Mars Exploration Rovers - Mars: The Water Trail
Thirty-eight years ago, on the eve of the first spacecraft flyby of Mars, everything we knew about the Red Planet was based on what sparse details could be gleaned by peering at it from telescopes on Earth. Since the early 1900s, popular culture had been enlivened by the notion of a habitable neighboring world crisscrossed by canals and, possibly, inhabited by advanced lifeforms that might have built them -- whether friendly or not. Astronomers were highly skeptical about the canals, which looked more dubious the closer they looked. About the only hard information they had on Mars was that they could see it had seasons with ice caps that waxed and waned, along with seasonally changing surface markings. By breaking down the light from Mars into colors, they learned that its atmosphere was thin and dominated by an unbreathable gas known as carbon dioxide.
The past four decades have completely revolutionized that view. First, hopes of a lush, Earth-like world were deflated when Mariner 4's flyby in 1965 revealed large impact craters, not unlike those on Earth's barren, lifeless Moon. Those holding out for martians were further discouraged when NASA's two Viking landers were sent to the surface in 1976 equipped with a suite of chemistry experiments that turned up no con-clusive sign of biological activity. Mars as we came to know it was cold, nearly airless and bombarded by hostile radiation from both the Sun and from deep space.
But along the way since then, new possibilities of a more hospitable martian past have emerged. Mars is a much more complex body than Earth's Moon. Scientists scrutiniz-ing pictures from the Viking orbiters have detected potential signs of an ancient coast-line that may have marked the edges of a long-lost sea. Today's Mars Global Surveyor and Mars Odyssey orbiters have revealed many features that strongly appear to have been shaped by running water that has since disappeared, perhaps buried as layers of ice just under the planet's surface.
Although it appears unlikely that complex organisms similar to Earth's could have exist-ed in any recent time on Mars' comparatively hostile surface, scientists are intrigued by the possibility that life in some form, perhaps very simple microbes, may have gained a foothold in ancient times when Mars may have been warmer and wetter. It is not unthinkable that life in some form could persist today in underground springs warmed by heat vents around smoldering volcanoes, or even beneath the thick ice caps. To investigate those possibilities, scientists must start by learning more about the history of water on Mars -- how much there was and when, in what form it existed, and how long it lasted.
One of the most promising ways to answer those questions is to look at the diverse clues that water has left on Mars. Besides the water-carved landforms visible for decades from orbiting spacecraft, many details of the story of water on the Red Planet are locked up in the rocks littered across its surface. Rocks are made up of building blocks known as minerals, each of which tells the story of how it came to be a part of a any given rock. Some types of minerals, for example, are known to form on Earth only submerged underwater, while others are profoundly altered when hot water runs through them, leaving behind residues. Up until now, it has been very difficult to get to know the minerals in martian rocks because we have not had the tools to unravel their mineralogies. By understanding Mars' rocks in a more complete manner, scientists can gain a better view into the history of liquid water on the planet. Like their prede-cessor mission, Mars Pathfinder, the Mars Exploration Rovers will pursue this goal by placing robotic geologists on the planet's surface -- ideally suited to "reading the rocks" to understand the still mysterious history of water, and even of life-friendly ancient environments.
Myths and Reality
Mars caught public fancy in the late 1870s, when Italian astronomer Giovanni Schiaparelli reported using a telescope to observe "canali," or channels, on Mars. A possible mistranslation of this word as "canals" may have fired the imagination of Percival Lowell, an American businessman with an interest in astronomy. Lowell founded an observatory in Arizona, where his observations of the Red Planet con-vinced him that the canals were dug by intelligent beings -- a view that he energetically promoted for many years.
By the turn of the last century, popular songs envisioned sending messages between worlds by way of huge signal mirrors. On the dark side, H.G. Wells' 1898 novel "The War of the Worlds" portrayed an invasion of Earth by technologically superior Martians desperate for water. In the early 1900s novelist Edgar Rice Burroughs, known for the "Tarzan" series, also entertained young readers with tales of adventures among the exotic inhabitants of Mars, which he called Barsoom.
Fact began to turn against such imaginings when the first robotic spacecraft were sent to Mars in the 1960s. Pictures from the 1965 flyby of Mariner 4 and the 1969 flybys of Mariner 6 and 7 showed a desolate world, pocked with impact craters similar to those seen on Earth's Moon. Mariner 9 arrived in 1971 to orbit Mars for the first time, but showed up just as an enormous dust storm was engulfing the entire planet. When the storm died down, Mariner 9 revealed a world that, while partly crater-pocked like Earth's Moon, was much more geologically complex, complete with gigantic canyons, volcanoes, dune fields and polar ice caps. This first wave of Mars exploration culmi-nated in the Viking mission, which sent two orbiters and two landers to the planet in 1975. The landers included a suite of experiments that conducted chemical tests in direct search of life. Most scientists interpreted the results of these tests as negative, deflating hopes of identifying another world on where life might be or have been wide-spread. However, Viking left a huge legacy of information about Mars that fed a hungry science community for two decades.
The science community had many other reasons for being interested in Mars, apart from the direct search for life; the next mission on the drawing boards concentrated on a study of the planet's geology and climate using advanced orbital reconnaissance. Over the next 20 years, however, new findings in laboratories on Earth came to change the way that scientists thought about life and Mars.
One was the 1996 announcement by a team from Stanford University and NASA's Johnson Space Center that a meteorite believed to have originated on Mars contained what might be the fossils of ancient bacteria. This rock and other likely Mars mete-orites discovered on several continents on Earth are believed to have been blasted off the Red Planet by asteroid or comet impacts. They are presently believed to have come from Mars because of gases trapped in them that unmistakably match the com-position of Mars' atmosphere as measured by the Viking landers. Many scientists questioned the conclusions of the team announcing the discovery of possible life in one martian meteorite, but if nothing else the mere presence of organic compounds in the meteorites increases the odds of life forming at an earlier time on a far wetter Mars.
Another development that shaped scientists' thinking was spectacular new findings on how and where life thrives on Earth. The fundamental requirements for life as we know it today are liquid water, organic compounds and an energy source for synthesiz-ing complex organic molecules. Beyond these basics, we do not yet understand the environmental and chemical evolution that leads to the origin of terrestrial life. But in recent years, it has become increasingly clear that life can thrive in settings much dif-ferent -- and more harsh -- from a tropical soup rich in organic nutrients.
In the 1980s and 1990s, biologists found that microbial life has an amazing flexibility for surviving in extreme environments -- niches that by turn are extraordinarily hot, or cold, or dry, or under immense pressures -- that would be completely inhospitable to humans or complex animals. Some scientists even concluded that life may have begun on Earth in heat vents far under the ocean's surface.
This in turn had its effect on how scientists thought about Mars. Martian life might not be so widespread that it would be readily found at the foot of a lander spacecraft, but it may have thrived billions of years ago in an underground thermal spring or other hos-pitable environment. Or it might still exist in some form in niches below the currently frigid, dry, windswept surface, perhaps entombed in ice or in liquid water aquifers. After years of studying pictures from the Viking orbiters, scientists gradually came to conclude that many features they saw suggested that Mars may have been warm and wet in an earlier era. And two currently operating orbiters -- Mars Global Surveyor and Mars Odyssey -- are giving scientists yet new insights into the planet. Global Surveyor's camera detected possible evidence for recent liquid water in a large num-ber of settings, while Odyssey's camera system has found large amounts of ice mixed in with Mars surface materials at high latitudes, as well as potential evidence of ancient snowpacks.
The Three Ages of Mars
Based on what they have learned from spacecraft missions, scientists view Mars as the "in-between" planet of the inner solar system. Small rocky planets such as Mercury and Earth's Moon apparently did not have enough internal heat to power vol-canoes or to drive the motion of tectonic plates, so their crusts grew cold and static rel-atively soon after they formed when the solar system condensed into planets about 4.6 billion years ago. Devoid of atmospheres, they are riddled with craters that are relics of impacts during a period of bombardment when the inner planets were sweeping up remnants of small rocky bodies that failed to "make it as planets" in the solar system's early times.
Earth and Venus, by contrast, are larger planets with substantial internal heat sources and significant atmospheres. Earth's surface is continually reshaped by tectonic plates sliding under and against each other and materials spouting forth from active volca-noes where plates are ripped apart. Both Earth and Venus have been paved over so recently that both lack any discernible record of cratering from the era of bombardment in the early solar system.
Mars appears to stand between those sets of worlds, on the basis of current yet evolv-ing knowledge. Like Earth and Venus, it possesses a myriad of volcanoes, although they probably did not remain active as long as counterparts on Earth and Venus. On Earth, a single "hot spot" or plume might form a chain of middling-sized islands such as the Hawaiian Islands as a tectonic plate slowly slides over it. On Mars there are apparently no such tectonic plates, at least as far as we know today, so when volca-noes formed in place they had the time to become much more enormous than the rapidly moving volcanoes on Earth. Overall Mars appears to be neither as dead as Mercury and our Moon, nor as active as Earth and Venus. As one scientist quips, "Mars is a warm corpse if not a fire-breathing dragon." Thanks to the ongoing obser-vations by the Global Surveyor and Odyssey orbiters, however, this view of Mars is still evolving.
Mars almost resembles two different worlds that have been glued together. From lati-tudes around the equator to the south are ancient highlands pockmarked with craters from the solar system's early era, yet riddled with channels that attest to the flow of water. The northern third of the planet, however, overall is sunken and much smoother at kilometer (mile) scales. There is as yet no general agreement on how the northern plains got to be that way. At one end of the spectrum is the theory that it is the floor of an ancient sea; at the other, the notion that it is merely the end product of innumerable lava flows. New theories are emerging thanks to the discoveries of Mars Odyssey, and some scientists believe a giant ice sheet may be buried under much of the relatively smooth northern plains. Many scientists suspect that some unusual internal process not yet fully understood may have caused the northern plains to sink to relatively low elevations in relation to the southern uplands.
Scientists today view Mars as having had three broad ages, each named for a geo-graphic area that typifies it:
Apart from that broad outline, there is lively debate and disagreement on the details of Mars' history. How wet was the planet, and how long ago? What eventually happened to all of the water? That is all a story that is still being written. In addition to studying the planet from above with orbiting spacecraft, NASA's Mars Exploration Program is putting robotic geologists on the surface in the form of instru-mented rovers. Both of the landing sites selected for the Mars Exploration Rovers show evidence of water activity in their past. The rovers Spirit and Opportunity will look at rocks to understand the types of minerals that they are made of, and hence the environments in which they formed. This, in turn, will offer clues about the environ-ment in which the rocks formed. Some types of rocks, for example, might be of types that form in running water, whereas others might be typical of the sediments that form on the beds of lakes.
Even if we ultimately learn that Mars never harbored life as we know it here on Earth, scientific exploration of the Red Planet can assist in understanding the history and evo-lution of life on our own home world. Much if not all of the evidence for the origin of life here on Earth has been obliterated by the incredible pace of weathering and global tectonics that have operated over billions of years. Mars, by comparison, is a compos-ite world with some regions that may have histories similar to Earth's crust, while others serve as a frozen gallery of the solar system's early days.
Thus, even if life never developed on Mars -- something that we cannot answer today -- scientific exploration of the planet may yield critical information unobtainable by any other means about the pre-biotic chemistry that led to life on Earth. Mars as a fossil graveyard of the chemical conditions that fostered life on Earth is an intriguing possibility.