraters to occur in. In fact the Opportunity site is the only area visited on Mars that has a wide, uninterruped expanse of sand to record such a microcrater-forming event. The sand and dust would also serve to wipe clean some areas, leaving a distribution of microcraters different from that created by the formation event.<br /><br />A coincidence that Opportunity traversed the area just after debris from a recent meteor breakup struck? Perhaps, but as my first paragraph shows, Opportunity's mission has already been blessed with amazing coincidences. <div class="Discussion_UserSignature"> </div>

 

[fossils]

Silylene,<br /><br />Well, first I think that these "micro-craters" ARE likely impact craters given that there are many many craters of all sizes, everywhere at Meridiani (not just on the ripples). And for example, the photo you posted is likely caused by thermal cycling and interaction with the underlying rock. But there are many aspects to the micro-craters that are interesting - and not just the ripple located micro-craters.<br /><br />For example:<br />http://marsrovers.jpl.nasa.gov/gallery/all/1/p/089/1P136103530EFF1500P2437L2M1.JPG<br /><br />It’s a possibility that melting and a surface wetting created the ripple micro-craters. I propose that the craters occur near the tops of the ripples (and not in low spots) because that is exactly where you might get an extra heating from the sun - due to the angle. You know like how the snow melts real quickly on the south sides of a mountain but can remain around for months on the north. On mars maybe we get a melt once in a while on the south but the north always evaporates.<br /><br />The coolest thing of all is the "melted" look to the "pool" of "sticky" soil that lies at the center of many of the micro-craters (and micro-channels).<br /><br />Maybe all it is, is the craters on the top south sides get kind-of glued in place with the surface crust. The surface crust might be stronger/thicker on one side of the ripple. Of course, this may or may not support the wetting hypothesis.<br /><br />The ripples look to have a layered structure.<br />http://207.7.139.5/mars/opportunity/pancam/2005-11-01/1P184113706EFF63%23%23P2413L2M1.JPG<br /><br /><br />Gentle meteors that create stains?<br />http://marsro

 

[fossils]

How many micro-craters are in this photo? I count seven disruptions that could be micro-craters.<br /><br />http://207.7.139.5/mars/opportunity/pancam/2005-11-01/1P184113706EFF63%23%23P2413L2M1.JPG

 

[thechemist]

JonClarke - <font color="yellow">Where did to get that great map? </font><br /><br />Actually, the map in my previous post is the latest Opportunity detailed transverse map released by JPL in Nov. 1st. <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[silylene old]

Well, some of the pictures you posted do look like aged micro-cratersm partially filled with dusts, or micro-craters formed in a slush. Interesting.<br /><br />Now since you stated you believe that the microcraters formed by impact (which is certainly possible), here is an odd micro-crater photo I found on the other board...which sure does look like an impact at an angle:<br /><br /> <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[silylene old]

And here are two micro craters I found from Oct 24 that don't look like impacts: <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[thechemist]

Silylene,<br />I am being patient until you reach 2048 posts and the board trusts you to approve your own images. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[fossils]

Last night, I dreamed about playing around at Meridiani. I dug around in the ripples and pitched rocks... I got real dirty and made huge clouds of dust that quickly blew away...<br /><br />I surmise that rocks (meteors) will usually disappear into the ripple dirt when they hit anywhere on the deep high area of a ripple. If the rock hits lower down on a ripple, near the underlying rock, it makes a depression (shallow crater) but does not disappear into the dirt.<br /><br />The rocks that hit high on a ripple bore a vertical tunnel (hole) of sorts down to where the rock comes to rest. As time flys by, the edges around the hole slum down into the hole and create a crater. The hallmark of these slum craters is the lack of a raised rim.<br /><br />Above a certain size (and velocity) meteors create a standard raised rim crater.<br /><br />An alternative is subatomic particles of anti-matter annihilating in a small bust of energy as they collide with the dirt of the ripple...<br />

 

[JonClarke]

Why don't you think these look like impacts?<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[silylene old]

Jon,<br /><br />Nearly all the micro-craters <i>do</i> look like impacts. The only ones which absolutely don't are the three footprint-shaped ones in the picture I posted first. The second picture I posted has microcraters which are filled in - either by wind-blown sand....or because they were formed by a sapping process.<br /><br />What I find unlikely is that there are so many of these microcraters. Why?<br />1) They are only at Meridiani (as best as I recall) - not at Gusev, or the Vikings, or Pathfinder. Admitedly the dunes are more likely to form craters from impact than harder ground.<br />2) I have a hard time believing microimpactors would survive passage through the Martian atmosphere, even at 8 mtorr, to hit the ground. After all, micro-meteors burn up all the time in Earth's upper atmosphere where the air is far thinner than the Martian surface.<br />3) What is the terminal velocity of a falling BB- to- pea-sized rock on Mars anyways? Is it fast enough to produce a microcrater in a dune? And not fast enough to produce a microcrater on more rocky ground such as Gusev or Vikings or Pathfinder? I couldn't find any studies on this either for or against.<br />4) One could argue that the microimpactors are secondary masses, like tektites, and moving slowly enough that they don't burn up. I agree, this is most likely of all the impact scenarios. Is there a fresh large impact nearby that may have generated these?<br />5) I think that the lifetime of a 5cm sized microcrater is less than 100 yrs given the brisk winds - and probably much less. This suggests either the impact event was very recent, or that these microcraters somehow form periodically in random locations. Actually, my impression is that the microcraters seem to have about 3 distinct ages...very fresh, mostly filled in, and 'fossil'. Maybe it is just the limited set I have seen, but they don't seem to show a continuum of ages. Or maybe I am wrong.<br />6) It strikes me as quite odd that the microcr <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[bonzelite]

i am glad to see you're questioning the entire impact scenario, with insightful info. it may be impacts, but it may not be. i am not entirely bought and sold on them all being of impact origin. but i'm not really against it per se. <br /><br />i've said this before, but many of the ones in the dunes appear literally a few days or weeks old. and their location atop crests attests to possible EM discharges seeking high ground. <br /><br />you mention that you doubt micro meteroites would survive entry into the atmosphere all the way to the ground, as this is most likely the size of the impactors were this the culprit, pea to sand grain sized. initially, i began to think this was the simplest way to explain it. but you have now made me doubt that again. <br /><br />and there is apparently no trace of a nearby larger impact, as there are no other debris remnants scattered about the area atop dunes or in troughs anywhere; it is a relatively clean terrain. i wonder if the rovers have seismic measuring abilities? <br /><br />i tend as well to not believe melting subsurface ice, ie, sinkholes. there is obvious displacement of material around the rim of some of the craters, as if it were pushed out and not sunken in. <br /><br />overall i have no answer. mysterious indeed. it is cool that planet mars is nearly entirely unknown as to what is really going on up there.