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The Search for Life
Editor's Note: This is the 12th in a series of essays
on exploration by Steven J. Dick.
Why do we
explore? As this essay series suggests, there are many answers. But since the
beginning of the Space Age one of the chief drivers has been the search for life
beyond Earth. Already in 1962 the Space Science Board of the National Academy of
Sciences declared the search for extraterrestrial life as the prime goal of
NASA's nascent space biology efforts. In doing so they realized not only the
scientific but also the philosophical import of what was just beginning to be
called "exobiology": "It is not since Darwin - and before him Copernicus - that
science has had the opportunity for so great an impact of man's understanding of
man." Taking a long view, the Board found the question of life the most profound
issue in the history of Western thought since modern science began with the
Scientific Revolution 300 years ago.
Image above: With a range of 13,600 km
and a sun angle of 29 degrees from zenith, this Mariner 4 image was the first
picture showing unambiguous craters on the surface of Mars. The area, 262 x 310
km, is a heavily cratered region south of Amazonis Planitia, centered at 14 S,
174 W. North is at about 11:00. (Mariner 4, frame 07B). Photo credit:
NASA.
NASA took this sentiment to heart with a series of missions
and research efforts that continue to this day. In this endeavor Mars has been
the focus of attention - the planet of imagination featured in H. G. Wells'
War of the Worlds and Ray Bradbury's Martian Chronicles, and the
planet that a century ago Pervical Lowell declared full of canals built by a
dying civilization. The Mariner 4 spacecraft in 1965 first revealed a Moon-like
Mars full of craters, dashing hopes for life. By 1972 Mariner 9 revealed a
different side of Mars: a panoply of geological features, including meandering
channels that appeared to be dry riverbeds. Hopes for life were raised, at least
past life or possibly microbial life.
This was the situation when in
1976, on the bicentennial of the United States, the two Viking landers set down
on the surface of Mars. In one of the great exploratory missions of all time,
these robotic surrogates, now representing humans "on Mars" for the first time
in history, returned reams of scientific data, including the results of three
biology experiments. The results of the biology experiments were open to
interpretation, and one of the principal investigators (Gilbert Levin) still
claims that his experiment found evidence of life, a case he and his colleagues
make in their book Mars: The Living Planet. But an experiment known as
the "gas chromatograph mass spectrometer" (GCMS) found no evidence of organic
molecules down to parts per billion.
Without organic molecules, the
building blocks of life, life cannot exist. The scientific consensus is that the
Viking experiments did not prove the existence of life on Mars, although the
sensitivity of the GCMS is still open to interpretation. Another of the Viking
biology principal investigators, Norman Horowitz (who died last month at the age
of 90), believed that his failure to find life on Mars justified a much broader
conclusion: "Since Mars offered by far the most promising habitat for
extraterrestrial life in the solar system, it is now virtually certain that the
Earth is the only life-bearing planet in our region of the galaxy. We have
awakened from a dream." That conclusion, found in his book To Utopia and
Back, is one that most would agree is too general based on what we know
about one nearby planet.
| Without organic molecules, the building blocks of
life, life cannot exist. | Mars held many
more surprises after Viking, one of which - the surprising claim in 1996 of
small fossils in the Martian meteorite ALH84001 - we will examine in another
essay. In terms of the exploration of Mars by spacecraft, after a twenty years
hiatus, and some failed attempts, the Pathfinder bounced to a landing on Mars in
1997. It carried no life detection experiments, but returned a raft of science
data ranging from Martian geochemistry to meteorology. In 2001 Mars Global
Surveyor revealed numerous gullies on Martian cliffs and crater walls, and
evidence of geologically recent liquid water. In 2002 Mars Odyssey gave strong
evidence that large quantities of water were present within three feet of the
surface of Mars at latitudes from the south pole to 60 degrees south. In 2004
the European Mars Express Orbiter returned data indicating the presence of
methane in the Martian atmosphere, possibly of biogenic origin.
Meanwhile, Opportunity, one of the Mars Exploration Rovers that began
operations in 2004 and still crawling around Mars today with its companion
Spirit rover, examined an outcrop of salt-laden sediment and found thin
intersecting layers interpreted as sand ripples, perhaps shaped by flowing water
in a huge shallow sea. What these spacecraft reveal is evidence of past, and
perhaps even present, water on Mars. And where there is water there may be, or
may have been, life.
In a surprising turn of events, some scientists
think the solar system now has even more potential for life than it did 30 years
ago. In 1996, just after the possible Mars fossil was announced, the Galileo
spacecraft confirmed what Voyager 2 hinted at already in 1979: that the Jovian
moon Europa harbored an ocean. Not even Arthur C. Clarke's science fiction had
dreamed of this scenario when NASA's exobiology program began in the early
1960s, though Clarke did broach Europa in his novel 2010.
Will we find
microbial or fossilized life on Mars? Will there be life in the oceans of Europa
or one of the other water moons of Jupiter? Or will we have to search the
atmosphere of another planet beyond our solar system to find other worlds with
signs of life? These questions, and many more, are the subject of intense
research in the field now known as astrobiology,
led by the 16 teams of NASA's Astrobiology
Institute and affiliates around the world.
Mariner, Viking, Mars
Odyssey, Mars Global Surveyor, Mars Express, the Mars Exploration Rovers and
their successors constitute a scientific detective story full of mystery, laden
with importance, and resonating with history. In short, a positive venture in
the best tradition of exploration, for the United States and for humanity at
large.
Further Readings: Bergreen, Laurence. Voyage to Mars: NASA's Search for Life
Beyond Earth (Riverhead Books: New York, 2000).
DiGregorio, Barry,
Gilbert Levin and Patricia Straat, Mars: the Living Planet (Frog:
Berkeley, 1997).
Ezell, Edward C. and Ezell, Linda, On Mars: Exploration of the Red Planet, 1958-1978 (NASA SP
4212: Washington, D.C, 1984).
Horowitz, Norman. To Utopia and Back:
The Search for Life in the Solar System (W.H. Freeman: New York,
1986).
Steven J. Dick and James E. Strick: The Living Universe: NASA
and the Development of Astrobiology (Rutgers University Press: New
Brunswick, N.J. and London, 2004).
Steven J. Dick, Life on Other
Worlds: The Twentieth Century Extraterrestrial Life Debate (Cambridge
University Press: Cambridge and New York, 1998).
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