Tuesday, November 17, 2009

Truly Trivial: How many planets are known to have liquid surface water?

NASA swimming poolImage via Wikipedia
Last Friday, NASA announced that its LCROSS mission -- also known as "let's slam a probe into the lunar surface while another probe films the action, Jackass-style" -- found significant amounts of water on the moon. This revved up interest in lunar colonization again, despite the fact that there isn't any pure, drinkable, liquid water on the moon.

Now, water is very useful for space exploration even if you can't directly drink it, as simple electrolysis lets you crack water into breathable oxygen and hydrogen rocket fuel. The moon just became much more interesting as a weigh station for spacecraft, as robot factories could be sent ahead to stockpile hydrogen, oxygen, and possibly even purified drinkable water for manned craft heading further out into the solar system. Moreover, water is a very effective radiation shield, provided you can encircle a crew cabin in a two-meters-deep tank of H2O on all sides. The "significant amounts" of lunar water means moonbases might just be able to extract the Olympic swimming pools' worth of water necessary to ward off the cosmic fry-daddy rays awaiting space travelers.

As Karl Schroeder points out, the main barrier to manned spaceflight isn't technology but the expense of hauling the necessary materials into orbit. Launching these materials off the moon requires climbing out of one-sixth the gravity well that exists on Earth, which means there's an economic argument to be made for gathering spaceflight materials from a lunar base rather than launching them from Earth with the astronauts. Besides, if we can perfect the robot factories on the moon, we can get serious about mining comets, which have a lot more water-ice mass per capita and a lot less gravity. Robot comet iceminers mean that manned spacecraft could have a plethora of automated air, fuel, and shielding depots scattered throughout the Interplanetary Transport Network.

Thus, the lunar water discovery is very exciting, but it's unlikely to lead directly to the holy grail of space exploration -- the discovery of extraterrestrial life. Virtually all life as we know it requires water to exist, particularly liquid water that appears on the surface of the world, where it can be exposed to nurturing (and mutating) solar radiation. We've found hundreds of extrasolar planets and thousands of extraterrestrial non-stellar bodies, but liquid surface water remains stubbornly rare.

How many planets besides Earth are known to possess liquid surface water?

To date, Earth is the only planet -- or celestial body of any kind -- known to possess the proper conditions for water to remain liquid on its surface. Earth is in fact very good at supporting surface water, as two-thirds of Terra's surface is covered in H2O. But Earth is so far unique in its combination of surface temperature, atmospheric pressure, and chemical composition that affords liquid surface water.

Even on Earth, water is a comparatively tiny fraction of the planet's makeup, with our entire hydrosphere (total surface, subsurface, and atmospheric water) representing less than 0.023 percent of Earth's total mass. Put another way, there are roughly 1.4 million trillion metric tons (1.4 quintillion tonnes, or 1.4 x 10^18 tonnes) of water on Earth, and only 20 trillion metric tons of that is water vapor, so our vaunted surface water is less than 1/5000th or the mass of our planet.

To be fair, the vast majority of known extrasolar planets are gas giants, which precludes the possibility of a conventional surface, let alone surface water. That's a product of the selection bias endemic to our planetary detection methods. Massive planets are easier to spot than Earth-sized worlds, and gaseous planets are far more massive than solid ones.

To date, we have detected only two exoplanets with water vapor in their atmospheres. HD 189733 b and HD 209458 b both not only show evidence of water, but methane, carbon dioxide and possible organic compounds in their atmospheres. Neither planet is particularly Earthlike, however. HD 189733 b is a hot Jupiter, which is to say a Jupiter-sized gas giant that orbits its star closer than Mercury orbits the sun. Astronomers nickname these planets oven roasters, as they endure extreme temperatures and pressures from their solar proximity, often to the point that their atmospheres are boiled away and they're reduced to chthonian remnants. HD 209458 b, meanwhile, is a high-density gas giant, almost 50 percent more massive than Jupiter but only two-third's Jupiter's volume, suggesting a high-pressure atmosphere. Neither planet is assured of having a surface, let alone surface water.

There are many celestial bodies that have known or suspected hydrospheres. Besides the aforementioned comets, which are often described as dirty snowballs, Mars has frozen icecaps. More importantly, the Jovian moons Ganymede, Callisto and Europa are thought to have subsurface oceans. Enceladus has active geysers that vent small amounts of water vapor. Jupiter's Titan and Neptune's Triton are also thought to possess hydrospheres, and now Earth's own moon is known to harbor water.

It is the moons which are particularly intriguing, as even if these massive extrasolar planets we find don't have water themselves, their moons might. It is quite possible that Earth is a rare standalone planet capable of supporting life as we know it, but the universe might be riddled with moons that are abundant with extraterrestrial lifeforms, all nurtured in the reflected light and heat of their massive parent planets. Learning about the water on our own local moon might help us find water on distant moons, and discover once and for all if life in this universe is common, or Truly Trivial.