Yeah -- that's a challenge. There are a lot of differences between obviously younger craters and ancient, degraded craters, including the old stand-bys of sharpness and blockiness. For example, Endurance and Victoria, while far different in size, look quite similar in morphology (at least from the orbital images) and might well be of similar ages. The cluster of which Erebus is a part is obviously a *lot* older and more degraded.

But, if I'm reading it right, it appears that the ancient cluster has left an uneven terrain that promotes selective wind erosion, while the impacts causing both Endurance and Victoria did not. That might be a matter of the size of the impacts in the cluster, or the interaction of multiple ejecta blankets (assuming the cluster was made all at the same time, which is not a very provable assumoption). But it also might be that the ancient craters struck water-laden target rock and the later impacts of Endurance and Victoria struck mostly dried rocks. My thinking is that the difference in mass of volatiles in the target rock would make a difference in how impacts make the landforms deform, and could cause the relief that has created the etched terrain appearance (explaining why ejecta blankets around ancient craters are still somewhat preserved in the etched terrain, while ejecta blankets around the younger craters have either been completely worn down or are in the process of being flattened out of view).

That's where I'm getting my (admitteldy WAG-ish) hypothesis that the ancient crater cluster was formed when the target rock was water-laden, and following it up with the further supposition that the resulting craters and ejecta were further modified by flowing and/or standing water by "cementing" the ancient craters' ejecta patterns with further overlays of evaporite that has "filled in the cracks" in the ejecta blankets.

I think maybe the *only* way we can establish this kind of stratigraphic sequence in this area is going to be determining if impacts occurred in wet or dry rock -- and especially at this site, where there has been so much erosion of what seems to be a *very* soft and erodable evaporite layer, that's going to be even tougher. But since I observe that the erosion patterns, as well as the erosion results, of obviously older craters seems so different than the erosion patterns we see at the younger, sharper craters, I can't help but come to the conclusion that such a difference in the groundwater state at the time of the impacts may well be a factor...

-the other Doug

I disagree that the rise ahead is Victoria ejecta. It's too widespread and doesn't appear to be situated right to simply be Victoria ejecta.

There is a crater roughly, what, four times the size of Victoria directly to the south of Erebus (the one I'm speaking of is very degraded, has a very dark duned plains floor, and probably looks a lot like Vostok in being eroded nearly to the ground). It, and a cluster of other craters similar in age and degradation (including Erebus at the northen border of the cluster) seem to sit along a ridge of somewhat higher elevation than the plains where we landed. It's possible that the rise in elevation we're looking at is an ejecta pile from that ancient crater cluster.

Also arguing against this rise being Victoria ejecta, in my opinion, is that Victoria seems to lie on a plateau of the same kind of very flat plains unit where we landed. The MOLA data shows that it's a little higher in altitude than the plains where we landed, but it looks very similar.

I'm guessing that the ejecta from the ancient crater cluster caused an initial landform rise, which was heavily modified by continuing epochal inundation by the brine sea we know was there. Once the sea receded for good, the relative roughness of the crater cluster terrain drove selective aeolian erosion patterns, which have resulted in the "etched" appearance we see today.

Finally, relating to the smoothness of the "plateau" on which Victoria is located -- if we postulate that the plains where we landed *and* the plateau on which Victoria is located were all formed by the slow wind erosion of a reasonably flat evaporite layer, and the etched terrain was formed by wind erosion of jumbled and water-modified ejecta blankets of a cluster of fairly large craters, then it stands to reason that Victoria was formed well after the altitude rise and the differentiation between flat plains and etched terrain had already developed. Otherwise, we'd have to dismiss ejecta roughening as the reason for the types of wind erosion we see in the etched terrain.

-the other Doug

I have a hard time believing the crater chains are simply lines of secondaries. The dynamics are all wrong.

I assume Moore et. al. are suggesting these crater chains / chasma are basically cracks caused by a large impact? Some of the chains are indistinct enough that I could believe it, but the younger ones (especially the one in upper left of the image being discussed) are obviously chains of circular depressions. And while diatremes could form a series of circular depressions along a fault line, how do you get diatremes without volcanic activity? I don't see any other evidence of cryovolcanism on Rhea (unlike what we see on Enceladus, for example).

Remember, a lot of people were absolutely convinced that the Davy crater chain on the Moon *must be* an endogenically-controlled chain of diatremes, but the data indicate that they'r really just a chain of impact craters. I think Occam's razor suggests these chains on Rhea are also chains of primary impact craters.

Both of my main concepts for how Rhea could develop such crater chains as direct primary impacts -- streams of outflung ring material and/or tidally shattered fragments -- can account for such chains seeming to lie radially away from a central larger impact. All you have to do is postulate a stream of roughly similar-sized fragments with one significantly larger fragment embedded within it.

Besides, it really looks to me like many of these crater chains are closer to parallel than being arrayed radially to a given point -- though until we get better global coverage, that's harder to pin down.

-the other Doug

Yeah -- we can make some pretty decent estimates of stratigraphic sequence, and we can try to gauge erosion to make a wild-ass guess on the amount of time a given object has been in its current position, sure. But any guess of this type would really be WAG-ish, with vast error margins.

Honestly, Viking looks a lot younger to me than Eagle, but significantly older than Fram and what Doug has named Mini-Fram. There is almost no sign left of Eagle's ejecta blanket, while Fram and Mini-Fram sport obvious debris aprons that seem to sit right atop the plains material (and even atop some duneforms). Viking's ejecta blanket seems to be sort of midway between the two -- there is a lot of jumbled evaporite lying out on the surface outside of the crater, but much of it has been filleted, partially buried and even eroded down significantly closer to flat than we see around Fram and Mini-Fram. And while Viking's floor is duned and completely filled with dark concretion materials (plus windblown basaltic dust), you can still see some relief created by the jumbled-rock floor that lies beneath the dust/concretion fill. Eagle's floor had completely smoothed out, with no sign remaining of the jumbled relief that must lie below its current floor.

I think one of the factors here is that Eagle is slightly larger than Viking, and probably sees a slightly different wind pattern (as is evidenced by the differences in duning between the plains around Eagle and those around Viking). But I also think that Eagle is significantly older than Viking, and the additional aging and erosion have smoothed out its features far more than has occurred at Viking.

Of course, there is always another possibility, one that has occurred to me more than once -- it may be that Viking, like Fram and Mini-Fram, is a true impact crater, while Eagle is a sinkhole (i.e., not formed by an impact event). That would explain the differences in the crater morphologies, as well. However, I think that it's more likely Eagle and Viking are both impact craters, and Eagle is just older.

-the other Doug

Yeah -- most (though not all) of the crater chains *do* seem to be organized along similar orientations relative to Rhea, it's true. And that does give them the appearance of being endogenically controlled.

I have an image, however, of some type of gravitational interaction flinging a stream of ring particles out of their orbits and out into the realm of the icy moons. Assuming there was no out-of-plane vector associated with the interaction, such a stream of particles would strike a moon along the ring plane, leaving chains of craters in its wake. The fact that these crater chains are no longer aligned with the ring plane could invalidate that theory, or could simply mean that Rhea's orientation to the ring plane has changed since a majority of the crater chains were formed.

On the side of structural control, we've seen these kind of "cracks" in the crusts of most of Saturn's icy moons, though in most cases they don't appear to be formed by chains of individual craters. And as they say, zero, one and many are significant findings, but "a few" means you probably don't have a complete dataset. In other words, if two other Saturnian icy moons display endogenic cracks modifying their surfaces, and we see what appear to be similar (though not identical) cracks on Rhea, the odds are high that the processes are interrelated, if not the same...

I think the jury's still out as to the formation of these crater chains, but the more data we collect, the closer we get to coming up with better theories...

-the other Doug

First off, a meteor the size of a car would blast a crater a mile or more wide. A meteor the size of a house would blast a crater something like five to ten miles wide. Viking was made by a meteor more the size of your fist (by the time it impacted, anyway).

Secondly, the rovers don't carry the kind of equipment needed to come up with wild-ass guesses as far as absolute ages of rocks are concerned. There is no way at all of even guessing how old that meteorite was, or how old any of the rocks that have been analyzed are. (There is a good thread in the MSL category on the kinds of equipment needed to date rocks even close to accurately.)

Finally, I don't know that anyone has ever calculated the difference in the number of Mars-orbit-crossing objects and the number of Earth-orbit-crossing objects, but I'd be willing to bet that the difference is fairly minor. Over billions of years, the population of objects thrown towards the inner system from the asteroid belt has probably evened out and is relatively constant from the inner edge of the belt all the way in to the Sun.

You do make a decent point that Mars has a thinner atmosphere than Earth's, and therefore more smaller objects reach the ground there than here. But remember that a vast majority of the meteorites seen on Earth are the size of a grain of sand (or smaller), most of which are the remnants of broken-up comets -- which will burn up in Mars' atmosphere just as easily as they do in ours. And while there more fist-sized objects that reach Mars' surface than ours, I'm sure, I doubt the rate of impact is any higher than we see, say, on the Moon. And the seismometers we left on the Moon showed that such impacts are pretty rare -- only a very few were seen in the years the ALSEPs were operated. And only a single impact of a car-sized object was recorded on the Moon in those years. (Not counting man-made objects, of course.)

We really don't have good enough empirical data to characterize the meteor flux at Mars. I'd be willing to bet, though, that any crater at Meridiani where the rocks of the exposed evaporite layer are semi-rounded and have shed a bunch of concretions (as Viking appears in the images) wasn't made in the last few hundred years. A few hundred thousand to a few million, I'd buy...

-the other Doug

More like fall and winter vacations. Considering how much larger the dunes become between here and Victoria, and considering how much of the evaporite layer is exposed (and just begging to be analyzed) within the etched terrain, my guess is that it will be September or October before we actually reach Victoria.

-the other Doug

These don't look anything like grabens to me. Yes, some are degraded (the larger craters tend to overly the crater chains), but many of them seem to be made of individual craters, lined up shoulder-to-shoulder for tens of kilometers, as sharp and fresh as any of the other craters on Rhea's surface.

Just so I know we're all looking at the same things, I drew red circles around what I'm calling crater chains. Note that most of them tend to be organized along the same general direction, with one obvious exception:



When I zoom in on this image, the linear features I'm calling crater chains really look to me like they're made up of tens of individual craters... and while degradation varies, especially in the uppermost feature, they seem no more degraded than the majority of the non-linear-aligned craters in their vicinity. (The "gash" I mentioned is the lowermost feature, halfway obscured by the terminator.)

But they don't resemble any graben I've ever seen.

So, what do y'all think -- endogenous or impact features? And if impact, how did so many align along the same general trend?

-the other Doug

I see a relatively large number of long crater chains -- including one just on the lighted side of the terrminator which looks like a huge gash in the side of Rhea.

I've seen a lot of crater chains on the other icy Saturnian moons, too.

Seems like we see more crater chains on these moons than we see on other solar system bodies of similar size. Including comparable Jovian moons.

My understanding is that such crater chains are thought to form when an impactor breaks up due to tidal forces (or due to an earlier encounter with a steep gravity gradient) but whose pieces fly along relatively closely-spaced until they impact another body. Rather like the way Shoemaker-Levy 9 broke into a "string of pearls" formation of comet chunks. Of course, there is always the alternate interpretation of such crater chains as endogenically controlled series of maars, though I'm not sure how that would work on an icy moon.

Any theories out there on why we're seeing more evidence of this kind of thing (i.e., the crater chains) in the Saturn system than we have seen anywhere else in the solar system?

-the other Doug

Honestly, I think a majority of Americans wouldn't care all that much. It's only a relatively small lunatic fringe here that is swayed by the anti-nuke Chicken Littles.

I may be a little overly optimistic, here, but I think a large majority of the general public understands that the risks of disaster are relatively tiny.

-the other Doug

Not if the launch goes as planned, no. But any time you launch a spacecraft with plutonium on board, a launch failure has the potential (however remote) of introducing said plutonium to the environment. The E.I.S. is just a safeguard to make sure NASA has taken reasonable precautions against any negative environmental impact from plutonium handling before, during and after the launch.

I'm not one of the anti-nuke whackos, believe me -- I believe NASA does, indeed, take all reasonable precautions. But I also think it's a good idea to hold *anyone* using or handling such dangerously toxic materials as plutonium to a very high safety standard.

-the other Doug

Cojones? I dunno -- it's a matter of risk vs. reward. If you flew one of those things, you got a shot at landing a LM on the Moon. That's a pretty decent risk investment, in my book... *grin*...

-the other Doug

Hey, yeah! The sand in the sun is a *lot* lighter than the sand in the shade...

Seriously, yes, I think there are some noticeably lighter grains in the sand here. Looking at the high-res orbital imagery, we should expect to see an increasing population of lighter-toned sand, preferentially distributed along what appear to be the leeward sides and bases of dune formations. A lot of the "etched" terrain detail appears to be made up of lighter-colored soils displayed preferentially along leeward dune sides as you get close to stretches of true evaporite outcrop. (Whether the lighter dust is displayed via depositional or excavative processes is hard to say, of course.)

I would think that the regolith in the area we're approaching (that looks considerably lighter in the orbital imagery) is of different composition than what we've seen thus far. Maybe it contains more evaporite erosion products than the soils on the flatter plains to the north? And a smaller proportion of concretions and concretion erosion products? That could point to either different erosion patterns (because of the relief from the large, old degraded crater cluster that includes Albert) or a change in concretion production from north to south.

Either way, it's just *fantastic* that Oppy is capable of driving several kilometers from its landing site in order to characterize such differences over distance. Brings tears to my eyes, I'm smiling so hard...

-the other Doug

Yes, it does, doesn't it?

"Fram-ish" could be better characterized, I think, as "a relatively young, fresh impact crater with blocky inner walls and local blocky ejecta arrayed roughly one crater diameter out from the rim."

I'm fascinated by the erosion patterns being defined by these crater morphologies. Obviously, Viking is an Eagle-sized crater and both seem to have been caused by very similar impacts. Viking, like Fram, has a more extensive exterior ejecta blanket and blocker inner walls -- it seems to be younger and fresher.

The lack of inner blocky walls in older craters is a no-brainer -- mass wasting and slow aeolian deposition means that rubble slides to the bottom and is then covered by wind-blown sediments. But the virtual erasure of the ejecta fields around Eagle-like craters shows just how easily eroded the excavated rock must be. Especially compared to the blueberries that resist erosion. Or even compared to basaltic rocks, like the ones we see at other landing sites -- at Gusev, for instance, the rocks haven't been eroded down to a smooth plain by billions of years of aeolian erosion, like they have at Meridiani.

I think there is a paper to be done, somewhere, on the erosion patterns of the concretion-rich evaporite rocks at Meridiani Planum. You'd almost think that, if aeolian erosion is the only factor in the reduction of the evaporites and subsequent paving of concretion materials, that you would only erode the evaporite down a centimeter or two, until it developed a thin layer of uneroded concretions that would protect it from the winds. Obviously, there has been significantly greater erosion than this, or else Oppy's tracks would be uncovering evaporite subsurface everywhere it went... so the evaporite must have continued to erode after it developed a thin layer of concretion materials.

I'm sure that aeolian dust deposition has something to do with the current state and depth of the regolith, as well -- but if we postulate that the depth of the regolith is primarily due to windborn dust deposition, you have to explain what "gardening" processes result in so many concretions sitting at the surface. And in any event, adding more covering material in the form of windborn dust deposition just reduces the evaporites' exposure to wind erosion even further, and there has to have been a *lot* of evaporite erosion to account for all those loose concretions.

I also have to wonder -- the composition of Meridiani soils and the evaporites are somewhat different, right? The soils seem to be basalt dust/sand (windblown deposition) mixed with concretion elements and *not* an unusually high concentration of the salts and sulphates that make up the evaporites.

So, when the evaporites eroded and left all these concretions littering the landscape -- where did the material go? Is it so thoroughly mixed with the global dust that it's one of the major constituents in the global dust's sulphate and salt content? Or are there large "traps" of eroded evaporite dust lingering somewhere nearby?

Just curious...

-the other Doug

I think you're right, people will eventually settle Mars. But they will have to get hardened to a lot of things we here on Earth would consider hardships.

One thing I keep thinking about, in terms of living on Mars (or even visiting it) is what we know of the composition of the soils. There are a lot of sulphates and salts of different kinds in the average Martian soils, plus a lot of good old fashioned rust.

I'm thinking that, once our intrepid Martian explorers come back inside their ship with their dirty boots and dirty suits, they're going to find that Mars really stinks. I may be wrong, but it seems to me that you put that much sulphur into the mix, you're going to get a broad range of objectionable odors.

Has anyone ever tried putting together a mix of elements to simulate Martian soils? If so, I'd be very interested to see what they say it smells like...

-the other Doug

Agreed! Steve Squyres is the best thing that ever happened to the Mars exploration program, as far as NASA PR is concerned. He is so unabashedly excited and emotionally involved in this work that he transmits the tremendous excitement of exploration to the general public.

If we want more public support for Mars exploration, we could do worse than have Steve Squyres up front as its public face. A lot more often.

-the other Doug

Oh, and BTW -- when I say "significant ridges" in the etched terrain, I don't necessarily mean *tall* ridges. Just significant in extent. The exposed rock may only stick up a foot or two from the surrounding surface. Or maybe even less. But I expect to see contacts between the impact-uplifted ridges, the surrounding evaporite/sandstone fill, and the dark regolith deposition.

In other words, a geologist's paradise. I'd really like to watch Squyres as Oppy rolls on into the etched terrain... *grin*...

-the other Doug

I've been looking at this image of Oppy's immediate destination:

http://www.msss.com/moc_gallery/r10_r15/me...15/R1500822.jpg

Staring at it without my glasses, slightly off-focus (a good technique, sometimes) I see that the etched terrain starts to resemble the splash-pattern ejecta blanket structure common to a lot of Martian craters.

There is a cluster of old, degraded craters just west of Victoria, of which Albert is one of the more distinct. The etched terrain seems to be patterned like splashed, semi-fluidized ejecta from these craters, which has been heavily eroded in the same manner and by the same processes that have eroded Albert and the other similar craters in the cluster.

I'd guess the timeline on the formation of the Victoria/Albert area is:

- Shallow seas form and evaporate over a few million years, laying down layers of sandstone and evaporite rocks.

- A cratering event makes a cluster of craters, including Albert and its neighbors. The target rock was either covered with ice or water, or had significant groundwater, resulting in the characteristic Martian splash-effect ejecta pattern.

- Seas continue to flood the area periodically for another few million years, eroding Albert and its neighbors, laying new layers of sandstone and evaporite on top of and within low spots within the craters and the ejecta blanket. This process comes close to smoothing the craters and the ejecta blankets out to a smooth plain, but remnants of the ridges in the ejecta and the crater rims are preserved.

- Mars gets really cold and dry for one or two billion years, and the blueberries (plus other dark minerals) erode out of the upper layers of the evaporites that cover this area. The landscape of exposed evaporite rock is slowly covered by a dark regolith made up of eroded blueberries and sand/dust imported via dust storms.

- The area around Albert and its neighbors consists of slightly bumpier terrain than that to its north, caused by the remnant ridges of crater rims and ejecta features sticking up over the final evaporite deposition layers. Wind erosion becomes preferential around these ridges, and the preferential deposition of the darker regolith forms the "etched" look.

- After most of these surfaces had become mature and resembled what we see today, Victoria was formed. Additional wind erosion has smoothed over most of Victoria's ejecta blanket, but since the target rock had lost most to all of its volatiles content, Victoria did not leave as noticeable of a splash-pattern ejecta blanket. It simply punched through the existing layers of sandstone and evaporites. The only deposition in and around Victoria since its creation has been aeolian.

This all suggests that we'll find some significant evaporite/sandstone ridges within the etched terrain, excavated by the Albert impact event and which predate the evaporite fill layers around them. It seems to me that, since Albert and the rest of the craters in this cluster are larger than any of the other impact features Oppy has visited, the rocks excavated up to its rim would be from the deepest layers of the sandstone/evaporite beds. Even though the craters have been heavily eroded, I'd think that, of the range of rocks available to you on the surface, you'd still find the oldest rocks from the deepest layers along the crater rim and ejecta blanket ridge lines in this old, degraded crater cluster.

We could also possibly find, in wind-sheltered strips along these ridges, a surface that still closely resembles the original evaporite surface that once covered the entire area -- the *original* dried seafloor. And we'll also find more robust dune development because of the wind-relief effects from the slightly raised rocky ridges.

What do y'all think?

-the other Doug

Looking at the stereo image carefully, I'm wondering if that is, indeed, a rock. It might be a rover wheel track cresting the ridge of one of the small dunes.

-the other Doug

Hissss.... Groannnn....

Seriously, though (and I'm delighted to be asking this seriously), anyone wanna wager on whether one or both rovers make it (in some kind of useful condition) to Sol 1000?

Of course, Steve Squyres might end up in the hospital from exhaustion by then... *smile*...

-the other Doug

It's my understanding, from the analyses of the rocks on the Gusev plains I've seen and been told about, that Gusev was filled with lava flows *after* the Noachian times in which it may have held a lake. This is especially suggested by the way in which the basalts of the plains embay those elements of the original sedimentary surface that were topographically higher than the mean level of lava fill.

In other words -- aren't the original delta materials, and any other lacustrine materials, out on the plains buried under tens of meters of basaltic lava flow? (I mean, it was my understanding that Bonneville proved to the geology team that the layer if basaltic lava was definitely thicker than Bonneville had excavated -- thought would be in the tens of meters, right?)

It would take a hell of a dust devil to get down to that layer, wouldn't it?

-the other Doug

I think a lot of the surficial layer's characteristics have been more affected by wind than by any other force. Both constructionally and destructionally. Yes, volcanic activity, ice and flowing water have obviously had a part in forming the landscape. But those forces have been dormant, or nearly so, over the vast majority of the planet for millions, if not billions, of years.

With what I am sure are extremely rare exceptions, the only things left that continue to shape the surface of Mars are the winds and the occasional impact. And while the air is thin, it can hold millions of tons of dust, enough to blanket the whole planet. That dust gets swirled around and dumped back out, then picked back up and swirled around some more, and dumped back out again... year after year for hundreds of millions of years.

No wonder that many of the smaller impact craters on the planet have been filled in with dust. The amazement would be if a lot of them hadn't been filled in already, I think.

Of course, let's not forget the one other active surface modifier on Mars -- the polar mechanisms. The seasonal freeze/thaw cycle at the poles must create a lot of small-grain particles as a by-product of the process, and of course entrained dust within frozen CO2 does get re-distributed when the dry ice sublimates in the spring... The polar mechanisms have a great effect on the arctic and polar landscapes, I'm sure, but they do also have a certain impact in the dust circulation cycle that affects the rest of the planet. Which, of course, are driven by winds -- getting us back to where we started.

-the other Doug

Since you don't want to get into Soviet/Russian lander failures, looking at just American attempts, there have been three loss-of-vehicle failures in thirteen attempts at lunar/planetary soft landings(*). 10-3 is a pretty decent track record.

Now, if you add in American attempts at hard landers, at which we're zip out of five(**), you're still talking seven failures out of seventeen attempts. 10-8 is still a winning record.

Finally, if you include American manned soft-lander attempts into the total -- one failure (in which the landing itself wasn't even attempted) out of seven tries(***) -- you achieve a whopping sixteen successes versus only nine failures. Winning nearly twice as often as you lose will get you thrown out of any casino in Vegas. And if you look just at American soft-landers, both manned and unmanned, the record is a phenomenal 16-4.

So, I'm thinking maybe NASA is a little better at landing probes on other planets than you suggest... *smile*... At the very least, "most landers haven't worked" is an inaccurate assessment of American lunar/planetary landing attempts.

All of that said, I agree with your main point. We learn from our failures just as much as we learn from our successes. In fact, we learn the things we want to learn from our successes, but we more often learn the things we need to learn from our failures...

* - American soft-lander failures: Surveyors II and IV, Mars Polar Lander. Successful soft-landers: Surveyors I, III, V-VII; Vikings 1 and 2; Mars Pathfinder; MERs A & B (Spirit and Opportunity).

** - American hard-lander failures: Rangers 3-5; Deep Space 2 (two hard landers). No successful hard-landers.

*** - American manned soft-lander failure: Apollo 13. Successful manned soft-landers: Apollos 11-12, 14-17.

-the other Doug

It occurs to me that there are two rather obvious mechanisms that could be limiting the obvious signs of cratering on Titan. The one that seems to get all the discussion is weathering/resurfacing -- it's common to date planetary surfaces by crater counts, after all.

But Titan has another mechanism going that we here on Earth are all too familiar with -- atmospheric destruction of impactors. Titan not only has an atmosphere, it's something like 1.5B at the surface, so it's even denser than our atmosphere. Result: a lot of the impactors that would leave visible craters on other planetary bodies burn up in Titan's atmosphere.

Also, while some reports are saying that the radar has discovered the only two craters found on Titan, I seem to see craters of varying sizes and levels of degradation in both the radar images and the ISS images. Circular features that remind me strongly of craters, at any rate. Not very many of them, but they're there, I think. So, there ought to be a way to use crater counting, even on Titan.

So, I guess my question is: Is there a correction algorithm of some kind that can account for the winnowing of impactors by Titan's atmosphere? If so, can it be applied to help us use crater counting to determine surface ages? And, I guess more basically, what's the current thinking on how important atmospheric winnowing is in the lack of numerous Titanian craters?

-the other Doug

I'll second that! Three cheers and a case of Mars Bars for Doug!

-the other Doug