We still don't have hard evidence for any life that's not of this Earth, but across our solar system there are some tantalizing possibilities for primitive life to find a haven. Some moons of Jupiter and Saturn are intriguing, and there's also the chance for some surprises somewhere on Mars. <p>Here are my top six candidates for the best spots to search for primitive alien life in our solar system. <p>FIRST STOP: Enceladus <p><i>Seth Shostak is a senior astronomer at the SETI Institute in Mountain View, Calif.</i>
Enceladus is just a small moon of Saturn, but contrary to historical assumption it's not simply another pretty ice-covered rock. <p>In 2005, NASA's Cassini spacecraft photographed geysers of frozen water spewing from cracks in Enceladus' southern hemisphere. The best guess is that reservoirs of liquid water lie beneath the frozen exterior, kept warm by gravitational interactions between this moon and other members of the Saturnian system. The necessities for life are there, and maybe Enceladans are as well. The moon has a mean radius of 156.6 miles (252.1 km). [<a href=http://www.space.com/13531-photos-enceladus-saturn-moon-cassini.html>Photos: Enceladus, Saturn's Cold, Bright Moon</a>] <p>NEXT STOP: Mars
Mars remains perennially popular for those hunting for otherworldly protoplasm. Particularly intriguing are the dark stripes that appear in the Martian summertime at Horowitz crater. These are likely to be salty meltwater only inches beneath Mars' dusty epidermis. A relatively simple probe could sample this muddy environment. Mars has a mean radius of 2,106.1 miles (3,389.5 km). <p>NEXT STOP: Titan
Enceladus' hefty Saturnian moon sibling is the only world in the solar system (besides Earth) known to sport liquid lakes. These are lakes of ethane and methane — liquid natural gas — endlessly topped up by a drizzle of hydrocarbon rain. But despite the odd ingredients and Titan's gelid temperatures (minus 290 Fahrenheit, or minus 179 Celsius), this is a world where chemistry's a happening enterprise. Titan possesses an equatorial radius of 2,575 km (1,600 miles), making it the second largest moon in our solar system.[<a href=http://www.space.com/12638-amazing-photos-titan-saturn-moon.html>Amazing Photos of Titan</a>] <p>NEXT STOP: Europa
Many would grant this Jovian moon a higher potential-life rating than I have, since there's probably more liquid water here than in all of Earth's oceans. The downside is that Europa's vast and salty seas lie beneath roughly 10 miles of ice that's harder than tensor calculus. Not only would it be a labor of Hercules to get a probe beneath this icy armor, but Europa's oceans are darker than a cave — which means photosynthesis won't work. However, something down there might subsist on geothermal heat or complex molecules from the surface. Europa possesses a mean radius of 970 miles (1,560.8 km). <p>NEXT STOP: Venus
A surprise entry in the exobiology sweepstakes is our sister planet, Venus, with its scorching surface temperatures (850 F, or 454 C). The planet is generally assumed to be as sterile as a boiled mule. But planetary scientist David Grinspoon, astrobiology curator at the Denver Museum of Nature and Science, points out that high in the Venusian atmosphere temperatures are refreshingly tolerable. Atmospheric sulfur dioxide and carbon monoxide might serve as food for floating microbes. Venus possesses a mean radius of 3,760.4 miles (6,051.8 km). <p>LAST STOP: Callisto and Ganymede
I considered these two moons of Jupiter together, as I feel they're neck-and-neck candidates for biology. Like their more celebrated neighbor Europa, they may have buried, liquid oceans. However, in the case of these two satellite siblings, briny deeps would underlie at least 60 miles (100 km) of rock. Finding inhabitants here is a shovel-ready project for our grandkids. Callisto possesses a mean radius of 1500 miles (2,410.3 km); Ganymede's mean radius is 1600 miles (2,631.2 km).