nprev - Umm, so you'll allow the Late Heavy Bombardment to temporarily warm up the climate (as independently suggested by the presence of clays in the most heavily cratered terranes), but not to grow spherules, even though spherical accretionary lapilli are typical of terrestrial impacts, and smaller glassy spherules are typical of lunar impacts - both, BTW, result from vapor condensation (so do rain and hailstones). Instead you'd prefer there to be two types of concretions, neither of which look like terrestrial examples? The ONLY thing that's unusual about the Meridiani spherules is their hematite-rich composition - otherwise they'd fit everyone's preconceived notions of what an impact should produce. And sorry, there's no Nobel Prize in geology.

Also, also just for fun, here's what soaking in a warm brine (such as the MER team proposes for Meridiani) might be expected to do to all those sulfates (keeping in mind that gypsum is the least soluble - the others should recrystallize even larger or faster):
http://www.crystalinks.com/mexicocrystals.html

Keep in mind that what that website says about "hydrothermal fluids" is wrong - the crystals just grew from warm groundwater at about 58 C (only somewhat warmer than the outdoor temperature in Phoenix today - see "In the News" below the main article). Any hotter and the gypsum (variety selenite, named for the Moon) would have dehydrated. Enjoy.

That's an extreme example, but if they were ever soaked in liquid water, as claimed, those allegedly windblown salts really should have recrystallized!

--HDP Don

Don, you missed this thread... pretty amazing...
http://www.unmannedspaceflight.com/index.php?showtopic=4383

Aussie - Some of these questions have been addressed in previous posts, but bear repeating.

Regarding Michelle Minitti's results - what happens if you roast basalt in a dry CO2 atmosphere is irrelevant - water (or steam) is the great catalyst. Without it, very little happens in the mineral world. Look it up. Also, believe it or not, large impacts heat things to many thousands of degrees - they separate metals from oxygen, at least transiently, and vaporize everything, without exception. Plenty of kinetics, infinitely more interesting than, say, volcanic processes - and equilibrium is hardly expected, with the rapid decompression and cooling. Consider, as a simple example, what might happen to hydrated Mg-sulfate, inferred to be the most common salt at Meridiani. Mg would separate from S and O and H. Then rapidly recombine into MgO, SO2, H2O, and O2 - plenty of oxygen to oxidize iron, and plenty of steam to catalyze the reaction (not to mention salts - why you don't want to buy a used car from a seaside car dealer - too much rust). Iron-containing impact products on Earth, other than the tiny iron condensation spherules found right next to Meteor Crater AZ, are fully oxidized - and the oxygen in the air has little to do with it. It's the oxygen in the target minerals. Also, believe it or not, it's rather difficult to model (or even imagine, for most people) such extremely rapid, disequilibrium processes. So observation of the products may be your best guide.

Regarding "small regions" - that description hardly fits Meridiani. Also, believe it or not, orbital infrared spectroscopy is an extremely crude tool - at best it detects what's abundant at the surface, and at worst, only thin coatings on something else, or nothing. It utterly failed to detect the 30% sulfate in the Meridiani rocks (even though sulfate vibrational regions are easy for IR in the lab), and was lucky indeed to detect the specular hematite in the spherules. In any region where the hematitic spherules were all small, or less abundant than about 15% (or coated with dust or buried under/mixed with sand or layered rocks), the hematite would probably be invisible to spectrometers in orbit. Ditto if the spherules had a different composition. So far, rovers have landed at only two places on Mars. In both places they have detected spherules (much smaller, less abundant ones at Home Plate), and at both places they have detected what look like salty surge deposits (essentially identical bedding structures, although the MER team seems extremely reluctant to admit this, and papers on one site never mention the other site). Ferric acid sulfates occur at both sites (jarosite is reported from Meridiani, unspecified ones at Gusev). If our impact hypothesis is correct, I might predict that future rovers that land in similar well-bedded rocks should find similar features: surge-type cross-bedding, impact-related spherules, and salts, possibly including acid salts. So far, we're two for two - so feel free to complain about the statistics of small samples.

The "indications of aqueous activity" that you refer to for Aram Chaos, and Ophir and Candor Chasma in Valles Marineris are inferences generally based on the presence of bedding and sulfate salts - also obviously expected for impact deposition (although I'll be the first to admit that water deposition also produces bedding, but the rocks don't look anything like those at Meridiani or Home Plate). BTW, our impact model depends on "aqueous activity" too - but in the form of steam. And we never claimed that the neutral salts don't indicate liquid water - just that that the liquid water could be older than the deposits being observed (see earlier posts). The highly unlikely mixture of soluble and insoluble salts at Meridiani also converted the MER team to this explanation, but they blamed the wind blowing across a long-vanished playa, instead of an impact. Then they wanted to soak the soluble salts many times in brine without recrystallizing them, have water flow briskly across a flat surface, and similar implausibilities.

Regarding "hydrothermal activity" and goethite, that has not been suggested for Meridiani by the MER team recently, so why are you bringing it up? (When the blue-gray or specular hematite IR signature was first detected, a super-giant hot spring system was inferred, but this idea was later quietly laid aside in favor of a giant metamorphosed iron formation, which now has been laid aside in favor of a giant field of sedimentary concretions.) That Catling and Moore (2003) paper is dates from the iron formation hypothesis. Frankly, instead of dealing with the largest hot spring in the solar sytem, or the largest iron formation, or the largest sedimentary concretion field, I find it much easier to blame everything on an ordinary garden-variety impact or impacts, for which there are so many possibilities that it is hard to know where to begin. But that's just me.

--HDP Don

You're right - thanks for the link.

--HDP Don

Elk Grove Dan - You're right - I made up an answer Centsworth II - From what I've learned about regional geology, the edge of the Meridiani hematite deposits is, in part provided by the sharp topographic edge of the plateau - wind erosion has preferentially removed the finer-grained material next to the heavily cratered hills. Having finer-grained material next to the hills is, incidentally, the opposite to the pattern you would expect had any of the Meridiani bedding been caused by water runoff (you'd expect coarse alluvial fans next to the hills instead). It is typical of surge deposits, though, as mentioned in previous posts. The rest of the "edge" may be merely a detection limit of about 15-20% spherules. In other words, a diffuse edge would probably be indetectable from orbit. (Anyone is welcome to correct this supposition, if they can do so authoritatively.). Finally, remember that Meridiani is not where the spherules are, it's where the wind blowing across a plain has left a lag deposit of spherules by eroding friable (poorly cemented) rock. Any spherules still in rocks, or buried beneath sand or dust, are indetectable from orbit.

Enough possibile answers?

--HDP Don

Bill - Umm, as a scientist I don't believe, I hypothesize, and absolute proof is impossible, by definition. All that a scientist can do is test various hypotheses for the same observations against each other, and try to decide which one best fits the data. Even for the best, there will always be holes and imperfections. In the Meridiani case, there are two other hypotheses. The first is volcanic surge, which is quite similar to ours, but hasn't gotten much traction, inasmuch as it has to call upon a vanished volcano. That little difficulty hasn't stopped the MER team from hypothesizing a vanished volcano for Home Plate, however. The second is the vanished playa hypothesis of the MER team, with which you may be familiar. That has obviously gotten a great deal of traction, even though there is no direct evidence for any of its complex individual parts, and considerable evidence against some of them, plus it contains many internal contradictions, and doesn't explain all of the observations (see my earlier posts). If you wish to "believe" in it, you are welcome, but that makes you a true believer, which is a religion.

If you wish to treat the impact surge hypothesis as science and not religion, I suggest you find some observations that it does not explain, or some internal contradictions within it, or some data that directly opposes it. So far no one on this forum has had much success, although it does have minor warts, as people delight in pointing out repeatedly (and believe it or not, I am extremely grateful - that's why I signed on). Mostly people seem to object because it runs counter to their beliefs and assumptions, or they have difficulty imagining parts of it. I continue to be amazed at how many people object to our proposing impact as an important process for a planet covered with craters and coarse impact debris.

Regarding the hematitic hailstones part, BTW, are you willing to grant me the very carbonate-rich accretionary lapilli of the Chicxulub impact that probably killed the dinosaurs, owing to the carbonate-rich target rocks (limestones), but unwilling to grant me very iron-rich accretionary lapilli for a probably iron-rich target rock on Mars? Wouldn't that make you somewhat miserly?

And hey, here in Arizona frogs (or at least toads) do spring spontaneously from mud, according to direct observation. Of course, I may have failed to observe to the mother toad laying eggs in that puddle late some night before it dried up.

Finally, if you wish to see possible direct evidence of impact surge, I suggest you look at some of the nearly eroded flower petal-like ramparts around Victoria Crater, beginning about one crater diameter out (anything closer is concealed under berries). See the July 5 HiRISE browse image, for example. Oppy must have driven right across these possible eroded surge deposits, without imaging anything different from the rest. I may have more to say about Vicky's surge deposits later. For tonight, that's all, because I'm writing this from home, and so lack access to any photos to post. Thanks for your brief comments - they gave me a great excuse to lecture about science (not that I ever needed one...).

--HDP Don

Dburt
I regret that I have grave reservations over your theory. I suspect that if I had presented your concepts and proofs for a PhD you would have torn me to shreds for lack of sustainable logic. Please do not take this comment as disrespectful but I suspect that your interest in the impact surge concept has become personal rather than scientific and your arguments seem centered on attacking the JPL theory without providing sustainable arguments for your alternative scenario. I do note that JPL considered the impact surge scenario as one of their favoured models and then rejected it based on tests and feedback from the rovers. Lets face it, Innocent Bystander raises significnt questions with respect to your theory.

We'll wait to hear your suppositions on the aerial photos...

--Bill

dBurt

"Can you explain that scour in terms of their model? They appear to have hypothesized a "supersurface" ("Wellington contact") of regional extent on the basis of a single large cross-bed with a large channel-like scour taken out of the top of it - which scour, on the face of it, makes their hypothesis untenable. Do you disagree? If so, why?"


Why not a channel cut or similar for the “scour” in the Wellington contact? If there was surface water in the area as the MER model states all you need is a small gradient and you have flow, which in turn could cause down cutting (I hear the surface rocks are easily-scoured sand). If there was a fluctuating water table at ancestral meridiani perhaps there was also topographic relief (even minor) just enough to generate flow once groundwater breaks through the ground surface (aka springs). Here is perhaps evidence in the shape of “scours” for a local surface hydraulic gradient at meridiani. Seems more plausible IMO (small wart) than a surge generated vortex (big wart). I would hold back on speculating a regional extent for the Wellington contact however. I’m still hoping to see a similar feature in Victoria.

Question – the surge model seems to apply more directly to the layers examined in the Columbia Hills then meridiani, yet there seems a lack of enthusiasm from you to really hit that hard. Are you taking one landing site at a time, or is there a more fundamental reason for that, or am I just completely out-of-the-loop crazy ?

I am going to do the same thing to this thread now as I have planned to do in another, similar thread - close it.

It's going around in circles, it's getting nowhere, it should have been healthy discusion, but for whatever reason it's not happened that way - it's frankly damaging to the forum.

dburt's written more than 41,000 words. If that's not going to get a point across, nothing will.

If people wish to discuss his Meridiani theory further, then please do so elsewhere or via email.

Posts in other threads on this subject will be deleted - as a subject it's been exhausted with this thread.

Doug

Absolutely right, EGD. Or even on a timescale of a few seconds, for a landslide or avalanche or flash flood. On Mars some of the most spectacular sudden erosion seems to occur in the vicinity of minor impacts, where shock waves toss dust and sand into the atmosphere, and then dust-storm-like super-fast density currents (a.k.a. impact surges) sweep radially across the ground surface and scour the subsurface (such scouring commonly reveals itself as cross-bedding in the sedimentary record). The aftermath of this scouring is clearly visible on orbital images, where all of the dust near very recent small impact craters has been cleaned out, leaving circular dark zones. Older, larger impact craters commonly display radial erosional grooves. (And apologies for pedagogical repetition; it's the professor in me .)

-- HDP Don

Thanks for the reference, PaulM. FYI, later that year, Ken Edgett published that work as an extended article (with numerous photos) in the Mars journal, web accessible to anyone:
http://www.marsjournal.org/contents/2005/0002/
As I recall without looking them up, most other estimates of erosion off the top of Meridiani are lower, on the order of (at most) tens of meters, rather than hundreds. The dense hematitic spherules, once eroded out, seem to have formed a "pebble armour" or "desert pavement" that slowed further wind erosion.

The Edgett paper says, AFAIK, nothing about clay deposits at greater depth, although he does refer to buried erosional channels, apparently lacking near the surface. (Note: not only flowing water, but also vortices in radially flowing surge clouds, can erode channels.) Deep clay-rich layers buried beneath Meridiani, if any, would presumably be older than those layers exposed near the surface, and therefore not directly related to them genetically.

Two quotes I originally enjoyed reading from that 2005 abstract (given our 2004 hypothesis that impacts alone could have deposited the Meridiani sediments imaged by the rover) include "These rocks are not superimposed on heavily cratered terrain, but rather are an integral part of it" (Introduction) and "Impact craters of diameters from < 50 m to > 100 km have been filled and buried by and within the strata." (p. 2). In other words, Edgett and Malin concluded, from orbital evidence (MOC images) alone, that Meridiani sedimentation exactly coincided with or at least heavily overlapped with impact cratering (a.k.a. Late Heavy Bombardment, LHB), although, unlike us, they did not hypothesize a cause-and-effect relationship between the two processes of cratering and sedimentation.

Furthermore, they noted a near-zero regional dip "All of the rock units that extend across Sinus Meridiani are horizontally bedded at regional scale" and "the [regional] dip [today] is only about 0.02 degrees" (p. 1). This important observation casts doubt on any hypotheses involving flowing or standing water, because water puddles or ponds, rather than flows, on a horizontal surface. Clay layers are characteristically deposited in ponds, which also characteristically lack cross bedding. The exposed Meridiani layers are cross-bedded everywhere and lack bedded clays, as well as obvious flow channels. To us, by far the simplest hypothesis that satisfactorily explains all of these features (as well as the uniform tiny spherules consisting largely of the high-temperature blue-gray form of hematite) is impact-related sedimentation.

Sorry, more pedagogical repetition from the professor.

-- HDP Don

Paul M, Regarding your post 406, Yes, that Edgett and Malin paper is an interesting read. I have been aware of their contention that Endurance and Victoria are exhumed craters for a couple of years and brought up their ideas on the other Mars blog. As I see it now the prediction that Victoria would be rimmed with layered material above the original rim has not been born out. The top layer around the present rim seems to be jumbled ejecta from the impact, not layered bedrock, suggesting that if Victoria has been overlaid with hundreds of meters of material this has now eroded down into the ejecta but not the pre-impact surface. The bright planar beds that surface most of Oppy's traverse seem to lie just under the ejecta and more layered bedrock just under that. Its looking like Victoria and Endurance have the simpler history, that is, they are eroded craters rather than filled, buried and exhumed craters. (I still wonder why some areas of the outer rim of Endurance look so much like bedrock but after seeing Victoria I am swayed to accept Endurance as having a similar history.)

In that paper Edgett and Malin never quite spell out what sort of sedimentation they are visualizing when synthesizing their stratigraphic sequences. In any case, their scenario failed to predict the strata around the rim of Victoria and cannot be taken to prove that Oppy's landscape has been exhumed from under 100s of meters of rock.

Maybe, but we're talking about the possible short term erosion of Rover tyre tracks here. Are you attributing this to impacts?

That's obviously where this discussion started, but it had wandered far afield before I contributed. No, I don't believe impacts have eroded rover tracks (at least, not yet).

-- HDP Don

Speaking as a geologist, if Cugel is looking for a single word describing the fact that wind is causing dust or sand to move, try "transport". For a sediment like sand, geologists commonly speak of "erosion" of some bedrock at the source (whether by sand blasting, impact, explosive volcanism, matrix dissolution, ice sublimation, or some other cause), "transport" of the sediment some distance from the source (possibly half way around the planet for wind-carried dust), and "deposition" of the sediment at some site where it accumulates for long enough and thickly enough to be described as a distinct unit (mappable layer) or even harden into a rock. On water-rich, geologically-active Earth, rapid preservation via burial beneath younger sediments and hardening (groundwater cementation) into a rock is generally assumed. On dry, frozen, geologically moribund Mars it is difficult to say for how long a given particle might blow around before being deposited somewhere (probably in a crater or other depression), and for how long such a deposit might be preserved. Hardening into a rock could involve ice as the cement, as for frozen dust layers at the poles, or sulfate salts as the cement, as was apparently the case at Meridiani (for whatever reason - acid playa lake or acid groundwater evaporation, fumarolic volcanism, sulfide oxidation, or salt/brine condensation from an impact cloud).

Volcanic (and probably impact) surge deposits, BTW, tend to become cemented as they are deposited ("instant rock"). Home Plate rocks, or at least the basal layers (and I hypothesize the upper layers too) have been described as a type of surge deposit (volcanic is assumed, probably unnecessarily).

-- HDP Don

So would I, although as mentioned above, occurrences of older clay-rich layers towards the base of the section would probably be unrelated to the younger cross-bedded layers observed by Oppy at the top. (For example, the many different rock layers exposed in the Grand Canyon, AZ are inferred to have formed in completely different environments that range from beach sands, to shallow seas, to river deltas, to desert dunes on land.) Also, I don't necessarily agree with the closing statement "The big picture appears to be a change from a more open hydrological system, with rainfall, to more arid conditions with groundwater rising to the surface and evaporating, leaving sulfate salts behind" because clays need not indicate rainfall and salts need not indicate liquid water evaporating. Clays and salts might form in a variety of ways, especially on planets other than Earth. In fact, rainfall (other than ephemeral condensation of water vapor released by major meteorite impacts) may never have been typical of Mars. That early Mars must have resembled present-day Earth seems to be an implicit assumption.

-- HDP Don

Inasmuch as I was unable to answer these questions by "other Don" last July, I'll touch on them now. Contrary to his claim, that scour is the right scale to be a vortex in a surge cloud - I saw and imaged several similar-scaled scours (1-2 m across) in volcanic surge deposits in Oregon last summer. I cannot see how it could be a water scour, if the planar upper surface marked the water table, as claimed (i.e., the whole idea is that wind or water erosion could occur down to the water table, but no further). If the water table afterwards dropped, before deposition of the overlying sandy layer, allowing water to erode a large channel, then why should the water stop at only one channel, why is the channel filled with sands that obviously were not deposited in water, and why didn't general erosion resume? In this regard, intricate braided stream patterns, not isolated deep channels, generally mark sand-choked streams flowing across very low gradients. I have looked at many such paleowater tables in eolian sandstones over the past few years, and have never seen such a scour in any of them. The boundary surfaces are always near-planar, with thin shale beds apparently marking low spots where standing water was locally present. We're still waiting to see the first shale bed on Mars. This could provide direct evidence for standing water in a dusty environment.

Regarding the Columbia Hills, slightly out-of-the-loop might be my vote. The hills appear to consist mainly of crudely bedded coarse ejecta (i.e., breccias), rather than finely bedded surge deposits. This is fairly obvious from outcrops, and was stated in previous posts, I believe. Members of the MER team seem comfortable with this interpretation, at least in part (although to fit their volcano interpretation, they would like at least some of it to be coarse volcanic ejecta, rather than impact ejecta).

-- HDP Don

Given that I can now do so (and I am grateful to Doug for the chance), I'll also reply to this July statement. First, as stated near the beginning of this thread, I have great respect for those who made what I might call the Cornell hypothesis, rather than "the JPL Theory" (my impression is that JPL press releases merely quote the PIs, in general). Based on their initial assumptions about an Earth-like watery past for Mars, and the need for readily studied terrestrial analogs, it was an entirely reasonable hypothesis. Most planetary people's ideas about impact deposits were shaped by studies of the Moon, a planetary body lacking an atmosphere or subsurface volatiles. In such a case, ballistic ejecta must predominate, because there is little or no opportunity for turbulent surge clouds to develop. Finely cross-bedded impact deposits therefore probably are lacking on the Moon (although microimpact surface gardening makes it impossible to know for sure), and all such deposits probably have been eroded on Earth. Given the lack of familiar examples, impact was rejected out of hand for the finely cross-bedded deposits at Meridiani, despite the abundance of impact craters of all sizes and relative ages in the vicinity, and the abundance of tiny spherules. If I hadn't spent several years studying volcanic surge deposits (some with spherical accretionary lapilli) in the 1970s, I'd probably have done the same.

Regarding "Innocent Bystander," that is, as I recall from last July, a piece of ejecta sitting on top of Home Plate that had been run over and partly crushed by the rover. It was interpreted to have formed originally by fumarolic or hot springs alteration (similar to the fragmental rocks of Silica Valley, and the high-silica rock "Fuzzy Smith" initially encountered on top of HP in 2006). Fumarolic and hot springs activity are believed to be common following major impact events on Mars (owing to the formation of impact melts and other heated rocks that can interact with groundwater or ice); they need not indicate volcanism. The isolated, fresh-looking high-silica blocks (as well as the blocks of vesicular melt) emplaced on top of Home Plate (of which "Fuzzy Smith" was the first found, apparently) must have been put there either by relatively recent impacts, or by relatively recent volcanic explosions. In other words, they all are ejecta blocks that originally came from somewhere else. As mentioned in previous posts, the highly vesicular melt blocks themselves could represent either impact melt or basaltic lava. Take your pick (at this point I have no valid basis for choosing).

HP Summary: Finely cross-bedded surge beds, fumarolic/hot springs alteration, ballistic ejecta, accretionary lapilli, and vesicular basaltic melt could indicate either volcanism OR impact cratering (or both). They are not unique to either. There are dozens of nice impact craters close to HP. Is there a nearby volcano? (Keep in mind that most exploding basaltic volcanoes are energy pip-squeaks compared to a decent impact, and so the volcano would probably need to be quite close indeed.) Lacking a volcano (which might be eroded or buried under sand), are there igneous dikes that could have fed a volcano? Rather than silica-altered pieces of ejecta, are there quartz veins or cemented alteration mounds that appear to have formed by hydrothermal activity in place? And so on. As I see it, the question of volcanism vs. impact is still completely open for HP, strictly speaking. However, the rocks are so similar to those at Meridiani, where explosive basaltic volcanism seems highly unlikely, and so like bedded rocks found to be extremely widespread in orbital images, in places where volcanism likewise seems unlikely, that impact deposition still seems to be the simplest general explanation (Occam's razor). Exceptions are possible, of course. Just show me the volcano (or dikes, or quartz veins, or whatever) at HP.

BTW, if you still believe that I have failed to provide "sustainable arguments for [my] alternative scenario" please cite some specific examples. As mentioned above, valid counter-arguments were what I was most hoping to obtain when I started this long and rather repetitious thread. Rejecting disproven hypotheses is the only way that science can advance.

And thanks for your valuable input.

-- HDP Don

So -- if there are nothing but volcanic and/or impact-emplaced materials anywhere in Gusev, including in the Columbia Hills, how does one explain the rather obvious large-scale landforms that argue very, very strongly that a river once flowed into the crater?

I'll also somewhat diffidently point out that Gusev began as a large crater, with a floor presumably paved with impact melt and breccia. Isn't it possible that the Columbia Hills are uplifted remnants of materials that once lay *below* a lakebed? The brecciated nature of at least some of the materials that make up the Hills would support this theory, I think.

Also, it's obvious that a layer of basalt was extruded onto the top of whatever materials made up the floor of Gusev prior to the lava emplacement. There are no obvious large flows that come from outside of Gusev (unless you want to argue that the river valley which debouches into Gusev is actually a lava channel) -- the lava must have escaped from vents within the crater itself.

I see a lot of large-scale morphological evidence for an early crater-lake which dried up and was then modified by relatively benign and non-explosive lava extrusion. Small-scale violent outbursts in the lava emplacement episode (possibly associated with hydrothermal effects) would seem to account for what we see at Home Plate, IMHO.

Then again, I'm just an amateur... *smile*... I think that what I'm seeing is consistent with the "Cornell Theory," here, but of course I could be wrong.

-the other Doug

Several interesting statements, here.

First off, the rocks at HP are quite dissimilar to those at Meridiani in everything except their fine layering. They are not shot through with concretions *or* lapilli, they are composed of hard-set basalt and not loose grains assembled into very friable rock by sulfur salts, and they have withstood erosive winds for far longer than Meridiani sulfate rocks have.

Secondly, and I know it's an imperfect analogy, our Moon is covered with tens of thousands of square kilometers of mare lavas for which there are no obvious volcanoes or even obvious vents to point to as origins.

Lavas of very low viscosity can seep up through a brecciated subfloor, "igniting" pockets of volatiles (such as ices) and creating localized violent explosions, but not creating cinder cones, volcanoes or other obvious signs of volcanic landforms. Our Moon is proof of that.

Show me all the dikes and quartz veins that "prove" that, for example, the Mare Imbrium lavas were emplaced by volcanic extrusion, and I'll take my hat off to you, sir...

-the other Doug

I thought that Irvine and Backstay were identified as probable dike rocks and that since differentiated volcanic rocks were not identified on the plains before the ascent of Husband Hill this almost necessitated a local magma source? Backstay was an essentially unaltered float high on Husband Hill and I thought that the concensus was that it was a local dike intrusion after uplift of the Hills. I can't find the reference but remember an article identifying Irvine as a surface exposure of a dike.

Indeed Irvine and Backstay were identified as dike rocks due to thier unaltered mafic nature

I'm getting the impression that the asking 'was it volcanoes or impacts' is a flawed approach. The only consensus being reached seems to be that there are any number of features which could have been formed by either, or both. The 'impacts only' argument seems to rest on the lack of an obvious nearby volcano. Dvandorn makes the good point :

.

The volcanoes only argument seems to over look the role impacts have almost certainly played in shaping every square foot of mars. If a large impact can cause volcano-like effects, and an obvious volcano is not needed for volcanism to occur then it seems that, in the absence of some other clue, both could have had equal responsibility. Given that a major impact may be capable of triggering volcanic activity the divide between the two ideas becomes still more blurred.

Dvandorn:
I believe that flood basalt vulcanism in india has been linked to the chixilub impact, on the other side of the earth.

To sum up, I very humbly suggest it would be worth considering a halfway house theory, taking into account both possibilities, as well as the possibility that the two could be intertwined.
Just for fun, and for volcano pele if he's following this thread Ive attached something on the possibility of impact induced vulcanism on io!

Sorry, I got behind on this old thread, what with spring semester classes beginning today. The basic problem at Home Plate may be that we are speaking of somewhat different things. The rovers and orbiters can see only the surface at Home Plate and elsewhere on Mars, and are therefore seeing only a record of the most recent events. (Note: the rovers and orbiters also happen to see things at vastly different scales.) The rock record of older events is buried, in general. I won't argue with you that Gusev was originally a large impact crater paved with impact melt and surrounded by planetwide ejecta. It then may have been filled with a frozen-over lake that deposited sediments, as indicated by the large channel that seems to drain into it. After the ice sublimed away, these lake or river sediments, if any, may have been covered with lava flows and injected with dikes, and these dikes may even have interacted explosively with wet sediments or groundwater (or ground ice) to form small pyroclastic surge deposits surrounding a maar (volcanic explosion crater). The problem is that the rovers and orbiters haven't directly imaged any of this at the surface. The rovers haven't imaged anything resembling lake beds at either landing site. Rather than intact lava flows or dikes, the Spirit rover has imaged only broken rocks. What broke them up and scattered them about, if not later impacts? Rather than typical pyroclastic surge (which, anywhere near a vent, should contain a major component of coarse material), it has imaged only layered fines (at Home Plate, anyway, just as at Meridiani). IMHO, these are most simply explained as the deposits of a distant impact or impacts, just as coarser broken material (Columbia Hills) may have resulted from closer impacts. The single bomb sag in HP, if that's what it is, is not diagnostic of either volcanism or impact.

Unlike Earth, with its extremely active tectonics (via plate tectonics), Mars (like the Moon) is tectonically nearly dead. Hills and mountains, such as the Tharsis volcanoes, can be built up or erupted, layer upon layer ("the hard way" with youngest on top), but not "lifted up" ("the easy way"), as you propose for the Columbia Hills (although an impact can deform and uplift rocks in and near the crater). One might argue for volcanism as the dominant recent event at Home Plate, but the evidence for late impact is far stronger, IMHO, in that craters are everywhere, everything is broken up (no intact lava flows or dikes) and an extremely heterogenous assortment of rocks is scattered across the surface, including the fines that make up Home Plate itself (and seem extremely common elsewhere). I realize that the surfaces of lava flows can themselves be broken up as a result of flow movement after cooling (e.g., aa in Hawaii), but such a process could not scatter basalt chunks across the surface of Home Plate (and nearly everywhere else, including the far Northern Plains of Mars).

I've personally looked at dozens of examples of pyroclastic surge deposits related to various types of small volcanoes, and, again IMHO, they don't closely resemble anything yet imaged by the rovers (either one). In fact, this very lack of close resemblance is usually used against us, especially for Meridiani. Of course, we are arguing for impact surge, not volcanic surge, but people seem to ignore this point whenever it suits their convenience. Again, a single bedding sag produced by a chunk of ejecta is not necessarily diagnostic of volcanism. I ask you, are any of the other features of Home Plate (and vicinity) necessarily diagnostic of volcanism?

Impact craters of all ages, up to and including the present, are the dominant geological features across most of Mars. Impact cratering appears to be the dominant "modern" process - the one that should be most represented in the modern (topmost or most visible) rock record. So far no mission has identified a single layer or type of sediment related to impact, other than perhaps small erosional remnants surrounding a single class of crater, rampart craters. Are isolated ejecta blocks otherwise the only rocks produced by impacts on Mars, or are important features still being missed by many in the planetary community?

Thanks for your comments. I could, of course, be wrong instead. Many here seem to think so.

-- HPD Don

Doug - I beg to differ about rocks at HP vs. Meridiani. I am not arguing that they are exactly the same (they are on opposite sides of the planet, after all), only that they appear to have formed by the same general process. That is, they resemble each other far more than they differ, mainly in bedding features. Also, HP rocks are likewise friable and salty (although not so sulfate rich as the high-albedo uppermost layers at Meridiani); they likewise contain no coarse material, except slightly coarser sand at the very base; they definitely are not "composed of hard-set basalt" although loose chunks of vesicular and non-vesicular basalt appear to be scattered around on their surface (chunks interpreted as impact ejecta). Spherules (called accretionary lapilli) have been reported from within HP proper and appear to be locally abundant in hematitic rocks in the immediate area (e.g., "King George Island" outcrop). The HP spherules are not reported as being rich in specular (hydrothermal or metamorphic shiny blue-gray) hematite, however. Most terrestrial surge beds contain no accretionary lapilli. The spherules at Meridiani do not appear to resemble actual concretions, other than being rounded, as enumerated many times above - wrong type of hematite, wrong trace element enrichment, wrong shapes, wrong size distribution, wrong spatial distribution in the rock.

Why do you then move the remainder of the discussion to features that might or might not be present on the Moon? How is this relevant to Mars? I will note that Mare lavas, other than obvious local impact-related disruptions, are all still "in place" (intact flows), unlike the wholly fragmental rocks so far found by Spirit. "Igniting" is perhaps an inappropriate word for explosive interactions of magma with water (phreatomagmatic being the technical term). These are mainly pipsqueaks geologically (the sorts of feature easily overlooked from orbit), but they do invariably leave a visible explosion crater that is usually called a "maar". AFAIK, these are utterly lacking on the Moon, owing to lack of water. They also appear to be lacking near HP, although they may be common in younger volcanic areas elsewhere on Mars.

Finally, almost no one argues that the lunar suface has been totally disrupted by impacts. Quartz veins are virtually impossible there, given the lack of water, but dike exposures would likely have been destroyed. It's a little unclear to me why many people are so unwilling to accept that impacts have similarly reworked the surface of Mars, and why many others discount the effects of the atmosphere and subsurface volatiles on the martian impact process (impossiblility of lunar-style microimpacts on Mars, for example).

Thanks again for the discussion.

-- HDP Don

Marsbug - Impacts vs. volcanism may be the wrong discussion for the Home Plate area considered in a vacuum (and for heavily cratered, obviously volcano-rich Mars in general), but is it the wrong discussion for the enigmatic finely layered rocks at the two rover sites considered together, or for similar finely layered, sulfatic rocks that orbital evidence suggests may be extremely widespread elsewhere in the equatorial regions of Mars? Impacts are fine for craters and their immediate vicinity, and volcanism for volcanoes and their immediate vicinity, or for plateau-forming lava sequences (such as Mare basalts on the Moon), but what are we to do with all the rest? I am arguing that the effects of major impacts are probably more planet-spanning than those of volcanism, and that volcanism on Mars has, over time, become so slow and so localized to specific areas, that it can safely be neglected in other areas, such as Meridiani. (In this I actually agree with the MER team.) As for HP, if impacts can explain everything you see, and volcanism can't, why do you need to invoke volcanism? I'm not saying that volcanism is impossible or didn't once occur, only that invoking it doesn't seem necessary (Occam's razor, a.k.a., the parsimony principle), especially for the finely cross-bedded layers of HP proper. Finally, if the HP layered rocks formed by the same process that formed very similar-appearing rocks at Meridiani, then volcanism can probably be excluded for both.

BTW, the linkage of major impacts to roughly coeval volcanism on the opposite side of the planet is easier to hypothesize than to prove. It has been hypothesized also for Mars (inamuch as Tharsis is roughly opposite Hellas), but is not yet proven, AFAIK.

Many thanks for your common-sense input.

-- HDP Don

Aussie - I feel an urge to point out that finding isolated chunks of dike rock, if that's what they are, is not the same as finding an igneous dike in place (i.e., they probably formed elsewhere from where they were found), and that even "dike rocks" in place could result from impact melts being forced up a crack (i.e., they might not indicate local volcanism). Thanks for remembering those two rocks, though. Solid dikes, BTW, commonly are more erosionally resistant than the rocks that contain them (i.e., they stand out clearly as linear ridges or slabs). Dikes have been identified from orbital images elsewhere on Mars, as I recall.

A pattern of resistant dikes radiating away from and/or concentric around a former volcanic center might be more convincing. The dike pattern is one way that geologists know that Shiprock, New Mexico, for example, was a former basaltic volcano, although the volcano itself (except for its neck) has been long eroded.

-- HDP Don

If you look at the arrangement of Irvine and the adjacent rocks akin to it,and also if you take into account Irvine's chemical makeup (mafic but less oloivine then plain basalts) and morphology, I have to say at least Irvine should be classified as a dike rock.

Furthermore, if Irvine were a product of impact melts forced into cracks, shoudn't it have a more different chemical composition than what has been detected?

McSween et al. (2006, published in JGR) are a little more cautious, stating in their abstract that Irvine and Backstay "are found as float on the flanks of the Columbia Hills" and "Irvine and Backstay are lavas or possibly dike rocks". In their text they say "More likely, the Backstay- and Irvine-class rocks were lofted onto the Hills as ejecta blocks or were intruded from below to form small dikes or sills." Their photos shows broken blocks crudely alligned horizontally, but no more than might be expected from multiple pieces of similar ejecta being emplaced together (i.e., definitely NOT "in place" to a geologist's eye - and many rocks of the Columbia Hills are similarly crudely layered or alligned). The lack of obvious vesicles (vapor bubbles) might argue for a dike origin, but many lavas lack vesicles too. Also, many dikes are visibly more coarsely crystalline than lavas, especially near the center of the dike, whereas no such wall to center zoning is visible for Irvine or Backstay. So I would would have to agree with the authors, no particular evidence favoring a dike origin, although it cannot be excluded. (Of course, dikes could be impact excavated too, if you want to have your cake and eat it too.) I found the complete JGR article here:
http://www.mars.asu.edu/christensen/docs/M...ev_JGR_2006.pdf

Many loose chunks of non-vesicular lava (or impact melt) are scattered across the top of Home Plate, along with chunks of highly vesicular lava (or impact melt). These all clearly are young ejecta.

As for the composition of martian impact melt, none has yet been identified as such, so I have no basis for including or excluding any particular chemical composition. Furthermore, millions or billions of years of soaking in an alkaline brine or of contact with crystalline salts might have altered these compositions before analysis, especially near the rock surface. FWIW, the above-discussed paper does note that Backstay and Irvine are unusually enriched in nickel, and then notes in passing "other rocks from the Columbia Hills have unexplained high nickel concentrations."

-- HDP Don

The authors do however consider that these rocks are volcanic from a local magma source.

Driebus et al indicated that the composition of Algonquin and Comanche resembled picrite which would seem to make them intrusive candidates? Bit on the Ni conundrum there too.
http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1649.pdf

Don Burt

One of the more thought provoking lines of conjecture for impact surge that you and P.K. mention IMO is the necessity for mechanical mixing of ancestral pre-meridiani sediments to arrive at the measured salt concentrations (as described in your 2004 GSA abstract). The beauty of the idea is its open endedness basically many observations at meridiani can be explained by seemingly predictable acts of fluid flow yet arbitrary, involving eutectic brines and assorted weathering processes - getting by the need for tight solubility calcs that JPL requires. My question, has this idea held up to the post 2004 information coming out of Meridiani (and Gusev)? I’ve included a few lines from that abstract for others:

“In this scenario, the chloride, sulfate, and bromide salts found by Opportunity were deposited mechanically along with the mainly basaltic materials, ice, and brine. Following emplacement, interactions of the fresh ejecta with the martian atmosphere over long times would allow the hygroscopic eutectic chloride and bromide salts to deliquesce and flow downwards, or be washed into the deeper regolith during ice melting to account for the vugs. Differential solubility can preferentially remove chlorides and bromides and leave sulfates. Features at the Opportunity site thus do provide additional evidence of an early hydrosphere on Mars, but it had already disappeared into the megaregolith when impacts produced this remarkable deposit.

"The other Don"

Aussie - As stated in a previous post, I have no problem with part or all of the floor of Gusev having once been covered with lava, from whatever source (given the lack of any direct indication of the possible location of a volcano or vent, the ambigous phrase "local source" must mean "down below somewhere"). Impacts could have excavated these and/or other rocks to form the Columbia Hills, and impact fines could have formed Home Plate. Impact melts or phreatomagmatic volcanism (explosive interactions with groundwater) might have been involved too - no way to tell at present (although there are multiple indicators of impact cratering and absolutely no indications of volcanic explosion craters such as maars or tuff rings).

The LPSC abstract on Algonquin Class rocks compares them to picrites or komatiites - both of them olivine-rich volcanic rocks officially (although picrite is ambiguous - the term has historically also been used for olivine-rich shallow intrusive rocks). The lavas of Mars have long been assumed to be olivine-rich, based on the high abundance of olivine interpreted from orbital spectra. The high Ni content (actually, high Ni/Mg ratio) is discussed, but not really explained, except to hypothesize vaguely that the martian mantle might be locally heterogeneous in regard to Ni, owing to bulk gravitational separation of olivine as cumulates. They neglect to mention that this process is difficult conceptually when the mantle itself consists mainly of olivine (how do you separate olivine from olivine?). They do mention that Ni enrichment in soils was earlier explained by a meteoritic component. The possibility of Ni-enriched impact melts is not discussed.

Thanks for your observations.

-- HDP Don

OD - Good question. When that abstract was written, Meridiani was being presented as something resesembling an evaporated sea or lake. The intimate mixture of highly soluble Mg-sulfates (and super-soluble chlorides and especially bromides) with nearly insoluble Ca-sulfates (as opposed to a regular bulls-eye pattern, with the more soluble salts concentrated in the center of the basin) almost demanded some sort of mechanical mixing. For us, impacts could have provided that mixing. A year after we had pointed this out in meeting abstracts, the official story was modified so that the salts were presumed to have been mechanically mixed by wind transport (even though terrestrial examples of such mixing were lacking - in rare terrestrial examples only the nearly insoluble Ca-sulfate gypsum is wind deposited). These granular salts were then presumed to have been soaked multiple times in flowing acid brines (of very special composition - simultaneously saturated with respect to every salt present, including jarosite) without in the least recrystallizing the salts, reducing the permeability, or preferentially leaching the more soluble Mg-sulfates (although some phase had to be leached to produce the cavities). These brines flowing through salts are also supposed to have uniformly deposited uniformly-shaped and sized tiny spherules of the high temperature (specular or blue-gray) form of hematite over an area the size of the state of Oklahoma, although hematite is also abundant in the matrix. (Accretionary lapilli or other spherules related to the impact or impacts was our hypothesis for the spherules, inasmuch as it satisfactorily accounted for their nature and broad distribution.)

Little new data on salts has yet been presented for Victoria Crater, and no comparisons have yet been made with the Endurance analyses, so there has been no reason to modify our original "brine splat" impact idea, allowing for post-impact weathering and diagenesis (including frost leaching of chlorides downwards and wicking of sulfates upwards, analogous to what occurs on desert mine dumps). We thought the argument might be over when nearly identically cross-bedded, salty, spherule-bearing rocks were discovered in the Home Plate area of Gusev, but we were wrong.

-- HDP Don

Edit: BTW, we had earlier (2002 and 2003) mentioned the possibility of impact mixing of salts in papers discussing the nature of martian subsurface brines.

Don Burt

For the sake of knowledge, I would like to know what's ur response on the nature of the festoon marks observed in Erebus class rocks in Meridiani planum;

The impact surge hypothesis claims the festoon marks to be product of aeolian erosion instead of flowing water in the past as concluded by the Athena science team.

On sol 632 and 633, Opportunity used its MImager to image the rock 'Kalavrita'(before and after grinding into it respectively). The rock had festoon marks on it and these marks persisted even after the rover ground into the rock. This shows undeniably that these marks are indeed due to flowing water and are not a product of erosion.

What does this mean to the validity of the Impact Surge hypothesis at Meridiani Planum?
(I hope I am not repeating a discussed issue).

Doc, regarding your post 335, and Dr Burt, please help me to understand the rover team model. I think that this is their scenario: Most of the layering in the bright surface layers (the top layers of bedrock at Endurance and Victoria and the surface rock along Oppy's traverse) indicates aeolian deposition, essentially a sand sheet. In a few places festoon cross-bedding indicates that moving surface water has rearranged the aeolian sediments. My question is this: Why aren't the two kinds of layering more obviously different? I am pretty sure that the water flow is not thought to have distorted pre-existing aeolian layering but rather would have eroded away the aeolian layers grain by grain, transported the sand some short distance and then deposited new layers with festoons indicating flow ripples. What I have trouble understanding is why the water deposited sediments have co-incidentally ended up looking so much like the aeolian sediments. The water-mobilized sediments might not have been layered at all or the layers might by clearly thicker or thinner or the grain textures might be clearly different, but the only difference seems to be a slight difference in curvature despite the two kinds of layering having been created supposedly by two completely different processes. Why have we not seen a contact between the two kinds of layering?

I attribute the festoons, if real, to minor variations in the impact-surge process. This explains why all the layering in the bright band shares many qualities and is seamlessly integrated together.

Doc - This HAS already been discussed extrensively above, and Kye just now brought up some relevant points. Also, your statement about "the festoon marks" being the product of eolian erosion is incomplete (see below).

You will not find the slang term "festoon" in many textbooks. The standard term is trough cross-stratification or small-scale trough cross-lamination (for smaller varieties). The "festoon" structure can only be seen in a cross section exposure nearly exactly perpendicular to the direction of flow, viewed looking down (or up) the direction of flow, and not in oblique sections such as you described. Except in vertical cliffs such as parts of the rim of Victoria, oblique sections are the rule in martian outcrops. Most, if not all, of what have been mistakenly called festoons on Mars are simple topographic artifacts of the oblique viewing angle caused by the elevation of the Pancam. As I stated near the beginning of this thread, picture yourself standing behind someone else, with both of you standing in water that comes up to below your waists. The horizontal water line on the other person will have the double trough shape "UU" as you look down, even though it is horizontal (I'm avoiding referring to human anatomy). You would need to have your eyes exactly at water level in order NOT to see this trough effect. The Pancam cannot bend down, and so it always sees this effect in near-horizontal beds, unless it is looking straight at a tall enough vertical cliff. The MI can bend down, but it also will always see this effect unless the surface it is looking at is perfectly vertical, and it is oriented perpendicular to that surface (its depth of field is so low that it can only view perpendicular to any surface; virtually all surfaces it has ever imaged were non-vertical). Even if a cliff is vertical, you still have to be looking exactly down or up the direction of flow to see the structure - random directions won't do.

If, despite this intrinsic problem with rover imaging of outcrops, you believe that some Mars "festoons" are real, consider the following: Although the migrating ripples that form "festoons" are widely believed to indicate flowing water, experts who have spent their professional life looking at volcanic surge deposits have found and photographed essentially identical features in those beds. After we showed some of these photos at LPSC, a non-volcanologist, who had never visited the localities in question, stated that these terrestrial deposits must be water-flow deposits, despite their occurrence in the midst of a pyroclastic flow deposit. If you choose to ignore the volcanologists (not me, but some of my co-authors) and want to believe this person about flowing water, consider this. Wet impact surge, and formation of accretionary lapilli (a.k.a. blueberries or spherules), implies condensation of steam. Condense a lot of steam and the result can be flowing water, either from rain (or snowmelt) or as excess moisture squeezed out very wet sediments. So there is no incompatibility between water flow and wet impacts (water is implicit in our initial "brine splat" term anyway).

That said, I would want some independent evidence of water flow across the horizontal surface at Meridiani, besides the enigmatic (and quite possibly non-existent) festoons. There are too many good arguments against water flow. For example, apparent festoons inexplicably occur in the midst of beds that have been called eolian, with no discontinuities (as pointed out by Kye). Everywhere else on Mars, channels are taken as evidence of water flow. So show me some water channels (keeping in mind that other phenomena, such as glaciers, submarine density currents, and vortices in expanding, decompressing surge clouds, can also scour channels).

Ultimately, you'll have to decide what the evidence supports. Mars, like the Moon and Mercury (and like Earth before the evidence was erased by plate tectonics), apparently was impact craterered to saturation about 3.8 billion years ago. That was THE major event in Mars history, as it was for the Moon and Mercury. That's when Mars lost 99% of its atmosphere and when clays apparently ceased forming by weathering. Since then, except for the wind, localized dying volcanism, local brine breakouts, ice redistribution and movement, and continued impact cratering, Mars has been a pretty cold and desolate place (life among the ruins, as it were). If you choose to ignore the evidence of impact cratering, then you may choose to believe the extremely complex Earth-like scenario that has been derived for Meridiani, one that is not applicable to similar beds elsewhere on Mars (and one that is not well supported by available evidence, IMHO). The impact explanation, whatever its loose ends (and it has a few), accounts for every observation made to date (by both rovers), is very much simpler, and potentially is much more generally applicable to the rest of Mars.

Apologies for getting so technical.

-- HDP Don

Time to close the playground again. It's a subject that has been done to death, into the undead, and now to death again. Everyone has had more than a fair chance to air the opinion, interpretation and respond appropriately. It's not being closed because of the subject, it's being closed because it's going in circles - tens of thousands of words of circles. Posts covering this ground elsewhere will simply get culled, we're done here. Thanks to all who've contributed from all sides - but I don't think anyone could say we're getting anywhere.

Doug