Callisto, ocean and possible life
The first indication that Jupiter's moon
Callisto might be of astrobiological interest
came in October 1998, with the publication of a paper proposing that Callisto,
like Europa, might have an underground ocean
of water and perhaps the basic ingredients for life. This suggestion followed
from measurements of Callisto's magnetic field by the Galileo
spacecraft. Krishnan Khurana of the University of California, Los Angeles,
and colleagues found that Callisto's magnetic field fluctuates in time with
Jupiter's rotation, suggesting that Jupiter's powerful magnetic field generates
electric currents in the moon which, in turn, give rise to the fluctuations.
In trying to understand how currents could flow within the body of Callisto,
Khurana et al hypothesized the existence of a subterranean ocean of salty
water which would effectively act like a giant battery. This idea was given
further credence by Galileo data which showed that the electric currents
within Callisto flow in different directions at different times.
|Cutaway diagram of Callisto, showing possible ocean
beneath the icy surface
Whereas Europa is heated internally both by tidal stresses and the
decay of radioactive elements, Callisto is only significantly heated by
the latter. Therefore, the outermost Galilean moon may not be quite as good
a prospect in the search for extraterrestrial life as its Jovian neighbor.
Nevertheless, it seems certain now that future probes will scrutinize both
Europa and Callisto for signs of subsurface water and prebiological or biological
activity. One possibility is that, if primitive life has evolved on these
worlds, it may be similar to the halophiles
found in highly saline environments on Earth. Plans to investigate the astrobiological
potential of the Galilean moons have also been given a boost by the discovery
of a large lake deep under the ice of Antarctica, Lake
Vostok, which may serve as a test-bed for instrumentation.
- Shapley, H. "Crusted Stars and Self-Heating Planets," Matemática
y Teorética, Serie A (Tucumán National University,
Argentina), 14 (1962).
- Andreyeschchev, A. and Scalo, J. "Duration and Habitability of
Brown Dwarf Planets." Bioastronomy 2002: Great Barrier Reef Conference
for an ocean on Callisto (Dec 2, 2001)
life on Europa
FORMS OF LIFE