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Martian Cave Probe?, Designs for the DEEP Search for Life
Shaka
post Nov 14 2007, 12:18 AM
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Does anyone consider it worthwhile to speculate on how we might explore Martian caves or lava tubes for the traces of life - past or present? I have not yet run across any 'official' proposals for "spelunker probes", so perhaps we could have some fun and get in on the ground floor with some feasible early designs. With the engineering and scientific expertise we have at UMSF we should be able to whittle down the possible features for such a rover to a practical core. If the planned surface scrapers and drillers don't turn up conclusive evidence to answer The Big Question, can we justify a search of the Martian Underground?

I can envisage a RTG-powered rover that enters a cave, or rappels down a skylight opening, leaving a base communication stage outside connected to it with a fiber-optic umbilical cable. Some form of laser or other illumination - in the visible and/or infrared - would presumably be required. How many of the MSL instruments could be included? What novel instruments would be appropriate? What is the optimal size and mobility design? 'Do we yet have 'hot' prospects for accessible caves? How should we choose the best candidates?

We can leave this to some JPL bright spark to develop, or we can dive right in. Any takers? smile.gif wheel.gif


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nprev
post Nov 14 2007, 12:52 AM
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Cool idea, but I don't think it'd ever fly unless there were some other mission objectives that could be added using the available spelunking features and/or instrumentation. First step here would be requirements development.

Couple of early concerns, here:

1. How much cable would we need? We don't know how deep many of these holes are yet.

2. A lot of the holes look like vertical drops, so the data cable would have to be a hoisting cable (with a hoist, and optocoupled slip rings, and all sorts of mechanical complexity) as well, something similar to that used on maritime vessel winches that have data wiring in the middle of the armor. Not light, and not cheap. Also, the surface vehicle would have to be either heavy enough not to topple or be dragged in, or anchor itself in some way.

3. During remote vehicle traverses, how to avoid cable snagging?

Still a cool idea, but I wouldn't want to try it... sad.gif


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dvandorn
post Nov 14 2007, 01:54 AM
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As far as I know, however, all of the lava tube caves we've seen from orbit occur in the Tharsis bulge, many along the flanks of the great shield volcanoes.

Those areas are significantly above "sea level" on Mars. Quite significantly. So much so that you have an even harder time landing anything there -- there's just not enough air to slow you down enough to be able to land anything as heavy as a MER, much less an MSL.

-the other Doug


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nprev
post Nov 14 2007, 03:27 AM
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Great point, oDoug. Hopefully <wish mode> we'll find some at much lower elevations that might be deep enough to have some significant (>20 mb, which we can probably find in some spots of Hellas with much less effort) atmospheric pressure in their depths </wish mode>. Short of that, hard to see how a caving mission would be worth the expense & risk. Have to be a deep cave indeed on the Tharsis Bulge to make interior conditions interesting enough to investigate.

End of reality check. Think that Shaka's in it for the enjoyment of conjuring up something innovative, and have no wish to rain on his parade! smile.gif I think that one must-have would be a radar or laser altimeter on the cave-crawler...


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Shaka
post Nov 14 2007, 06:20 AM
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You get my drift, nprev. Elevation could be a crucial criterion (krikeys! A Crushingly crucial criterion!), but so could be the proximity of geothermal warmth. There must be at least a few caves down amongst all those canyons and rills and badlands and chaotic terrains that could represent a final refugium for the never-say-die remnants of a past Martian biosphere. A refuge from the withering surface assaults of impact barrage, radiation, desiccation, freezedrying, peroxidation etc ad nauseam. Indeed, if there has been the vast underground labyrinth of water/ice systems, subject to sudden outfloods, drainage, rising and falling water tables etc, there MUST be a Martian underground. The remnants of Martian life may have abandoned the hostile surface so long ago that they don't even remember that the surface exists. Drilling down a meter or two may be no more productive than drilling into a Sahara dune. We may have to ferret out the ventilation shafts, the skylights, that offer access to the 'command bunker' of Martian life. (Disable poetic license)

I think it's fair to assume, for the mental exercise, that openings to the underworld have been or will be found. We won't be waiting for the first astronauts to set foot on Mars, and tell them "Just climb down there and see what you find." Even if we wait for man to reach Mars, we will first explore those hazardous nether-regions only with robot probes. So why wait for man? Let's at least put on paper, or the web, a scheme for exploring a cave. (If it's fruitless, Doug can always delete it.) smile.gif

Happy Thanksgiving Day Parade to all!


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Cugel
post Nov 14 2007, 01:20 PM
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Elevation is only a problem if you come in on a chute. If you have a nice powerful descent rocket you can land anywhere on Mars. I know this will do something ugly to your budget but powered EDL on Mars must be developed for manned missions anyway. So, in true Zubrinian style, let's just suppose we have that technology and somebody has payed for it.
The good thing is that the landing ellipse will now be down to a few meters or so. We can do a pinpoint landing right next to the hole in the ground.
So, maybe we don't even need a rover type lander (to get to the hole), just a big boom might be sufficient to deploy the probe. (This is the optimistic part)
Also, I don't see the cable problems nprev mentioned before. If you can lower something many miles into an ocean on Earth it surely must be possible to do that in a cave on Mars.
The whole shebang (lander + probe) can be solar powered, so accept for the EDL hardware I don't see why it would cost many billions (MSL class maybe?).

Another idea if you don't like that cable: just drop a battery powered probe into the hole and dangle an antenna which is connected to the lander a meter deep or so in the cave . That would be good enough for a radio link. With airbag technology the fall must be survivable for the probe (which could be pretty small and simple anyway).
Probably carry more than one of these probes anyway. The probes could be deployed into the hole by shooting them out of an airgun, so the lander could actually be quite some distance from the rim.
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centsworth_II
post Nov 14 2007, 03:59 PM
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If you're looking for microbe-friendly habitats, I think resources would be better
spent developing a means of drilling deep beneath the surface at an area of your
choosing, not limited to specific cave locations. Sending a rover into a cave would
be incredibly cool, but I wonder at the energy budget for providing enough light
to take the cool pictures.
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nprev
post Nov 14 2007, 04:12 PM
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QUOTE (Cugel @ Nov 14 2007, 05:20 AM) *
Also, I don't see the cable problems nprev mentioned before. If you can lower something many miles into an ocean on Earth it surely must be possible to do that in a cave on Mars.


I was thinking more about what happens after the crawler reaches the bottom & begins to move horizontally. Ocean trawlers on Earth rely on brute force to overcome obstacles, yet still they break cables with tensile strengths well over 10000 lbs & lose gear (been there, done that, with plankton collection gear & scallop survey trawls when I worked for NOAA; reterminating those cables is NOT fun). Cable-linked oceanic ROVs almost always steer well clear of the bottom & are controlled in real time to avoid obstacles. Doesn't take much for a mission-ending snag in the Martian spelunking scenario.

Now, if you're just talking about lowering an instrument package vertically down a hole, that's of course much less risky. Have to ask, though, what sort of useful data would be acquired? Only thing I can think of is to lower a methane-sniffer, and it seems that we'd do better to land a more complete instrumentation suite at other, more accessible surface sites.


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Shaka
post Nov 15 2007, 04:56 AM
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Thanks, guys, for your input. Clearly this is not in the easy-as-rolling-off-a-log category of engineering challenges!
I'm wondering, for starters, if anyone has yet seen, from orbit, openings like a cave entrance other than the few lava tube skylights at high elevation. Something we can throw a methane probe into. Maybe that's the best available, for now. sad.gif


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Cugel
post Nov 15 2007, 11:59 AM
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QUOTE (nprev @ Nov 14 2007, 05:12 PM) *
I was thinking more about what happens after the crawler reaches the bottom & begins to move horizontally. Ocean trawlers on Earth rely on brute force to overcome obstacles, yet still they break cables with tensile strengths well over 10000 lbs & lose gear (been there, done that, with plankton collection gear & scallop survey trawls when I worked for NOAA; reterminating those cables is NOT fun). Cable-linked oceanic ROVs almost always steer well clear of the bottom & are controlled in real time to avoid obstacles. Doesn't take much for a mission-ending snag in the Martian spelunking scenario.


You're right. The bottom of a cave might not be the ideal surface for a wheeled vehicle. Too much debris, cracks and other nasty things. But as Shaka said, simply characterizing the environment would be very interesting. Temperature, atmosphere composition, radiation levels, some remote spectrometry... it would tell you whether the cave is habitable.

Another question (of a more general nature) is this: when you detect methane in a cave, is it possible to make a distinction between a biologic and a volcanic origin of the gas? I know they did something similar with Huygens on Titan, by comparing isotope ratios or something but I don't know if this would work with the extremely low methane levels on Mars.
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Shaka
post Nov 15 2007, 08:21 PM
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Good fundamental astrobiological question, Cugel. A quick dip into Google Scholar suggests that the answer is "Yes, maybe." Mars Methane Sources and see also:Carbon AND Hydrogen Isotopes
Of course the hope would be that if organisms in the underground are the source of the atmospheric gases, the concentrations inside the cave would be significantly higher, hence easier to quantify isotopically. Given the current drive toward miniaturized spectrometers (for military, if not scientific applications!), we may not be far away from an astrobiology lab in a shoebox or better. Weight may not be the challenge.

The bottom of a cave or lava tube may well be a severe challenge to rover mobility. Wheels may have to give way to legs - with snowshoes! - in order to clamber over fallen rocks and dust puddles.

Edit: I just had a spine-tingling mental image of a mechanical SPIDER! Instead of spider silk paying out the back end, you would have fiber-optic cable! tongue.gif


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dburt
post Nov 15 2007, 10:22 PM
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QUOTE (Shaka @ Nov 13 2007, 11:20 PM) *
...the proximity of geothermal warmth. There must be at least a few caves down amongst all those canyons and rills and badlands and chaotic terrains that could represent a final refugium for the never-say-die remnants of a past Martian biosphere. A refuge from the withering surface assaults of impact barrage, radiation, desiccation, freezedrying, peroxidation etc ad nauseam. Indeed, if there has been the vast underground labyrinth of water/ice systems, subject to sudden outfloods, drainage, rising and falling water tables etc, there MUST be a Martian underground. The remnants of Martian life may have abandoned the hostile surface so long ago that they don't even remember that the surface exists. ...


With regard to moisture and geothermal warmth, why restrict yourself to a cave? The climate inside a cave, unless it is a very deep one indeed, represents the average of the climate at the surface (averaged over day/night and winter/summer) so that any Mars cave is likely to be extremely cold inside, and not necessarily a promising place for life. For me the best refuge for life might be near the vent of one of those very long-lived but largely quiescent Tharsis volcanoes - potentially lots of warmth, moisture, and energy-yielding chemistry, persisting for billions of years at the same spot (no plate tectonics to move it around). Aperiodic eruptions of basalt need not wipe out life for more than a meter or two away from the vent contact, and it could easily re-establish itself afterwards.

Given that drilling to any significant depth is likely to be prohibitively expensive and fraught with mechanical difficulties, why not plan to land our magic rocket in the summit caldera of a volcano (perhaps not one of the tallest ones) and lower our cable of instruments down an old vent or old fumarole, if one could be located? Alternatively, look among the local throat-clearing volcanic ejecta or caldera collapse breccias around the rim for signs of past life. On Earth the closest analog for such a target might be the active biological communities first discovered during the late 1960's near "black smokers" or submarine volcanic vents (above sea floor spreading centers between separating tectonic plates or in regions of extension behind island arcs). Just another idea (one that I've mentioned previously, I believe). What do you think?

-- HDP Don
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Shaka
post Nov 15 2007, 11:09 PM
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For sure, Prof Don! Any kind of vent or other opening into a volcano would be sweet meat for our 'spelunking spider' (should we name her Shelob ? tongue.gif )
If such openings exist (and I would imagine someone at HiRISE is already looking), then they would be an absolutely top-drawer priority to the geologists as well as the astrobiologists.


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nprev
post Nov 16 2007, 01:13 AM
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Like the spider, still hate the cable. I wonder if it's technically feasible to build a compact ultrasonic sonar transducer to periodically (and redundantly) vibrate into solid rock with enough juice for it to be picked up by the surface vehicle. The bit rate would be lousy, though, even if we used a combination of modulation methods...maybe a couple of distinct channels would help, like multiple ISDN lines for videoteleconferencing.

EDIT: Ha! Got it! The spider rides down to the bottom on a little descent platform, which is equipped with a simple RF transceiver. The spider calls back to the platform (which stays there), which in turn relays data up to the surface lander and new commands to the lander. Piece of cake, except for all the multipath problems in a cave...redundancy & freq agility should help that a lot, though.

If the cave's just too rough to traverse, you could haul the spider back up again & use it to explore the surrounding territory (always looking for risk mitigation strategies, here!)

BTW, the host platform pretty much has to be a rover...doubt very, very much that we could ever put a fixed lander near enough to a cave/fumarole/vent to get the spider where it needs to go.


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Shaka
post Nov 16 2007, 02:39 AM
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Don't understand your problem with Shelob's silk. I'm thinking not so much of oceanographic MOCNESS cable, but more like the stuff that pays out behind a guided torpedo or a TOW missile or maybe one of those retrievable towed RF decoys. Think of the bandwidth! And if it can be smoothly payed out and reeled in and support Shelob's weight, then there's no need for a separate lowering platform or limited communication ability. Shelob just walks away from her rover/ lander/base station (which anchors itself with some augers) and heads toward the opening. If the floor drops away, Shelob just pays out cable and drifts down until she regains footing and resumes walking. Equip her with a compact RTG generator, and she should have enough juice to light things up with white or infrared LEDs, take pictures, zap the walls with her chemcam laser, and scare the almighty crap out of Marvin the Martian! laugh.gif


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