Forgetting for a moment the two high-profile mission failures that recently marred NASA's Mars program, 1999 was actually a remarkable year for discovery about Mars, many leading planetary scientists say.
Despite the costly and embarrassing loss of the Mars Climate Orbiter, the Mars Polar Lander and the two
robotic microprobes that were to dispel much of the mystery of Mars' polar climate and terrain, the year has still been a productive one for Mars science.Evidence is steadily streaming in from the Mars Global Surveyor that the planet may have been underestimated. Far from being the dead, cold, simple planet it has often been thought of, Mars seems to have had a complex, dynamic past, with an active iron core that produced a strong magnetic field, and a surface covered with huge oceans and perhaps even hot springs.
Surveying the Mars Global Surveyor
In March the Mars Global Surveyor finally began its two-year imaging and mapping assignment. The mission was to begin in early 1998, but a handful of problems kept the satellite from its proper orbit for more than a year.
During the past nine months, though, the spacecraft returned more than 40,000 images, mapped the planet's topography, measured its gravity and magnetism, and sent back spectral information about the composition of material on the ground.
"The spacecraft really gave us a very comprehensive view of the planet," said Maria Zuber, a geophysicist at the Massachusetts Institute of Technology, and a member of the team that interprets data from the global surveyor's laser altimeter. The altimeter, which can accurately determine the elevation of any point on the surface within about 40 centimeters (16 inches), has been used to build detailed topography maps of the planet.
"We now have a very good idea of the thermal properties of the planet, its global magnetism, global topography and global gravitation," Zuber said.
Rebuilding the wayfaring observer
The surveyor's success has been all the more appreciated because it had been so long in coming; the spacecraft's five scientific instruments were replacements of devices that vanished with the $1 billion
Mars Observer in 1993. Controllers lost contact with that satellite just days before it was to have entered orbit around Mars. They have never discovered what went wrong.The global surveyor was essentially a stripped-down version of the observer. Built for $154 million, and launched with an additional $65 million, the surveyor flies copies of five of the observer's seven main instruments.
By the time the spacecraft began performing its duties, many of scientists and engineers who built instruments for the spacecraft had been working on them since the beginning of the Observer project in the early 1980s.
1999 will be remembered as "the year that we actually finally got there," said Mike Ravine, a member of the team that built spacecraft's camera that is our only eye on Mars. "After a decade and a half of work, a decade and a half of struggle... it felt really good to actually have it be there, where it was supposed to be, taking the images that it was made to take," he said.
The camera has both wide- and narrow-angle lenses, allowing it to take pictures of broad swaths of the planet, and to make isolated high-resolution images of small spots all over Mars. In the high-resolution images, a viewer can recognize surface features the size of automobiles.
Calling the imaging mission an "amazing success," Ravine said that the surveyor's pictures may keep scientists busy well into the next century. "When you think of the fact that the narrow-angle images are these little tiny postage stamps stuck all over Mars, assimilating that into some view of what it's actually telling us is going to take a number of years," he said.
Just one of the interesting characteristics that the camera has already revealed is the layering of dust and ice that cover the planet's poles, MIT's Zuber said. Almost 20 years ago, pictures from the Viking Orbiters showed large scale layers of ice and dust -- layers that appeared to be hundreds of meters thick. The Global surveyor showed the alternating layers were visible on the scales of less than 10 meters, which raised the intriguing and still unanswered question of whether the deposition of the ice and dust is occurring on an annual cycle.
While the images are the most immediately gratifying to the general public, many scientists argue that the real achievements have been in the data from the laser altimeter.
A northern ocean on Mars
Some of the most dramatic findings of the year came from the Mars Orbiter Laser Altimeter, said Allan Treiman, a planetary geologist at the Lunar and Planetary Institute in Houston. The altimeter revealed that Mars' northern lowlands are flatter than most scientists imagined, a measurement that seems to support the argument that there was once an ocean covering much of the planet's northern hemisphere.
"It really, truly looks like there really was a large standing body of water across the northern hemisphere," Treiman said. "That, I think, has really turned around the community's thinking about Mars."
While the idea is still controversial, much of the data from the Mars Orbiter Laser Altimeter does support the possibility of a
hemispheric ocean in the north. That was the main argument in a paper by planetary geologist James Head and a group of colleagues at Brown University, published earlier this month in the journal Science."It's so flat. It's so close to a level, or shape, that an ocean would have that it's hard to imagine anything else could possibly do that, except sedimentation in some kind of liquid," Treiman said.
The flatness of the northern lowlands was completely unexpected, Treiman said. Prior to the Global Surveyor, the martian topography was not well measured.
Previous measurements, made by the Viking orbiters in the early 1980s, were inaccurate, often by a few kilometers, said Sean Solomon of the Carnegie Institution of Washington's Department of Terrestrial Magnetism, and a member of the surveyor's laser altimeter team. This is because the Viking orbiters weren't well equipped to make accurate map-quality measurements.
Mars' ancient magnetic field
In April, leaders of the Global Surveyor's magnetometer team announced that while Mars does not have an active magnetic field, the spacecraft detected strong magnetic bands in the southern hemisphere.
The magnetic field experiment measured magnetic stripes of alternating polarity across much of the southern highlands. An average of 125 miles (200 km) wide, the longest of the stripes extended more than 1,250 miles (2000 kilometers) along an east-west line. Mars' magnetic characteristics were discussed in two papers published in the April 30 issue of Science. The lead authors were Jack Connerney and Mario Acuna of NASA's Goddard Space Flight Center.
The findings stirred up the scientific community's interpretation of Mars and renewed speculation that the planet may once have had an era of plate tectonic movement in the crust.
Similar magnetic striping occurs on Earth, but on much smaller scales. The feature is measured on the sea floor where hot rock from the planet's interior is slowly deposited as continental plates move apart.
Hot rock is magnetized by the Earth's magnetic field, and as the material cools, it becomes a frozen record of the planet's polarity.
But because the Earth's magnetic field reverses every few tens of thousands of years, new rock is magnetized in opposite directions on the same time scale. Thus, scientists find magnetic striping spreading along lines of sea floor. Once magnetized, rocks' polarity remains unchanged, as it is not heated above about 900 degrees F (500 C).
What happened on Mars to create such huge magnetic bands is unknown, but some scientists are looking into the possibility that some kind of plate tectonics may have once occurred on the planet.
Another explanation offered is volcanism. Perhaps deep in Mars' history, giant volcanic events occurred in which the crust cracked and thick bands of volcanic material intruded to form the broad magnetic stripes, Zuber suggested.
There is a great deal of guessing and argument about what caused these features. Whatever mechanism caused them, though, the features do indicate "that Mars once had a very vigorous magnetic field, that was shut off, very early in the planet's history," Zuber said.
The magnetic field is crucially important, because it tells scientists when the planet was active. A magnetic field is caused by convection of iron within a planet's hot interior. This circulation creates an internal dynamo that drives the magnetic field. That is tied up with questions about when and how Mars could have been the kind of warm wet place favorable to the development of life.
If Mars ever did have a thicker atmosphere that could have maintained temperatures where liquid water held temperatures, it was almost certainly at a time when the planet was volcanically active. Gasses, such as water vapor and carbon dioxide, would have been released from the planets interior during ages of active volcanism.
Furthermore, a magenetic field forms a kind of protective bubble around the planet, shielding any atmospheric gasses from the sun's intense charged particle wind. Unprotected by a magnetic field, water vapor and other atmospheric gasses would be rather quickly stripped away from the planet, Allen Treiman said. Thus, prior to this year, many people thought that Mars had likely always been a cold, dry place without a magnetic field that could have protected an atmosphere.
"Early on if (Mars) did have a very strong magnetic field, that fits together with an early environment that could have supported water on the surface," Treiman said.
How long such an environment could have been stable is also a key question.
Data from the Global Surveyor's magnetic fields experiment are helping scientists determine just when the planet's magnetic field shut down. When giant meteorites smashed into the planet, the impact would have heated rock to intense temperatures that would then have been magnetized by any magnetic field.
But in their analysis of the Surveyor's magnetic measurements, NASA's Connerney and Acuna found that two of Mars' largest and oldest craters -- the Hellas and Argyre basins, estimated to be about 4 billion years old -- lack any evidence of crustal magnetism.
This implies that Mars' magnetic field already died out as early as 4 billion years ago. The estimate throws a lot of established thinking about Mars on its head, Zuber said, because until recently, the predominant view of Mars was that it accreted cold, and only heated up in the interior fairly late in its evolution.
Evidence for extinct hot springs
One of the least-recognized instruments aboard the Global Surveyor is the Thermal Emission Spectrometer (TES), an instrument that measures the heat-retaining characteristics and spectral signature of various materials on the surface.
Scientists have been using it with a keen eye to searching for minerals that might be associated with water. The strategy is to find regions most likely to have supported life sometime in the planet's past.
The most important work yet, according to Phil Christensen, the instrument's principal investigator, is the discovery that a large dark region near the equator contains between 10 and 15 percent of a mineral called hematite - a form of iron oxide. Christensen, a geologist at Arizona State University, said the mineral could be evidence for hot springs that existed on the surface for thousands of years.
"It forms in a few unique environments," Christensen said. "Most of them involve hot water. It either precipitates out of water or it forms around hot springs and hydrothermal systems."
Christensen announced his research earlier this month at the American Geophysical Union's fall conference in San Francisco. It will be published in the Journal of Geophysical Research early next year.
"These (hematite minerals) would provide the first evidence that water was stable near the surface of Mars for a long period of time," he said in an interview. "While you can look at pictures and see evidence for river channels and other features, some of those channels could have formed in very catastrophic events. Maybe the water flowed across the surface for a few weeks. The minerals that we see take thousands of years if not longer to form."
Many scientists now agree that this hematite region, which covers an area hundreds of kilometers across (100 kilometer is 62 miles), is one of the most interesting landing sites for future Mars missions. It is actually one of the few prime landing sites for the Mars Surveyor 2001 mission.
The hematite work is only one of the many discoveries this year that came from the Thermal Emmission Spectrometer, Christensen said. During the year, about 20 scientific papers based on TES data were published, he said. Within the next few months, the TES team will be producing the first global maps of the composition of the martian surface.
The planet's anomolous gravity
Another
mapping "first" was achieved this year by the Global Surveyor's radio science team. Using extremely sensitive radio measurements that can detect minute changes in the spacecraft's speed, the group produced the first global maps of the planet's gravity.When the satellite passes above dense regions of the planet, the pull of gravity is ever-so-slightly greater than normal, causing the spacecraft to dip a bit in its orbit. This extra pull makes the craft speed up. Measuring this minute wobble in velocity, scientists mapped the strength of gravity over the surface of Mars. The results were announced in a paper written by David Smith and a group of colleagues, and published in the Oct. 1 issue of Science.
Gravity measurements alone tell very little, though. The real work lies in comparing the gravity and topography maps. The topography data says something about the surface, while the gravity measurements reveal a great deal about what lies beneath it. Comparing the two allows scientists to probe into the history of the planet's crust and the outermost layers of the interior.
"It allows us to say things about how thick the crust is, and about how mechanically strong the interior has been at different times," said Solomon, who has also been working on the gravity data, along with many others. The mechanical strength is related to the heat coming out of the interior. If a lot of heat is moving from the interior into the crust, the crust will be relatively soft and will respond to topographic loads such as large surface volcanoes or craters.
The weight of volcanoes would tend to push down on a weak crust, while the crust would rise against an area on the surface where an impact excavated millions of tons of rock. A cold or very thick crust would relax very little in response to changes in the surface.
During the past few months, Zuber and many others have been comparing the gravity and topography data and will publish the results in an upcoming issue of Science.
The year in review
While the Mars Global Surveyor has not provided the sort of quantum leap in the understanding of the Red Planet that the first orbiter, Mariner 9 delivered, the spacecraft has been extremely important, said Joseph Boyce, NASA's program scientists for Mars Exploration. Its contribution is this: it has refined what we know, revealing how much there is yet to learn about Mars, he said.
"From my perspective, and looking at the MGS (Mars Global Surveyor), there have been some discoveries that are just mind-boggling from the standpoint of observation. There's really no good way to explain them," he said. The best explanations about the magnetic striping, the possible existence of oceans, the apparent hematite region are all just educated guesses, Boyce said.
"The kind of fun, exciting thing is that a lot of what (the Mars Global Surveyor) tells is really very much unexplained. Scientists are going to have to go back now and really consider strongly what these data mean," Boyce said.
Whatever the final understanding that comes out of further study, the face of Mars has been changed by the Global Surveyor, said Solomon.
"Mars is turning out to be a wonderfully complex planet, more like the Earth than anything we appreciated," he said.
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