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Posted by: vexgizmo Apr 6 2006, 02:24 PM

OK, it's time to have it out. Is Enceladus really spewing water, or are its fractures effectively sublimating warm ice like a comet?

Have a careful read of the Enceladus Science papers (specifically Porco et al vs. Spencer et al.) and you will see that the evidence for water is equivocal, and arguably circular. The prime piece of evidence for liquid water (Porco et al) is the inferred high ice/vapor ratio of the plume (top of p. 1398). This is inferred from scattering models and assumptions of plume particle sizes and argued unlikelihood of particle entrainment in sublimating gas (explained briefly in their note 30, and into p. 1399). Should we hang our conclusions, exploration strategies, and hopes for life on moels of ice/vapor ratio, particle size assumptions, and inferred difficulty of entraining particles in sublimated gas?

Instead (Spencer et al), the fractures of Enceladus may simply expose warm (T ~ 180K) ice which sublimates like a comet (p. 1405).

Show me the water.

Posted by: ugordan Apr 6 2006, 02:27 PM

To be fair, isn't the evidence of water on Europa circumstantial as well? Granted, it's very strong evidence, but one could still also say about Europa: "Show me the water".

Posted by: The Messenger Apr 6 2006, 02:54 PM

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Circumstantial, and a little curious: Shallow craters. Assuming they were originally much deeper, and backfilled with liquid water, why do they contain the central peaks characteristic of 'solid' crater floors? I've never seen ice bunch up at the center of a frozen lake.

I'm also curious about the assertion that water vapor sublimes away from comets. Haven't we observe rather discreet jets on both Wild 2 and Tempel 1? What am I missing?

Posted by: djellison Apr 6 2006, 03:23 PM

Look at the phase diagram of water

http://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/cha/T073590A.gif

At deep space pressures ( i.e. very near zero - and indeed anything below 6mb ) - water doesnt exist at any temperature.


Also - what's wrong with a central peak in a crater that is then backfilled with water?

Doug

Posted by: vexgizmo Apr 6 2006, 03:29 PM

It is H2O gas that sublimates away--that is what is intended and implied. Cometary jets are presumed to be places where ice is exposed at the surface, rather than mantled by non-ice debris.

Posted by: Bob Shaw Apr 6 2006, 03:38 PM

The impact event happens, the rebound of the central peak happens, then the crater floods with meltwater, which freezes over quickly enough not to sublimate away.

Bingo!

Bob Shaw

Posted by: The Messenger Apr 6 2006, 04:49 PM

That works well, if the original central peak was above the backfill level - which I assume is what you are telling me. If the original crater was a factor of 10 deeper, and the top ~70% was water, I would think that the original central peak would be small, since all of the water on the surface at the time of the impact would have vaporized. This is what I am trying to understand: How deep do they estimate the crater was before the backflow, and how high should the central peak be?

Posted by: volcanopele Apr 6 2006, 05:18 PM

Getting back to Enceladus, the case for liquid water is based on the amount of solid particulates in the plume compared to the amount of water vapor in the plume as seen by UVIS, the lack of ammonia within the plume as measured by INMS, and the appearance of the plume (collumated jets, extended plume shape) in ISS images. The calculated abundance of water ice particles in the plume is based on the assumption that the effective size of the particles is 1 micron. Where that number came from, I am not sure. The particle size distribution was calculated such that the number of particles above 2 microns would match the abundance found by CDA.

Now, there are a few problems I can see, admittedly, but there are ways Cassini can plug those holes. First, the effective particle size is assumed to be 1 micron. I am not sure where that number derives from. However, using multispectral observations of the plumes, using both VIMS and ISS, I think that the effective particle size can be determined as the mission runs along. Maybe VIMS already has that from their Rev18 data. According to http://photojournal.jpl.nasa.gov/catalog/PIA06443 , the average particle size is 10 microns...

The second problem that I see is the use of data from different time periods. All the CDA, INMS, and UVIS data was taken during the Rev11 flyby of Enceladus while the ISS data is from Rev18 in late November. The MIMI instrument (Jones et al. 2006) determined that there is likely significant variability in the amount of outgassed material. UVIS observed indirect evidence for a major outburst in early 2004. So perhaps the data from Rev18 and Rev11 can not be directly compared. Combined observations more directly linked in time may be needed to resolve this issue, perhaps during Rev61. The Rev61 encounter is a much closer encounter with Enceladus (currently slated for a 23 km close approach distance). Observations by CDA can be used to determine particle abundance at 2 microns. ISS possible could image "glowing" plumes when Enceladus is in eclipse (much like Io).

So, I understand where vex is coming from. The case for liquid water is not a done deal, but based on our current data, it is better than the alternative, sublimation model. It isn't enough to drop Europa in favor of Enceladus, yet, but data gathered over the next few years, I think, can more solidly make the case.

Posted by: scalbers Apr 6 2006, 05:42 PM

Or would you want to more "liquidly" make the case?

Actually the cometary jet question is a good one in terms of whether comets can make discreet jets without liquid water.

I wonder also how a glowing effect might be produced in the event of an eclipse?

And, would a hypothetical 10 micron mean particle size (vs 1 micron) imply a greater ice abundance, and better changes of liquid water in the geysers?

Posted by: hendric Apr 7 2006, 02:40 AM

How much of a difference in plume velocity would there be between warm ice sublimation and liquid water evaporation? Enceladus is much larger than the average comet, but the 212 m/s escape velocity isn't that high.

Would warm ice sublimation (WIS) tend to be a self-limiting effect, as dust etc covers the top of the ice, a la comets or glaciers? With liquid, dust etc could get blown off easier or end up sinking in the water.

Could warm ice continually sublimate for long periods? IE, how fast would the warm ice have to be moving towards the surface to keep sublimating at the same rate, given an estimated crack length and width? It would be easier for a reservoir to feed liquid vs solid.

Posted by: Richard Trigaux Apr 7 2006, 07:46 AM

Even if it is "only" ice sublimating like on comets, it is still very interesting. It is well known that comets release a quantity of dust, even if it is mainly from sublimation of ice (dust particules would be released when the surrounding ice disappears).

That makes that, even in the hypothesis of sublimation, if there are biological molecules or particules, it is still possible to catch them with the equivalent of the stardust mission to Saturn passing through the plumes.


http://www.msnbc.msn.com/id/11744581/


http://www.unmannedspaceflight.com/index.php?showtopic=2396 with my proposition of an effective trajectory to catch Enceladus dust and bring it back to Earth.

The above idea would be much cheaper than a complex orbiter/lander project like an Europa lander, so that it does not come at the expense of such a large mission. At at least it would give us hard evidence in only some years.

(Please if you want to switch the discussion on this idea, go to the appropriate thread. This one is about liquid water or not)

Posted by: The Messenger Apr 7 2006, 05:31 PM

Spawling occurs in nozzle throats when there is differential heating within the carbon cloth / phenolic resin matrix. It is thought to happen directly beneath resin-rich pockets in-between the plies. Since carbon cloth conduits heat much better than the resin, the area under a resin rich region will be super-heated from the sides inward, rather than directly above. In addition, resin rich regions are less porous, therefore any volatiles generated are easily trapped and build up pressure.

I mention this seemingly unrelated topic because of the observational results: High speed video of a spawling nozzle shows round, ice cream scoop-like pockets of nozzle throat material blasting into the gas stream, and leaving crater-like pockets. The video resembles what was observed in the jetting on Wild 2.

I can see a strong analogy with what might be happening on and in Enceladus: Water underneath the ice is differentially heated from the bottom up, leading to explosive bumping of water vapor, cracking the ice and propelling the water vapor/particles to escape velocity. What I am suggesting is that the venting proceeds like superheating: Once a critical point is reached, there is a cascading effect that propagates outward and upward in a conic circle, directing the plume upward.

Posted by: tty Apr 7 2006, 06:45 PM

Well lets see now... Warm active glaciers move up to a meter per hour, but that is mostly basal sliding, which requires liquid water. Cold-based glaciers which move by internal creep are much slower, say from a centimeter to a meter per day. For ice at 180 K which would be quite stiff I think a centimeter per day is high, but let's assume it.

The amount of H2O emitted has been quoted as 150-450 kg/s which comes to about 1.3 - 3.9 x 10^10 cm^3 per day, i e 1 cm/day over 1.3 - 3.9 x 10^10 cm^2 = 1 to 4 square kilometers, or a 200 to 800 km long and five-meter wide crack.

It sounds possible, but perhaps a bit on the high side.


tty

Posted by: edstrick Apr 8 2006, 09:58 AM

Considering geysers vs sublimating ice on Enchiladas <grin>...

Imagine ice with a high thermal gradient... cold at the surface, warming rapidly with depth, becoming plastic when the temperatrue approaches melting, then with liquid water, perhaps in a pocket or linear body caused by an opening caused by fracture at depth.

Imagine the ice is indeed fractured, which may have been intruded by water, but is currently not open to water at the bottom, due to ice creep or whatever reason.

The cold ice at the surface doesn't sublimate. Go down into the crack.. the surfaces are facing each other more than they are sky, and can get warmer due to the heat flow from under the surface than bare sky-looking ice with the same heat flow. The warmer ice will tend to sublimate, possibly some tending to freeze on the edge of the crack where it opens on the surface and the ice is colder. Deeper in the crack the sky is less exposed, you are deeper in the regional heat gradient and possibly closer to a local heat source.... the ice is warmer and sublimates faster, vapor escaping upwards into the colder crack above...

Perhaps there is some equilibrium between heat from warm vapor and the cold ice near the surface, the crack may narrow due to icing, but not close completely. The warm ice zone at the bottom of the crack may tend to open wider and deepen ... retreating into the thermal gradient.... eventually breaching into liquid water, causing intense geysering into vaccuum until much of the water in the pocket re-freezes due to evaporative chilling and easy access of liquid water to near vaccuum is choked off.

We have very non-linear processes here with lots of feedback... I'm 100% positive what's going on in and at those tiger stripe cracks is *COMPLICATED* geometrically and thermally and geologically.

Posted by: Richard Trigaux Apr 8 2006, 01:19 PM

Yes, ery non-linear stuff, with short eruptions of water when a crack opens, and then this water freezes in the crack, keeping it more and more open, like in the oceanic ranges where basalt dykes open the cracks and repell the continents.


In the very place where this process takes place, we could see a very difficult terreain, with a kind or karst* formed by sublimating ice. With alot of holes and cracks. Worse, we can assume that most of the snow formed by evaporation of liquid water, just falls back and cover all the previous holes, turning the ground into a rover nightmare. Add to this large boulders as those already seen in other parts of Enceladus... A landing around may be much more complicated than on Europa. Perhaps a lander may have very large inflatable tyres, or crawl like a caterpilar on very large skis, to avoid to sink into snow and underlying cavities. Better: a snow bike!

In facts we really don't know how the vents look like, and it would be a good bargain if Cassini could image them precisely in the close pass at 25kms. For this it may require to tilt, in order to compensate for motion blur (at this stage of the mission, taking some risks wil be worthy)




*karst, geological forms like cracks, caves, shafts, formed by dissolution of limestone by water.

Posted by: tty Apr 8 2006, 04:46 PM

Ice karst actually exists on Earth. It is called "thermokarst" and is found particularly in parts of northeastern Siberia where there are "edoma" deposits. This is an eolian deposit that something halfway between loess and glacier ice. However in principle it can develop anywhere where there is permafrost.

tty

Posted by: volcanopele Apr 10 2006, 05:22 PM

Electron impact excitation of the Nitrogen in the plumes may produce emissions at 391 nm and 428 nm. Long exposures during eclipse may allow us to catch these.


Not quite sure. I'll have to take another look at the equations used.

Posted by: BruceMoomaw Apr 12 2006, 10:38 PM

Does it really make any difference? Where both the possibility of life and the possibility of detecting its remains on the surface are concerned, the real key question is: does Enceladus have a subsurface ocean -- or at least a south-polar subsurface sea -- of liquid water between its ice layer and its rocky core?

If it does, then how the water gets to the surface doesn't really make any difference. If Enceladus lacks actual vents punching from the liquid-water layer all the way up through the ice layer to the surface, then what happens -- similar to Europa -- is that the top of the liquid-water layer freezes onto the bottom of the ice immediately above it, and then solid-state convection of the warm ice slowly transports the ice (and any biological remains frozen into it) up to the surface, where it sublimates into space. Fully 99% of the water expelled by Enceladus falls back onto its surface elsewhere as snow -- and the weight of that accumulating snow makes the ice underneath it slowly sink down and creep along sideways until it contacts the layer or pocket of subsurface liquid water, at which point it melts and the whole cycle begins again. Enceladus is estimated to expel 15 metric tons of water from its south pole per second -- which is enough to totally recycle its entire worldwide supply of ice every 130 million years.

And in either case, if the underground sea contains life, the frozen remains of that life can almost certainly be found in the surface ice at the venting area -- or even in the ice particles expelled from the surface, where they can perhaps be scooped up by a sample-return flyby. This is especially true since Saturn lacks Jupiter's malignant high-radiation environment which quickly destroys any biological remains that reach Europa's surface.

So the only relevant question is: how likely is it that Enceladus DOES have a subsurface sea, instead of just perpetually recirculated warm but solid convecting ice all the way down to its rocky core? Matson et all think it's virtually certain. http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2219.pdf : "Models presented so far show that it is difficult, if not impossible, to produce the observed [south polar heating] power if [tidal] dissipation occurs only in the ice." They are convinced that the south pole is heated by tidal dissipation in a big chamber of magma in the rocky core -- initially melted by a concentration of Al-26 during Enceladus' earliest days, and sustaining its heating since then through tidal friction (with an actual feedback mechanism operating, as the BBC article says, to keep the frictional heating at a high enough level to keep the magma molten). And the heat from that magma pocket, in order to drive such energetic venting, must melt the ice next to the rocky core into liquid water.

Moreover, we have the methane and nitrogen in the plume. What's maintaining their supply? Those gases don't freeze at the temperature of the Saturnian system; once they leave Enceladus they're gone forever -- and unless the venting started less than 130 million years ago (which seems unlikely), then it would already have completely purged any methane and nitrogen that had been frozen into Enceladus' ice at its formation. So an additional supply of those two gases must be coming from somewhere else -- specifically, from warm, or downright hot, geothermal vents out of Enceladus' rocky core and coming into contact with its ice/water mantle. Quoting Matson et al again: "It is interesting to note that the amount of N2 detected in the geyser (~4%) corresponds to the solubility of N2 in water for a temperature of 25 deg C and a pressure of 1 bar (~1 km depth). This is a further constraint on the mechanism responsible for the geyser."

Matson does think that the nitrogen is produced underground, by all of Enceladus' interior ammonia being broken down while it's still in the rocky core's vents -- which does raise the question of why there is some contrary evidence that Titan IS still releasing ammonia from its interior (as I noted in the "Great Ammonia Mystery" thread). But it turns out that Raul Baragiola -- whose own ideas about Enceladus' ammonia I and Frank Crary have seriously misunderstood, according to a recent E-mail from him -- has an alternative theory which may explain that puzzle completely, which I'll talk about once I get a little additional information from him.

At any rate, it seems to me that Bob is raising an unnecessary panic. If it turns out that Enceladus does just have rapidly sublimating surface ice instead of actual liquid-water geysers, it probably doesn't make any difference to either the chances of Enceladan life, or to our ability to detect it with (relatively) easy surface studies.

Posted by: BruceMoomaw Apr 12 2006, 11:17 PM

While we're on the subject of the plume: Hunter Waite's March 10 "Science" article on Cassini's mass spectral analysis of it (which Bob helpfully sent me) suggests that not only has probable evidence of acetylene and propane (<1% each) been found, but there are some weaker possible indications of smaller amounts of ammonia and HCN in it (<0.5% each). Obviously we need to analyze a denser part of the plume, which of course is one of the prime goals of the very low 2008 flyby.

Posted by: BruceMoomaw Apr 16 2006, 01:03 AM

Raul Baragiola confirms that Frank Crary and I misunderstood his theory at the Europa Focus Group meeting. His theory is that Enceladus may actually have ammonia -- but NOT, as we thought, that a substantial amount of it is releaesd in the vapor plume but is then broken down virtually completely into nitrogen by Saturn's charged-particle radiation before it can reach Cassini's mass spectrometer. he agrees that that is impossible during the brief 15 minutes of transit from surface to Cassini.

Instead, he agrees with Bob that Enceladus may not have liquid geysers at all, and that its vapor plume is instead released from the surface of warm, sublmating ice. But while Dennis Matson and comapny think that Enceladus' internal ammonia is broken down into N2 and H2 by the hot geothermal processes occurring in the moon's rocky core before it ever gets into the ice mantle, baragiola thinks it may actualy be incorporated into the ice as ammonia, and get slowly carried to Enceladus' surface by solid-state ice convection -- but that, within just a few days of its reaching the surface, Saturn's radiation breaks it down completely into N2 and H2. And he thinks that this happens at a rate much faster than the rate at which the water vapor sublimates off the surface of the warm ice -- so that, by the time the vapor lifts off from Enceladus' surface, it already contains only a small trace of ammonia, with most of it having already been turned into nitrogen and hydrogen that was temporarily trapped in the ice after its creation.

He also thinks there's a real chance that there is no subsurface liquid-water sea inside Enceladus at all, and that we may just be seeing continuous slow solid-state convection of solid ice contacting the warm top surface of the rocky core. This, however, is not necessary to the rest of his theory -- as I said earlier, there could be a subsurface water sea with ammonia mixed into it, freezing at its top with the ice gradually convecting to the surface from there.

Posted by: The Messenger Apr 16 2006, 01:48 AM

??? What is Saturn radiating that would zap ammonia so completely that it only shows up in the plume in trace quantities? Shouldn't we expect at least a measurable quantity of ammonia in the form of ammonium hydroxide?

Posted by: BruceMoomaw Apr 16 2006, 03:53 AM

All I've got on that is so far is what he says in his EGU and AGU abstracts: "We will present results from laboratory studies on the radiation effects on ammonia–water mixtures pertaining to the environment of Saturn’s icy moon Enceladus. We show that ion irradiation destroys ammonia efficiently, and produces N2 that could be the source of N+ that has been detected in the exosphere. Warming the irradiated mixtures we observe outbursts of water and ice grains at temperatures much lower than those needed for sublimation of water ice. These radiation processes may explain the plume of water vapor and grains observed by Cassini at Enceladus."

...and a barebones PowerPoint presentation of his to the Cassini Plasma Spectrometer team which I've just discovered: http://caps.space.swri.edu/caps/TeamMeetings/MtgMinutes31/IcePhysicsOnEnceladus.pdf

Posted by: The Messenger Apr 16 2006, 05:54 AM

Thanks - I still don't see data supporting copious amounts of moisture (edit)...I mean ammonia. Could Enceladus be a more perplexing puzzle?

Posted by: edstrick Apr 16 2006, 09:20 AM

Saturn has nowhere near the hellish radiation belts Jupiter has, but it has (compared with Earth) intense radiation belts. Significant trapped radiation starts somewhere around Dione's orbit and is very strong at Mimas' orbit and inside to the outer edge of the A-Ring. As I recall, the dominant flux is high energy <MeV> electrons. Not very extended exposure with minimal shielding would be lethal to humans (a few hours, maybe?, compared with about 1 min at Io).

The radiation belts are truncated at the outer edge of the A-Ring, where particles spiraling along magnetic field lines between Saturn's poles suddenly run into the orbiting ice and get absorbed after very few attempts to pass through the rings. Pioneer 11 was the first spacecraft (and till Cassini, the only one) to observe this, as it's periapsis was well within the outer edge of the A-Ring. One quote I recall from one of the charged particle instruments was that ionizing radiation observed over the rings was lower than test data taken in the laboratory before launch.

The biggest source of radiation over the rings was something called CRAND.... Cosmic Ray Neutron Albedo Decay. Very high energy cosmic ray penetrates the magnetosphere, hit's a ring particle, and knocks out neutrons. These decay with neutron's characteristic half-life and the decay products were detected by Pioneer as it flew over or under the rings.

Posted by: vexgizmo Apr 16 2006, 06:15 PM

There's no need to panic, Bruce--it's just science!

Posted by: BruceMoomaw Apr 16 2006, 11:15 PM

Well, yeah, but my point was that Baragiola doesn't seem to have any concrete reason for believing the latter -- just that it's a possibility -- and that, if Matson & company are right, the heat level contacting the bottom of the ice is so high that there MUST be a liquid water layer.

The difficulty is how to determine this one way or the other, if there AREN'T liquid-water vents coming all the way to the top. Enceladus' ice layer is probably too deep for a Europa-type radar sounder to punch through all the way to a liquid-water layer -- we might have to depend on very sensitive gravity mapping, of the sort that only an orbiter could do. And in that case, it might be more scientifically cost-effective to jump directly to a lander, since even if an orbiter confirmed the existence of liquid water that would provide no evidence that it contained biologically interesting material (which is the central purpose of Enceladan exploration).

Posted by: JRehling Apr 17 2006, 05:01 PM

I'll re-mention my mega-strategy for a Europa mission that makes some sense for Enceladus as well:

Put a lander with a seismic package into an area of key interest. Shortly after it lands, have an impactor land a short distance away, creating a seismic thump of known dimension. That should map the local crustal depth quite accurately. The impactor would return Ranger-style imagery as it closes in. Of course, the lander would return descent/surface imaging of its own site. Meanwhile, a flyby craft images the impact plume and comes back to Earth with a sample from the plume.

I think that would make a great Europa suite which, yes, would be very expensive but less than JIMO (or a war), and would utilize almost every component for double-duty of some kind.

Enceladus doesn't quite require the plume since it makes its own, but the suite might perform very well there, too.

Posted by: Bob Shaw Apr 17 2006, 05:51 PM

The 'Europa Suite' mission could use a spacecraft bus design similar to that used by Pioneer Venus for the entry probes, perhaps then using it as the impact craft while short-lived probes do their thing. Put an RTG/Ion drive combination on it and use the spent upper stage as added mass, too (as with LRO etc) - split the vehicles in 'bomb runs' as the armada sails in a la Galileo and the Jupiter Entry Probe, trajectories tweaked as they go but quiescent, then brake the final stage like the Advanced Mariner would have done back in the 60s so that the Lander (etc) precedes it to target.

Might cost a bit, though!

Bob Shaw

Posted by: BruceMoomaw Apr 17 2006, 07:02 PM

I did some fat-chewing with a couple of people at the Europa Focus Group meeting about this idea (which Edwin Kite once pushed as well). There are big problems.

First: if you drop your survivable seismic lander from a Europa flyby craft rather than an orbiter, you've got to have a BIG retrorocket for it. And it's open to question whether it's worth doing that (or even dropping a small survivable lander from orbit) just for seismic data on the ice layer's thickness -- that's not really relevant to deciding whether and where to land a biological lander (unless the ice crust is so thin that an orbiter's or flyby's radar sounder could pierce it in any case); and when you DO land a biological lander, a seismometer can be extremely easily added to it as a small extra instrument to get that data as well.

Second: an impactor that can send back really high-resolution closeup images of the details of Europa's surface before impact, Ranger-style, will hve trouble returning any that are better than those which an orbiter or flyby craft could get (of much bigger areas) just by carrying a big HiRISE-type camera -- unless the impactor either carried a very big camera itself or could return last-second images at a very high rate. (The last images of the Moon's surface from the Rangers were no higher-resolution that those which HiRISE could get from the orbit planned for Europa Orbiter. This, in fact, is the same reason why they cancelled the Block 4 Rangers and the descent cameras on the Surveyors -- their views would have been no better than those from the Lunar Orbiters, and covered tremendously smaller areas.) The one way to do better is to have a descent camera on a little lander that brakes itself to a slow enough speed that it can survive landing and play back its final images afterward -- but then you have that problem with the big hulking retrorocket on the lander capsule.

I suppose it might be worthwhile to carry out your proposal with the "seismic capsule" carrying several other instruments, such a descent camera and some simple nonbiological surface-analysis instruments -- that is, a version of Europa Orbiter's proposed "bowling ball" lander, but equipped with a much bigger retrorocket. But this kind of mission (including the impactor and the Stardust-type sample-return flyby craft) is really getting up there in complexity and cost to the same levels as Europa Orbiter. Would its total science return be as good? Especially since it couldn't do nearly as much inspection of Europa's surface as EO could to find good landing sites for a big biological lander, and since there's a good chance that the tiny Stardust-type sample of ice which it returned to Earth would be too small (or too chemically modified by its high-speed impact into the aerogel) to contain recognizable biological-type material? (Bob Pappalardo has successfully dampened my previous enthusiasm for a Europa Ice Clipper-type mission -- although Charles Hibbitts and I spent some time discussing that possibility at the meeting.)

Posted by: dvandorn Apr 18 2006, 06:15 AM

There is, however, a good reason for a lander to have descent imaging capabilities, especially if there is no HiRise-style camera orbiting its destination.

Context.

Granted, a Ranger-style series of images with steadily decreasing coverage isn't that much good for observing large trends and larger context. But if your eventual landing site is visible throughout the descent, then you have context imaging that lets you observe from a relatively low-resolution level (good for identifying large trends) all the way down to the finest detail visible from the surface.

I am convinced that the DIMES imaging was very, very useful in the determination of geological context on the MER landings. Similar imaging would have been useful for the Surveyor landings, I believe, and I think we missed out on something when the imaging was canceled.

-the other Doug

Posted by: djellison Apr 18 2006, 07:14 AM

Problem with DIMES was that they downsampled it quite a lot, so the actual res wasn't any better than follow on MOC data. It was still interesting though. MARDI on Phoenix will be very very interesting.

I wonder why they didn't keep the full res DIMES image on board - would have been usefull - but I supose given the motion of the entry 'complex' - it would probably have been blured to the point of matching the downsampled res anyway.

Doug

Posted by: Bob Shaw Apr 18 2006, 01:01 PM

other Doug:

If there *had* been descent imaging on Surveyor (Surveyor 1 actually flew the camera!) then it'd put Phil out of a job.

No, come to think of it, it'd give him *another* job!

Bob Shaw

Posted by: BruceMoomaw Apr 18 2006, 09:38 PM

Yes, but that's for a SURVIVABLE lander -- in which case the value of descent imaging is obvious anyway, during the last moments of the descent (with the images played back post-landing). In the case of an Impactor, the science value of descent imaging is less as compared to the context you get anyway from a HiRISE-type camera combined with a wider-angle one.

I think a case can now be made that the most valuable thing you might be able to do with a little piggyback rough lander on Europa Orbiter is descent and surface imaging; we badly need to know just how rough that surface is on fine scales, so that we can design the later expensive Astrobiology Lander in a way that will give it the best chance of success. (I gather that, in retrospect, the designers of the 1970s Mars program very much regretted not outfitting the 1971 Mariners with a higher-resolution camera system that would have allowed them to do a better job of selecting safe and interesting landing sites for the Vikings.)

The second most valuable thing would be some way for the lander to obtain more information on the physical structure of the subsurface -- both the depth of the radiation-scrambled impact regolith (so that we'll know how deep the Astrobiology Lander needs to drill or lower a sampling cryobot), and the amount of rock and salt mixed in (so that we'll know just how difficult such sampling will be). A properly designed radar system on the Orbiter, however, MIGHT be able to do that, maybe with some help from surface composition mapping instruments.

Posted by: edstrick Apr 19 2006, 08:18 AM

Mariner Mars 71 had relatively poor stability, and the cameras had a slow reset/expose/readout/erase cycle of about a minute each. While they did candidate Viking landing site imaging during the extended mission, typical narrow angle landing site mosaics were a raggedly overlapping 5 frames or so. A narrower angle camera would have only further exposed the limitations of that spacecraft for potential landing site mapping.

The Viking Orbiter's twin narrow angle cameras were designed for rapid-fire alternating exposures at something like 1.2 seconds with 15% or so overlapping fields of view. Spacecraft motion in the primary mission orbit with a 1500 km periapsis shifted the field of view by enough so the next frame from a camera would have a 15 or 20% or so overlap with the previous frame, so you'd get a nice continuous 2 frame wide strip mosaic across a landing site up to some 10 or 15 frame-pairs long. Typically they'd take a series of 3 or 4 or so strips as the spacecraft went through periapsis, slewing the 2 degree of freedom scan platform so one strip would overlap some 15 or 20% with the previous one.

Nowdays, there's enough computing power to do full resolution mosaics of Viking orbiter images, but there wasn't back in the late 70's. They would certainly add up to impressive tens-of-megapixel mosaics, since each single frame was about one megapixel.

Posted by: BruceMoomaw Apr 19 2006, 03:02 PM

Interesting. Boeing proposed a Martian version of Lunar Orbiter for the job, and presumably -- with its motion-detection sensor -- that WOULD have had sufficient stability for the job. But it would presumably have had to carry regular TV cameras as well for wider coverage of the planet, and saved its film only for the areas considered worthy of detailed coverage.