Liquid water gets astronomers excited these days as the search for life elsewhere in the universe has become something of a unifying underlying theme of planetary science. No life on Earth exists without water. Of course, finding water elsewhere says little about the possibility for life except that, given our experience with terrestrial life, it probably can’t be ruled out. Hence the excitement over the recent discovery of the so-called “Earth-like” extra-solar planet, which in all likelihood is probably less Earth-like than Mars. Gliese 581c, five times more massive than Earth, orbits its star (Gliese 581) in only 13 days and is more than 10 times closer to it than we are to the Sun. It manages to keep an average surface temperature amenable to liquid water because Gliese 581 is a stellar wimp. Gliese 581 is an M-type star, which is a spectral type whose technical definition is “cold”. The spectral types of normal stars in order of decreasing temperature are OBAFGKM (“Oh, be a fine girl/guy: kiss me!” is the classic mnemonic). Gliese 581 is at the cold end of the distribution (the Sun is a G-type star; I make a living looking at hot O and B stars with the Cassini UVIS instrument). Its close proximity to the star means that the planet may be tidally locked to the star, keeping one side in perpetural daylight and the other in constant darkness. If the planet has a global ocean, sub-surface currents could lead to a layer in the ocean that is amenable to life. A planet this size likely has the raw materials (in the form of heavier elements) for life. Its high temperature means it has both energy available, and possibly liquid water. These are three key ingredients astrobiologists look for.

Closer to home, new work is exploring the water vapor geysers on Saturn’s moon Enceladus. Previous studies posited a near-surface reservoir of liquid water as the source of these geysers. A new paper by Francis Nimmo and Bob Pappalardo suggests that liquid water is many km below the icy surface and that the proximate source of the geyser material is heating produced by friction in the moon’s so-called “tiger stripes”. These roughly parallel grooves centered on the south-polar region are the apparent source of Enceladus’ geysers. Tidal flexing of the moon could cause blocks of ice to rub against each other, heating them enough to allow sublimation of ice (the direct evaporation of water from ice to gas). The tidal flexing is due to Enceladus’ slightly eccentric (non-circular) orbit around Saturn: when it is closer to Saturn the moon gets stretched slightly more than when it is a little further away. This leads to flexing or distortion of the moon as it orbits Saturn. Nimmo and Pappalardo calculate a half-meter slide of ice in the tiger stripes, and that much flexing requires that there is liquid water in the interior (if it were solid all the way through, the amount of flexing would be smaller and would not generate enough heat to produce the geysers). The JPL press release can be found here.

While Enceladus may have liquid water below its icy crust, unlike Gliese 581c it is poor in the energy and raw materials department. Future mission planning for the Saturn system is still focused on Titan, and Jupiter’s moon Europa which possibly harbors a sub-surface ocean, tops the list of icy worlds for future missions.