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By Emily Lakdawalla


LPSC: Thursday: The Moons of Jupiter and the future of Outer Planet Exploration

Mar. 17, 2006 | 22:07 PST | Mar. 18 06:07 UTC
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I said earlier I was going to cover the poster sessions next, and there are some cool things that I want to write about, but I thought I'd better get to something a bit more topical a bit sooner: Europa and the other Galilean satellites, and when (if!?) we'll be exploring them again.

After checking out Knauth's talk in the Mars session I went over to the one session on the Galilean satellites of Jupiter. I took some notes, but frankly I'm not sure they're of too much interest to most people. It's not because Io, Europa, Ganymede, and Callisto are boring -- far from it. It's just that Galilean satellite science has matured to the point where it's less about discovery and more about models, which are a little more difficult to explain. It seems to me that the outer planets community has basically finished cataloguing the types of things that you can see in the Galileo images. We know how many craters there are, and their sizes and distributions. We've described and organized all the types of ridges and cracks on Europa and Ganymede into classification schemes, and mapped how they're oriented. We've figured out the temperatures of the eruptions on Io. A lot has been done.

So what's left to do? Well, we're still far from being able to explain most of that stuff. And even if there are explanations that seem to work to explain a feature generally, when you get into specifics or variations you have to develop explanations for why this or that feature is locally different. So there were talks on trying to figure out just how fast the volcano Pillan on Io erupted (Ashley Davies concluded that when Galileo saw Pillan erupt, that was probably the biggest volcanic eruption ever witnessed in recorded history). There were talks attempting to explain the patterns formed by the cycloidal cracks on Europa (obliquity could have an interesting effect, but questioners pointed out that true polar wander was probably more important). Several people were looking at how stresses in Galilean satellite ices might change the grain size, and potentially the viscosity, of the ice, which could answer questions about just how vigorous a process convection is inside these bodies. I listened to some of it, but didn't find much that I could explain very readily, so I decided to take a break, post a blog entry, go buy some tacos, and return for a lunch meeting on the future of outer planet exploration.

The meeting was organized by Torrence Johnson, Bill McKinnon, and Bob Pappalardo, who loosely represent three professional generations of outer planet scientists. Torrence opened the session, which was attended by maybe 75 or so people: "This is a kickoff meeting to see where we are at the moment." He showed a graphic with time on one axis and missions on another, with each mission represented by a green and a red bar. The green bar began when the mission first started being talked about, and had an asterisk in it where NASA actually funded the mission start. The green bar changed to red at launch, and projected out until the end of the mission, with markers for planetary encounters. There's a short but distinguished list of past or current outer planets missions: Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Cassini-Huygens, and New Horizons. (I would very much like to reproduce this graphic but don't have the time to hunt down the "first started talking about it" and "NASA started funding" dates -- I'd love help.)

Pointing to the diagram, Torrence said, "It's very easy to concentrate on minimizing trip time. In fact, if you look at this graph, a good half of the amount of time that a mission is being considered by a community, it isn't in flight at all." Indeed, the green bars and red bars show a strong tendency to be of roughly equal length. "It takes a long time to get unity in the community on these," by which Torrence meant goals, instrument suites, mission profiles, etc. "If you want to get something going, you have to get it started, talk about it, work with it, and persist. That's been reasonably successful for the last 30 years. Because we've leapfrogged things, we've had a staggered approach that's taken into account long trip times. The result is that if you draw a line down in any given year, there has usually been a mission in flight, with data coming down, or about to come down at any time." Indeed, the green bars and red bars have overlapped quite systematically for the last 30 years. In flight doesn't necessarily mean that a mission is returning data, but even when a mission is just in the cruise phase it's generating a lot of activity among scientists as they prepare their instruments and their theories for a new encounter with an outer planet.

Of course, Torrence was leading up to a point that you probably predicted already, if you've been paying attention to what's been going on at NASA lately. "At this point, we would normally have started" the next mission. Torrence had a couple of abortive bars on the graph representing future missions. There's JIMO, which has a short, dead bar. There's "Europa Orbiter," which has a slightly longer bar but is also dead. There is Juno, which is moving forward. But clearly there is a gap, where we should have started the next big mission, and we just haven't. "We've got nothing beyond those missions, and we've got nothing building on the Galileo and Cassini discoveries at Saturn and Titan."

Torrence went on to refer to something that had happened earlier in the week that has really stirred up the outer planets community. Titan researcher Jonathan Lunine gave an invited talk after lunch on Monday (when I and apparently every space journalist except Leonard David was watching the Stardust press conference) in which he, too, discussed the future of outer planets exploration, and seemed to advocate the consideration of a mission to Titan and Enceladus next rather than a mission to Europa. While such a suggestion should of course be at least spoken about in light of Cassini's discoveries there, in the context of the slashing of science funding and the cancellation of the Europa mission by NASA, many in the outer planets community fear that by making such a suggestion in a public forum, Jonathan ran the risk of giving the budget wonks at NASA the ammunition they need to say, "look, you guys aren't ready to start a flagship mission, you still can't agree where you want to go." As a result, it seemed this week that conflict might open up between Europa and Titan advocates. I'll be returning to this later, but let me continue with Torrence's point about the lack of current planning for a future outer planets mission.

"It's what I call the CEV gap," Torrence continued. "Budgets back at NASA are frozen until we get the CEV going. There isn't much hope for new starts in this time period. So even if you are optimistic, you are talking about no launches until 2020." In the spirit of optimism, Torrence discussed what is being done to look forward to future outer planet missions. In particular, he highlighted the activity of the Outer Planets Assessment Group, or OPAG, which is a body that holds meetings open to the community (there are equivalent groups for the Moon, Mars, and Venus, called ILEWG, MEPAG, and VEXAG), and NASA supports these. He asked Curt Niebur of NASA, who's in charge of OPAG, to stand up and talk about that.

Niebur began with a clear reference to the conflict raised by Jonathan Lunine's talk. "We will all either hang together or we will, assuredly, hang separately," he said, and there was murmured agreement. "Fran Bagenal [who runs OPAG with Niebur] has started a Europa focus group chaired by Ron Greeley [an outer planets researcher of Torrence's generation]. That group works directly with mission designers at JPL. We spent about $500,000 of HQ money on a very simple study, which was: how much mass could we throw into Europa orbit? And the answer was, a lot of mass. So what this group has been doing is going through what kind of goals we could achieve with that mass." He finished by asking the room for further input, saying, "What studies should we be doing? What technology should we be developing?"

Torrence then gave the floor to Bob Pappalardo to talk about another group that is planning for the future of outer planets exploration, called the "Europa Focus Group" (not to be confused with the Europa focus group within OPAG). "This is a child of the NASA Astrobiology Institute," Bob began -- I'll remind you here that astrobiology funding has been slashed 50% in the fiscal year 2007 NASA budget. Bob showed several slides detailing an overarching goal, and the main science objectives that OPAG has developed for a Europa Orbiter mission. The goal is, "Explore Europa and Determine its Potential for Life," with the following detailed science objectives:

  • Characterize the ocean through its effects on potential fields and its dynamic relationship with the ice shell.
  • Characterize processes operating within the ice shell, and the nature of ice-ocean exchange.
  • Determine surface compositions and chemistry, especially as related to habitability.
  • Understand the formation of surface features, including sites of recent or current activity, and identify candidate sites for in situ exploration.
  • Characterize the magnetic environment and moon-particle interactions.
  • Determine how the components of the Jovian system operate and interact, leading to potentially habitable environments in icy moons.


Establishing objectives such as these is a necessary first step in organizing a mission; along with the mass and time constraints, it helps to organize what suite of instruments and what mission profile will be necessary. Bob also mentioned that in addition to OPAG and the Europa Focus Group, there are also discussions taking place between NASA and ESA scientists. "The Europeans have an opportunity to propose including outer planets in their Cosmic Vision, with a launch somewhere in the 20-teens," Bob said. He then gave the floor over to Bill McKinnon, who was representing the Division of Planetary Sciences of the American Astronomical Society (DPS).

"The DPS committee was faced with some very distressing budget news recently," Bill said. "Our overall strategy is to support planetary science in general, and we are faced with rather catastrophic cuts to research and analysis. My personal opinion is that I think that there is a lack of appreciation at a high level for us." Basically, he said, NASA HQ (or perhaps he was referring to the Administration, I am not sure) seems to see scientists as no different from skilled workers in other fields. In the last couple of decades it's become common to lay off skilled workers when times are bad, and then re-hire them all a couple of years later when the economy approves. Bill said that, by contrast, scientists are generally "in it for life, or not at all." However, he added, "I think that working with the good people at Headquarters, and there are good people at Headquarters, this problem is on the road to being solved. Our overall strategy is that we have to save the field itself, not just a particular mission."

He mentioned the testimony given by Wes Huntress to the House Science Committee, which -- when forced to consider the NASA science budget as a zero-sum game -- ranked research and analysis absolutely first in importance (to maintain the science community), then the least expensive missions (Explorers and Discoveries), then medium missions (New Frontiers), and then, sadly, lastly, flagship missions. This was not to say that flagship missions aren't being advocated for, Bill said: "We are very supportive of trying to get a start for Europa. We are pursuing multiple strategies."

Then it was my turn, as the representative of The Planetary Society and the public, to stand up. I reminded the room that Wes is also our President, and that his and others' advice is guiding our own activities. I told them about our campaign that began just as saving the Europa mission and is now broadening into saving science at NASA in general. I told them that New Horizons has demonstrated that the public can be a very important voice in helping to overcome obstacles to missions, if they want a mission to happen; and I invited everyone in the room to talk with us and participate in our advocacy activities in general and in our current campaigns to advocate for restoring science funding to the NASA budget in particular.

At that point, the floor was open to questions. Bill asked me whether the public wants a Europa mission. I answered, definitely. When you have such a young world concealing a dynamic environment of briny oceans and the potential for life, the public is definitely interested -- they've even seen an IMAX movie about it.

Jeff Moore asked if the pain of a flagship mission could be reduced by giving it a longer but flatter funding profile that might hit NASA for only, say, a hundred million dollars a year. Torrence answered that that just wouldn't work; in the end it would cost more, and that no matter what, a flagship-sized mission would need to ramp up to 300 or 400 million for the year or two before its launch. (I asked Torrence later whether we could orbit Europa or Titan for the cost of a New Frontiers mission, and he said it was pretty much impossible; you really have to spend more than a billion to make it work.)

Galilean satellite researcher Francis Nimmo stood up and confronted the gorilla in the room, which was Jonathan Lunine's Titan advocacy. (Jonathan, sadly, wasn't there to participate; he'd already gone home.) "The very last thing we need is to be tearing ourselves apart," he said. "Can we avoid a civil war between Europa and Titan?"

"Internecine warfare is not what we need," Torrence said. "In a logical world, this would not be a problem; we are just beginning to talk about Titan and Enceladus, we would be starting Europa [mission planning] now" if they could, but NASA is not cooperating. In other words, if there were one or two flagship missions planned per decade (and until last year NASA's future planning included two per decade), it would be obvious to everybody that Europa is more ready and should be next, and Titan would probably come after that. "But we have to live with realities. And we also have to live with the fact that people are going to be excited by the latest thing," meaning Titan and Enceladus.

Titan researcher Ralph Lorenz then asked the room (in his typically piquant way), "what would it take to convert the Europan fundamentalists to Titan? Would the discovery of an active cryovolcano do it?"

Torrence replied that there was already an active cryovolcano in the Saturn system, not at Titan, but at Enceladus. Because Jonathan's talk was clearly a focus of such debate, Torrence decided to go ahead and acquaint the room with exactly what Jonathan had said, in case anyone missed it -- and since I missed it, this was great.

According to Torrence, Jonathan said: "There are three strategic elements for solar system exploration -- understanding origins of planets and the solar system, their evolution, and their capability for supporting life. The current suite of missions (New Horizons and Juno) do a good job of the first two (origin and evolution of planets), but not the third (life).

"He also pointed out that as a practical matter, Europa has had 3 strikes." In other words, Jonathan apparently said that since Europa has been canceled three times, it may just be a politically difficult goal, and politics should force you to consider alternative missions. I should mention here that it's this statement that has apparently riled up the Europa folks most as being rather absurd, in their point of view (and, I might add, Lou Friedman's as well). And anyway, JIMO wasn't even their fault, it was obviously doomed from the start. But I won't get started on that now. Torrence continued summarizing Jonathan: "Next is new data form Saturn pointing to Titan and Enceladus as important elements, and it's conceivable that they could be easier to explore."

Now, I think, Torrence shifted away from summarizing Jonathan and returned to his own opinions. "Whenever you have this type of issue come up, you come back to fundamental principles at how we have been successful at guiding policy. At the moment, our established set of objectives go back to COMPLEX reports of 1969 and 1970, up to most recently the Decadal Survey [which was in 2003]. You will find a heck of a lot of consistency. Consistency to objectives is important. Inconsistency gets Congress and everyone else on your back really quickly. Note that consistency to objectives is not the same as loyalty to a destination or mission. All of these things must be updateable and upgradeable in that they need to be responsive to new discoveries."

When it comes to figuring out which mission to do next, it's not just science that's important, Torrence continued. "Strategic planning is less pure. What do we do first? Exploration strategy must incorporate several factors. The science objectives from the Decadal Survey, et cetera, determine destinations, priorities, and mission objectives. But technological readiness determines launch order. Then there a whole bunch of other things called programmatic realities. What is the public excited about? What sort of money is available when? What is the mission category balance?" meaning small, medium, and flagship missions.

"I agree with Jon on the basics. At Europa, the astrobiology goals require a precursor orbiter (even with a simple lander) to know how to address habitability issues. We don't know how to get to the ocean! My personal opinion is that you can't solve the problems of Europa with one more mission." In other words, an orbiter is necessary first to truly understand Europa's icy geology and geophysics. Only then will we be ready to send some kind of subsurface explorer that's capable of reaching Europa's oceans.

"For Titan and Enceladus, the Cassini data will provide enough information to inform the next stage of in situ exploration." In other words, Cassini is probably all the precursor that's necessary for a follow-up in situ mission to Titan's surface (or, alternatively, its atmosphere, with an aerobot or balloon.)

Torrence finished by saying that the room was getting concerned about the wrong problem. "It's not Europa versus Saturn. Both targets address the highest priority goals." Torrence explained this through analogues to habitable environments on Earth. "Titan is a pre-biotic Earth, with complex, exotic Earth-like processes. Europa is like a mid-ocean ridge hydrothermal vent system. Enceladus has hot springs and geysers, hydrothermal systems like Yellowstone. If you wanted to search for life on Earth, and needed to pick one, the answer is both Titan and Saturn -- you might not know which one to go to first. Perhaps you'd go to the easiest."

All else being equal, Titan may well be an easier target than Europa (whether it's a more desirable target now is a debate that has hardly begun). But all else isn't equal. "The programmatic realities are that we have had many years of investment in Europa, and we think it can be implemented now. At the same time, we have an asset in the Saturnian system to follow up on Saturnian discoveries. If we right now turned the switch and proposed a new Saturn mission, realities would limit you to a less exciting payload than we have there already! Even an optimistic scenario requires several years to get to phase A plan for post-Cassini Saturn."

Jeff Moore asked if this message -- that the outer planets community is still ready to begin a mission to Europa now -- has been delivered to NASA Headquarters recently. Curt Niebur of OPAG stood up and said "This issue came to OPAG last year, and OPAG replied that Europa is number 1" about 8 months ago. Additionally, he made the interesting comment that "There is a desire at NASA to get a mission going as a first priority, and where it's going is a second priority."

At the very end of the meeting, Bob Pappalardo closed by remembering a fellow researcher -- a student, younger than him, younger than me, who was an undergraduate at Brown at the same time that Bob was there as a postdoc and I was there as a grad student. Jiganesh Patel died tragically last week at the age of 28, and Bob asked everyone to remember his contributions to the study of the Galilean satellites. To that, Torrence added, "Bob has reminded us that what we do is a profoundly human activity. Get that message out to the public."

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