It's fun to speculate about life on Mars, on Jupiter's watery moon Europa, or around other stars. But if we expect too much too soon by way of actual discovery, the fun could turn to boredom.
That would be bad news for the US National Aeronautics and Space Administration, which is counting on public interest in life "out there" to support its space science program. Now, hype about possible extraterrestrial life outruns realism.
For example, scientists with NASA's Galileo Jupiter exploration mission say recent images of Europa suggest there's liquid water beneath the moon's icy surface, water that is warmed by an internal heat source. Under reporters' prodding at a press conference, Ronald Greeley of Arizona State University in Tempe noted this is consistent with, but not direct evidence for, the possibility that some kind of primitive life may have evolved on Europa.
He added that, since organic compounds are widespread in the solar system, Europa has "a high potential" to meet the three main criteria of a possible life site - namely, "the presence of water, organic compounds, and adequate heat."
That's a lot of speculation spiced with a few images showing what appear to be icy water flows driven by internal heating. Yet it's what news reports and NASA's own press release emphasized. Actually, the criteria for a habitable moon are stringent and hard to meet, as a team at Pennsylvania State University at University Park explained recently in the journal Nature.
Profs. James Kasting and Richard Wade and graduate student Darren Williams note that the alien planets discovered so far are giant gas bags like Jupiter. We can't yet detect Earth-sized planets orbiting other stars. Since the gas bags probably don't support organic life, it's worth looking at what one of their moons might do.
Mr. Williams explains that, to support long-term evolution of life, a moon needs to sustain liquid water on its surface. It and its planet must be close enough to their star that the water isn't all frozen but not so close that stellar heat drives it off.
The moon also must be large enough to hold an atmosphere that prevents water loss. Then, too, it must withstand the attack of energetic particles trapped by the planet's magnetic field. These speeding particles can strip off an atmosphere. In general, Williams says these requirements call for a planet-size moon somewhat more massive than Mars with a magnetic field to fend off the trapped particles.
Of the near-dozen alien planets found so far, only two come close to meeting such criteria, the Penn State scientists say. They are called 16 Cyg Bb and 47 Uma B in the constellations Cygnus and Ursa Major, respectively. In our own solar system, a moon like Europa might also support primitive life in water protected by ice if there is enough internal heat to keep some water liquid.
The challenge is to find out if suitable sites for life actually exist. Commenting on the paper of Williams and his colleagues, planetary scientist Christopher Chyha of the University of Arizona in Tucson notes that, to pin down the prospects for habitable sites around other stars, astronomers need to detect Earth-like planets as well as gas giants. And that capability "is years, and possibly decades, away," he says.
Likewise, it will take future missions to probe Europa and return samples for analysis to determine its potential for supporting life. Meanwhile, hot debate continues as to whether the deposits found in a meteorite from Mars are really fossilized bacteria. Here again, it probably will take extensive exploration and sample analysis to assess prospects for past or present Martian life.
The bottom line is that we know enough about the requirements for extraterrestrial organic life to know some of the kinds of sites to look for. But we still are far from being able to find hard evidence that such sites actually exist.