Arthur C. Clarke: 2010 Odyssey Two (1983).
Callisto makes a big splash - Marshal Space Flight Center Space Science News
The most distant of Jupiter's Galilean Moons, Callisto shows the highest density of impact craters in the Solar System, but harbors no volcanoes or even any large mountains. It is thought that the surface is billions of years old. The first hint that something interesting might be happening beneath the surface came from Galileo's measurements of Callisto's magnetic field. Dr. Krishan K. Khurana of UCLA and colleagues discovered that the magnetic field fluctuated in time with Jupiter's rotation. The best explanation was that Jupiter's powerful magnetic field was creating electrical currents somewhere within Callisto, and those currents in turn created a fluctuating magnetic field around Callisto.
This contrast-enhanced image of Jupiter's moon Callisto was captured earlier this year by NASA's Galileo spacecraft. Callisto is approximately the size of the planet Mercury, making it the third largest moon in the Solar System, after Ganymede and Titan. Its icy surface is billions of years old, lacks any sign of volcanic activity, and is densely covered with rifts and craters. Scientists studying data gathered by the Galileo spacecraft now believe that Callisto's heavily cratered surface may overlie a salty liquid ocean.
But where could currents flow on Callisto? The icy surface is a poor conductor and the atmosphere is negligible. Dr. Kivelson suggests that "there very well could be a layer of melted ice underneath [the surface]. If this liquid were salty like Earth's oceans, it could carry sufficient electrical currents to produce the magnetic field."
Lending further credence to the premise of a subsurface ocean on Callisto, Galileo data showed that electrical currents were flowing in opposite directions at different times. "This is a key signature consistent with the idea of a salty ocean," Khurana added, "because it shows that Callisto's response, like Europa's, is synchronized with the effects of Jupiter's rotation."
These microbes called archaeabacteria, or simply "archaea", constitute a third branch of life on Earth, along with prokaryotes (normal bacteria) and eukaryotes (plants and animals). Like prokaryotes, the genetic material of archaeabacteria float freely throughout the cell -- they are not contained within the cell nucleus like eukaryotic organisms. However, the DNA of archaeabacteria more closely resemble that of plants and animals than normal bacteria. They are truly in a class by themselves, and if life is discovered elsewhere in the solar system it may be similar to the archaeabacteria of Earth.
Europa may still be a better prospect for extraterrestrial life than Callisto simply because it's warmer. "The basic ingredients for life -- what we call 'pre-biotic chemistry' -- are abundant in many solar system objects, such as comets, asteroids and icy moons," explains Galileo Project Scientist Dr. Torrence Johnson. "Biologists believe liquid water and energy are then needed to actually support life, so it's exciting to find another place where we might have liquid water. But, energy is another matter, and currently, Callisto's ocean is only being heated by radioactive elements, whereas Europa has tidal energy as well," from its greater proximity to Jupiter.
The strongest clues to life on Callisto and Europa may lie right here at home. In 1996, radio sounding and altimetry measurements revealed the the presence of an underground lake in Antarctica near the Russian Vostok Station. Lake Vostok is overlaid by about 3,710 meters (12,169 ft) of ice and may be 500,000 to 1 million years old. Since the discovery, drilling has gone slowly while procedures are worked out to keep it pristine. No one has seen or sampled the lake - the deepest ice sample is from 100 meters (328 feet) above the liquid surface - nor is anyone sure why it is liquid, hence the scientific curiosity. Scientists are hopeful that Lake Vostok can one day serve as a terrestrial laboratory to help us understand better the oceans on the distant moons of Jupiter.
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Jet Propulsion Laboratory, Pasadena, CA
October 21, 1998
The Galileo findings, to be published in the Oct. 22 issue of the journal Nature, reveal similarities between Callisto and another of Jupiter's moons, Europa, which has already displayed strong evidence of a subsurface ocean.
"Until now, we thought Callisto was a dead and boring moon, just a hunk of rock and ice," said Dr. Margaret Kivelson, space physics professor at the University of California at Los Angeles (UCLA) and principal investigator for Galileo's magnetometer instrument, which measures magnetic fields around Jupiter and its moons. "The new data certainly suggest that something is hidden below Callisto's surface, and that something may very well be a salty ocean."
This premise was inspired by Galileo data indicating that electrical currents flowing near Europa's surface cause changes in Europa's magnetic field. "This seemed to fit nicely with other data supporting the idea that beneath Europa's icy crust, a liquid ocean might be serving as a conductor of electricity," said Kivelson.
Armed with that information, Kivelson and UCLA colleagues Drs. Krishan K. Khurana, Raymond J. Walker, and Christopher T. Russell set out to test a similar theory about Callisto, "although it seemed far-fetched at the time," Kivelson said. The team went back and studied data obtained during Galileo's flybys of Callisto in November 1996, and June and September of 1997.
Kivelson and her colleagues found signs that Callisto's magnetic field, like Europa's, is variable, which can be explained by the presence of varying electrical currents associated with Jupiter that flow near Callisto's surface. Their next challenge was to discover the source of the currents.
"Because Callisto's atmosphere is extremely tenuous and lacking in charged particles, it would not be sufficient to generate Callisto's magnetic field; nor would Callisto's icy crust be a good conductor, but there very well could be a layer of melted ice underneath," Kivelson said. "If this liquid were salty like Earth's oceans, it could carry sufficient electrical currents to produce the magnetic field."
Lending further credence to the premise of a subsurface ocean on Callisto, Galileo data showed that electrical currents were flowing in opposite directions at different times. "This is a key signature consistent with the idea of a salty ocean," Khurana added, "because it shows that Callisto's response, like Europa's, is synchronized with the effects of Jupiter's rotation."
Although scientists consider the possible presence of an ocean on Europa as one factor hinting that life could have developed there at some point, it is doubtful that Callisto could harbor life, according to Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.
"The basic ingredients for life -- what we call 'pre-biotic chemistry' -- are abundant in many solar system objects, such as comets, asteroids and icy moons," Johnson explained. "Biologists believe liquid water and energy are then needed to actually support life, so it's exciting to find another place where we might have liquid water. But, energy is another matter, and currently, Callisto's ocean is only being heated by radioactive elements, whereas Europa has tidal energy as well," from its greater proximity to Jupiter.
Galileo flies by Callisto four more times between May and September of 1999, which may yield more clues about the possibility of a Callisto ocean. However, Kivelson said that scientists will rely heavily on theoretical models to test their interpretations about Callisto.
Kivelson and her team also are reexamining magnetometer data from Jupiter's largest moon, Ganymede, to address the tantalizing concept that Callisto and Europa may not be the only moons of Jupiter with subsurface oceans.
The latest Galileo exterior images of Callisto, released on Oct. 13, and a new artist's concept of a cutaway view of the moon's interior are available on the Internet at the Galil+eo website.
Galileo has been in orbit around Jupiter, studying the huge planet, its moons and its magnetic environment, for over 2 1/2 years. It is currently in the midst of a two-year extension known as the Galileo Europa Mission. Galileo is managed by JPL for NASA's Office of Space Science, Washington, DC. JPL is a division of Caltech, Pasadena, CA.
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