There has been lots of discussion of a mission to Europa in the excellent thread on the Juno mission. I thought that since a Europa mission seems to be once again becoming a possibility, it deserved its own thread for news, updates and discussion. I thought I'd kick things off with a summary of past efforts on a Europa mission, and on where things stand now. If I make a mistake, please correct me!

In the course of its prime and extended missions, Galileo found evidence of liquid water under the icy surface of the planet. Planning began on a Europa Orbiter mission, with a projected arrival date of 2008, to confirm the presence of the ocean, characterize the thickness of the icy crust and identify places for a future landing. One thing to note about these earlier plans: they included a direct trajectory to Jupiter, presumably to minimize mission duration and qualms about RTGs re-entering Earth atmosphere after some (highly unlikely) targeting mishap. But NASA lacked a nice category of missions to place the Europa Orbiter in. Eventually it got lumped together with Pluto Express and Solar Probe in a Outer Solar System program labelled "Fire and Ice", a term which also got applied to the Galileo Europa Mission extension. Without a solid program to support it, (like Mars Exploration, Great Observatories, or Discovery) the mission looked like an orphan.

As Bruce Moomaw has well documented, attempts to kill off the Pluto mission led to a tug of war between NASA, the planetary scientists and the public, resulting in Congressional directives to NASA. Pluto Express became the Pluto/Kuiper Belt Explorer and then New Horizons and New Frontiers 1. (New Frontiers 2 is of course Juno.) But the cost for the Europa mission continued to rise, and the launch date recede, as the difficulty of radiation shielding and the large delta-v requirements hit home, and the mission's public profile fell. The launch date moved to 2010 and the costs moved over a $1b. Then along came Sean O'Keefe and JIMO, a justification for the Prometheus program through developing nuclear electric propulsion, not with RTGs, but with an in-space fission reactor. Launch got moved to 2011, then 2012, while the cost went even further through the roof.

With the arrival of Mike Griffin, JIMO was cancelled. As Griffin said to Congress, "It was not a mission, in my judgment, that was well-formed." But interest in a Europa mission remained and remains strong. In 2003 the National Academy of Science's Decadal Survey flatly stated that a Europa Orbiter was the top priority for the next Large scale (aka Flagship) mission. (See page 196 of the report.) NASA's current Solar System Exploration Roadmap reaffirmed a Europa orbiter as the next flagship mission. The question as always is money. As Administrator Griffin said, "The Science Mission directorate wants to do a Europa mission, the National Academy of Sciences wants to do a Europa mission, I want to do a Europa mission. When we can afford it in the budget, we'll do it."

Evidence of that support beyond rhetoric and reports trickled out with a letter from Andy Danzler, NASA's Solar System chief, to the Outer Planets Assessment Group (OPAG). He reported that he had "funded a team to take a quick look at the boundary conditions of a mission to Europa, that is, how much power, mass, travel time, etc. for various realistic scenarios. For planning purposes, this group is looking at launch dates in the 2012-2015 range, although the later dates are more likely in terms of funding." For funding details however, we have to wait for the FY 2007 budget.

OK, now the good stuff.

The latest meeting of OPAG included reports on a Reference Design for the mission. A kind of first draft which establishes a baseline which can be tweaked and modified to extract the best science return.

There are many things to like about this draft design:
* The mission is now permitted to use Earth flybys, and uses a proven trajectory, the same as used by Galileo (Venus-Earth-Earth Gravity Assist). This allows a BIG increase in the available mass.
* The orbiter uses RTGs, but not super advanced ones that require further years of development.
* The orbiter is similar to Cassini in appearance, with 2 engines, a cylindrical tank structure, RTGs at the base, the magetometer boom at the top, and space for a lander bolted to side. The similarities may make it easier to convince Congress that this is something NASA knows how to do. The most obvious configuration change is with science payload and HGA having switched places, and the addition of a radar array. And there looks like a camera the size of MRO's HiRISE!
* The mission is definitely Flagship in scope with a launch mass of over 7000 kg on a heavy lift launch vehicle. For comparison Cassini was 5712 kg at launch on a Titan IV, and Galileo was 2223 kg when launched using the Shuttle and an Inertial Upper Stage.
* There is a good opportunity for ESA participation with the lander and science instruments. NASA/ESA co-operation is on the agenda for the next OPAG meeting.
* The mission does not assume big upgrades to the Deep Space Network. If the Next Generation DSN does come along, that's just gravy.
* Despite the Europa focus, the mission appears to give at least part of a Galilleo II style tour with multiple flybys of the outer Galileans over 18 months. Only Io will have to wait.

The OPAG Europa working group is also expected to present further work at the next meeting in October. More details will emerge then. I think there is room for cautious optimism on this mission. While we won't be seeing a mission launch for at least another 7 years, the combined weight of the planetary science community does tend to get it's way in the long run. I think the momentum is finally starting to build.

Very nice work. (My saying this, by the way, has no connection whatsoever with your praising my work on the Great Pluto Probe War...) Absolutely the only error I can detect -- and I've long been obsessed with Europa exploration -- is that the original design for Europa Orbiter DID have a big radar array that in fact looked very much like this new one; it was just located at a different place on the craft.

And, yep, they seem determined now to add a very big HiRISE-type camera, in addition to the much smaller one they planned from the start. Not only are high-res shots of Europa important for understanding its surface processes; they're crucial for figuring out how to safely land spacecraft on what looks like a VERY rugged surface. While I'd love to see a small lander (if properly designed) added to this mission, however -- as would Jack Farmer -- it is very much up in the air whether they'll have the money to do so. (I'm currently planning a future article arguing that the best possible design by far for a small lander on this mission would be a penetrator rather than a surface lander.)

I'm all for the Penetrator.

Yeah. We could use the spare ones the Japanese have...

45% to 50% of the cost of the Europa orbiter mission is the cost of mission design and test.I would build 4 more space craft of identical design except for the mass set aside for the lander/probe.this mass would be used by a payload best suited for the target planet. targets? saturn(a cassini follow on) uranus and neptune. the 4th spacecraft would be insurence or a mission of oppertunity.

It might be neat to buffer one up and put a more appropriate instrument sweet on it, and send it on a tour like that of Galileo's later years (in other words, flying by Io repeatedly and occasionally other moons). With a Hirise likecamera, it could study Io's temporal activity, with closeup coverage every few weeks.

Europa/Io complete mapping is a must.


What a disappointment with mapping from Galileo. Even with the extended mission Europa is still poorly mapped.

The thing that worries me is that we are, by necessity, playing a game of Twenty Questions with Europa, and a big, battlestar-galactica craft asks a lot of questions at once, meaning that some of them may be mis-asked. (Like the fluid-probing instruments on Huygens.)

A big camera plus a possible lander could serve purposes, for sure, and if they came for free, who could complain? But look at how many missions we're using to pursue Mars exploration... Given that Europa is also going to take a lot of missions to crack (surely spread over a much longer span of time), a smaller scope might be called for.

Note that with a powerful camera, not much of the surface gets mapped: MOC on MGS will end up mapping only a few percent of Mars's surface (it was have been about 1.5%, IIRC, in the nominal mission). The kicker is, a Europa Orbiter won't live for a decade like MGS, but a month, so *very* little of Europa's surface will benefit from the camera's work. Granted, an attempt to image representative sites (both typical and the odd, atypical feature) should return a weighty fraction of the scientific knowledge that a comprehensive high-res mapping would, but the value of the camera still has to be weighed against that. I suppose the idea would be to produce *final* imaging of potential lander sites, and you have to do that sooner or later, so why not now? Well, the answer might come once the other instruments have had a look.

For a lander, that goes in spades. If we find something out from the hi-res camera, that could really affect lander design.

I think it'd be wiser to launch a probe with radar, a good-not-great camera, no lander, and have a quicker turnaround til the next mission. Europa's not going anywhere. Let's be methodical instead of extravagant.

Well, actually, in its low resolution channel, MOC has mapped the planet many times over at resolution of a few hundred meters. If the big camera comes with something like CTX, it could be quite useful. I also think that we are in a bit of a better positon with regard to a priori knowledge at Europa than at Titan. With a lander however, that is not the case. I don't think we know about the surface at a fine scale well enough for good site selection, although a relatively simple penetrometer wouldn't hurt - I just think it is early.

I think you have to balance the size and capability of the spacecraft against the permissible frequency of visits. Because of Europa's distance, and the large delta-v required to go into orbit, a sequential program like that for Mars is not going to be feasible. So there is more demand for the number of question-answer cycles to be kept to a minimum, even if that means more capable, and hence more expensive missions.

When it comes to any form of lander, I think anything complex will send mission risk and cost too high. But penetrometers may be vulnerable to being axed once the squeeze begins for spacecraft resources and funding, even if we are looking at 7 tonnes for the mission. The Decadal Survey *did* identify a Europa orbiter and lander as separate missions, after all. If the lander is an international contribution, that would make it more secure.

The HiRISE style camera is interesting. Certainly the 30 day prime mission is way too short a time to return the amount of data involved in mapping at that kind of resolution. Since the mission will have a wide angle camera for the global mapping, the question would be where to aim the big mirror. One aspect of the mission that would help is the many flybys and steady final approach to Europa before orbit insertion, which would give lots of opportunities for preliminary surveys. Also, if the mission carried a lot of onboard memory, then once in orbit thumnails could be sent, and then selected detail returned. But at that point the mission team would have to make up its mind *fast* (i.e. on a daily basis) on what was to come back in high resolution.

With a HiRISE style camera, you can do an enormous amount of high quality "raster" style mapping of moons from a distance. Either with a framing camera or a pushbroom sensor (I'd go with framing cause of low light levels), you couild build up low-distortion gigapixel mapping mosaics before the spacecraft moves a lot or the moon rotates a lot.

Somebody do the math and tell me for Galileo-type orbits, what resolution you get 6, 12, 24 and 48 hours from a flyby of the 3 ice moons, and what resolution <km/pixel> you get on Io.

Would a soft lander be possible given the allocated space (and mass) for it on the craft ? I assume there's no real atmosphere to parachute into, so i'm afraid the answer is NO.

http://www2.jpl.nasa.gov/galileo/europa/hst.html

"Europa's oxygen atmosphere is so tenuous that its surface pressure is barely one hundred billionth that of the Earth,"

I guess my dream of having a stereo camera with 360 PAN capability on Europa shatters here. A soft lander seems impossible for now......

Even a simple very small lander (a kilogram of instruments) would be useful for learning some things that we'll need to know for a more complex lander/rover. The Hi resolution camera won't be able to resolve things better than a few meters, and will be unable to actually test the surface composition.

But soft landing without an atmosphere isn't possible. At least not for the allocated mass of the lander.

I was thinking more in the lines of a Melt its way threw type probe.


Or how about this!

What the h... is that ?

Ever seen the sequel to 2001: A Space Odyssey?



It actually looks both plausible and Soviet.

Marcel:

Let's think outside the box...

...Ranger-A plus airbags?

Bob Shaw

I suppose that IF we could make airbags that strong, it would easily thump! back into space far beyond the escape velocity of 2,2 km/sec.....it would buy us about a nanosecond on the surface

I'm 30 now, I hope that before I kick the bucket that I'll know for sure if there is a Subsurface ocean or not.

Life or No life.

Just the fact that a ocean other than our own is out is very cool.

A subsurface/orbiter probe should be Top Priority. *Ducks at Tomato's*

If the orbiter released a craft from low orbit and the craft had a small rocket to de-orbit and layers airbags intended to pop on impact you might sufficiently decelerate a fairly hard set of instruments (perhaps 60 to 100 g's).

I'll have to do a few quick calculations to see if this is reasonable. The big doubt item is whether anything useful can be put in a small enough lander.

They had airbags on Luna 9.

An alternative is to be resigned to the fact that Europa exploration is going to take a lot longer than Mars exploration. The trade-off, simply put, is: Do we want to get the most bang for our buck but have it take more time to fly all the missions we want, or do we want to get the science sooner and risk some missions/instruments that end up missing the point due to some yet-unknown characteristic(s) of Europa?

Don't kick me out of the enthusiast club, but I can't fabricate a case for urgency here. If it takes 8 billion-dollar missions to reach a certain level of understanding, vs a quartet of 3-billion-dollar missions completed in half the time, how do you explain (to the public??) that getting the answers sooner is worth the extra $4 billion? Assuming a fixed budget for exploration, this also means the rest of the solar system gets gyped out of many missions. There is opportunity cost.



I think a smash-and-grab mission that uses an impactor to blast some ice up to a collector that is on a free-return trajectory to Earth has to be considered.

As I see it, the lander concept comes down to two main investigations: What is the composition of the non-H2O stuff? Is there a seismic/thermal/magnetic indication of the structure/activity of the crust and subcrust?

An orbiter can start to speak to the magnetic and probably thermal (by scanning the nightside and eclipsed-dayside in IR) issues. Smash-and-grab would give us a point sample of composition.

I think a very strong candidate plan for the first two missions would be an orbiter that performs detailed surfacing mapping, including scrutiny of whether or not the non-ice component is the same compositionally everywhere. This mission would screen for the best possible locations for any future surface mission, whether it be smash-and-grab, a Pathfinder-style lander, or a penetrator-lander. It is certainly risky to launch a lander of any style without having that basic reconnaisance completed.

The case for the second mission being a lander seems elusive to me. The magnetic and thermal questions will be addressed in part by the orbiter (of course, note: the conditional nature of that statement is already evidence that the second mission should be designed around the results of the first). A smash-and-grab mission would not provide the seismic data of a lander, but would provide infinitely better analysis, in earthly labs, of surface samples -- for far less delta-v.

The combo strategy I have mentioned before for the first landed mission would be to have a lander with seismic capabilities touch down (or penetrate) first, then have an impactor (with its own camera, of course) strike the surface nearby shortly thereafter, providing a known seismic event that would probe the crust fantastically. That same impactor could be the one spraying particles up to the catcher's mitt on the free return trajectory. In all, three elements involving the surface, designed according to the results of the orbiter mission, with a broad wealth of returned data: seismic, magnetic, and thermal data from the landed probe at one location, closeup imagery of a second location, precise seismic data which would give excellent data on one location in the crust, and samples for earthly labs!

Seen this way, the great upside is not to link Mission 1 and Mission 2 to the same launch, but Mission 2 and Mission 3.



I agree that sophisticated regimes for selecting imagery returns are called for. I don't see why such a mission could not have truly massive memory (cmon, that's light), and the ground crew would have the entire duration of the mission to request imagery for downlink -- imagery taken the first day should still be available for downlink on the last day. Store everything, or at least a heck of a lot. It's a nice thought that the orbiter could have a great set of high resolution imaging in its memory, and the ground crew could peruse the low resolution map, and then request detailed observations in terms of a downlink, as opposed to in terms of a new, future observation.

Such a mission was developed called Europa Ice Clipper. A 50-pound ball would be slammed into Europa by a flyby probe, which would fly through the debris cloud, grab some samples of Europa, and return them to Earth.

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

I've been thinking for some time about a modified version of Ice Clipper, in which the spacecraft would be based not on Stardust (with a small impactor) but on Deep Impact (with a much bigger Impactor kicking up a much larger amount of debris, and kicking up almost all of it from depths far below the radiation-modified upper layer). The Impactor's camera could also get extremely high-res final photos which could provide additional valuable information on small-scale surface ruggedness for the purposes of lander design.

I've even wondered if it might be advisable to launch such a mission BEFORE the Orbiter; a high-res camera and near-IR camera on the main craft, coupled to a very high-capacity and high-speed data recorder, could get high-resolution terrain and compositional data on quite a respectable part of Europa's surface just from a flyby (like the "Firebird" Io flyby once proposed as a Discovery mission). One possible motivation for such a mission flying first has disappeared, though: Janus Eluszkiewicz's argument that Europa's upper layers might be riddled with large cavities that would seriously interfere with the depth penetration of a radar sounder -- making it advisable to test the effectiveness of Europan radar sounding from a flyby first -- has come under very serious fire on the grounds that he simply assumed that such cavities could exist when the physical evidence is against it: http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2346.pdf .

And the big problem with a smash-and-grab mission remains: given the very small amount of surface material that it would collect, could even supersensitive Earth-based labs properly inspect the sample for biological evidence? (Especially since it's quite possible that the heating the samples would inevitably undergo as they plowed through the aerogel collector layer would break down organic compounds.) if so, it might be preferable to initially analyze Europa's ice using in-situ instruments, even given their greatly reduced sensitivity and flexibility, simply because they could analyze much bigger amounts of material. JPL's own design study for an initial lightweight Europa soft lander ( http://www.lpi.usra.edu/opag/jun_05_meetin...ssion_Study.pdf ; http://www.lpi.usra.edu/opag/jun_05_meetin..._Trace_OPAG.pdf ) calls for such an organic-isotopic analysis -- using a combined liquid chromatograph and mass spectrometer -- as one of the two top priority instruments for a Europa lander, the other being a seismometer for data on ice-layer total thickness. I myself would regard organic analysis as even more important.

The problem is collecting a big enough sample for such analysis on a small lander -- and collecting it from a fair depth, below the radiation-scrambled surface layer, using a lightweight sample-collection system. The JPL study (which focuses on a surface lander, just because that's the one design it was contracted to examine) expresses concern about this, but doesn't mention specific solutions. A penetrator would seem to be the logical solution. The "Polar Night" Discovery mission proposed to analyze lunar polar ices -- which could well end up as the second in the new series of US lunar exploration probes -- called for three penetrators, each weighing only 30 kg, surviving a crash into the surface at 75 meters/sec and burying themselves 1-2 meters deep ( http://www.nrl.navy.mil/techtransfer/exhib.../PolarNight.pdf ; http://www.mae.usu.edu/faculty/tmosher/Gen...edia/Mosher.pdf ). They would each carry a neutron spectrometer (not necessary for Europa) and a mass spectrometer, and impact tests in which these instruments were fired into a 2-meter layer of plywood and exposed to 1200 Gs (four times their planned load) showed them surviving just fine.

Again, though, if interesting compounds are seriously diluted in the Europan ice, the problem is acquiring enough of them to analyze -- which might require a heated probe to melt its way down through several dozen meters of ice and filter diluted compounds out of the resulting large amount of meltwater ( http://lasp.colorado.edu/icymoons/europacl...ps_EurAbode.pdf ). But such a probe would almost certainly be too big to carry as a piggyback on Europa Orbiter.

If a small penetrator COULD have a chance of analyzing enough material to be worthwhile, however, it would seem vastly preferable to a surface lander as a piggyback on Europa Orbiter in almost every way. It would easily dig below the radiation-modified surface layer (unlikely to be more than a meter or so deep); it would be much lighter than a surface lander; it could land on virtually any terrain, no matter how rugged; it would bury itself and thus provide its own shielding from Jupiter's radiation (which is otherwise a major problem for a moderately long-lived lander); and it would couple its seismometer to Europa far more rigidly than a surface lander. It would probably be unable to obtain post-landing terrain photos, but it could record descent photos during the last few seconds before impact and play them back later for almost equally good imaging data.

Stop press! While poking around on the Web for the above note, I've just found that Paul G. Lucey -- the Principal Investigator for "Polar Night" -- is also working on "Thunderbolt: In-Situ Detection of Biotic Compounds on Europa" ( http://www.higp.hawaii.edu/cgi-bin/higp/di...ame=PaulG.Lucey ). This is surely a Europa penetrator, and I intend to talk to him about it immediately.

Not being familliar with Luna-9, I checked it out and.....did the engineers include something of an easter egg in this image?

I don't think so... ...there was a big insulating cover over the lander - is that what you're thinking of?

Penetrators have been proposed for many missions, but they always suffer from the same problems: very limited room for sophisticated instruments (big difference between a spectrometer that can detect water and one that can unambiguously classify organic molecules) and the need for entry into the surface to be near vertical. At the same time, there are some nice features to penetrators for Europa: they did beneath the surface ice (good for sampling) and have a meter or two of ice shielding them from the radiation (good for a longer life). A potential issue: if the Europeans do the lander, I don't think they have much experience with penetrators, but could be quite wrong on this.

Someone mentioned an impactor - how about using a small stream of impactors to blast successive craters, and at the end of the stream would be the instrument-laden lander itself? It might still need to drill, but not nearly as much.

Uh-uh -- you'd need a huge weight in impactors to blast a hole of any significant depth, whereas you could achieve much greater penetration for tremendously less weight just by making the probe a melt probe (or giving the surface lander a longer drill). To say nothing of the gargantuan targeting difficulties...

Back in 1998 I initiated a discussion list for landing a probe on Europa to explore its subsurface global ocean. Named Icepick, the discussion lasted until just a few months ago.

http://www.klx.com/europa/

You can read the discussions here. I think we hit on many if not most of the scenarios for making this mission plan a reality.

http://www.mail-archive.com/europa%40klx.com/

If someone wants to revive the Icepick list and get discussions going again, I would be most grateful. Jeff Foust ran the intial list and Web site.

Yep, that's the site where I got my start as a space commentator -- and where Simon first ran into me. Sad to see that it's finally disappeared. Maybe I should have hung around there, but I've been juggling several plates at one time for the last few years and just never got around to dropping back in. It starts to look as though the discussion site for Europa exploration may migrate over here.

Apologies if this has already been discussed, but on the OPAG site, there's a fascinating report on Europa Surface Science options. It runs to 86 pages and covers radiation issues as well as landing methods. It was based on the JIMO as the mothership, but much of the discussion is still relevant, I think. I haven't had the chance to go through it in detail, but one point caught my eye: for 375 kg, you can soft land 167 kg on the surface using powered descent. For comparison, the Huygens probe had a mass of 320 kg.

Ok, a melt probe. RTG powered I assume? Just did a quick search.....one page says this of Cassini's RTG's:
"The alpha particles naturally heat the pellets to 572 degrees Fahrenheit (300 degrees Celsius)."
Not too bad at all, more than I expected actually. That'd definitely make a hole....though I'm just thinking now, it'd encase itself beneath the ice. The water above would likely refreeze fairly quickly, even with a toasty robot beneath it. So the little meltbot would be sealed under the ice rather quicly. What would it use for communication? A fiber optic line would be risky (might get tangled), and would add weight. And I don't know how well radio waves penetrate ice.

A fiber optic cable is probably the best bet, using a floating transmitter/receiver at the end of the line to keep it above the meltwater until it refreezes. Radio would require repeaters to go through the ice, which is possible, but you'd have to power them somehow, and keep them from melting down (or up!) when they activate.

Jeff: I saw that OPAG report -- and the two more recent papers att the OPAG site that I mentioned previously elaborate on it somewhat.

Hendric: The idea of a fiber-optic line for communications -- originally the favored idea -- got the boot several years ago, both because of weight problems and becuase the slow but steady ductile sliding of Europa's ice layers would almost certainly snap it. The current plan is to have the probe carry a stack of tiny disk-shaped radio repeater packages powered by tiny RTGs, and release one every kilometer or so that it descends -- so that they're close enough to pick up each other's radio signals through the ice and thus chain-link the signal from the melt probe all the way to its surface carrier.

I should add that the heat from the extremely tiny RTG that each such package would require would not be nearly enough to melt the surounding ice and make it sink deeper.

Wouldn't it? Let's say you want a 10W transmitter. You will need at least, I don't know, ~15W total for electronics and losses and such..? The abysmal efficiency of RTGs meas you will need at least a ~100W heat source to power the thing......ice is a very good insulator.....

I would very much like to see a plot of EM wave attenuation vs frequency for ice so that any "windows" could be identified and the necessary transmitter power could be constrained with higher confidence.

Hmm this looks interesting...

There are two relevant JPL Technical Reports on this design. Unfortunately, JPL's technical-report server seems to be offline for now, so I've attached both reports.

Actually, each transceiver would use a mere 0.12 W power source, hooked up to a capacitor to allow periodic bursts of 1.3 W transmission power. So that's why there's no RTG ice-melting problem.

And here's the other JPL report.

hmmm! very interesting thank you!

Was anyone at the recent (October) OPAG meeting where the new Europa Orbiter was due to be discussed?

The October report and documents aren't up yet, but surely it can't be too long now. Wonder how the talks with ESA went? Hope there was some more support for the 2013 opportunity, given the extra dry mass that could be delivered (probably enough for the soft lander studied by Balint, Nov. 2004).

Roly

I wasn't able to make it to OPAG, and have been monitoring their site for news on the presentations and final report from the October meeting. They haven't turned up yet, but I expect them soon.


http://www.lpi.usra.edu/opag/meetings.html
http://www.lpi.usra.edu/opag/reports.html

Why not ultrasonic communication? Sounds transmit well and far in solid mediums, and there would be no need of lines, repeaters and the like. I see well the penetrator having an array of piezoelectric crystals on its top, it would even be directive. On the countrary a radio link could not work if there is a fault or ice layer at 0°C soaked with salty water.
After, if the penetrator has a density between water and ice, it would float at the bottom of the ice and collect many molecules with a filter.

And with no added cost this sonar (call it by its name) would be a wonderfull mean to probe the ice crust, faults and galeries, and above all to probe the ocean itself, its depth, eventually layers and currents, floating objects, and the ocean floor...

fascinating: an ultrasonic "image" of volcanoes from the rocky core of Europa... "black smokers" (thermal vents) would be already fine.

More and more: the penetrator abandons its floater, and sinks to the rocky floor itself, and takes photos of it, eventually showing weeds and living forms...

Wow!



And on the surface?
A repeater left on the surface could send enough power (like Huygens did) to be picked from Earth (with a slow bit rate. Of course if there is an orbiter to relay data it would be better). If the repeater is buried in ice, just its radio antenna out, it could work for months and years, much compensating for the slow bit rate.

The presentations from the third OPAG meeting have just arrived: http://www.lpi.usra.edu/opag/oct_05_meeting/agenda.html . (The final report from the meeting isn't available yet, though.)

Though this may sound terribly obvious, any Europa landers/ocean explorers had better be designed to last a long time if their main goal is going to be the search for life on that Jovian moon.

Unless the ice crust is encrusted with dead microbes or such similar creatures and their alien jellyfish counterparts are saturating Europa's ocean, I do not want to end up with the same situation as the Viking landers, who were stuck on two tiny spots on Mars and had scientists and the media declaring the Red Planet a dead world (again) when a few scoopfuls of dirt revealed no native microbes. Europa will require a long exploration.

To add: The global ocean on Europa is estimated to be *60 miles* deep. Thankfully in one sense the moon's much smaller mass makes the bottom water pressure on Europa no "worse" than that found in the deepest parts of the Pacific Ocean (7 miles down), but if there are black smokers and alien versions of red tubes worms and giant crabs living around them, can we design a probe that could make it all the way to the bottom of the Europan Ocean and return the data to Earth?

Another question: Life may be able to survive on Europa in its present state, but based on what we know, could it ever have gotten off to a start in the first place?

The reply to the second question is simply: we don't know. We don't know whether life could have evolved out of prebiotic molecules on Earth had the water been as acid and/or saline as Europa's appears to be; there has been at least one abstract I've read expressing doubt, but given our stupefying continuing level of ignorance about how the chemical process occurred on Earth itself, we just don't know.

As for the first question: even if we don't get all the way down to isolated "smokers" on the floor of Europa's ocean, we should be able to detect microbes (or their remnants) from such locations spread uniformly through the ocean water -- after all, that's how life on Earth gets transferred from one isolated smoker to another and so survives after the first smoker finally goes out. And if the alternative theory is true that Europan microbes may derive their nourishment instead from chemicals manufactured by radiation in Europa's upper ice layer and then gradually transferred down to the ocen by geological processes in that ice layer, the principle is even more true -- in fact, in that case the life would probably be concentrated at the TOP of the liquid-water layer.

This also leaves the question of whether we can find evidence of microbes in Europa's liquid ocean without even having to bore down through the ice layer to the ocean, by instead analyzing the surface ice itself to look for such remains transported up to the surface by those same slow geological processes in the ice layer. The consensus seems to be that this is a real possibility -- but, since there's a thin layer of brittle supercold "nonconvective" ice 1-3 km thick on top of the warmer main ice layer which slowly convects (and which even perhaps carries pockets of still-liquid brine upwards), we are going to have to be careful to choose landing sites that look likely to have had buried material erupted all the way up to the surface. (There are several types of Europan surface features that show promise of this, which is another reason why we do need a Europa orbiter first to pick out good landing sites.)

Yes, good approach in a first time, before trying to reach the bottom of the ocean (what I think possible but more complicated). But the very upper layer of ice (about 1m) is exposed to strong radiations, and thus sterilized, and anyway not representative of the global ice chemistry. So we need to drill from the very first landing, even if only 1-2m. For this reason penetrators were proposed (sticking themselves in ice like an arrow, which solves the problem of soft landing) or using a more classical drill, or a heat source able to melt ice. (RTGs were proposed, but RTGs are weak, a chemical source would perform better for this very purpose). A tip would be to place all the electronics into the drill, so that it is protected from radiations and it can last for much longer, very useful if it carries a seismometre.


Also a RTG was proposed to melt the ice down the ocean. But are RTGs powerfull enough for this? I would rather see a bot with a screw-shaped nose and a body with fins, like in sci-fi novels, running with a high gear rate, it would be much more power-efficient and faster than just melting ice. We have plenty of places on Earth to test this, in the Antarctic ice shelds. If there are interesting results from a surface examination, there will be a strong support for the idea of looking at the botton of the ocean.