Mars is 700 billion billion tons of iron and rock, wrapped in an unfamiliar landscape of canyons, craters and calderas.  Nonetheless, the most compelling thing we could find on this enormous, orange orb would be a microgram of wet chemistry able to reproduce, move, grow, and evolve.

Our fascination with life on other worlds is an extension of our interest in life on Earth. "Animal Planet," National Geographic, and zoos are all reflections of our curiosity about other forms of life.  Much of that curiosity is simply a primeval, hard-wired interest in things we could eat, and things that might eat us.  After all, life is highly competitive, and it pays to know the competition (unless, of course, you're General Motors).

So curiosity about extraterrestrial life is both understandable and long-standing.  Today, many of our efforts to find DNA's alien equivalents focus on distant locales; for example, the use of space-borne telescopes to sort through the spectra of extrasolar worlds, looking for atmospheric gases that would be evidence of biology.

But there's also a backyard effort: the hunt for life nearby.  Life in the solar system.  Life whose remove is measured in hundreds of millions of miles - not hundreds of trillions.

The idea of looking for proximate protoplasm is old. In the nineteenth century, Mars, Venus, and even the pulverized, dusty crust of the Moon were all considered possible loci for life.  Such optimism soured in the early space age, when probes revealed Mars' dry, ultraviolet-stung surface and Venus' autoclave temperatures (the Moon had long been out of play).  By the 1980s, many scientists believed that the worlds beyond our own were just inanimate balls of rock, whirling silently about an indifferent Sun.

That pessimism now seems, if not quite quaint, at least questionable.  The number of nearby worlds where biology might arise is on the uptick. 

Life requires a few basic ingredients: (1) raw materials, (2) a solvent (water being first choice), and (3) energy to drive it all.  The first is probably not a problem on any rocky hunk of junk in the solar system.  The handful of elements necessary for life is available just about everywhere.  The latter two requirements are coupled, since energy is necessary to keep fluids liquefied.  The outer realms of the solar system are cracking cold, and in these remote places the Sun's feeble rays are insufficient to keep water from freezing granite-hard.  For that reason, researchers have usually opined that any solar system body skulking in the dark expanses beyond Mars' orbit will be in perpetual, frozen rictus. 

But science grooves on surprises. And one of the big surprises of the past few decades is the discovery of worlds that are warm despite being situated in places where the Sun doesn't shine. Consequently, there are now more than a half-dozen objects among Sol's minions that are considered possible hosts for life.  As a handy guide to these neighborhood biomes, useful for impressing relatives or strangers on the bus, we offer the following inventory, thoughtfully separated into two categories: the sunny, and the not so sunny.  Starting with the former, we begin, most obviously, with:

Earth.  Our world is the poster child for solar-powered planets.  Most flora and fauna on Earth - with some important exceptions such as the bacteria that live in deep rock - are ultimately animated by the roaring nuclear fusion taking place in the Sun's heart.  On Earth, it's usually chlorophyll that converts this radiant energy into chemical compounds to energize our existence (or bulge our waistlines).

Venus.  Despite the fact that Venus, our sister planet, has been described as purgatory personified, there are some researchers who still hold out hope for life there.  David Grinspoon, at the Southwest Research Institute, notes that the thick, sulfuric acid-ridden clouds of this planet might be a stable environment for floating life.  Venusian acidophiles - analogs to a type of bacteria that can withstand highly acidic environments on Earth - might eke out an existence there.  "It's a long shot," Grinspoon admits, but he insists that we shouldn't rule out life on this nearby world.

Mars.  Then and now, everyone's favorite, inhabited extraterrestrial planet.  While Mars' highly reactive and powder-dry landscape is practically guaranteed to be sterile, there is indirect evidence for watery aquifers a few hundred feet beneath the surface.  If these liquid reservoirs exist, life may have found refuge within.  Today's Martians - if any - would be alive thanks to internal, geologic heat sources that keeps these putative aquifers warm.  Nonetheless, we classify the Red Planet as a Sun-powered world simply because any life would presumably have arisen during those long-gone salad days when liquid water pooled on the surface.

Titan.  This large moon of Saturn, revealed in detail by NASA's Cassini mission, and subject to shameless examination by the Huygens probe, is far too cold for liquid water.  But its air is thick with hydrocarbons.  David Grinspoon has suggested that the Sun's weak ultraviolet light might rip apart some of these atmospheric compounds, producing acetylene.  Falling into the liquid lakes of methane and ethane below, this gas (best known for firing blowtorches on Earth) could serve as a food for microscopic life.  Unlikely? Yes. Impossible? No.

Turning to the not-so-sunny abodes for possible life, we find they are all airless moons at the distance of Jupiter or beyond.  Reference to our own natural satellite would suggest that there's nothing quite as desolate and dead as a moon without an atmosphere.  After all, small bodies cool fast, and in the more-than-four billion years since their birth, the satellites of the outer solar system - where the Sun's warmth is meager - should have cooled to temperatures beyond a penguin's worst imaginings. 

However, moons are plentiful around the gas giant planets (where at least 98 percent of all satellites in the solar system hang out), and multiple moons interact in ways that can heat them up.  In these systems, sibling satellites engage in gravitational tugging matches that cause them to be squeezed and stretched by their planetary parent.  The resulting friction can produce everything from warm oceans to active volcanoes and geysers.  (The energy ultimately comes from the rotation and the orbital motion of both moons and planets.)  Known as "tidal heating", this warming of moons seems to be a common occurrence.  Indeed, it's something we might have noticed centuries earlier if we didn't live on a planet whose moon is an only child. 

The best known of these tidally heated satellites are:

Europa.  There's good evidence, mostly from its changing magnetic field, that this ice-covered world orbiting Jupiter has an ocean lying 10 miles or so beneath its crusty exterior.  At the bottom of this vast, cryptic sea, volcanic vents might be spewing nutrients and hot water into a cold, dark abyss, providing both the food and energy for simple life.

Ganymede and Callisto.  Both of these jovian moons show magnetic field variations similar to those of Europa, suggesting that they, too, might be hiding large, watery oceans.  Given their thicker ice skins, finding that life - if it exists - would be even more daunting than for Europa.

Enceladus.  In the news recently, this Saturnian satellite seems to be a giant Slurpee - an icy moon that, thanks to tidal heating, is spouting geysers of water into space.  An unexpected entry in the horse race of habitability, Enceladus is the first other world for which we have convincing evidence of liquid water.  And where there's liquid water ...

Bottom line?  We can now list eight worlds (including Earth) in a nine-planet solar system that are possible places for life.  This is not quite as plentiful as the ancient Greeks believed - after all, they assumed that everything they could see in the sky was populated, including the stars.  But our knowledge of both the requirements for life and the conditions of the solar system far exceeds that of Aristotle and his buddies.  And in fact, our count of habitable worlds may still be low.  For example, there's Neptune's moon Triton, on whose surface the Voyager 2 spacecraft discovered geysers.  Perhaps Triton is also a candidate for life.  Then there are the short-period comets, which are routinely warmed by passage close to the Sun.  They, too, might surprise us with habitable environments.

The bald truth is that we still haven't found a single shred of extraterrestrial life.  But it's remarkable, and heartening in a perverse kind of way, that the places worth searching now greatly exceed our abilities to do so.  There's plenty of frontier left, even nearby.

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