Did Earth’s biosphere emerge as a homegrown phenomenon, as scientists have long suspected? Or is it possible that early microorganisms, which by 3.5 billion years ago had grown to fill the oceans, sprang from a seeding by life forms from another world? The latter hypothesis, called panspermia – or, more accurately, transpermia – does not attempt to explain how life originated, but merely moves the origin to another locale. Still, it is an intriguing possibility. Data from orbital probes sent to Mars and from NASA's Mars Exploration Rovers show that water has flowed, at least intermittently, on the Martian surface in the past. And so, it is not unreasonable to hypothesize that life existed on the Red Planet long ago, and perhaps continues to this day.
Ancient Mars: Habitable and Perhaps an Abode for Life
Planetary scientists have learned that early in its history our solar system could have included many worlds with liquid water, the essential ingredient for life as biologists know it. Data from NASA's Mars Exploration Rovers corroborate previous suspicions that water has flowed, at least intermittently, on the Red Planet in the past. It is not unreasonable to hypothesize that life existed on Mars long ago and perhaps continues there. Life also may have evolved on Europa, Jupiter's fourth-largest moon, which appears to possess liquid water under its icy surface. Saturn's biggest moon, Titan, is rich in organic compounds; given the moon's frigid temperatures, it would be highly surprising to find living forms there, but they cannot be ruled out. And last year, based on information transmitted from the Cassini space probe, scientists determined that the Saturnian moon, Enceladus, may have a lake or ocean beneath its ice, like Europa, and so it too may harbor life.
As strange as it may sound, life may even have gained a toehold on torrid Venus. With temperatures in excess of 900 degrees and atmospheric pressures 90 times that of Earth, the Venusian surface probably is not habitable today, yet conceivably the planet could support microbial life high in its atmosphere. And, most likely, the surface conditions on Venus were not always so harsh. Venus may have once been similar to early Earth
A question therefore beckons: With so many places in the Solar System where life might exists, could life forms move about, jumping from planet to planet?
The Case For Transpermia: How Scientists Know That Materials Travel From Mars To Earth
Over the past two decades, researchers have learned that 34 meteorites found on Earth actually came from the Martian. The Mars rocks are listed for the public on the website of NASA's Jet Propulsion Laboratory.. Their Martian origin is not in doubt, because characteristics of gases trapped within match precisely with atmospheric measurements that NASA’s Viking landing craft took in the 1970s.
From simulations of comet impacts on Mars, scientists know that rocks can be launched into space, on trajectories which could deliver them to Earth. Usually, such interplanetary hitchhiking is slow, taking million of years. However, as this writer and his co-author, Ben Weiss, noted in their article "Did Life Come from Another World?" (Scientific American, 2005), a small fraction of Martian rocks arriving on Earth have made much swifter journeys. Following an impact event on Mars, some material can make the trip in merely a year, and approximately ten fist-size rocks weighing more than 100 grams will reach Earth within just three years! Add pebbles, dust particles, and other small debris, and the volume of Martian material traversing quickly through the interplanetary realm, with its harsh radiation environment, increases further.
Could Living Organisms Survive A Journey From Mars?
By studying magnetic properties of certain Mars meteorites, Weiss, of MIT, and other investigators, such as H. Jay Melosh, of the University of Arizona, have learned that rocks can be catapulted from Mars without being heated more than a few hundred degrees Centigrade. This is important when considering the possibility of transpermia, because there are microorganisms on Earth that can survive at such temperatures, but not much higher. Scientists also have determined that a baseball-size rock entering Earth’s atmosphere following a voyage from Mars is heated by friction only about a centimeter or so into the rock. This is good news for any cells, or for suspended, dried out life packages known as spores, tucked away in the rock’s cozy interior. Good news, that is, if the life forms completing the journey from Mars have not been fried along the way by the interplanetary space radiation.
In Cooperation with Russia, the Planetary Society will Investigate Transpermia
While the journey part of the scenario remains untested, as part of an initiative of the Planetary Society, a group of investigators led by this writer are preparing a collection of life forms to be carried within a Russian Federal Space Agency probe, known as Phobos-Grunt. Once launched on a 34-month round trip through interplanetary space, the organisms will pass through an environment where space radiation is most severe, beyond the Van Allen belts. These belts, also known as the magnetosphere, constitute a magnetic shield, protecting life, including human beings, from charged particles, one of the worst components of cosmic radiation. While the experiment, designated by the acronym LIFE, for Living Interplanetary Flight Experiment, cannot prove whether terrestrial life actually originated on Mars, it may provide insight as to whether a transpermia scenario is possible, and whether life, once initiated, can spread with ease throughout the cosmos.
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