Life Traveling In Space: A Story Of Panspermia

Imagine tiny microscopic specks of life, on your computer screen, in the dirt, inside deep ocean trenches--everywhere. Sometimes, they seem so vulnerable and insignificant, yet precisely the opposite is true. Microbiota may have had an ecological role larger than even the most devoted early microbiologist could have realized. They may have seeded life itself across space.

The idea that life can be distributed throughout the universe, from planet to planet, is called panspermia. Some lifeforms, particularly the extremophiles, are hardy enough to survive the extreme conditions of space. Meteorites and comets are the most discussed methods for transportation, and the story is fascinating.

Lithopanspermia is the name given to the transfer of organisms across space using rocks. Transporting life-containing rocks from the ground to another planet might seem like a conundrum at first, but the mechanism is simple enough. During the impact of an asteroid or a comet with a planet's surface, the pressure is so great that the material forming the impact crater is ejected at a speed reaching escape velocity, allowing chunks of rock to escape the confining threshold of the atmosphere. Although a great portion of the impacted area experiences high enough temperatures to easily destroy most life, a significant fraction of the rock may experience the lower temperatures and pressures required to permit the survival of amino acids and microscopic organisms. Once in space, these microbes face the task of surviving the ''vacuum, weightlessness, temperature extremes, cosmic rays, and, for those close to the surface of the rock, ultraviolet radiation from the sun'' (Lunine).

We know that some organisms do possess the ability to survive these conditions. For example, tardigrades, also known as "water bears," have been shown to survive exposure to solar radiation while in space. However, organisms well beneath the surface of the rock fare the best chance of survival because they are shielded from ultraviolet radiation. Bacteria that form spores, which are extremely tolerant of low temperatures and normally dangerous conditions, could survive within rock for millions of years. Once our rock reaches the end of its journey and plunges into the atmosphere of another planet, the organisms within may survive, provided the rock is large enough to offer the necessary heat shielding. Without it, any chances of life inside it would be burned up and destroyed. It's in this way that interplanetary travel of microorganisms is thought to occur over billions of years. They're a bit like geological spaceships, if you will.

Does this mean that panspermia could have seeded life across the galaxy? Not quite. There's the enormous distance between stars, and a timescale in the tens of millions of years for a trip. The longer the period of time, the less likely life is to survive a trip to another world. In the mind of mainstream science, this makes interstellar panspermia an unlikely occurrence. Fortunately, we can still wonder about it within our own solar system.

For example, if cosmic dispersal has been occurring for so long, one wonders if natural selection still occurs in space. If so, then might the descendants of space-faring life would be the most hardy, radiation-resistant, long-lasting, and efficient? If these critters are capable of surviving in space, then why haven't we noticed any yet? One might expect to find the descendants of planetary impact survivors across the solar system. These are the questions Caleb Scharf, director of Columbia University's multidisciplinary Astrobiology Center has posed, and he suggests a few answers to this paradox. It could be because we haven't searched hard enough, or because the environmental and biochemical limits of the critters make them less hardy than previously believed. Perhaps the answer lies closer to home: on a familiar red dot in the sky. The idea that life could have hitched a ride across from Mars to Earth, or vice-versa, is thought to be a much more plausible phenomenon than others.