The First Europa Lander, What can be done first, cheapest & best? Options

[nprev]

I think that many people in this forum would agree that somebody's going to have to land on Europa someday before the rather elaborate schemes to penetrate the outer ice layer will ever fly, if for no other reason than to get some relevant ground truth before committing to such an elaborate, expensive, and risky mission.

EO seems to have ruled out any surface science package for that mission (though it would be nice to change their minds! ), but I think that there is a valid requirement at some point to directly assess the surface properties of Europa in an inexpensive yet creative way. Some candidate instrument payloads might be:

1. A sonar transducer/receiver set embedded within a penetrometer to determine crust density and examine the uniformity of the ice layer within the operational radius of the instrument (looking for cracks and holes, in other words).

2. A conductivity sensor again embedded inside a penetrometer to measure the native salinity of the surrounding material and possibly derive some constraints on the composition of metallic salts in the European crust (saltiness has a major effect on ice properties, in addition to the obvious need to derive the salt content of any underlying ocean).

3. A seismometer for all sorts of reasons.


How does this sound? Any critiques, additions, or subtractions? I omitted a surface imager not only because of bandwidth/extra complexity considerations but also because it seems desirable to penetrate the crust in order to minimize as much as possible reading any contaminants from Io during surface measurements. The orbiter data could be used to sense and subtract this from the penetrometer readings.

[Steve G]

I think that many people in this forum would agree that somebody's going to have to land on Europa someday before the rather elaborate schemes to penetrate the outer ice layer will ever fly, if for no other reason than to get some relevant ground truth before committing to such an elaborate, expensive, and risky mission.

EO seems to have ruled out any surface science package for that mission (though it would be nice to change their minds! ), but I think that there is a valid requirement at some point to directly assess the surface properties of Europa in an inexpensive yet creative way. Some candidate instrument payloads might be:

1. A sonar transducer/receiver set embedded within a penetrometer to determine crust density and examine the uniformity of the ice layer within the operational radius of the instrument (looking for cracks and holes, in other words).

2. A conductivity sensor again embedded inside a penetrometer to measure the native salinity of the surrounding material and possibly derive some constraints on the composition of metallic salts in the European crust (saltiness has a major effect on ice properties, in addition to the obvious need to derive the salt content of any underlying ocean).

3. A seismometer for all sorts of reasons.
How does this sound? Any critiques, additions, or subtractions? I omitted a surface imager not only because of bandwidth/extra complexity considerations but also because it seems desirable to penetrate the crust in order to minimize as much as possible reading any contaminants from Io during surface measurements. The orbiter data could be used to sense and subtract this from the penetrometer readings.

The all ice surface would facilitate the use of penetrators instead of more complex landers. Ideally, look for vented material that came to the surface from a fissure, and maybe there would still be some organics left over. But yes, I would want a seismometer!

[Guest_BruceMoomaw_*]

JPL's study of useful instruments for a small Europa lander listed, in order of priority:

(1 & 2): A surface GCMS (or, better, a mass spectrometer combined with some kind of liquid chromatography) to look for organics and other interesting compounds; and a seismometer to try to sound the thickness of the total ice layer.

(3) A magnetometer to make induced-field measurements simultaneously with an orbiter to obtain data on the thickness of both the ice layer AND the underlying liquid-water layer.

(4) A surface panoramic camera.

There was, as I've noted, a knockdown debate on this subject at the November COMPLEX meeting -- including an unbelievably dreary and nitpicking debate over the usefulness of seismometers and magnetometers, culminating in the general conclusion that you'd need at least 1 or 2 weeks of data from them, which requires powering the lander with a small RTG rather than batteries.

But the trouble, again, is that the most important measurement by far would be a search for possibly biological organic compounds buried deeply enough in the ice that Jupiter's radiation won't have scrambled them unrecognizably -- and such a small lander probably can't drill the 1 or 2 meters necessary for that purpose. Any of the other measurements that a small lander could make can just as well be delayed and then put on the first dedicated larger Europa Astrobiological Lander mission to look for near-subsurface organics, at a site selected by Europa Orbiter.

The obvious possible solution to this problem would be a penetrator, and in fact at the meeting I spoke up and suggested just that. (It has other major advantages, too -- elimination of the need for a heavy final braking or shock-absorption system, shielding of the lander from Jupiter's destructive radiation by the ice itself, and better coupling of the seismometer to Europa's surface.) Unfortunately, Torrance Johnson was one step ahead of me. He said that just this possibility was examined in depth by the JIMO science definition team, and it turned out that penetrators on airless worlds have one huge Achilles heel -- there's no airflow to keep their nose pointed straight in their direction of travel when they hit the surface. So you need to add a big and complex attitude-control system to do this, or your penetrator will hit the surface slightly skewed, at which point you're screwed.

Still, Paul Lucey of the University of Hawaii is proposing "Thunderbolt" -- a Europa penetrator to look for subsurface organics (a descendant of his "Polar Night" Discovery proposal that would launch three small penetrators from a lunar orbiter to look for polar ice). The COMPLEX people actually suggested that I should get in touch with him to see if he's found a possible solution to the attitude-control problem, and while I haven't been able to contact him yet I intend to do so after New Year.

By the way, I'm telling you guys all this because every crumb of it had to be cut out of the shortened version of my upcoming article on the COMPLEX meeting for "Astronomy".

[JRehling]

Still, Paul Lucey of the University of Hawaii is proposing "Thunderbolt" -- a Europa penetrator to look for subsurface organics (a descendant of his "Polar Night" Discovery proposal that would launch three small penetrators from a lunar orbiter to look for polar ice).

An interesting factoid is that penetrating into lunar ice would be operationally very similar to penetrating into Europan ice -- if there is lunar ice on the surface of comparable purity (I suspect it's dirtier). That is, the orbital-->impact delta-v would be very similar. The main difference is that a lunar ice mission would end up in darkness. Of course, the instrument selection might be quite different as well.

[edstrick]

as Bruce points out.. hit the surface slightly off dead-on, and you're screwed.. and slightly off is very damn little, I think. NOT trivial.

Regarding the lunar polar ice. I think the best evidence is fairly clear. Despite the weak claims of a signal of ice in bistatic-radar data using the prospector or clemantine (forget which) data, earthbased radar reveals no trace of depolarized backscattered signals from low attenuation, thick ice deposits in the lunar cold-traps. They see it on Mercury's polar ice, the martian poles, and icy galillean satellites. Not the Moon.

This agrees with (I dont' remember who's) model for lunar volatile sources and sinks, combined with models for loss mechanisms from polar ices, including radiation sputtering, UV photodissociation (interplanetary and interstellar Lyman Alpha), impact gardening etc. The conclusion was that the most plausible model had many percent but not tens of percent ice mixed with regolith in the polar cold traps to depths of meters or more. This would be rather hard to spot optically, except maybe by scanning with infrared lasers, one in a strong ice absorption band, the other just outside the band.

A real resource, but not one you'd want to squander as hydrogen for rocket fuel, etc., except on the shortest term.

[Bob Shaw]

There are *natural* penetrators as well as man-made ones. All we do, is look for a nice fresh impact, fissure, mini tiger-stripe or whatever, and land there. Turn a few ice boulders over, and there's your reasonably deep, reasonably fresh sample.

An orbiter with *good* imaging would be required, plus a smallish lander (perhaps attached to the orbiter, or not - the orbital dynamics might make it cheaper to have survey orbits/descent profiles which are best served by two vehicles).

Bob Shaw

[gpurcell]

I think it is going to be pretty difficult to argue for/design a complex lander on Europa without establishing ground truth with a simpler lander. If there is no lander on the next mission, then I suspect it will be two missions beyond before we see a very capable lander.

[nprev]

Re the penetrometer descent alignment problem: Wouldn't a "nose-heavy" penetrometer with an elongated body tend to align itself to local vertical during descent regardless of the presence of an atmosphere? If I'm not mistaken, a variation of this sort of tidal stabilization is used on terrestrial comsats for attitude stabilization up at GEO. Even though Europa's gravity gradient is much gentler, a penetrometer launched with a sufficiently low delta-V with respect to Europa should therefore have enough time to align itself properly during the descent.

[Guest_BruceMoomaw_*]

Uh-uh. An object that short would take weeks -- and maybe months -- to align itself, and then it would point itself straight downward instead of in the direction of motion. There is no simple solution to this problem.

As for a smaller lander to provide ground truth for the later bigger ones, there are two possible ways in which this could be useful -- but there are cheaper alternatives to both. First, of course, we need really high-resolution pictures of Europa's surface to see what kinds of landing hazards exist -- especially since the data we have up to now suggests that the surface may be extremely rugged, laced with crevasses and small ridges. But Europa Orbiter is now virtually certain to carry a big MGS-type camera for very high-res photos of some patches of the surface from orbit.

Second, it might be wise to get a measure of the salt content of the ice before sending a big lander that would probably obtain its data by releasing a short-distance Cryobot to melt its way 100 meters or more into the surface. (We want to be sure of getting below the upper layer of radiation-modified surface material -- which may have been gardened by impacts a fair distance into the surface if we're unlucky -- and Chris Chyba has also pointed out that a Cryobot may be the only way of gathering and filtering enough meltwater to have a good chance of detecting small amounts of biochemicals.) But high salt content might jam up a Cryobot by causing it to build up more and more concentrated brine in front of its nose until you had a block of salt which it could not melt through -- so it may be necessary to add a mechanical grinding head to chew down through this at the same time that you melt the ice. However, EO is virtually certain to carry a good near-IR spectrometer -- and a mass spectrometer to analyze the molecules of Europa's surface sputtered into the space above it by Jupiter's intense radiation -- and these together should be able to answer that question. (They may even add an X-ray spectrometer for direct element measurements.)

I've wondered, though, if it might be worthwhile to put a small sterilized impactor on EO, equipped with a camera, to be released during the last flyby of Europa which the craft will make to almost match orbits with the moon before it finally fires its rocket engine on the next pass to enter orbit around it. This could provide us with very close-up pre-impact photos that might provide more data on surface roughness -- and the Orbiter might be able to fly through the cloud of debris thrown up by the impact (Ice Clipper-style) to do a better mass spectrometric analysis of Europa's main surface constituents. And such an impactor would of course be much lighter than a flat-out lander, especially since EO wouldn't need to carry the fuel to brake its mass into Europa orbit. I still imagine the data from this would not be worth the monetary and mass cost, but I DO intend to ask the mission's designers about the possibility.

[nprev]

Hmm. How about a "semi-hard" lander al a Ranger instead? (...without the balsa wood, of course!)

Seriously, if the velocity differential is small enough during the final flyby, maybe something like a minature version of the MER EDL system without chutes but augmented by a retro could drop a nice little instrument suite on the surface with a high probability of success.

[tty]

The obvious possible solution to this problem would be a penetrator, and in fact at the meeting I spoke up and suggested just that.  (It has other major advantages, too --  elimination of the need for a heavy final braking or shock-absorption system, shielding of the lander from Jupiter's destructive radiation by the ice itself, and better coupling of the seismometer to Europa's surface.)  Unfortunately, Torrance Johnson was one step ahead of me.  He said that just this possibility was examined in depth by the JIMO science definition team, and it turned out that penetrators on airless worlds have one huge Achilles heel -- there's no airflow to keep their nose pointed straight in their direction of travel when they hit the surface.  So you need to add a big and complex attitude-control system to do this, or your penetrator will hit the surface slightly skewed, at which point you're screwed.

Two wild ideas:

1) Europa has an atmosphere though it is extremely thin. Might some kind of large drag chute or balloon give enough drag to stabilize a penetrator? A penetrator would be long and thin, so it would not require much drag to align it.

2) Armor piercing projectiles use a "sleeve" of softer material (e g lead) to hold and align the penetrator on contact with the armor.
However the ice may be to soft to "strip" the sleeve and in any case I think it would only work if the penetrator comes in more or less vertically.

tty

[ermar]

Europa has an atmosphere though it is extremely thin. Might some kind of large drag chute or balloon give enough drag to stabilize a penetrator? A penetrator would be long and thin, so it would not require much drag to align it.

Europa's atmospheric pressure is something like 10^-7 bar... which, according to Wikipedia, is 10 times less than the pressure on Earth at which aerodynamic surfaces cease to function. (Europa's atmosphere is oxygen gas, so density for a given pressure should be similar). The drag is enough that you couldn't orbit at 1 m above the surface for too long, but methinks it would be a little low for penetrator alignment.

[tasp]

Perhaps a future mission would drop an impactor at high velocity to vaporize some ice and a carefully timed following craft would decel in the plume . . . .


{dramatic to watch, but would it work?}

[ljk4-1]

Perhaps a future mission would drop an impactor at high velocity to vaporize some ice and a carefully timed following craft would decel in the plume . . . .
{dramatic to watch, but would it work?}

It was called the Europa Ice Clipper:

http://www.astrobiology.com/europa/ice.clipper.html

[tty]

It's too bad there is no GPS system on Europa. The new US/Swedish Excalibur guided artillery shell actually has a miniaturized GPS/INS guidance package and solid rocket attitude control system that is not only ruggedized to survive being shot out of a 155 mm gun and small and light enough to fit inside a 155 mm shell, it's also dead cheap compared to normal space hardware and built to stand long-term storage without maintenance.

tty