After Earth, Mars is the planet with the most hospitable climate in the solar system. So hospitable that it may once have harbored primitive, bacteria-like life. Outflow channels and other geologic features provide ample evidence that billions of years ago liquid water flowed on the surface of Mars. Although liquid water may still exist deep below the surface of Mars, currently the temperature is too low and the atmosphere too thin for liquid water to exist at the surface.
Mars, Water and Life
Why Explore Mars?
What caused the change in Mars' climate? Were the conditions necessary for life to originate ever present on Mars? Could there be bacteria in the subsurface alive today? These are the questions that lead us to explore Mars. The climate of Mars has obviously cooled dramatically. By studying the reasons for climate change on Mars, which lacks the complications of oceans, a biosphere, and industrial contaminants, we may begin to understand the forces driving climate change on Earth. As we begin to explore the universe and search for planets in other solar systems, we must first ask the question 'Did life occur on another planet in our own solar system?' and 'What are the minimal conditions necessary for the formation of life?'
What Are We Looking For?
The planet Mars landed in the middle of immense public attention on July 4, 1997, when Mars Pathfinder touched down on a windswept, rock-laden ancient flood plain. Two months later, Mars Global Surveyor went into orbit, sending back pictures of towering volcanoes and gaping chasms at resolutions never before seen.
In December 1998 and January 1999, another orbiter and lander were launched to Mars. And every 26 months over the next decade, when the alignment of Earth and Mars are suitable for launches, still more robotic spacecraft will join them at the red planet.
These spacecraft carry varied payloads, ranging from cameras and other sensors to rovers and robotic arms. Some of them have their roots in different NASA programs of science or technology development. But they all have the goal of understanding Mars better, primarily by delving into its geology, climate and history.
With the announcement in 1996 by a team of scientists that a meteorite believed to have come from Mars contained what might be the residue of ancient microbes, public interest became regalvanized by the possibility of past or present life there. The key to understanding whether life could have evolved on Mars, many scientists believe, is understanding the history of water on the planet.
Mars Exploration: Fundamental Questions
What is the meteorology and climate history of Mars?
- What are, and where are, the reservoirs of water and carbon dioxide on Mars?
- What is the process of climate change including behavior of the polar caps?
- What does the history of climate change on Mars tell us about Earth?
Has there ever been life on Mars?
- What is the evidence for, and timing of, warmer, wetter past conditions?
- Where is the evidence for past life likely to be found on Mars?
- How do we recognize evidence of past life and sample Mars properly?
What is the geology and inventory of resources on Mars?
- What is the interior structure of Mars and is the planet active today?
- What do the global topography and geologic structure tell about the planet's evolution?
- What are the global inventory and distribution of near surface materials and volatiles?
Should Mars be the next destination for human exploration?
Mars And Water
Mars perhaps first caught public fancy in the late 1870s, when Italian astronomer Giovanni Schiapparelli 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 convinced him that the canals were dug by intelligent beings - a view which he energetically promoted for many years.
By the turn of the century, popular songs told of sending messages between Earth and Mars 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 first flyby and orbiter missions showed a desolate world, pockmarked with craters like Earth's Moon. The first wave of Mars exploration culminated in the Viking mission, which sent two orbiters and two landers to the planet in 1975. The landers included experiments that conducted chemical tests in search of life. Most scientists interpreted the results of these tests as negative, deflating hopes of a world where life is widespread.
The science community had many other reasons for being interested in Mars apart from searching for life; the next mission on the drawing boards, Mars Observer, concentrated on a study of the planet's geology and climate. Over the next 20 years, however, new developments in studies 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, NASA's Johnson Space Center and Quebec's McGill University that a meteorite believed to have originated on Mars contained what might be the fossils of ancient microbes. This rock and other so-called Mars meteorites discovered on several continents on Earth are believed to have been blasted away from the red planet by asteroid or meteor impacts. They are thought to come from Mars because gases trapped in some of the rocks match the composition of Mars' atmosphere. Not all scientists agreed with the conclusions of the team announcing the discovery of fossils, but it reopened the issue of life on Mars.
Other developments that shaped scientists' thinking included new research on how and where life thrives on Earth. The fundamental requirements for life as we know it are liquid water, organic compounds and an energy source for synthesizing complex organic molecules. Beyond these basics, we do not yet understand the environmental and chemical evolution that leads to the origin of life. But in recent years it has become increasingly clear that life can thrive in settings much different from the longheld notion of 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. Life might not be so widespread that it would be found at the foot of a lander spacecraft, but it may have thrived billions of years ago in an underground thermal spring. Or it might still exist in some form in niches below the frigid, dry, windswept surface wherever there might be liquid water.
NASA scientists also began to rethink how to look for signs of past or current life on Mars. In this new view, the markers of life may well be so subtle that the range of test equipment required to detect it would be far too complicated to package onto a spacecraft. It made more sense to collect samples of Martian rock, soil and air to bring back to Earth, where they could be subjected to much more extensive laboratory testing with state-of-the-art equipment.
Mars and Water Mars today is too cold, with an atmosphere that is too thin, to support liquid water on its surface. Yet scientists who studied images from the Viking orbiters kept encountering features that appeared to be formed by flowing water - among them deep channels and canyons, and even features that appeared to be ancient lake shorelines. Added to this were more recent observations by Mars Pathfinder and Mars Global Surveyor which suggested widespread flowing water in the planet's past. Some scientists identified features which they believe appear to be carved by torrents of water with the force of 10,000 Mississippi Rivers.
There is no general agreement, however, on what form water took on the early Mars. Two competing views are currently popular in the science community. According to one theory, Mars was once much warmer and wetter, with a thicker atmosphere; it may well have boasted lakes or oceans, rivers and rain. According to the other theory, Mars was always cold, but water trapped as underground ice was periodically released when heating caused ice to melt and gush forth onto the surface.
In either case, the question of what happened to the water remains a mystery. Most scentists do not feel that Mars' climate change was necessarily caused by a cataclysmic event such as an asteroid impact that, perhaps, disturbed the planet's polar orientation or orbit. Many believe that the demise of flowing water on the surface could have resulted from gradual climate change over many millennia as the planet lost its atmosphere.
Under either the warmer-and-wetter or the always-cold scenario, Mars must have had a thicker atmosphere in order to support water that flowed on the surface even only occasionally. If the planet's atmosphere became thinner, liquid water would rapidly evaporate. Over time, carbon dioxide gas reacts with elements in rocks and becomes locked up as a kind of compound called a carbonate. What's left of Mars' atmosphere today is overwhelmingly carbon dioxide.
On Earth, shifting tectonic plates are continually plowing carbonates and other minerals under the surface; heated by magmas, carbon dioxide is released and spews forth in volcanic eruptions, replenishing the carbon dioxide in the atmosphere. Although Mars has no known active volcanoes and there are no signs of fresh lava flows, it had abundant volcanic activity in its past. However, Mars appears to have no tectonic plates, so a critical link in the process that leads to carbon dioxide replenishment in Earth's atmosphere is missing. In short, Mars' atmosphere could have been thinned out over many eons by entrapment of carbon dioxide in rocks across its surface.
That scenario, however, is just a theory. Regardless of the history and fate of the atmosphere, scientists also do not understand what happened to Mars' water. Some undoubtedly must have been lost to space. Water ice has been detected in the permanent cap at Mars' north pole, and may exist in the cap at the south pole. But much water is probably trapped under the surface - either as ice or, if near a heat source, possibly in liquid form well below the surface.
The Mars Surveyor 1998 Missions were designed, and their payloads selected, to address the science theme "Volatiles and Climate History" on Mars, thereby directly addressing the climate-history and resource themes of the Mars Surveyor Program, while supporting the life-on-Mars theme through characterization of climate change and its evolving impact on the distribution of water.
Mars Surveyor 98 Science Goals
Additional details here.
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