Faint Echoes, Distant Stars

by Ben Bova

  The most spectacular grand canyon in the solar system is Valles Marineris (named after the Mariner 9 orbiter that discovered the valley), a split in the Martian crust deep enough to swallow the Alps and longer than the distance between New York and San Francisco.

  Mars has a weak magnetic field, further evidence that its metallic core is either very small or too cold to remain liquid—or both. However, data from the Mars Global Surveyor spacecraft in orbit around the red planet indicate that Mars’ core may be at least partially still molten, according to a report from Charles Yoder and his colleagues at NASA’s Jet Propulsion Laboratory.

  Mars has been battered by meteoroids, which is hardly surprising since it orbits much closer to the Asteroid Belt than Earth does and its atmosphere is too thin to erase meteor craters through weathering, although there are seasonal sandstorms that sometimes engulf the whole planet and could erode landscape features over long periods of time. Interestingly, the southern hemisphere of Mars is much more heav-ily cratered than the northern, which—together with other hints of evidence—has led some astrobiologists to speculate that Mars’ northern hemisphere may have once been covered by lakes or even a sizable sea of liquid water.

  A WARMER, WETTER MARS?

  Most researchers believe that there must have been running water on Mars in the past. Spacecraft photos show sinuous trails that look very much like dried-out riverbeds (not like the straight-line “canals” that Lowell thought he was seeing). The Pathfinder/Sojourner craft of 1997 saw that the Ares Vallis region in which they landed had been swept by floods some time in the past. Imagery from the Mars Global Surveyor, which went into orbit around Mars in 1997, has shown exciting evidence of erosion caused by liquid flow that strongly indicate that water was flowing slightly beneath the Martian surface as recently as 100,000 years ago—an eyeblink in geological time.

  Geologist Phil Christensen, head of Arizona State University’s Mars Space Flight Facility, believes that snow packs are responsible for much of the erosion seen on Mars. Spacecraft photographs have shown snow accumulations in craters. Christensen believes that the bottom-most layers of the snow pack melt, and the water is kept liquid by the pressure of the snow above it. This liquid water carves the gullies that seem so recent, he asserted in a report in 2003.

  In 2002, the Mars Odyssey spacecraft established itself in orbit around the planet and detected strong evidence of water ice buried a meter or so beneath the surface at both the north and south poles of Mars. Odyssey’s instruments (one measures gamma rays emitted from the surface and two pick up neutrons) actually sensed hydrogen; scientists infer that the hydrogen comes from water molecules locked in the soil. They could not tell how deep underground the ice may reach, but they estimate that in some places up to half the subsurface material could be water ice.

  In 2003, Odyssey’s infrared camera discovered water ice along the edge of Mars’ south polar cap, where it is usually overlain by frozen carbon dioxide that evaporated in the Martian summer. This was the first direct evidence for water on the red planet.

  Lesser amounts of water ice apparently lie just beneath the surface elsewhere on Mars, as well, frozen into permafrost.

  Most astrobiologists are convinced that Mars was warmer and wetter in the past than it is today. How long ago? That remains to be determined. Did a “warmer, wetter” Mars support life? That, too, is as yet unknown.

  A FROZEN DESERT

  Mars’ thin atmosphere has determined its fate. The Martian atmosphere, thinner than Earth’s high stratosphere, cannot hold much heat. At noon in midsummer along the Martian equator, the ground temperature might rise to a comfy 23°C. But the air temperature at nose level above the ground would be about -18°C, and on that same night the temperature would plummet to -75°C or lower. Whatever solar heat the ground absorbs during the day radiates away into space; the atmosphere stores almost none of it.

  Mars’ atmosphere is only about a hundredth the density of Earth’s: equivalent to the air pressure at more than 30,000 meters’ altitude on Earth. Even if it were pure oxygen it would be too thin for a human to breathe. It is mostly carbon dioxide, with just 0.2 percent oxygen. Yet that oxygen content in the atmosphere means that human explorers could extract oxygen from the Martian atmosphere, condense it, and use it for life support.

  Thin though the Martian atmosphere is, it still blows dust storms that sometimes cover the entire face of the planet. Wind velocities of more than 160 kilometers per hour have been seen, although since the atmosphere is so thin, there is very little force in the wind. A 160-kilometer-per-hour wind would have the impact of a gentle breeze on a spacesuit-clad astronaut standing on the surface of Mars.

  Liquid water would immediately boil away in the near vacuum of Mars’ surface, despite the low temperature. The water present on the planet is frozen into ice at the poles or belowground. The polar caps also contain dry ice: frozen carbon dioxide. During springtime, the icecap shrinks; the Viking landers, thousands of kilometers from the north polar cap, photographed frost on the ground—evidence that the ice at the poles sublimes (goes directly from solid to gaseous state) and is then carried toward the equator by the seasonal winds. Although the north polar cap is larger than the southern, it sometimes disappears completely during the northern hemisphere summer.

  As we have seen, there is strong evidence that ice exists below the ground, too, as permafrost. If true, there should be ample water available for human explorers. And perhaps for local life-forms.

  The thin Martian atmosphere is usually so clear that Mars receives about the same amount of solar energy on the ground that Earth does, despite its being farther from the Sun. The atmosphere contains about 0.03 percent water vapor, just about the limit it can hold, considering how cold it is. Analyses published late in 2001 show that up to one-third of all the carbon dioxide on Mars is cycled each year from the polar caps into the atmosphere and back again. Frost occasionally mantles the ubiquitous rocks, and clouds, fogs, and hazes have been observed, particularly in the rift valley complex that makes up Valles Marineris.

  Mars is a cold, dry desert. Its red color comes from iron; the Martian sands are oxides of iron. Rust.

  ONCE AND FUTURE LIFE?

  But Mars was probably not always the way it is now. In earlier times, eons ago, Mars might have been considerably warmer, with a thicker blanket of atmosphere than it now holds. Liquid water may have flowed across its surface or just below it. We know ice ages have existed on Earth, when glaciers covered much of Eurasia and North America. Perhaps Mars is now undergoing its version of an ice age. Perhaps there is life buried deep underground, or nestled inside rocks as the Antarctic lichen called cryptoendoliths are on Earth.

  Considering the extremophiles that exist on Earth, especially the deep, hot biosphere that lies beneath our feet, could similar organisms live deep below the harsh surface of Mars without needing photosynthesis to generate their energy? They would depend on heat welling up from the planet’s core, though, and the available evidence (lack of plate tectonics, lack of a planetary magnetic field) points strongly to the conclusion that Mars’ core is almost entirely cold.

  What turned the iron sands of Mars into rust? The sand grains are mainly iron ores that have been highly oxidized. Where did the free oxygen come from? On Earth, vast deposits of banded iron formations were created by early cyanobacteria giving off oxygen as part of their metabolism: Their “waste” oxygen combined with iron deposits at the bottom of the sea. Is Mars’ rusty iron the result of ancient life processes?

  Perhaps life once existed on the surface of Mars and was wiped out, completely eradicated from the entire planet by a cataclysm similar to the meteoroid strike that extinguished Earth’s dinosaurs some 65 million years ago.

  We have seen that Mars has been more heavily bombarded by meteoroids than Earth. The impact basin called Hellas Planitia (the Plain of Hellas) is a gigantic crater, nearly 2,000 kilometers wide and several kilometers deep. On the opposite side of the planet lies
the Tharsis Bulge, where the great volcanoes have arisen. Did a massive meteoroid smash into Mars hard enough to gouge out Hellas Planitia, send a shock wave through the solid body of the planet, and raise the Tharsis Bulge and its volcanoes? If so, the shock of such an impact might have been so severe that it blasted most of Mars’ atmosphere out into space or caused a planet-wide “nuclear winter” type of dust cloud that blotted out the Sun for months or years.

  Either way, such a meteor impact could have wiped out whatever life might once have existed on Mars. Biologists surmise that Earth may have been sterilized more than once in its early history. The same could have happened to Mars. Perhaps the first human expeditions to Mars should include paleontologists.

  THE “WHITE MARS” HYPOTHESIS

  Or perhaps all of these speculations are nothing more than wishful thinking. There is an alternate explanation for the appearance of river-like channels and other evidence that liquid water once raced across the surface of Mars.

  Nick Hoffman of La Trobe University in Australia is one of a small group of planetary scientists who are skeptical of the idea of a “warmer, wetter” Mars. He believes Mars has always been cold and dry, too cold for liquid water to exist on its surface or even underground.

  What about the evidence for vast floods and erosion caused by liquid flow? Hoffman believes that liquid carbon dioxide caused them.

  Under ordinary terrestrial conditions, carbon dioxide cannot exist in liquid form. Carbon dioxide is either gaseous or solid (dry ice) on Earth. Dry ice sublimes, goes directly from solid to a gas, with no liquid phase in between. On Earth.

  On the surface of Mars, carbon dioxide behaves the same way: It does not melt, it sublimes. But a short distance below the surface, Hoffman points out, the pressure is high enough (and the temperature low enough) to allow carbon dioxide to be liquid. Giant runoffs of liquid carbon dioxide created the erosion features seen on Mars’ surface, in his view. Flowing like water underground, liquid carbon dioxide undermined the surface to produce the gullies, arroyos, and flood plains that spacecraft cameras have revealed.

  Hoffman has one bit of substantive evidence that favors his “white Mars” hypothesis: Two of the meteorites that have fallen to Earth after being blasted off Mars have been found to contain minute droplets of liquid trapped inside them—liquid carbon dioxide. We shall see more of the Martian meteorites shortly and examine the possibilities that they contain fossils of once-living microscopic organisms in them.

  If the “white Mars” hypothesis is correct, liquid water has never existed on Mars. And life has never arisen on the cold, dry, red planet. Hoffman’s ideas are not accepted by most astrobiologists. They far prefer to think that Mars was once warmer and wetter, capable of bearing life.

  VIKING’S ENIGMATIC RESULTS

  By the 1970s, despite the disappointing results of the various Mariner missions, Sagan and the “pro-life crowd” had succeeded in convincing NASA to send a pair of Viking spacecraft to Mars, still hoping to find some signs of life on the red planet. Each Viking craft consisted of two components: an orbiter that would photograph the planet’s surface and a lander that would touch down softly on the rust-red sands of Mars. Both landers carried three automated experiments designed to detect possible Martian life:

  1. Label release. Radioactive carbon-14 was introduced to samples of soil scooped up by Viking’s robot arm, remotely controlled from JPL. The soil sample was dumped into a nutrient mixture designed to feed any microorganisms that might be in the sample. If there were organisms present, the gases given off by the sample would show a high percentage of carbon-14.

  2. Pyrolitic release. A soil sample was mixed with gases that simulated the Martian atmosphere (mostly carbon dioxide), dosed with a slight amount of radioactive carbon-14 as a “tracer,” and then exposed to a fluorescent light that simulated sunshine. If there were any Martian organisms present that depended on photosynthesis, they should have multiplied vigorously in such an environment. After five days the gases were flushed out of the chamber and the soil was baked at a temperature of 625°C. If the baked soil released a significant amount of carbon-14, it would be coming from organisms in the soil that took in the carbon dioxide.

  3. Gas exchange. Soil samples were “incubated” in a water-rich nutrient bath (which the Viking scientists called “chicken soup”) and an atmosphere of carbon dioxide, krypton, and hydrogen. Living Martian organisms, if anything like those on Earth, would convert some of the carbon dioxide to free oxygen.

  Each of the three biology experiments showed results that seemed to indicate life—at first.

  The gas exchange experiment showed a sharp increase in the amount of oxygen given off by the soil samples, although this soon leveled off. Both the pyrolitic release and the label release experiments also gave strong positive returns at first, followed by lower returns afterward.

  In short, all three Viking experiments gave results that were neither straight inorganic chemistry nor obvious Earth-type biology. However, the Viking landers also carried gas chromatograph/mass spectrometers, instruments designed to detect organic, long-chain carbon molecules in the Martian soil samples. They failed to detect any evidence of organic molecules, down to a level of one part per billion. As far as the GC/MS was concerned, Mars’ regolith was barren.

  Most investigators eventually concluded that the Martian soil is lifeless. The surprising positive results from the biology experiments were attributed to superoxides in the soil, an excess of volatile oxygen molecules. The Martian soil, apparently, is something like powdered bleach. Even though a few scientists insisted that their colleagues interpreted Viking’s results much too conservatively, the consensus was that life does not—and cannot—exist on the surface of Mars.

  On the other hand, researchers in Antarctica used a technique similar to Viking’s label release experiment to search for life in the near-freezing water of Lake Vostok, which is perpetually covered by an ice cap 4 kilometers thick. They not only obtained positive results, much as Viking did, but they also pulled living microorganisms out of the ice: cold-adapted extremophiles.

  Has the scientific community written off the Viking results too soon? With 20/20 hindsight, it seems clear that the Viking experiments were aimed at finding the kind of life we are familiar with here on Earth. None of those experiments could have detected truly alien life-forms, because no one knew what to look for except Earth-type biology. Their failure should have been no surprise. Sagan insisted that “absence of proof is not proof of absence” of life on Mars. He knew that Viking was merely one early (and fairly primitive) step in our investigation of the red planet.

  The politicians in Washington felt otherwise. Once it became clear that Viking had not found life, enthusiasm for further space exploration dwindled to the vanishing point in the nation’s capital. Viking had cost nearly $1 billion (1970s dollars, at that). No matter how Sagan, the Jet Propulsion Laboratory, or anyone else worked to persuade them otherwise, the politicians would not vote for that kind of funding for many, many years.

  VISITORS FROM MARS

  But while space scientists found it increasingly difficult to get political support for returning to Mars, others realized that Mars had sent visitors to Earth.

  Researchers have identified thirteen meteorites that originated on Mars and landed in Antarctica, Africa, Europe, and the Americas. The largest such meteorite is named Zagami; it was discovered in Nigeria in 1962. It weighs 18 kilograms.

  Scientists call them the SNC meteorites after three of the locations where they have been found: Shergotty (India), Nakhla (Egypt), and Chassigny (France). The Nakhla stone allegedly killed a dog when it struck the ground in 1911.

  The SNC meteorites are known to be from Mars because the gases trapped within these rocks have the same ratios of noble gases (argon, neon, etc.) as the atmosphere of Mars, which was measured by the Viking craft. While Viking was considered a disappointment, even a failure as far as the search for life is concerned, it did return
invaluable information about the red planet.

  The meteorites were blasted off the surface of Mars by the impact of much larger meteoroids, and thanks to Mars’ relatively light gravity they were hurled completely off the planet to wander through space for millions of years. Eventually they were caught by Earth’s gravity field and crashed into our planet after a fiery plunge through our atmosphere. Each of the “Martian 13” came from below the surface of Mars; they do not exhibit the kind of weathering that surface rocks would show.

  FOSSILS OF MARTIAN LIFE?

  In 1996, a team of NASA and university investigators shocked the world by announcing that they had found possible fossils of ancient bacteria in one of the Martian meteorites. ALH84001 is a 1.9-kilogram rock about the size of a hefty potato, discovered in 1984 in the Allan Hills ice fields in eastern Antarctica. The NASA team, led by David McKay of the Johnson Space Center near Houston, made their announcement at a news conference before the publication of their findings in a scientific journal. This led to some criticism by more conservative scientists, but as NASA chief Daniel Goldin remarked, this news was too big to wait for ordinary scientific protocol.

  Not everyone agrees with McKay and his team. Scientists have hotly debated the idea that the nanometer-scale structures found deep inside ALH84001 are the fossils of once-living Martian bacteria. Similar structures have been found in a few of the other Martian meteorites.

  Radioactive dating has shown that ALH84001 is 4.5 billion years old, which means it dates back to the very beginnings of the solar system, the time when rocks first crystallized on Mars from their earlier molten state. It was blasted off Mars some 14.4 million years ago and, after a long, meandering sojourn through interplanetary space, was pulled down to Earth and hit the Antarctic ice about 13,000 years ago, to be found in 1984 by NASA geologist Roberta Score, who was on her first meteorite-hunting trip to Antarctica.