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Callisto makes a big splash

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Callisto makes a big splash


October 23, 1998: Until now most scientists thought Jupiter's moon Callisto was a dead and boring moon, an unchanging piece of rock and ice. Data reported in today's issue of Nature could change all that. It appears that Callisto, like another of Jupiter's moons Europa, may have an underground liquid ocean and at least some of the basic ingredients for life.

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."

Life under the ice?

Callisto is the second moon of Jupiter thought to harbor a sub-surface ocean. The other is Europa. As evidence mounts for at least one and possibly two liquid oceans in the Jovian satellite system, scientists are cautiously optimistic that life could exist there. Europa and Callisto aren't the only places in the solar system where the building blocks of life have turned up. Scientists have recently discovered water on the moon, and right-handed amino acids in carbonaceous chondritic meteorites. If these basic ingredients have indeed combined somewhere in the solar system to produce extraterrestrial life forms, the conditions that they live in are likely to be harsh compared to the gentle climes of Earth.

In recent years researchers have discovered a new class of micro-organisms here on Earth that can live or, at least remain viable, under very extreme conditions -- from volcanic vents deep in ocean trenches, to ice more than 400,000 years old, to Siberian permafrost more than 5 million years old.

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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.

Near the shore of Owens Lake, CA. The pink coloration is caused by halophilic (salt-loving) bacteria living in a thin layer of brine on the surface of the lake bed. The gleaming white material in the foreground is soda ash (sodium carbonate), once harvested from evaporation ponds by the Pittsburgh Plate Glass factory seen in the distance. The plate glass factory is no longer in operation. Links to a related story and more pictures of halobacteria in Owens Lake.

If the putative oceans on an alien planet are truly salty, then the microscopic lifeforms that live there might be similar to salt-loving extremophiles here on Earth. The picture left shows the nearly-dry lake bed of Owens Lake, California. The pink colors are caused by a bloom of halophilic ("salt-loving") bacteria in the muddy brine. Conditions in the lake bed, where the water is saturated with salt, are truly harsh. The air temperature at the surface is over 100 deg. F and the water just below the salt crust is 130 - 150 degrees F. Yet Owens Lake is teeming with life.

Dr. David Noever, a member of NASA's Astrobiology Institute, had this to say about the possibility of a salty sea on Callisto: "One way to to visualize Callisto's proposed ocean is to imagine a battery, which is run by electrochemistry (meaning charged ions like sodium and chloride), that can generate currents and thus drive magnetic fields. That's the potential physics. As for the potential biology, who knows? All we know is that there are examples of salty brines in warmer spots in the solar system, Earth, that are teeming with life where it is least expected."

Europa may still be a better prospect for extraterrestrial life than Callisto simply because it's the warmer of the two satellites. "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 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.

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Lake Vostok at a Glance

  • It was discovered in Antarctica in 1996 by Russian & British scientists
  • The lake lies 3700 meters beneath the icy surface
  • The composition is believed to be fresh water based on estimated density
  • The lake bed is 710 meters below sea level
  • It is 125 meters in depth, on average.
  • more information & image credits
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.

Web Links

Galileo- Europa home page at JPL

Jet Propulsion Laboratory home page

Callisto's moon may hide a salty ocean - JPL/Galileo story posted Oct 21.

More NASA Science News


Related Stories:
Exotic-looking microbes turn up in ancient Antarctic ice

Space Tether may be best way to explore Europa

Clues to possible life on Europa may lie buried in Antarctic ice


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Author: Dr. Tony Phillips
Production Editor: Dr. Tony Phillips
Curator: Bryan Walls
Responsible NASA official: John M. Horack