These days funding for missions to some of the most interesting places in the Solar System is much in question. But sooner or later we’re going into the outer system to investigate the possibilities for life on worlds like Europa, Enceladus or Titan. The case for Europa seems particularly compelling, but we have to be careful about our assumptions. When the Europa Orbiter Science Definition Team developed a strategy for Europan exploration in 1999, it was generally believed that any Europan ocean would be covered by a thick and impermeable layer of ice. Life, then, might exist around deep sub-oceanic volcanic vents if it existed at all.
Thus the strategy for Europan exploration that evolved: Three missions, beginning with an orbiter, followed up by a lander and, finally, a third mission that would drill down through the presumably many kilometers of surface ice to explore whatever lay beneath. Even in more financially optimistic times, that strategy didn’t get us into Europa’s ocean until well after 2030, and today we struggle to come up with a date for the Europa Jupiter System Mission (surely later than 2030 just for an orbiter), with a timetable for ocean exploration that is without doubt set back by decades.
Re-examining Europan Exploration
Why not then, says Richard Greenberg, take time to re-evaluate our entire Europan strategy, factoring in all the work that has been done sine the late 1990s? Greenberg (Lunar and Planetary Laboratory, University of Arizona) is the author of Unmasking Europa (Springer, 2008), which sharply critiques the ‘thick ice’ assumption by pointing to many instances of Galileo imagery showing what appears to be young and constantly resurfacing ice. Cracking and melting of a thinner ice sheet could actually expose the ocean, and make the job of studying it far easier, while demanding considerable caution in terms of possible contamination from terrestrial organisms.
Image: A representation of possible subsurface structures, prepared by the Jet Propulsion Laboratory for the recent Europa Orbiter Science Definition Team (SDT-2010), shows a very different picture from the version of a decade earlier. According to SDT-2010, ‘‘The NASA Jupiter Europa Orbiter will address the fundamental issue of whether Europa’s ice shell is ~few km (left) or >30 km (right), with different implications for processes and habitability.’’ The thick ice as shown on the right extends tens of kilometers down toward the rocky interior. The model with thinner, permeable ice is now considered on par with the earlier canonical model of thick, impermeable ice. Credit: Richard Greenberg/Astrobiology.
In a new paper, Greenberg notes that the recent Joint Jupiter Science Definition Team identified the thick vs. thin ice question in 2010 as a key objective of Europa exploration. The thick ice model, in other words, is no longer the only game in town, raising real questions about how we proceed in long-term planning. Greenberg reviews the case for thin ice, especially in places like Conamara Chaos, where rafts of displaced crust can be seen lodged in what appears to be lumpy, refrozen ice, with new cracks changing the terrain yet again and suggesting melt-through. Unmasking Europa has that story in detail, but the paper offers a helpful summary to get you up to speed.
I think Greenberg’s case is strong, but I want to focus on the implications of thin ice rather than the case for it. For if we do determine that the Europan ocean is accessible, our mission focus shifts to exploiting the terrain to find the best place for surface operations. From the paper:
Rather than focusing on the daunting, perhaps impossible, task of drilling down to the ocean, we should consider how to take advantage of the biosphere’s natural accessibility. With the rapid resurfacing, almost any europan landing site might provide oceanic samples; the issue will be how to find the freshest ones. When chaotic terrain forms, it replaces a section of crust with frozen ocean. Ridge formation squeezes out ocean material. Fresh oceanic material may be exposed at the surface as gaps open up and create the dilational bands. Any of these processes could be laying out biological samples on the surface.
Thus the need, says Greenberg, to ‘land smart,’ picking the optimum landing site to avoid the need for drilling through the ice in the first place. Reassessing a Europa orbiter, then, should involve a key objective: Identifying the most likely sites where the underlying ocean may have been recently exposed. A lander integrated with the orbiter mission would have the chance of finding extraterrestrial life near or even on the surface, as the frozen remnants of materials that have been briefly exposed through surface shifting of the ice. Greenberg thinks such a strategy could give us an answer on Europan life within the lifetime of many adults living today, as opposed to pursuing what is essentially a holding strategy as we develop a thick ice drilling model.
Ice and Contamination
An overriding concern is protecting Europa from life that has hitched a ride from Earth, a problem that looms large with the thin ice model, whereas with thick ice and an isolated ocean, the chances of contamination seemed more remote. The NRC Task Group on the Forward Contamination of Europa (2000) adopted a standard for protection that says the probability of contaminating a Europan ocean with a viable terrestrial organism must be less than 10-4 per mission, referencing a 1964 resolution that Carl Sagan had a hand in fashioning with regard to the exploration of Mars.
But the Sagan formulation, developed with Sidney Coleman, is deeply flawed. Here is part of Greenberg’s critique:
The basis of the calculation was that the probability of contaminating Mars during these missions should be very small for at least the duration of the specific envisioned exploration campaign. In other words, the underlying premise of that study was that the purpose of planetary protection was to protect the interests of then currently active scientists, rather than future generations of scientists or of alien organisms themselves. They arbitrarily selected a value of 0.1% as the maximum acceptable probability of contamination. On that basis, they calculated that each spacecraft launched to Mars had to be sterilized enough so that there was less than a 0.01% chance of having any organism on board. This calculation depended on specific assumptions about unknown conditions relevant to Mars and on guesses about the future exploration program.
The international Committee on Space Research (COSPAR) accepted the Sagan/Coleman recommendation in 1964, even though its selection of a level of underlying risk was arbitrary and relied on assumptions about the survival of terrestrial organisms in space that were, in Greenberg’s view, crude. Its ethical premises were also shaky, with the assumption that we need to preserve a biosphere only long enough for the current scientific community to finish exploration. What about future generations of researchers, and what about extraterrestrial life forms themselves? This COSPAR resolution was a key source for the 2000 NRC task group report on Europa at a time when a thick ice model featuring an isolated ocean was still favored.
We need to reconsider such questions, and thankfully, a new National Research Council study on planetary protection has been commissioned by NASA, although at the moment its mission is to develop contamination standards for icy moons that are still based on Sagan/Coleman. Thus Greenberg’s call for a new analysis. And the scientist has an interesting suggestion involving what he has previously called a ‘natural contamination standard.’ It goes like this:
…exploration would be acceptable if the probability of humans infecting other planets with terrestrial microbes is smaller than the probability that interplanetary contamination happens naturally. Such a foundation principle would be ethically defensible and could be translated into specific, research-based, quantitative standards.
Greenberg also thinks the NRC panel should come up with practical sterilization criteria so that NASA can draw on an independent analysis as the basis of building Europan exploration craft. The NRC 2000 report left such matters in the hands of NASA and its contractors. The questions this raises are too significant to go unanswered. If Europa does have permeable ice, there is a distinct possibility that its biosphere extends near to the surface, raising the odds for contamination. This calls for a new look at planetary protection whatever the time frame involved, and a tightening of standards that need to move beyond those of Sagan/Coleman.
The Greenberg formulation, based as it is on a natural contamination standard, would depend on the rate of spacecraft arrivals on Europa because it would be keyed to the natural rate of interplanetary transport. It is, at least, one way of looking at contamination that takes in current data, which include the distinct possibility that we are dealing with thin and permeable ice on the distant moon. And there is a philosophical issue that trumps the proceedings. Do we have an ethical imperative to protect indigenous life forms from contamination wherever we go in the universe, and should this imperative be factored in to every mission concept we create?
If so, we need to think long and hard about the potential for thin ice on Europa, because getting the contamination question wrong would compromise both our scientific and moral objectives. The paper is Greenberg, “Exploration and Protection of Europa’s Biosphere: Implications of Permeable Ice,” Astrobiology Vol. 11, No. 2 (2011). Abstract available.