Most scientific groups to examine this question have concluded that it is -- certainly a detailed survey of Europa's entire surface would be immensely useful for these purposes. And if the money is available for it, it should certainly be flown. But given the expense of this mission, and the fact that NASA will be continue to be seriously strapped for cash throughout the foreseeable future, perhaps we need to examine whether it is really absolutely necessary.

Back at NASA's Third Astrobiology Science Conference at Ames Research Center in 2004, Christopher McKay delivered a report on the findings of the science definition team for "JIMO" (Jupiter Icy Moons Orbiter) -- the gigantic reactor-powered, ion-engine-equipped "battleship"-class probe that then Administrator Sean O'Keefe had been unwisely persuaded to support, to orbit Europa and two of Jupiter's other moons sequentially.

The team concluded that a small piggyback Europa lander was highly desirable for JIMO in the event that this multi-billion-dollar behemoth was actually flown. But, according to McKay, they also concluded that it was not strictly necessary for JIMO to extensively survey Europa to find a good landing site before releasing the lander, even if its experiments did include a search for actual biological evidence on Europa -- because some highly promising sites (such as "Castalia Macula", a very dark, smooth spot apparently covered with nonice material relatively recently erupted onto Europa's surface) have already been identified from the Galileo craft's photos as likely fitting the desirable criteria of "youthfulness, presence of non-ice materials, and the likelihood of material exchange with the subsurface" -- as well as sufficient smoothness that a lander's chances of actually surviving its touchdown would be maximized.

Now, the proposed later Europa Astrobiology Lander (EAL) would be a much bigger, much more expensive lander, and one that would have to probably have to carry out a full-fledged soft landing (whereas a smaller lander could use airbags or similar techniques to survive a hard landing in rather rugged terrain).

It would be very unwise to pick a desirable first landing site on Europa for such a mission just from Galileo's photos and near-infrared surface compositional maps. But the question does arise: do we strictly need to fly an actual Europa orbiter to pick out a very promising landing site for it -- or could a "Galileo 2" craft that just orbited Jupiter and made repeated flybys of Europa (and perhaps Jupiter's other big moons), with improved instruments and a radio system capable of returning far more data than Galileo's largely crippled system, map enough of Europa well enough to pick out a satisfactory site for the first EAL?

The Europa Explorer orbiter actually has two main goals. The first is to confirm, once and for all, that there actually is a subsurface liquid-water ocean beneath the moon's ice crust. This is obviously biologically important (although Europa could be of intense scientific interest even if it had such an ocean during its early years which has since frozen solid). But the Galileo mission has already confirmed the existence of such an ocean almost beyond doubt by making several measurements of Europa's "induced magnetic field" -- as the moon plows through Jupiter's magnetic field, that field is inducing electric currents in some kind of highly conductive near-surface substance which in turn generate a weaker local field that is always exactly opposite the current direction of Jupiter's field, and a large subsurface layer of liquid water is the only realistic candidate.

Europa Explorer would nail down the ocean's existence beyond doubt both through more magnetic measurements, and by using a laser altimeter to measure the very slight up-and-down tidal flexing of Europa's ice crust as the moon orbited Jupiter at different distance -- as one might expect, an ice crust with a liquid water layer sloshing around underneath it would flex a lot more than just a layer of solid ice sitting on rock.

A Jupiter orbiter that just made multiple Europa flybys, instead of orbiting the moon for a long period, probably could not do accurate enough laser altimetry for that last test, and its ability to make induced magnetic field measurements would also be seriously limited compared to an orbiter. But, just by making 10 or 20 more Europa flybys with a magnetometer onboard, it could probably confirm Galileo's data beyond the point of practical doubt.

Europa Explorer's second main goal is to find the best possible landing sites for Europa biological landers, using high-resolution cameras (to locate safely smooth areas and also spots where there seems to have been relatively recent upwelling geological activity), a near-infrared surface composition mapping spectrometer (to look for the highest concentrations of interesting nonice substance, including organics), a thermal infrared mapper (to look for still-warm spots on the surface where surface eruptions of liquid water might have occurred within the last few thousand years), and a long-wavelength radar sounder to peer down through the top few kilometers of Europa's ice to look for near-surface pockets or fissures of liquid water (as well as further looking for spots where eruptions might have occurred in the fairly recent past).

A flyby craft would, of course, be far more limited in the amount of Europa's surface that it could photograph and map at high resolution -- if the Cassini spacecraft's flyby mapping of Titan is any guide, Galileo 2 might be able to acceptably map at most 10-20% of Europa's surface for this purpose. But this just might be enough -- given both that the Galileo craft's limited coverage has already given us a set of particularly interesting-looking possible sites that could be reexamined in more detail by Galileo 2 when it made close flyovers of them, and that the first flybys by the Galileo 2 craft could be devoted to looking for additional promising landing spots which it could then fly over later for more detailed study.

In particular, it could carry the same type of very powerful narrow-angle camera -- approaching the meter-scale resolution of the "HiRISE" camera on the Mars Reconnaissance Orbiter -- which will likely go on Europa Explorer, to provide us with an idea of the fine-scale roughness of Europa's surface even without landing, in order to let us properly design the Europa Lander.

One area in which a multiple-flyby Galileo 2 inspection of Europa might be particularly weak is the radar sounder planned as one of the highest-priority instruments on Europa Explorer. Such a device can only provide a depth cross-section of Europa's subsurface ice along a narrow strip directly underneath the craft, and then only at close range -- which a flyby craft, unlike an orbiter, would only be at briefly on any flyby.

Such a sounder, however -- if aimed at specific interesting preselected targets -- could still tell us a lot about the subsurface structure of Europa and the way it tends to be reflected in features visible on the subsurface; a radar sounder is considered one of the highest-priority instruments for any future mission making a flyby of Saturn's moon Enceladus, for just this reason.

And there's another possibility for a Europa flyby craft: "synthetic-aperture radar" (SAR), of the type used by the Magellan craft to map Venus and by Cassini to map Titan. Such radar provides 2-D photograph-like images of the surface topography of a world, rather than providing a depth profile of its subsurface structure. But the right frequencies, if they were included in it, might penetrate only a short distance below Europa's ice -- say, 100 or 200 meters -- and thus provide a map, covering quite a wide area, of the location of subsurface liquid-water pockets near Europa's surface, even though it could not tell us their precise depth. (A SAR system like this is also being seriously considered to map Mars' ancient geological features beneath its upper layer of windblown dust and sand.)

Since one of the primary factors in trying to pick a good site for the Europa Lander is trying to locate liquid-water pockets near the surface, this alternative form of radar might actually be more useful than a radar depth sounder in picking such landing sites. (Even a radar depth sounder, by the way, cannot penetrate deeply enough through Europa's ice to detect the main subsurface ocean and tell us the total thickness of the ice crust -- unless that crust is less than 5 kilometers or so thick, which is now considered unlikely -- but that particular piece of information, while scientifically important, is not necessary to pick a good landing site. Indeed, the Europa Lander -- in addition to its biological instruments -- will almost certainly carry a small seismometer to tell us how thick the total ice crust really is.)

It should be emphasized, again, that an actual orbiter of Europa would be far superior to a Galileo-2 type craft to examine Europa in detail, and inspect all of its surface for the best landing sites. If NASA continues to be seriously strapped for science money, however, the much cheaper Galileo 2 mission might be worthwhile as a fallback to precede the Europa Lander -- especially since its mission could be designed to also inspect Jupiter's other three big moons in important new detail using the same instruments (although with fewer flybys than those the craft would make of Europa).

Indeed, one NASA team is currently studying an alternative to Europa Explorer as a Flagship-class mission to the Jupiter system -- a "Jovian System Observer", which would probably end up orbiting Ganymede instead of Europa and studying that moon in detail after making multiple flybys of Europa and Callisto. (It might carry a multiple-aperture "MIDAS" telescopic camera -- a new concept just now being designed -- to observe Jupiter's other moons in sharp detail even from a long distance away.)

This mission, however, is not likely to cost much less than the Europa Explorer would -- it would differ solely in needing somewhat less radiation shielding, and there is already more than enough of a power margin on Europa Explorer's Delta 4 launch booster to handle the Explorer's added shielding weight -- and, while Ganymede is geologically a very interesting world, it lacks the biological interest that raises Europa to the top ranks of NASA's future targets.

So, if the next mission to Jupiter's moons is not the full-fledged Europa orbiter, I feel that the much lighter and cheaper Galileo-2 type mission would probably be the best alternative choice as a prelude to the eventual Europa Astrobiology Lander (or as an advance scout to discover whether Europa is worthy of an expensive biological lander at all). At the moment, however, NASA does not yet seem to be doing any detailed study of such a possible New Frontiers-class Europa mission. It is, however, doing two studies of possible New Frontiers-class missions to two more distant targets -- Saturn's moons Titan and Enceladus -- and I'll be describing those next.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends. Email This Article

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