by Ben Bova
There is no major disagreement among the scientists about the rock’s origin or age; that much seems firm. But the evidence for life is more circumstantial.
No one realized that ALH84001 was from Mars until 1993, when its reddish color and internal composition identified it as an SNC meteorite. That is when the NASA/university scientists began slicing into the rock and examining its microthin samples in electron microscopes. They found five kinds of evidence to support the idea that living organisms once resided inside ALH84001 when it was part of Mars’ crust.
1. Globules of carbonates—compounds of carbon and oxygen—of a kind similar to carbon compounds produced by bacteria on Earth. The carbonates have been reliably dated at 3.9 billion years old.
2. Iron sulfide and iron oxide minerals similar to forms produced by bacteria on Earth. These are magnetic minerals; on Earth, some varieties of bacteria produce tiny filaments of similar magnetic minerals, which allow them to sense their orientation in their watery environment.
3. Within the carbonate globules are complex molecules of polycyclic aromatic hydrocarbons (PAHs), which organisms on Earth often produce when they decay. Remember that PAH molecules have also been detected in interstellar dust clouds, comets, and carbonaceous meteorites.
4. The carbonate, iron sulfide, and iron oxide minerals are not a stable mixture; they require some form of nonequilibrium environment to exist together. Living organisms could provide such a nonequilibrium environment.
5. Nanometer-sized spheroidal, rod-shaped, and filamentary “structures” that may be fossils of nanobacteria. The first Martians we meet (if they really were once alive) are no bigger than a billionth of a meter.
Put all these points together and you have a picture of bacterial life existing underground on Mars some 3.5 billion years ago, similar to the time when life was getting started on Earth.
The discovery of extremophiles and the deep, hot biosphere on Earth helped to explain what may have happened early in Mars’ history, when the red planet was presumably warmer and wetter than it is today. Some enthusiasts even wondered hopefully if similar organisms might not still be living beneath Mars’ cold, bleak surface.
Each of their points is debatable, McKay and his coworkers admitted; the minerals could have been produced by nonliving, abiological causes, for example. But, they argued, what are the chances of having abiological processes causing all of these features in the same stone? In particular, how do you explain the “structures”?
Scientists can be a combative lot, and there have been plenty of alternative explanations offered by scientists who disagree—sometimes vehemently—with the McKay team’s conclusions. The dissenters maintain that the “structures” are just mineral deposits, not fossils of nanobacteria. Besides, those structures are too small to be any form of organism; most terrestrial bacteria are many times larger. The PAHs and other minerals could have been deposited by abiological means, the dissenters insist.
The issue is still in doubt. Although nanometer-sized bacteria have been discovered on Earth, most scientists either disagree with the McKay team’s conclusions or have decided to withhold judgment until more and better evidence comes to hand.
Cynics claimed that the ALH84001 was nothing more than a ploy by NASA to increase its dwindling budget. Sagan died in 1996 but not before he saw the United States and several other nations commit themselves to at least a dozen new robotic missions to Mars. ALH84001 certainly helped to focus the political decision-makers on returning to Mars. NASA administrator Daniel Goldin was also a prime force in this new wave of exploration.
THE MARS LAB
Meanwhile, teams of researchers are living, working, and searching for life in the bleak, windswept, frozen, dry valleys of Antarctica’s Ross Desert, which are as close to the barren and bitterly cold conditions on the surface of Mars as it is possible to get on Earth.
With less than 2 centimeters of snow per year, this frigid desert often reaches temperatures of -60°C or lower. The air is so dry that the rare snowfall does not melt, it evaporates directly into water vapor and is driven away by the fierce winds sweeping down from the Transantarctic Mountains. Yet life exists there—inside the rocks that clutter the mountain slopes and desert floor.
In the 1980s, biologist Imre Friedmann discovered the tiny organisms that he named cryptoendoliths, meaning “hidden within the rocks.” The cryptoendoliths are tiny colonies of lichen16 that have made homes for themselves in the cracks inside the Antarctic rocks, a few millimeters below their surfaces. There they receive enough sunlight for the algae to make photosynthesis work, and they are protected from the harsh winds and even warmed by the sunlight that slightly heats the rocks. They obtain water from the occasional snow or frost that accumulates on the rocks. They survive the months-long Antarctic night. Organisms like them might even be able to live on Mars, perhaps.
The dry Antarctic valleys are dotted with some seventy subglacial lakes, which are covered year-round by thick layers of ice. The biggest and deepest of these is Lake Vostok. Roughly the size of Lake Erie, Lake Vostok is sealed over by a 4-kilometer-thick ice cap. Below the ice, the lake’s water is nearly 600 meters deep. Yet even there biologists have found bacteria, fungi, and algae in samples they have pulled up from cores drilled 2.5 kilometers into the ice. At that level the ice is about 400,000 years old and has been sealed off from the outside environment for that length of time. The microbes found in the ice are not from our world, they are extremophiles that have existed within the ice for millions of years.
Smaller lakes have been found to harbor thick mats of spongy algae on their bottoms, surviving on the dim sunlight that penetrates their ice covers.
Antarctica is a happy hunting ground for meteorite seekers, as well. The dark stones not only stand out clearly against glacial ice sheets, but they are relatively uncontaminated by terrestrial organisms, since they have been kept in a natural deep freezer since they fell to Earth.
A few of those meteorites originated on Mars. At least two of them, NASA scientists maintain, contain microscopic fossils of nanobacteria that lived on Mars 3 billion years ago.
If the frozen Ross Desert and windswept glaciers of Antarctica harbor life, what might we find on Mars?
THE NEW WAVE
The first of the new wave of Mars explorers was a bouncy little probe called Pathfinder. Launched December 4, 1996, Pathfinder exemplified Goldin’s insistence on finding “better, faster, cheaper” ways of exploring the solar system. Two factors were behind Goldin’s thinking:
First, electronic miniaturization had reached the point where sensors and computers could be made tiny enough to be carried by much smaller and lighter spacecraft. Yet the information handling capabilities of these smaller packages were better than their bigger forebears.
Second, Goldin could fit more smaller, cheaper spacecraft into the NASA budget than big, ornate programs such as Viking that took decades to bring to fruition.
Pathfinder seemed to justify Goldin’s approach. It literally bounced onto the Ares Vallis region of Mars on Independence Day 1997, encased in an assemblage of air bags that cushioned its fall and were “lighter, cheaper, better” than retrorockets would have been. Millions watched on TV as the anxious JPL scientists and engineers exploded into a noisy celebration when Pathfinder landed safely and began returning data.
The earlier Viking landers each weighed 600 kilograms, plus the weight of their landing rocket’s propellants. Little Pathfinder weighed 360 kilograms and carried an even smaller rover, Sojourner, that weighed only 16 kilograms. The Pathfinder program cost $250 million, a quarter of Viking’s cost. As one JPL scientist put it, Pathfinder cost less than the motion picture Titanic.
For eighty-four days—well beyond its planned lifetime—doughty little Sojourner trundled up to rocks that the JPL crew nicknamed Yogi, Chimp, Darth Vader, etc. and made measurements with its X-ray spectrometer of the rocks’ compositions. Both the rover and Pathfinder sent back spectacular photos of the lands
cape and butterscotch-colored Martian sky. The imagery showed clearly that the Ares Vallis territory had been swept by massive floods sometime in the past, further support for the supposition that Mars had once been warmer and wetter than it is today.
THE “FACE” ON MARS
Mars Global Surveyor took up orbit around the red planet on September 12, 1997, and began mapping the surface in unprecedented detail. As far as the public is concerned, however, its biggest achievement was something of a letdown.
In 1976, one of the thousands of photos that the Viking orbiters had taken showed a rock formation that looked uncannily like a human face. Some enthusiasts, including many UFO aficionados, leaped to the conclusion that intelligent Martians had carved a monument similar to the carvings on Mount Rushmore. Skeptics wondered how Martians could carve a likeness of a human head, but the enthusiasts began to see not only “the face” in Viking photos, but pyramids and whole cities, as well.
Global Surveyor’s sharper cameras showed that “the face” on Mars was actually nothing more than a heavily eroded mesa. The enthusiasts were not pleased.
FAILURES AND BACKLASH
Pathfinder and Global Surveyor were bright successes in an otherwise dark picture. Of the thirty missions launched in the twentieth century to the red planet by the United States and Russia (nee USSR), only ten returned usable data. Most crashed on launch or went defunct as they neared Mars. From 1962 to 1996, the Russians doggedly sent twenty spacecraft to Mars. Only three of them returned any usable information: Mars 3 crash-landed and sent back data for twenty seconds, Mars 4 and 5 briefly transmitted pictures from orbit. Five of the Russian probes failed at launch.
Some space scientists are half convinced that Mars may be inhabited after all—by evil gremlins.
In 1992, Mars Observer mysteriously went silent three days before it was to enter orbit around the red planet. But the year 1999 was particularly bad for Mars explorations.
In September, NASA’s Mars Climate Orbiter burned up in the thin Martian atmosphere, apparently because the spacecraft’s builders (Lockheed Martin Astronautics) used English measurements to define the spacecraft’s momentum while its operators (NASA’s Jet Propulsion Laboratory) thought that the figures were expressed in metric units. As a result, Climate Orbiter was inadvertently programmed to dip too deeply into the red planet’s atmosphere; it burned up instead of establishing itself in orbit at a safe altitude.
Then, in December 1999, the Mars Polar Lander simply disappeared. After a near-perfect flight to Mars, the spacecraft’s radio signal blinked out when it was commanded to land at Mars’ south pole. Was it swallowed up by dust or did it crash on landing? Did irate Martians smash it in protest against uninvited snoops from the third planet? No one knows, although later photographic inspection of the polar region by the orbiting Mars Global Surveyor showed that the landing area was much rougher and rockier than the mission planners had believed.
After the Bay of Pigs fiasco in 1961, President John Kennedy said, “Success has a thousand fathers; failure is an orphan.” In space operations, however, failure often has a thousand investigators (or more!) trying to find who is at fault. The failures of Mars Observer, Climate Orbiter, and Polar Lander forced an examination of Goldin’s “faster, better, cheaper” policy. Critics complained that there had been too much time pressure and too little funding for the failed Mars missions.
Goldin’s policy recognized from the outset that all rocket launches and space missions are fraught with risk. By opting for many small missions instead of a few large ones, that risk is spread. But the successes of Pathfinder/Sojourner and Global Surveyor were overtaken by the later failures of Climate Orbiter and Polar Lander. Investigating committees began to look for reasons, and the consensus was that planetary missions must be better funded and less time-pressured.
Meanwhile, from its orbit around Mars, Global Surveyor has observed gigantic dust storms that sometimes envelope almost the entire planet. One such storm was raging across much of Mars as the Mars Odyssey craft approached the planet in late 2001. Many in NASA worried that the storm would interfere with plans to have the spacecraft dip into the upper fringes of the Martian atmosphere so that it could use aerodynamic drag to lower Odyssey’s orbit. After the failures of the two 1999 missions, NASA was under intense pressure to succeed with Odyssey.
In October 2001, the Mars Odyssey spacecraft reached the red planet after a six-month flight and NASA drew a collective sigh of relief. Odyssey began sending back preliminary photos of Mars as it started the aerobraking maneuver that established the spacecraft in a circular orbit at 400 kilometers’ altitude. In February 2002, Mars Odyssey began its primary mission of studying the composition of the Martian surface, searching for signs of water, and measuring the radiation reaching the ground from outer space.
The future exploration of Mars depends more on political whim than technical capabilities. The United States, the European Space Agency, Japan, and Russia all intend to send robotic probes to Mars. Their sensors and instrumentation are being adapted in light of the findings of ALH84001 and the discovery of terrestrial extremophiles. Some of the missions being planned include returning samples of Martian surface material, rock, and deep borings.
It may well be, though, that definitive answers to the question of life on Mars will not be found until human explorers reach the red planet with equipment that can drill kilometers into the crust. No government has shown any interest in funding such a program, which would cost hundreds of times more than the robotic probes now underway.
14
Hotworlds and False Assumptions
God is subtle but he is not malicious.
—Albert Einstein
AT FIRST GLANCE—and even second or third—Mercury and Venus seem unlikely planets to search for life. But as Einstein pointed out, we must keep in mind the subtleties that often elude first impressions.
Mercury is the closest planet to the Sun and Venus the second closest. Both planets are hot, with surface temperatures far above the boiling point of water. But there the similarities between them end. Even their high surface temperatures are due to completely different causes.
Both these inner planets were greatly misunderstood by astronomers for centuries. It was only in the middle of the twentieth century, when radio telescopes began to study them, that their true natures became apparent.
ELUSIVE MERCURY
Named after the messenger of the gods because it flits around the Sun so quickly, Mercury resembles the element of the same name, which is also called quicksilver: elusive, puzzling, hard to pin down.
The closest planet to the Sun, Mercury is a rocky, dense, sun-scorched world. Earth-based astronomers have a hard time seeing Mercury because it is always so close to the Sun’s glare. This led to one of the great mistakes of astronomical history.
Since Mercury is so difficult to see, for centuries astronomers made their observations when the planet was at its farthest point from the Sun (which astronomers call its greatest elongation). What they saw in their telescopes was a small, fuzzy image that seemed always to present the same markings. They concluded that Mercury turned on its axis once in the same period as its orbit around the Sun, just under eighty-eight days. Thus, they reasoned, Mercury’s rotation is locked, just as the Moon’s is; Mercury keeps one side always facing the Sun and one side constantly in darkness.
For more than a century astronomers accepted the “locked” Mercury, where one half of the planet was always broiling beneath a mammoth blazing Sun while the other side was always in sunless darkness, its temperature near absolute zero.
But it wasn’t so. In 1962, radio astronomers were surprised to find that Mercury’s allegedly “dark” side was quite hot.17 In 1965, using the giant Arecibo radio telescope to transmit radar pulses, Cornell University astronomers Gordon H. Pettengill and Rolf Dyce were surprised to see that instead of a sharp return pulse from Mercury’s rocky surface, they got a slightly smeared “echo.” This could only h
appen if the surface being hit by the pulses was turning faster than expected. They checked and rechecked, because “everybody knew” that Mercury’s rotation was locked and its rotation rate (its day) was exactly the same as its revolution around the Sun (its year).
After exhausting all the other possibilities, the astronomers concluded that the radar observations of Mercury showed that the planet’s rotation is not locked. Mercury spins slowly on its axis, taking 58.6 Earth days to make one rotation. Later observations confirmed their conclusion.
This leads to a strange situation. Mercury’s rotation rate of 58.6 days is exactly two-thirds of the planet’s eighty-eight-day year. Because Mercury’s orbit is so far from circular, if you were standing on the surface of Mercury (well protected inside a spacesuit, of course), you would see the Sun moving from east to west across the dark airless sky, but it would slow down noticeably as you watched, then reverse its direction and head back east for a while before resuming its westerly motion. At some locations on Mercury the Sun would rise briefly, dip below the horizon, and then rise again for the rest of the “day.” After sunset the Sun would peek back up above the horizon before setting for the “night.”
If you measured a Mercurian “day” from the time the Sun appeared directly overhead (local noon) to the next time it reached that point, it would take 176 Earth days. From the standpoint of noon-to-noon, the Mercurian “day” is longer than its year!
The Sun looms large in the Mercurian sky: It appears twice as big as we see it from Earth when Mercury is farthest from the Sun (called the aphelion point, about 70 million kilometers from the Sun) and three times larger when it is closest (perihelion, 46 million kilometers).
