By: Michael H. Carr
A S THIS ISSUE OF NATURAL HISTORY WENT TO
PRESS, AT LEAST SIX SPACECRAFT WERE ALREADY OR-
BITING—OR SPEEDING TOWARD A RENDEZVOUS WITH----- THE PLANET MARS.
I n the vanguard of this wave of martian exploration are two NASA orbiters, the Mars Global Survejor, in orbit since 1997, and the Mars Odyssey, in orbit since 2001, which have by now collectively observed the planet for eight years. The two have already returned an enormous amount of data ahout Mars: its topography, which reflects a surprisingly complex geological history, incorporating thick stacks of layered sediments and seemingly recently waterworn gullies; its ancient magnetic field, now vanished because its core has cooled, but still traceable in the magnetization of ancient rocks; its surface chemistry and its primarily basaltic mineralogy; and its fine—scale surface structures, sculpted by wind and ice. The data from the two orbiters have also been crucial for planning the other missions now approaching Mars, particularly in helping planetary geologists pick exploration sites that are both scientifically interesting and relatively free of hazards to landing.
First among the approaching missions is another orbiter, Nozonmi, launched by Japan’s Institute of Space and Aeronautical Science in 1998. It is due to arrive in January. Nozorui will examine the interaction of the planet’s upper atmosphere with the so—called solar wind, made up of highly energetic partides from the Sun. Since Mars has no magnetic field, it is constantly bombarded by the solar wind. The particles carry enough energy to break molecules in the tipper atmosphere into their atomic constituents. Some of the lighter resultant elements get carried away in the solar wind, and so the planet is gradually losing its atmosphere. Knowing how fast that is happening today will enable scientists to estimate how thick the atmosphere was in the past, and so—because of the greenhouse effect of an atmosphere—how warm the planet may once have been.
This past June the European Space Agency launched the Mars Express, made up of an orbiter, the eponymous .Mars Express, and a lander known as the Beagle 2. Mars Express will go into orbit this Christmas Day, 2003, minutes after Beagle 2 is scheduled to land on Isidis Planitia (see map end of article) The lander is to measure surface and atmospheric properties, and will probe as deep as five feet into the martian soil. Its onboard instruments will seek bulk organic matter, as well as the isotopic signature of the biologically important element carbon. Most elements occur in nature as a mix of isotopes of slightly differing atomic weights . On Earth, some biological processes preferentially utilize certain isotopes of some elements, so that the carbon isotopes that occur in organic molecules, for instance, have diff-erent weights than the ones that occur in inorganic compounds. Measuring the isotopic ratios on Mars will provide cities about possible biological activity.
The orbiter Mars Express has numerous instruments for analyzing the surface and atmosphere, including a high—resolution stereo camera and instruments for measuring surface composition that complement the ones on Mars Global Surveyor. Mars Express also has a radar device for detecting water more than a mile below the surface.
Finally, this past summer NASA launched two Mars rovers, which will join the two U.S. spacecraft already examining the planet. Spirit, the first rover, is scheduled to land on the surface on January 4, 2004, Opportunity, the second, will land on January 25, 2004. . The two rovers will land on opposite sides of the planet and investigate the geology of regions where liquid water might once have been present. The targets of their searches will be water—bearing minerals and sediments laid down by water.
The two rover missions, along with the other four, constitute by far the greatest assemblage of spacecraft people have ever sent to Mars. Their presence will dramatically pick tip the tempo of the research begun by the Viking missions and, more recently, by the 1997 Pathfinder rover. Those missions failed to find any evidence of life on the martian surface. Yet of all the extraterrestrial bodies in the solar system, Mars is still the most likely place where conditions might have been hospitable for life . If Spirit and Opportunity successfully carry out
their missions, planetary scientists will have a much better idea of whether some form of life evolved on Mars in the past, and of where we might best go to look for it, or for its remains.
The modern roots of people’s fascination with Mars extend at least as deep as the late eighteenth century. By that time observations had already revealed that Mars has sonic remarkably Earth—like qualities: polar caps, seasons, clouds, a day that lasts roughly twenty—four hours, and even, it seemed, oceans. On the basis of those observations, the contemporary English astronomer William Herschel speculated that life existed on Mars.
It wasn’t until the late nineteenth century, however, that the public became caught up in what quickly grew to be a frenzied discussion about the ways of martians. The dynamo behind the popular hysteria was a nineteenth-century American namedPercival Lowell. Lowell, a scion of a prominent Boston family, was a devotee of Asian culture and an accomplished amateur astronomer. The main evidence” Lowell offered for his speculations about life was an elaborate network of “canals” that had been observed and mapped by the Italian astronomer Giovanni Schiaparelli. Lowell suggested that intelligent martians had built the “canals” to transport water from the polar caps to the equatorial deserts. Other observers failed to see the waterworks, but the possibility of civilizations populated by martian little green men led to a torrent of writings about martian invasions, the potential colonization of Mars, and the threat of interplanetary wars.
In spite off Lowell’s claim to the contrary, little can be seen of Mars’s surface features through a telescope; the planet is just too small and too far away. The sightings of the canals proved to be imaginary, the result of too much striving to make out features at the limits of telescopic resolution. Scientific interpretation of the martian surface did not realistically begin before observations could be made from spacecraft. And for those hoping to confirm Lowell’s ideas, the first such images, obtained in the 1960s by NASA’s Mariner 4, were deeply disappointing. The small areas photographed showed no canals, no oceans, no oasis.
B T WATER ON MARS STILL SEEMED A REAL POSSIBILITY.
The Mariner 9 spacecraft revealed a complex surface geology: volcanoes, canyons, dry valleys, lava plains, and, most intriguingly, flood channels. The dis-covery of the flood channels led to tantalizing visions of running water-and it even went almost without saying that where water flows, there could be life. The data were returned to Earth in 1972, just as NASA was preparing the Viking missions. The discoveries were timely because the main emphasis of those missions was to search for life. Once again, however, the outcome was disappointing: Viking did not even find organic molecules suggestive of the presence of life on the planet’s surface—much less life itself.
After the Viking program, the pace of Mars exploration slowed. The focus shifted from the direct and rapid detection of life to acquiring a better understanding of the planet. That still meant looking for water, or at least for where it might have been. In the meantime, public attention drifted elsewhere, until two events renewed wider interest in Mars.
The first event was the announcement in 1996 that a meteorite from Mars contained evidence—possibly fossilized bacteria—suggestive of ancient life. The second event was the extraordinary success of NASA’S Pathfinder rover in 1997. The martian meteorite that caused such a fuss in 1996 is generally no longer considered to contain any fossils, and nonbiological explanations of the observed mineral formations now seems more appropriate. Yet the search for water—and life—on Mars has hardly been abandoned. The new convergence of spacecraft is proof enough of that, all of theii~ trying to help answer essentially the same questions that fired the imaginations of Herschel and Lowell: Has liquid water ever been abundant on the martian surface? And if so, has it enabled the planet to support life?
T HE GEOLOGY OF MARS IS A SPECTACLE TO BEHOLD.
The planet’s southern hemisphere bears the scarring of heavy bombardment by meteorites; the craters, much like the ones that pockmark the highlands of our Moon, clearly date to the era, sometime before 3.8 billion years ago, when all the bodies of the inner solar system were subject to heavy meteorite bombardment.
The northern martian hemisphere, however, is sparsely cratered, indicating that the old cratered surfacc there has been buried by younger materials. What are these materials? They could be volcanic, but Timothy J. Parker and his coworkers at NASA’s Jet Propulsion Laboratory in Pasadena, California, have speculated that they are sediments in what were once ocean basins. Their elevations are some three miles lower than those of the cratered southern uplands. Perhaps, then, the old, cratered surface is partly buried by marine sediments. But what exactly caused the northern depression is unknown.
Straddling the boundary between the northern plains and the southern highlands is Tharsis, a broad dome more than 3,600 miles across and more than six miles high at its center . The dome is comprised mostly of layers of volcanic rock, which can be seen in the walls of canyons on the dome’s eastern flank. On the Tharsis dome are several huge volcanoes, the largest being the 370-mile-wide, fourteen-mile-high Olympus Mons.
But Olympus Mons is not the most spectacular surface feature of the planet. That distinction probably belongs to the Valles Marineris, a system of interconnected canyons extending 2,000 miles east-ward from the summit of the Tharsis dome to a low region called the Chryse Planitia, which adjoins the northern plains. The canyons in the Valles Marineris are typically 120 miles across and between 3.7 and 6.2 miles deep. Their origin is unknown, but faults that radiate outward from Tharsis can clearly be seen in the canyon walls, suggesting that stresses caused by the Tharsis bulge may have fractured the crust and formed great rift valleys. Once the rift valleys formed, landslides and water erosion probably enlarged the rifts to create the canyons that we see today.
The canyons themselves are not entirely the product of erosion (as is, for instance, the Earth’s Grand Canyon). But they still preserve evidence of a wetter Mars. Layered deposits, which may be the remnants of sediments suspended in long—dry lakes, cover some of the floors of the canyons. Near the east end of the system of canyons are some areas of collapsed ground, out of which rise several huge, seemingly waterworn flood channels. Other flood channels emerge from the east end of
the canyons as well, possibly as a result of the sudden release of water from lakes within the canyons.
In the effort to explore Mars for water, one of the most perplexing and important issues to address is its climate. Today the planet is inhospitable to any life even resembling the life on Earth. The atmosphere, mostly carbon dioxide, is thin: its surface pressure is less than 1 percent that of the Earth’s atmosphere.
Such a thin blanket of air provides almost no greenhouse warming, so surface temperatures average -67 degrees Fahrenheit at the equator, and less than -l 03 degrees Fahrenheit at high latitudes. It is so cold that carbon dioxide condenses out of the atmosphere each winter to form thin but extensive polar caps. Their retreat each summer exposes water—ice caps more than a mile and a half thick. Geologists have known about that abundant ice since 1976, but liquid water must be quite rare.
The scarcity of liquid water on Mars today is not easy to square with the abundant evidence that large volumes of water flowed on the planet in the past. In addition to the channels left by large floods, dry valleys that appear to have been cut by slow— moving water also meander across much of the old cratered terrain in the southern highlands.
Dry river valleys also occur occasionally on younger surfaces, particularly on volcanoes. Their presence and their distribution on the surface strongly suggest that warm climatic conditions prevailed at times in the past, particularly early in martian history, when most of the valleys were formed. Perhaps early Mars had a thick atmosphere that was subsequently eroded by large impacts and by the solar wind, or was destroyed by chemical reactions with the surface.
But not everything on Mars conforms with that picture of a warmer, wetter planet in the past. Under warm, wet conditions, rocks weather to produce salts, such as carbonates, and hydrated minerals, such as clays. Those minerals have not been detected by the orbiters. Moreover, computer simulations suggest that the greenhouse effects of a carbon dioxide atmosphere could not have created a wet climate: no matter how thick it was, it could not have trapped enough solar energy to stabilize liquid water.
Yet geomorphologists insist that the evidence on the surface for running water is unequivocal. The salts and clays, they argue, must be hidden from view, and some factor must be missing from the computer simulations. Climatologists are equally adamant that warm conditions were unlikely, particularly in the planet’s early history. At that time the Sun’s energy output was likely to have been lower. But if Mars was never warm and wet, the prospects that some form of life once flourished
there become very dim.
T he need to study the history of water on Mars has heavily influenced NASA’s choice of landing sites. But mission scientists had to balance that objective with a large number of engineering criteria: a site’s altitude must be at least 0.8 miles below the martian “sea level,” so that there is enough air for the parachutes to work. The site must be low in latitude as well, so that solar panels can get the most intense sunlight possible. And it must not be too windy, too rocky, too dusty, too rough, or too cold at night.
Two landing sites were eventually chosen . The first, for Spirit, is on the floor of an ancient, ninety-five-mile-wide impact crater called Gusev. A broad, 540-mile-long channel known as Ma’adim Vallis cuts through the southern rim of Gusev and extends deep into the southern highlands. Within the crater a group of hills stands at the mouth of the channel, which could be the remnants of a former delta. If the channel was cut by water, the water must have pooled within Gusev before exiting slowly to the north, and much of the material displaced by water erosion would thus have settled out where the water pooled.
Windblown sediments, ash from a large volcano some 150 miles to the north, and lava eruptions within the Gusev crater itself may also have helped fill the crater. Layered deposits have been partly eroded by the \vind in sonic places, exposing an etched surface. Elsewhere, dunes are common. Sedinients deposited by the water may also have been brought to the surface by the meteorite impacts that gave rise to the many craters visible today . If Spirit can find such materials, they would help show whether a lake once existed within the Gusev crater, and tinder what conditions the sediments were deposited. The size of the particles, their shape, their comp -osition, variations from layer to layer, and the presence or absence of a cement will all provide clues to answer such questions as: What were the climatic conditions when Ma’adim Vallis was cut? What is the composition of the highland rocks? Was the early martian climate ever really warm and wet?
T he second landing site, for Opportunity, is in Meridiatii Planum, which lies on the side , opposite the Guscv crater. The Meridiani site represents a different line of attack in the search for water----a mineralogical rather than a geomorphological approach. Gray hematite, an iron—bearing mineral that normally, but not exclusively, forms in a wet environment, was detected there by the orbiting Mars Global .Survcyor.
The hematite lies in the uppermost layer of a geologically complicated region. That top layer is part of a series of layered deposits partly overlying the ancient cratered surface, which has been cut by river valleys. The hematite layer appears to be thin, and the underlying layers poke through it to the surface in many places. The rover Opportunity should be able to sample both the hematite—bearing layer and the layers below.. It may also be able to collect samples of the ancient cratered surface, because meteorite impacts may have excavated stich material and thrown it into the site.
How the hematite was deposited presents an intriguing puzzle . It is unlikely that the layers of sediment formed in a lake, because no basin is present. Instead, they were probably deposited from the air, perhaps as volcanic ash. The hematite could have formed from iron—rich materials in the on final layers of sediment, or it could have been deposited from iron—rich water percolating through the sediments. The rover Opportunity will seek to determine how the layers were laid
down, and look for evidence of water from hot springs, which could arise out of local volcanic warming. On Earth, such springs, as in Yellowstone, commonly support hardy organisms . Perhaps they did on Mars, too.
O perating the rovers on Mars will be a demanding task for those of us who control them from Earth. Every day will begin with an assessment of the data from the preceding day. We’ll interpret new images or spectra and determine the new position of the rover as quickly as possible. Then, within two hours of receiving the new data, all the project scientists will meet to discuss the data and what to do next: Shall we do more analyses on the rock we examined yesterday? Shall we get a more detailed look at the cliff a hundred yards away? Shall we move to a new location, and if so, where?
After settling on the broad plan, the various scientific groups—chemists, geologists, mineralogists, and the like—will disperse to draw up a wish list of observations. Two hours later we’ll reconvene, reconcile our differences, and make a plan roughly consistent with the resources available: time, power, data bits for transmission, and so forth.
A long and tedious process translates the plan into specific commands that are finally sent to the spacecraft. The rover will carry out the program, and the next day the process will start all over again.
The availability of solar power limits each rover’s lifetime to just a few months. The goal is for each to travel at least 600 yards, but the actual distance will depend on the site, the ease of travel on it, and the scientific interest of the terrain around d the landing site. At this stage we can only hope we have chosen the sites wisely. But if we have, and if our good fortune continues, in just a few more months some of our questions about Mars’s ancient past will be answered, and we will have a better understanding of the role of water in the evolution of the planet.
Cover Story December, 2003
Volume 112 - Number 10. (Pgs. 32-38)
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