The search for water in the solar system
Observer's Notebook for October, 2000
by John Rummel
Less than 100 years ago, there was at least a rough consensus in the scientific community that life in our solar system was plentiful. Lowell's Martian canals were widely believed to be the construction of an advanced civilization trying to move water from the polar icecaps to the middle latitudes. Others wrote of the inhabitants of the moon and even the sun. While many scientists even were skeptical of such claims, hard evidence either way was hard to come by.
We've come a long way in the 100 years since. Humans have explored the moon directly, and camera/instrument packages have visited the sun, all the planets except Pluto, comets and asteroids. Needless to say, no advanced races have been located or photographed, and no remains of ancient cities have been uncovered.
The search for life these days boils down to the search for water. Everything we know about Earth-based biology speaks to the necessity of liquid water for life. No water, no way. Therefore, the search for liquid water in the solar system is now the focus of astrobiologists.
Currently, there are two possibilities for water in the solar system: Mars and Jupiter's moon Europa.
Considerable geological evidence of abundant liquid water in the past, and the presence of ice water on the surface today (surface temperatures range from -100s C to -8s C). Mars shows evidence of volcanoes active in the recent geological past, and therefore geothermal warming. Under the frigid surface, liquid water could certainly still exist. Add to that the 1996 announcement of fossilized microbial life forms in a Martian meteorite, and you have a very compelling case for life on Mars, at least in the distant past. Those microbial fossils are still considered very controversial, and the number of scientists who accept them to be actual fossilized life forms is small. Despite that, Mars remains the best bet for locating life off the Earth.
NASA/JPL are currently in the planning stages for a number of Mars missions. For details and specific science objectives, see
For lesson plans, readings and activities for students, see:
Thursday's Classroom: Divining Water on Mars, Part 1
Thursday's Classroom: Divining Water on Mars, Part 2
Thursday's Classroom: Divining Water on Mars, Part 3
The environment around Jupiter is very hostile. Average temperatures on the surface of its moons (other than Io) are estimated to be in the range of -160 degrees C. At first glance, this would be one of the last places you'd think of to search for liquid water. All that changed when the first exploration of Jupiter's moons took place starting in the late 1970's. Europa surprised everyone by having a surface made up almost entirely of water ice. Speculation about what was under that ice was intriguing, but evidence was hard to come by.
By the late 90s the evidence for liquid water under the ice of Europa is still circumstantial, but it grows more convincing with every new image and data-set sent back by NASA's Galileo probe. Intricate patterns of cracking and shifting ice on the surface may point to current or recent upwellings of warm liquid from far below, but only actual pictures of current changes in the ice field patterns would constitute definite evidence. An actual photo of a water geyser would be even better, but so far none have been seen.
In January of this year, NASA/JPL scientists found another indirect way to hunt for liquid water - and scored a big hit. By studying Europa's magnetic environment with sensitive instruments on board Galileo, significant shifts were found as Europa's magnetic field interacts with Jupiter's much stronger field. The pattern of interactions is strongly suggestive of a liquid, salty electrical conductor beneath the moon's surface - salt water. Scientists were quoted in the media saying that this is the strongest evidence yet for a liquid water ocean on Europa.
Europa is not the only Jovian moon currently suspected of hosting water. Callisto is also a good candidate for having liquid water under the surface, but most scientists prefer to focus on Europa only because the evidence there is so much more abundant.
NASA currently has a Europa mission in the planning stages. For information see:
For more general information on the search for life beyond the Earth, and related links for teaching materials, see:
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