Is Alien Life Surfacing Nearby?
by Seth Shostak, Senior Astonomer
Extraterrestrial life may be showing up in some obvious places. No, this is not about hairless aliens that have come to Earth in saucer-shaped craft, but less sophisticated life just next door.
A century ago, scientists believed there was only one obvious stomping ground for alien biology in our solar system: Mars. Because it was reminiscent of Earth, Mars was assumed to be chock-a-block with animate beings, and its putative inhabitants got a lot of column inches and screen time. Red Planet residents were generally assumed to be similar to us: size-wise, technology-wise, and wise-wise. By 1900, astronomer Percival Lowell was energetically shopping the idea that canals laced the martian surface, the handiwork of aliens desperate to irrigate a dry world.
However, sophisticated inhabitants fell out of favor (with scientists, if not with film makers) once spacecraft revealed the landscapes of Mars to be desiccated deserts, efficiently sterilized by deadly ultraviolet light from the Sun. The surface was inhospitable, to put it gently. Nonetheless, it was still possible that microbial Martians were living a few hundred feet underground, where watery aquifers could shelter life happy to do its thing in the dark.
Consequently, expert opinion shifted. Our best chance for finding Martians was not to sit behind a small telescope in Flagstaff, Arizona as Lowell did, but to send drilling apparatus to Mars that could suck muck from far beneath the surface and examine it microscopically. That's a tough task, of course. It hasn't been done - or even planned in detail.
Mars still remains the astrobiology community's number one choice for "nearest rock with life," but there are many researchers who argue that the moons of Jupiter are better bets. In particular, Europa, Ganymede, and Callisto are all thought to hide vast oceans of liquid water beneath their icy, outer skins. Europa is the most promising case, and has the thinnest skin. The best known approach to examining this moon's watery habitat envisions a robotic probe that would melt a hole through 10 miles of granite-hard ice, and lower some sensors to look around. Again, not yet on the drawing boards.
But either way, our favored approach to finding biology beyond Earth involved drilling down deep - either through rock or ice.
However, discoveries bandied about at the American Geophysical Union meeting held last week in San Francisco have revised the game plan.
Consider Mars ... a poster child for geophysical inactivity, or so it was thought. The Red Planet has long been assumed to be as inert as medieval peasantry, but closer examination shows that it's less torpid than believed. Orbiters have photographed features known as "dark slope streaks" extending down the sides of crater and canyon walls. These sinewy stains are about the width of your living room, and grow longer as the sun and summer raise the surface temperature. They sometimes extend a half mile or more, and come and go with the seasons. The obvious - and most plausible - explanation is that these streaks are caused by mineral-laden ice just under the surface, melting in the warmth. It wets the landscape as it runs downhill.
Mind you, the idea that these streaks are salty water (which helpfully has a lower freezing point than pure water) is based on circumstantial evidence only. Pictures, in other words. But if true, it suggests that the quickest way to find Martians might be to land a rover on the streaks, scoop up the wet dirt, and check for microbes living in their own miniature martian spring. No need for the deep drilling project: life may be right there for the finding - in damp dirt. That's a real game changer.
The other news concerns Europa, the albino ice-ball-of-a-moon in orbit around Jupiter. The Hubble Space Telescope has found a cloud of what seem to be dismembered water molecules a hundred miles or so above Europa's south pole. The likely scenario here is that liquid is being spewed into space from the ocean below as Jupiter pulls and tugs on Europa's frozen skin. The geysers seem to be located in cracks in the surface ice.
Almost a decade ago, Hubble found watery plumes shooting out of Saturn's moon Enceladus, so this phenomenon isn't new. But Enceladus is a runty orb, so the water erupting from its frigid epidermis dissipates into the vacuum of space, and is gone for good. Europa is a beefier satellite, and can pull the material shot up from the cracked-and-crazed polar region back down to pile up on the surface.
Consequently, if there's any life holed up in the Stygian waters beneath Europa's glistening exterior, then bits of biology might be just lying in handy heaps right there on the icy, south-polar landscape.
It's good news for mankind, or at least for that fraction of it that would be interested to know if there's life beyond our world. For years, astrobiologists have been speculating on the possibility of finding some sort of small, squirmy critters on Mars or Europa. But in neither case did they have reason to think that the proof could be found on the surface of these worlds, within easy robot reach. Now that's changed.
Carl Sagan once said that "somewhere, something incredible is waiting to be known." "Somewhere" may be a landing spot only a short rocket ride away.