The Case for Mars (8 page)

Today, most researchers—but definitely not all—feel that the
Vikings
did not find evidence of life. Instead, they have drawn a picture wherein the Martian soils are rich with peroxides and superoxides. According to this theory, the results from at least two of
Viking’s
experiments were the signature of chemical reactions involving these peroxides. The failure of the GCMS to detect any carbon at either site fit neatly with the peroxide/superoxide theory because peroxides destroy organic matter with abandon. But not everybody buys this, with some suggesting that perhaps the GCMS was not sensitive enough to detect vanishingly small amounts of organic material, that is, life. When cultured in
Viking
, it is conceivable that such sparse spores could quickly multiply into a population large enough to give off the positive signals. Similarly, the abrupt end of the signals displayed by the biology packages, easily explained by the chemistry side as the exhaustion of the peroxide supply, could also be explained by an over-multiplying population of organisms in the soil sample poisoning itself with its own wastes. Gilbert Levin, principal investigator for the biology test package called the Labeled Release experiment, to this day believes passionately that his equipment detected evidence for Martian life. A decade after the
Viking
landings, Levin would write “. . . after years of laboratory work trying to duplicate our Mars data by non biological means, we find that the preponderance of scientific analysis makes it more probable than not that living organisms were detected in the LR [labeled release] experiment on Mars. This is not presented as an opinion, but as a position dictated by the objective evaluation of all relevant scientific data.”
²
A scant twenty pages earlier in the same volume, another biology team member, Norman Horowitz, writes, “For s
ome Mars will always be inhabited, regardless of the evidence. . . . One does not have to search far to hear the opinion that somewhere on Mars there is a Garden of Eden—a wet, warm place where Martian life is flourishing. This is a daydream.”
³

My own feeling is that Horowitz is too harsh in his assessment of the possibility of Martian life and Levin a bit too enthusiastic. The best bet is that the
Vikings
did not detect life
in the surface soil of Mars.
The reason for this is that there is no liquid water there, and virtually no organics, and so while abstract arguments can be made for the “sparse spore” hypothesis, it seems almost impossible to construct a rational theory explaining how the life cycle of these putative Martian surface organisms would function. Furthermore, since Mars has very little in the way of an ozone layer in its atmosphere, the surface is bathed in ultraviolet light of sufficient intensity to do a pretty good job of sterilizing the planet’s surface of microorganisms. However, regardless of Horowitz’s opinion, this does not rule out the possibility of a microbial “Garden of Eden”
below
the surface. In fact, if terrestrial life has taught us anything, it’s that life flourishes not only in “Garden of Eden” environments, but in hellish ones as well. Indeed, there are families of bacteria known as chemotrophs that derive their energy from various inorganicmicals as opposed to sunlight (like plants) or organic nutrients (like us). A small group that is adapted to temperatures of 70° to 90° centigrade and lives happily by oxidizing sulfur for their energy requirements would probably feel right at home in some underworld environments that probably exist on Mars. Across our own globe, in the most extreme environments imaginable, scientists have discovered life tenaciously hanging on, making do with scant resources. In the Antarctic, colonies of lichen flourish within surface rocks, protected from the harsh environment by a centimeter or so of porous sandstone. Vast colonies of microorganisms thrive around the mouths of deep-sea vents that spew founts of boiling, mineral-rich water. There are organisms that thrive only in heat, others only in cold; some that grow only in alkaline conditions, others only in acidic environments; some that feed on sulfur, others on iron, others on hydrogen. Not only can life survive extreme environments, it appears it can also survive over unimaginably long time spans. In the late 1980s a researc
h group in Britain discovered that a group of salt-tolerant microbes called halobacteria could become trapped within rock salt and survive for months at a time in their tiny, briny homes. Intrigued, the group set out and collected samples from a natural subterranean salt deposit that dated from the Permian period, more than 230 million years ago. Again, they discovered tiny, fluid-filled cavities within the rock salt, and within a small fraction of these cavities (6 out of 350) they discovered viable halobacteria that could be cultured in the laboratory after a time span of more than 200 million years.
⁴

All creatures great and small surviving in extreme environments have one thing in common: their environment includes a source of water, however meager. The fact that Mars shows a remarkable amount of evidence of both surface and subsurface water in its distant past argues for the possibility of life in the past or perhaps even now in an unexpected “Garden of Eden.” Host environments to such life could be thermal hotspots, such as hot springs; subsurface hotspots; subsurface permafrost deposits; subsurface or near surface brines, or perhaps even areas with evaporite deposits, such as the salt formation that was home for millions of years to earthly bugs. Many geologists believe that Mars does have a liquid water table, at least in certain places, perhaps a kilometer or so beneath its surface. Perhaps life which evolved on the surface of the planet in the distant past when it was warm and wet has retreated there. Recently, investigators in the state of Washington discovered a species of bacteria living deep underground, subsisting on the chemical energy derived from the reaction of cold ground water with basalt. There does not seem to be any particular reason for believing that similar organisms could not survive equally well in the subsurface environment that is hypothesized to exist on Mars. The point is that life is tough, even if it may be hard to find on Mars. No one expects to find herds of six-legged Barsoomian thoats thundering across Martian dunes. But life on the level of microorganisms, living in sheltered environments, that’s another matter. It could be there now, or may have been there once. To find it will take more than robotic probes with limited mobility, dexterity, and perception.

AFTER VIKING

The
Viking
orbiters and landers kept on with their science observations long after the biology experiments came to a close. Orbiter 2’s last transmission came on July 25, 1978, followed by the demise of Lander 2 on April 11, 1980, nearly two years later. Orbiter
1
sent its last signal on August 17, 1980, while Lander
1
signed off on November 5, 1982.

The Soviet space program attempted two launches in 1988 to explore Mars and its moon Phobos that met with disappointment, continuing a streak of bad luck that has plagued every Soviet or Russian Mars mission. (Out of more than fifteen attempts, none has been successful.) The United States’ Mars program has also had to deal with failures. The
Mars Observer
spacecraft carried seven instruments intended to investigate Mars over the course of a Martian year. The mission would “rewrite the books” on Mars, or so researchers hoped. But just days before the spacecraft was due to enter orbit around the Red Planet, it fell silent. In attempting to reconstruct what may have happened, engineers have surmised that a fuel line ruptured as the spacecraft prepared to fire up its engines to slip into Mars’ orbit. Whatever the cause, after a seventeen-year hiatus, America’s exploration of Mars appeared headed for the deep freeze.

Fortunately, instead of using the demise of
Mars Observer
as a pretext for thrashing NASA’s Mars exploration budget, members of Capitol Hill looked kindly upon continuing the legacy of exploration that the
Vikings
exemplified, though with a twist. With a new focus on “faster, cheaper, better” methods of accomplishing planetary exploration, NASA has fashioned a decade-long program of Mars exploration out of the
Mars Observer
failure. Instead of launching a single massive spacecraft to the Red Planet, American plans now call for a series of small spacecraft to orbit and land on Mars. This program started in late 1996 with the launch of the
Mars Global Surveyor
spacecraft and the
Mars Pathfinder
mission. About half the size of
Mars Observer
, the
Surveyor
began mapping the Red Planet from polar orbit in January 1998. Among its discoveries to date have been altimetry data revealing a large basin of depressed and relatively uncratered terrain in Mars’ northern hemisphere that is flatter than anything on Earth except for the sea bottoms, indicating the previous
existence of a northern ocean.
Mars Pathfinder
landed on Mars on July 4, 1997, with the help of parachutes, braking rockets, and airbags. Surviving several 40 to 60 miles per hour bounces along the surface,
Pathfinder
opened up and released a tiny rover (dubbed
Sojourner
after antislavery heroine Sojourner Truth).
Sojourner
then traveled for two months about the Ares Valles runoff channel landing site, collecting geological information and making the notable water-indicative discoveries of rounded cobbles and conglomerate rocks. An additional lander (targeted to the south pole) and orbiter were launched to Mars in 1998, and plans call for additional spacecraft launches in 2001, 2003, and 2005.

While the U.S. robotic Mars exploration program is moving ahead, budgetary difficulties and bad luck have thrown the Russian program into chaos. Russia’s latest attempt, entitled
Mars 96
, aimed to place a spacecraft in orbit around Mars, as well as two small science stations and two ground penetrators on the Martian surface, but was thwarted by a launch vehicle failure in the fall of 1996. This caused the indefinite postponement of a second mission,
Mars 98
, which was to deliver an orbiter, rover, and balloon to the planet. The Russian
Marsokhod
rover would have dwarfed the American
Pathfinder
rover and, instead of venturing just 10 meters away from its landing site, could have logged nearly 50 kilometers. Trailing an instrument-laden “snake,” the balloon, a product of the French space agency CNES, was designed to soar as high as 4 kilometers into the Martian atmosphere during the day, but settle toward the ground during the Martian night. Designed for a ten-day flight, the balloon would have been able to rac several thousand kilometers in its windborne wanderings across the Martian surface. With Russia’s economy continuing to lurch, however, it is questionable whether this mission will ever fly.

While the Russian planetary exploration program may be dying, new players are entering the field. Europe will send its first interplanetary mission,
Mars Express
, to the Red Planet in 2003. An ambitious mission,
Mars Express
includes both a French-made orbiter and the British lander
Beagle II
, which will carry experiments to search the surface for life. France has also signed a bilateral agreement with the United States to team on a 2005 Mars Sample Return mission, and offered to make spare payload capacity on its new Ariane 5 launch vehicle available to support Mars “mic
ro missions,” as well. Italy has proposed a Mars communications orbiter, while Japan has launched a radar probe, called
Nozomi
, which will reach Mars via a roundabout orbit in 2002.

Among missions currently under discussion in the United States is the Mars Aerial Platform mission, or MAP, developed by myself and others at Martin Marietta. MAP is a conceptual design for a low-cost mission that would return tens of thousands of high-resolution photographs of the Martian surface, analyze and map the global circulation of the atmosphere, and examine Mars’ surface and subsurface with remote sensing techniques. At the heart of the mission is a hightech approach to a very low-tech concept—balloons.

This is how MAP would work: A single Delta-class booster would launch the MAP payload on a direct trajectory to Mars. The payload would consist of a spacecraft carrying eight entry capsules, each capsule packed with a balloon, deployment equipment, and a gondola carrying science instruments. Ten days prior to arrival at Mars, the spacecraft, now spinning like a top, would release the capsules, casting them off in directions that would ensure their entry at widely dispersed locations. As each capsule began its descent through the atmosphere, a parachute would deploy to slow the capsule to the point where a balloon could be inflated. Each would be made of a commercially available material known as “biaxial nylon 6” that is just 12 microns thick—one third the thickness of a standard plastic trash bag. Though seemingly made of gossamer, these balloons will be surprisingly tough. The material’s manufacturing process guarantees that it harbors no pores, which means that balloons made of this nylon simply will not leak and, therefore, can be expected to remain inflated not for days, but years. Following inflation, the parachute, capsule, and inflation equipment drop away, carrying a meteorology package to a soft landing on the Martian surface. Free of extraneous equipment now, each balloon begins the first of perhaps hundreds of days of roaming the highways of Mars, the planet’s eternal winds.

The 18-meter-diameter balloons will cruise over the surface of Mars at an altitude of 7 to 8.5 kilometers and, unlike the French balloon that had been planned for
Mars 98
, will maintain these altitudes day and night. They will be able to do this, because with their new material and compact configuration (enabled by a very lig
htweight gondola) the balloons will be strong enough that when their gas pressure is increased by the heat of the day they can just hold it in without venting. Since they won’t vent gas by day, these “superpressure” balloons have no need to drop ballast at night, and therefore can fly almost forever at constant altitude. Current models of Mars’ atmospheric dynamics suggest that the winds will carry the balloons primarily in a west-east direction on the order of 50 to 100 kilometers per hour. At these speeds, each balloon could circumnavigate Mars every ten to twenty days and, assuming a conservative average mean time to failure of a hundred days, we can expect each balloon to circumnavigate Mars at least four times. Each balloon will carry an 8-kilogram instrument package containing atmospheric science instruments, data recording and transmitting equipment, a rechargeable battery, a solar array, and an imaging system, the heart of the package. Comprised of two sets of optics—one for high-resolution images, another for medium-resolution images—the imaging system will significantly advance our understanding of Martian geology while allowing us to identify landing sites for future missions and possible areas that offer promise for investigations of past or present Martian life. The best
Viking
orbiter images reveal surface features the size of a baseball diamond;
Mars Surveyor
images show surface details as small as a mid-sized car; the cameras MAP carries will be able to reveal surface features the size of a cat (which is not to say that they will reveal Martian cats). Every fifteen minutes during daylight hours, the cameras carried by each balloon will take two simultaneous photographs: one a black-and-white, high-resolution photograph and the other a color, medium-resolution photograph centered on the area imaged by the high-resolution camera (the latter photograph can be used to determine the location of the high-resolution photograph on a map of the planet). MAP will return a staggering number of photographs. For each hundred days an eight-balloon fleet sails the Martian winds, MAP will return 32,000 high-resolution photographs along with an equal number of context photographs at resolutions superior to the best
Viking
images.