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Life on Other Moons Apparently No Terrestrial Clouds Green with Microorganisms 10 ? Christopher F news and views Exobiology airborne ecologies are known on Earth: if such ecologies are possible, why are there Life on other moons apparently no terrestrial clouds green with microorganisms 10 ? Christopher F. Chyba Less speculatively, it is known that deep subsurface microbial ecologies exist on hat does it take for a world to be ~ Earth. If Earth is in fact home to a 'deep, hot Whabitable? As with so many other ~ biosphere'11 not dependent on solar energy, questions in exobiology, Carl ;i' such biospheres might exist on other worlds Sagan helped pioneer this topic1, and in an well outside the surface-liquid-water zone article last year2 he presented a masterly - unless surface liquid water or surface and cautionary review of the changing energy sources (principally solar ultraviolet) scientific fashions on the question. Now are required for the origin oflife. that extrasolar giant planets (or brown In our own Solar System, circumstantial dwarfs) have been found around nine evidence for a liquid subsurface ocean on main-sequence stars, Williams et al., on Jupiter's moon Europa is growing, and there page 234 of this issue3, extend the discus­ could be regions ofEuropa where conditions sion to the habitability of moons around lie within the range of adaptation ofAntarc­ these worlds. tic terrestrial organisms12• Although such What it takes for a world to be habitable speculations will remain unresolved until depends on who will be doing the inhabiting. the question ofa europan ocean is settled, it is Clearly, the requirements of humans differ striking that Europa, one of two prime can­ from those offorests4 , which in turn are more didates for a second habitable world in our stringent than those of, say, green slime. Europa, as seen by the Galileo spacecraft. Europa, own Solar System, nevertheless lies well Habitability for putative extraterrestrial Jupiter's second moon, is one of the most beyond the surface-liquid-water zone, has technical civilizations may be the most inter­ promising sites for life in the Solar System, for only about a tenth of Mars's mass, and has esting kind to consider. Yet since the history despite having a frozen surface there is evidence almost no atmosphere. ofsurface life on Earth is largely the history of of an ocean below. But moons around the giant Our ignorance is sufficient to allow even green slime, its needs must remain ofspecial planets in other solar systems may be more like more speculative environments within our interest to exobiologists. Earth, with surface liquid water and moderately Solar System13• As Sagan pointed out, if even Recent work has defined 'habitable zone' thick atmospheres. a conservative definition of habitable zone as the range oforbital distances within which suggests many locales for life around other worlds can maintain liquid water on their planets, but current observing techniques stars - a conclusion to which Williams et al. surfaces5'6• This reflects the utter dependence cannot discover companions much below lend support- then "whatever we have of terrestrial life on liquid water7• The giant the mass of Jupiter, which is widely deemed ignored could only serve to broaden the planets orbiting the stars 16 Cyg B and 47 unsuitable for life. We therefore now have an biological arena"2• UMa may by this definition lie within their argument for habitable moons that parallels As it stands, Earth is the only known stars' habitable zones. But Williams et al. the previous one for planets: our giant plan­ habitable world. Investigating the condi­ argue that these worlds (like the gas giants in ets appear to be typical; they have moons that tions required for worlds to be habitable, our own system, whose solid cores lie at might be suitable for life (were they more naturally reminds us of our responsibility to depths where pressures and temperatures massive and within the Sun's habitable zone, our own. Here, too, Carl Sagan's voice will be are extreme) probably lack a solid or liquid by the arguments ofWilliams etal.); so extra­ dearly missed. surface suitable for life. solar giant planets (in habitable zones) prob­ Christopher F. Chyba is in the Department of So Williams et al. instead consider the ably do as well. It is certainly of the first Planetary Sciences, University ofArizona, Tucson, possible moons of the extrasolar giants, and importance for exobiology that we now have Arizona 85721, USA. show that some ofthem might provide habi­ evidence that planetary formation is com­ I. Shklovskii, T. S. & Sagan, C. Tntelligent Life in the Un iverse tats for extraterrestrial life. They find that mon. Nevertheless, ultimately what is need­ (Holden-Day, San Francisco, 1966). such a moon would need to be a bit more ed is a means ofd etecting Earth-sized worlds 2. Sagan, C. in Circumstellar HabitableZo11es (ed. Doyle, L. R.) 3- 14 (Travis House, Menlo Park, CA, 1996). massive than Mars (about 0.1 Earth masses) - a capability that is years, and possibly 3. Williams, D. M., Kasting, J. F. & Wade, R. A. Na ture 385,234-236 in order to retain a substantial atmosphere decades, awal. (1997). (essential for surface liquid water) for It should be emphasized just how conser­ 4. Heath, M. in Circumstellar Habitable Zones (e d. Doyle, L. R.) billions of years. The moon might also vative the definition of habitable zone used 445-457 (Travis House, Menlo Park, CA, 1996). 5. Kasting, J. F., Whitmire, D.P. & Reynolds, R. T. lcarus 101, 3 need an appreciable magnetic field to protect by Williams et al. (and most other workers) 108- 128 ( 1993). its atmosphere against loss due to sputtering really is. Carl Sagan was famous for his con­ 6. Kasting, J, F. in Circumstellar Habitable Zones by charged particles. Jupiter's moon cern that our notions ofthe conditions need­ (ed. Doyle, L R.) 17- 28 (Travis House, Menlo Park, CA, Ganymede, the largest moon in our Solar ed for extraterrestrial life are cautious to the 1996). 7. McKay, C. P. lcarus91, 93- 100 (1991). System, has a magnetic field, but is only point of being chauvinistic. In his recent 8. Burke, B. F. (cd.) TOPS: Toward Other Planetary Systems (NASA, 2 0.03 of Earth's mass. More massive moons piece on habitable zones , he raised a series 1992). may well exist. of objections to the 'surface liquid water' 9. Sagan, C. & Salpeter, E. E. Astrophys. /. Suppl. Ser. 32, 737-755 There is a certain irony in postulating definition ofhabitability. ( 1976). moons around the newly discovered planets For example, Sagan and Salpeter have lO. McKay, C. P. in Van Nostrand Reinhold Encyclopedia of Planetary Sciences and Astrogeology(cds Shirley, J. H. & as sites for life. Before the recent flurry ofevi­ speculated on possible ecological niches on Fairbridge, R. W. ) (Van Nostrand Reinhold, New York, in the dence for planetary systems, the argument giant planets: perhaps water clouds, simple press). for extrasolar habitable planets ran like this: organic molecules and abundant energy !!.Gold, T. Proc. Nat/ Acad. Sci. USA 89,6045-6049 (1992). our star appears to be typical; it has planets sources are enough for life, and no solid or 12. Reynolds, R. T., Squyres, S. W., Colburn, D. S. & McKay, C. P. lcarus 56, 246- 254 (1983). 9 suitable for life; so other stars probably do as liquid surface is required • To this, the 13. Chyba, C. F. & McDonald, G. D. Annu. Rev. Earth Planet. Sci. 32, well. We now know that other stars have counter-objection has been raised that no 215- 249 ( 1995). NATURE/ VOL 385 /1 6 /ANUARY 1997 201 .
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