The baffling case of the tiny craters on the Meridiani dunes

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JonClarke

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Remember that the regolith at Meridiani is very different to anything we have seen at the other foru landing sites. Basically ripples and small dunes of sand and gravel compared to rubbly basalt and mixed sand and silt sized particles. It may be that the craters are better expressed in the well sorted sandy material than in the others. It certainly makes sense that they are.<br /><br />I've seen a lot of holes and pits formed by collapse into underground cavities, we seen some of these elsewhere at Meridiani. These don't look anything like them - far too circular and no sign of structural control. So some form of sapping or piping seems ruled out, to me at any rare.<br /><br />I suspect that most of the micrometeorites do burn up in the atmosphere, We are only seeing the results of the few that don't. Also note that there is a minimum suize as well, the craters don't extent down to the microscopic level, as they do on the moon. This is consistent with a micro meteorite origin.<br /><br />If these pits are craters, they tell us that the dunes have been there for quite some time, and may even provide a means of roughly dating them, given the fact that the micrometeorite flux at Mars would be approximately known (though not by me).<br /><br />Jon<br /><br />Added it edit: terminal velocity can be calculated using the formula Vt = sqrt ( (2 * m * g) / (Cd * r * A) )<br /><br />where m = mass, g = gravitational acceleration, Cd = drag coefficient , r = gas density, A = cross sectional area. See http://exploration.grc.nasa.gov/education/rocket/flteqs.html <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>
 
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fossils

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"If these pits are craters, they tell us that the dunes have been there for quite some time..." - Jon<br /><br />Jon, I wrestle with this thought. - - How long - - how long does the Meridiani landscape take to change in a significant way, like a dune ripple moving along, a "berry" to erode out? The photos of blowing sand at Eagle seem to indicate that it is not too slow. The ripple craters seem to indicate that it is a very long time indeed.<br /><br />Do significant changes occur at Meridiani only every 51,000 years during a point in the axial tilt precession? Maybe this is true and only infinitesimal dust blows by in the mean time. Maybe a dust storm can bring change, it would be nice to know. <br /><br />It is a rhetorical question - how much time - how fast, oh how slow is the change?<br /> <br /><br />More craters and stains, and eroded craters?<br />http://207.7.139.5/mars/opportunity/navcam/2005-11-04/1N184210049EFF63%23%23P1605R0M1.JPG
 
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bonzelite

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the only point i have a problem with is the age of the pits and dunes: i don't think they or the dunes' immediate near surfaces are really so old. using the "kiss method," you can see they're nearly new looking. mars has a dynamic eroding atmosphere, very active and alive. <br /><br />what is your idea of old? we may have similar or different notions of age in this context. for example, it would not surprise me if much of the sand dune structure throughout mars is resurfaced regularly by dust storms, if not wholly, at least in large part. maybe every few years or much less. <br /><br />a question, though: has a major global dust storm happened during the rovers visits yet? i don't recall one happening. at least not yet. i do think i remember that they had to shut one of them down for a while, like a week or so, because of a local dust storm event. for that time i [think] i remember there were no raw images downlinked.
 
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JonClarke

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That's a very interesting question. The otehr is, are the dunes the net result of the very slow processes we see at present, or are they a fossil landscape from an episode of much greater movement in the past?<br /><br />If these are micrometeorite impacts, their size-frequency distribution will give us a rough idea. The fact that there are no large craters suggests that the dunes are comparitively young. <br /><br />However calculations will be complicated in that we are not dealing with hyper velcity impacts, like on the moon. The objects are travelling fast, so that the sand sprays out forming a crater, but not enough to vapourise the impactor. The heatshield meteorite shows that such bodies are travelling comparatively slowly when they hit the ground, and they survive impact. Also craters don't form at all in the more heterogeneous and cohesive regolith at the other landing sites. This points velocities of no more than a 100 m/s, I suggest.<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>
 
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bonzelite

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understandably, the air density of mars is very thin. winds blowing at 100mph on mars would not carry the same punch as they do on the earth. despite this, given enough time and volume of airborne sand, frequency of massive sand storms, dunes will erode and form and shift nonetheless. this effect will be further extremely pronounced, regardless of air density, if the storms, as in dust devils, are electrical in nature. displacement of loose sand would be a cake walk were this the case. electric field gradients of up to about 25 kV/m have been estimated in Martian dust storms (compared to only10 kV/m in terrestrial dust devils). in an electrical storm with separation of charge across miles of sand, movement of particulate matter is very easy and frequent. <br /><br />given martian propensity for occasional global dust storms, at the very least numerous local ones regularly, i highly doubt the sand dunes are ancient formations. even if the underlying sand is extremely old, the near surface features are probably very new. as in months new. at most, i would bet not any more than 5 or 10 years. <br /><br />http://aoss.engin.umich.edu/faculty/nrenno/mars.html
 
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fossils

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"A question, though: has a major global dust storm happened during the rovers visits yet?" - Bonzelite<br /><br />There is a dust storm right now on mars that you can see with a home telescope! See the many news articles about it. It's possible that the dust will reach Meridiani, but for now only a very slight high altitude obscuring has occurred over Meridiani. A big dust fall could render the MER inoperative.<br /><br />Lets talk dust. What do we mean by dust? Do you see lots of dust at Meridiani? I don't. In fact there is an amazing lack of expected dust. No dunes of dust piled on the leeward sided of rocks, no dust covering the rocks. Endurance was particularly clean.<br /><br />What dust there is exceedingly light. Otherwise there is sand, hematite thingies armoring the surface, and stuck together crusted-over "soil". There is light sand/dust between the rocks in and around Eagle and Endurance - sometimes "pooled" in places like a fluid. But in other places the "pooled" dust is tightly compacted or even crusted over.<br /><br />Any new dust (what little there is since everything seems so glued down) quickly blows away across the plane, maybe to be deposited in dunes to the north/west.<br /><br />When the MER digs a trench, a very fine whitish powder dust separates from the sand and then either blows away or collects in and around the trenching. This whitish powder is presumable the pulverized sulfate rock.<br /><br />So, really from what we experience on Earth, Mars is very clean, and most of the "dust" is crusted down. Dust devils and strong winds break some free - into dust storms - at times. But is a Martian dust storm really that dense with dust?<br /><br />This is not to say that if a rover or you were to kick around there would be no dust, on the contrary, if we had a movie of the MER driving we might see huge plumes of dust quickly trailing away in the thin fierce wind that constantly blows at Meridiani (and likely Gusev).<br /><br />Things change very ve
 
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bonzelite

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i tend to feel that surface modification is ancient, old, new, and very new on mars.<br /><br />dust, as well as larger sand particles, covers the entire planet in varying amounts depending on region and topography, anywhere from sheer rock to a sand pit. the martian wind is so dust laden that the atmosphere is typically rust-colored during the day, with the twilight of pre-dawn and after sunset lasting for up to two hours on mars, as light is scattered high in the hazy sky.<br /><br />the basic underlying structures of sand dunes being loosely cemeted in place for thousands of years, "fossilized," nothwithstanding, not all dunes are this way. and constant fallout of atmospheric suspension of particles is perpetual and ongoing 24/7. some regions that were dark 20 years ago, when Viking first mapped mars, are now bright red; some areas that were bright red are now dark. winds move sand and dust from region to region.<br /><br />http://www.news.cornell.edu/Chronicle/99/8.19.99/Mars_pix.html<br />http://mars.jpl.nasa.gov/mgs/msss/camera/images/top102_Dec98_rel/dunes/<br /><br />meridiani planum is replete with dust, everywhere. the rock abrasion tool markings throughout the mission, particularly the very shallow or surface-only abrasions, in context with surrounding rock, reveal how entirely covered every square millimetre of the surface is with fine dust. in this manner, the actual color of the rock is revealed. <br /><br />insofar as density in the storms, it is more an issue of particle acceleration and displacement due to separation of electrical charge (initiated by cold and warm air fronts as seen in earth-based electrical storms): the larger (+) particles will settle to the bottom, whilst the smaller (-) ones will rise up in gigantic colums of fast moving vortices, some as fast as 300m
 
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JonClarke

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I agree that the dunes look pristine. Their lack of impact craters of any size larger than the craters we are discussing supports the idea they are very long in Martian terms. This might still mean they are thousands or even millions of years old. Even on earth there are land surfaces that looks pristine but are millions of years old, for example in the Antarctic dry valleys or the Atacama desert. <br /><br />If these tiny craters are not due to impact, then I would be inclined to think these dunes are hundreds or thousands of years old, and probably still moving during big storms. if they are impacts, then I would be inclined to saying they are thousands to millions. But that is just a guess.<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>
 
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JonClarke

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I think you are right about the absence of dust. It is worth remembering that Meridiani Plnum used to be called Terra Meridiani. It's one of the classic dark markings of Mars. We now know these are areas that are generally swept free of dust. the fact that the rovers are repeatedly cleaned is consistent with this (Gusev is in another one of the classic dark areas).<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>
 
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silylene old

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Here is an interesting new photo frmo Opportunity showing a larger microcrater and two possible smaller ones (ref Atomoid on unmannedspaceflight.com). The larger one does look like an impact crater.<br /><br />What is unusual is the zig zag in the foreground. I am unsure frmo the shadows if the zig zag is a raised ridge, or from sapping into cracks in the underlying bedrock.<br /><br />The original photo has more interesting features:<br />http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=2462 <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>
 
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thechemist

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Also the area we are in, the Mogollon ridge, has dark cobbles/material scattered all around. <br />I can't tell whether we look at shadows, holes or just dark deposits.<br />Argh, put me in a spaceship and send me to Mars, asap <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>
 
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silylene old

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The so-called Ultreya area in Gusev has some good dune fields which should be good substrates for showing off microcraters (assuming microcraters are formed by impacts). I wonder if we will find any of the enigmatic microcraters there? I sure hope Spirit goes to check! (I am predicting 'none')<br /><br />Overhead view (vertically stretched 4x, from Ultrax):<br />http://i16.photobucket.com/albums/b14/ustrax3/4x.jpg<br /><br />Side view:<br />http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=1354<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>
 
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bonzelite

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i hope when MRO is online, it photographs the Viking lander sites. i'd like to see what they look like today. buried in sand or the same? <br /><br />
 
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silylene old

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Well, Spirit is now in the "Inner Basin", a sandy area with loose windblown sands in many spots.<br /><br />So far, I have seen no microcraters. Please keep a watch for microcraters in Spirit pictures as it traverses this sandy area.<br /><br />An absence of microcraters in sandy areas of Gusev would be interesting, and perhaps suprising if microcraters are formed by 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>
 
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silylene old

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For the last 10 days Spirit has been moving through an area with patches of nice soft drifting sand. The terrain ought to be ideal to form "microcraters", if those craters were caused by impact.<br /><br />After examining hundreds of images carefully, I have seen zero microcraters so far.<br /><br />This means that:<br />1) Microcraters from impact are a local feature of Meridiani only (local secondary impacts?)<br />2) The wind erosian rate at Gusev is far more than Meridiani, and any recently formed microcraters from impact were erased (somehow, this doesn't seem right).<br />3) Microcraters are formed by impact. But, so far, we were just unlucky and haven't stumbled on any microcraters at Gusev.<br />4) Microcraters are not formed by impact, and are a local phenomena to Meridiani. <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>
 
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JonClarke

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why doesn't 2 seem right to you?<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>
 
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centsworth_II

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The patches of sand seen by Spirit add up to a very tiny fraction of the area looked at by Opportunity. Simple statistics would probably show that the probability of seeing a microcrater even if they occured at the same rate as in Meridiani is low. All in my humble opinion, of course.<br /><br />Could the dark cobbles seen with increasing frequency by Opportunity be connected to the micro craters? Was each dark lump lying on the sulphate rock surface previously buried, with the microcrater it created above it? Are the cobbles from Victoria Crater, or farther? Will there be a dark band in the wall of Victoria corrosponding to the dark cobble material? <div class="Discussion_UserSignature"> </div>
 
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silylene old

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#2 could be right, don't know! ....I thought the number of cleaning events experienced by Spirit were comparable to Opportunity, so I assume the wind erosion rates in both locations are rather similar. Is there data to the contrary?<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>
 
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silylene old

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<font color="yellow">The patches of sand seen by Spirit add up to a very tiny fraction of the area looked at by Opportunity. Simple statistics would probably show that the probability of seeing a microcrater even if they occured at the same rate as in Meridiani is low. All in my humble opinion, of course. <br /></font><br />Agreed....I am still looking, and I hope to see a microcrater.<br /><br /><font color="yellow">Could the dark cobbles seen with increasing frequency by Opportunity be connected to the micro craters? Was each dark lump lying on the sulphate rock surface previously buried, with the microcrater it created above it? Are the cobbles from Victoria Crater, or farther? Will there be a dark band in the wall of Victoria corrosponding to the dark cobble material? </font><br />I have wondered the same.<br /><br />++++<br /><br />Another microcrater idea: Perhaps Mars passed through a narrow intense stream of a comet tail, and had a very short-lived meteor storm, and this caused a high density of micrometeorites to impact only into the Meridiani locale. <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>
 
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bonzelite

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<font color="yellow"><br />Could the dark cobbles seen with increasing frequency by Opportunity be connected to the micro craters? Was each dark lump lying on the sulphate rock surface previously buried, with the microcrater it created above it? Are the cobbles from Victoria Crater, or farther? Will there be a dark band in the wall of Victoria corrosponding to the dark cobble material?</font><br /><br />this is interesting. <br /><br />it made me think, too, that the microcraters may be sinkholes. <br /><br />
 
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siriusmre

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I predict that when the microcraters are examined, their floors will be shown to be glassified. <div class="Discussion_UserSignature"> </div>
 
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centsworth_II

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<font color="yellow">"it made me think, too, that the microcraters may be sinkholes.'</font><br /> <br />This be more plausable if they occured in areas of thin sand cover. Many of them occur high up the sides of drifts leaving, it seems, a thick layer of sand between the bottom of the microcrater and the rocky pavemant. The small diameter of the microcrater would indicate that its depth never reached to the pavement where the supposed sinkhole opening would be.<br /><br />Summing my general thoughts: The microcraters must be a surface phenomenon caused by impact. The question is, impact by meteor or secondary debris from other large impacts (or volcanic eruption?). <div class="Discussion_UserSignature"> </div>
 
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silylene old

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Odd. So far no microcraters anywhere in Gusev yet. At the frequency we saw them in Meridiani, I was hoping we'd seen one or two by now. Perhaps Gusev is much windier?<br /><br />Not even on this big dunefield (many more great pictures on Exploratorium): <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>
 
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centsworth_II

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Opportunity traveled over many pristine sand drifts before seeing it's first microcraters, didn't it? There were other features that appeared to be associated with cracks, depressions, or holes in the pavement on which the drifts occur, but I don't think these are associated with the microcraters.<br /><br />Opportunity did not see microcraters over much of the drift area it traversed, but has seen several in its recent trecks. <br />Spirit has not seen microcraters in the drift areas it has looked at.<br />I think these observations together boost the possibility that the microcraters seen by Opportunity are due to a single, recent event. <div class="Discussion_UserSignature"> </div>
 
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