2001: A Mars Odyssey
NASA returns to Mars with an orbiting spacecraft that will search for water, past and present
An artist's impression of the Odyssey spacecraft in orbit around Mars includes the long boom of the gamma ray spectrometer and the thermal blanketing covering even the high-gain radio antenna dish.
On 7 April, a probe will set out for Mars that could be pivotal in the search for water--and life--on the red planet. The 2001 Mars Odyssey orbiter will also be attempting to erase memories of the failure of the last two NASA missions to that destination.
The Odyssey's mission is to map the planet's geology, paying particular attention to the role of water, both past and present. Recent results from the Mars Global Surveyor (MGS), in orbit since 1997, have brought interest in water there to fever pitch. Last June, pictures from the MGS indicated that brief flows of liquid water could have been present on the planet as recently as any time between yesterday and two million years ago. In December, the discovery of layered deposits of rock at multiple locations on the planet suggested sedimentation left by ancient lakes.
An artist's impression of the Odyssey spacecraft in orbit around Mars includes the long boom of the gamma ray spectrometer and the thermal blanketing covering even the high-gain radio antenna dish.
Mars is very dry, much drier than the most arid desert on Earth. But a consensus exists among planetary scientists that liquid once flowed on its surface. Dried-up river channels and flood plains are clearly visible in the photographs from the Viking missions of the 1970s. Could these river channels and flood plains have been carved by a mixture of dry ice and rocky debris? If they were formed by water instead, how long was the water present? Were infrequent flash floods provoked by volcanic eruption or meteor impact or did seas and lakes once cover the planet? Different theories exist to explain how Mars is what it is today, with very different implications for the two questions that are at the heart of the search for water there: did life arise on Mars and is it still there now?
To find some answers, the Odyssey will carry an infrared and a gamma ray spectrometer, plus a radiation detector. And to prevent a repeat of earlier losses, its design has been revised.
The hunt is on
One of three experiments on board, the THermal EMission Imaging System (Themis), should provide a wealth of information about the history of Martian water from its examination of the planet's present-day mineralogy.
"The beauty of mineralogy, versus looking at an image, is that in an image I can see a gully," explained the instrument's principal investigator, Philip Christensen of Arizona State University, Tempe. "But I don't know from looking at the image if that gully was carved in a very brief flash flood of water that ran for a week and evaporated or froze [or if it was carved out by a relatively long-lived river]."
Mineral deposits take thousands of years to form and finding minerals associated with water would indicate the presence of standing water for significant periods of time, said Stephen Saunders, the mission scientist at NASA's Jet Propulsion Laboratory (JPL), in Pasadena, Calif. Such water on Mars "must have contained a lot of dissolved mineral matter, just as water on Earth does. When it evaporated, it would have left that matter behind as salts. On Earth that process creates a whole variety of salts and we should be able to see that with Themis."
Themis is a successor to the Thermal Emission Spectrometer (TES) currently on board the Mars Global Surveyor. Both are infrared spectrometers (except that, unlike TES, Themis can also image in the visible wavelengths). Although a cold planet, Mars still emits infrared radiation. A lot of it fits an idealized black body curve for any radiating body at a given temperature. Researchers will use this curve to determine Martian temperatures from Themis data; but of greater interest is where the infrared spectrum deviates from the black body curve.
Each type of molecule has a characteristic infrared emission spectrum that modifies the black body curve. When different molecules are intermingled, as in the atmosphere or in most minerals, the emitted spectrum is a linear combination of their individual spectra. Observed spectra can be decomposed and compared against a library of laboratory spectra, so as to identify molecules and minerals.
In contrast, the X-ray spectrometer attached to the Sojourner rover of the 1996 Pathfinder lander could determine only which elements were present at the Pathfinder site, allowing considerable room for interpretation as to the actual compounds and minerals made out of those elements.
Swords into plowshares
Themis is constructed differently from the earlier TES instrument. Its ancestor is "actually a military M-16 rifle night scope," said Christensen [see illustration]. However, there were difficulties in adapting the military technology for space research--the electronics had to be redesigned and radiation hardened.
Themis outdoes TES in its spatial resolution. Whereas the earlier spectrometer averaged spectra over spots 3 km in diameter, Themis has a spatial resolution of 100 meters when imaging in the infrared. "Instead of [TES] seeing minerals on the kilometer scale, now we can see them at football field scale," said Christensen. "To a geologist, that makes a huge difference. You average a lot of rock types together when you're looking at a 3-km spot."
The new instrument also looks at a narrower bandwidth of infrared light. As the most fruitful region of the infrared spectrum has already been identified by TES, the Odyssey instrument will concentrate on that region by examining 10 spectral bands between 6.5 and 14.5 µm, in contrast to TES's 650-µm range.
Themis can also image in visible light with a resolution of 20 meters, providing a useful complement to the camera onboard the Global Surveyor, which sees details as small as 1.4 meters across. Mapping Mars at these high resolutions necessitates a narrow field of view; so the more cameras in orbit, the faster finely scaled maps can be produced.
The ability to detect water-related minerals at high resolution is important to Christensen, who would dearly like to get a better look at a number of interesting features on Mars. "TES found three sites on the equator that have an [iron-based] mineral called hematite, which is very indicative that there was water there. I believe these were probably ice-covered lakes," he said.
However, finding evidence of standing water is only one of Christensen's goals. He wants to understand the geology of the entire planet. "What we've seen with TES is that the Martian crust is composed of two types of volcanic rock," he said. The planet's northern and southern hemispheres look very different [see illustration]. The surface of the southern hemisphere is rugged and is covered with basalt--"the black stuff that comes out of volcanoes like those in Hawaii." The northern hemisphere is smooth and covered with a volcanic mineral called andesite. "On Earth it forms in relatively few places. It forms all the picture postcard volcanoes like Mount Fuji," continued Christensen. Andesite on Earth forms when rocks melt in the mantle in the presence of a small amount of water.
The surface of the northern hemisphere is also younger than in the south, as determined from the relatively fewer meteorite craters in the north. The implication is that "the [underlying] interior of Mars either changed over time or is different in the southern hemisphere versus the northern hemisphere." This information imposes tight constraints on scientists trying to model how the interior of Mars evolved.
Other minerals observed by TES also raise questions. "There's always talk about water everywhere on Mars, but we're seeing minerals that are totally unweathered...minerals like olivine," said Christensen. On most of the Earth, olivine weathers down to clay minerals within a few thousand years; but on Mars "we're seeing [rocks with olivine] that are supposedly a few billion years old and still pristine."
To Christensen, this indicates that the climate of Mars has been mostly cold and very dry throughout its history. "When some people...start thinking of lakes and oceans on Mars, they immediately imagine a clement planet with palm trees and beaches...but it was more like the Antarctic. If there was an ocean, it was probably ice covered. If you leave a volcanic rock sitting out in Antarctica and return a few thousand years later, it won't be weathered."
Surveying for the future
Themis will also be used to help evaluate future lander sites, in particular for NASA's rover missions, slated for 2003. Photographs can be deceptive--even at the MGS orbiter camera's 1.4-meter resolution, what appears to be a smooth plain can be filled with jagged rocks.
Saunders explained how Themis can help by measuring the thermal inertia of a site. Thermal inertia is a measure of how rapidly a surface adjusts to changes in ambient temperature. "Dust cools very quickly and rocks hold heat. The ratio of rocks to dust largely determines the thermal inertia of a spot." Measuring the temperature pattern over a possible landing site during the course of a sol (one Martian day) can yield an estimate of the number of exposed rocks and their size distribution--and hence reduce the likelihood of the lander coming down on a lethal jumble of boulders.
Prospecting for water
While Themis may point to where water once was, "with the Gamma Ray Spectrometer (GRS) [another on-board experiment], we might be able to see water distribution today," said Saunders. This experiment also includes a neutron spectrometer (NS) and a high-energy neutron detector (HEND). The GRS will be able to construct a map of the distribution of atomic elements in the Martian surface and will even be able to probe beneath the surface.
Accordingly, GRS will measure gamma rays emitted by atomic nuclei that have been struck by cosmic radiation, which is constantly bombarding Mars. Some of this radiation excites atomic nuclei in the surface. When an atomic nucleus returns to its normal, or ground, state, it emits a gamma ray with an energy characteristic of that element. Additionally, gamma radiation can penetrate the surface, and so the orbiting GRS can detect the elemental composition of the upper 1020 cm of Mars with a spatial resolution of about 300 km.
The neutron detectors included in the GRS experiment use a related phenomenon to dig even deeper--detecting hydrogen, an indicator of water, up to a meter below the surface. The incoming cosmic radiation also creates fast neutrons as a byproduct of its interaction with nuclei. These fast neutrons then strike other nuclei and are slowed in the process. The mass of the hydrogen atom and the mass of a neutron are very close and hence the neutron is slowed considerably by hydrogen--it becomes a low-energy, thermal neutron. The ratio of fast to thermal neutrons from the surface indicates how much hydrogen is present.
William V. Boynton at the University of Arizona's Lunar and Planetary Laboratory in Tucson is the principal investigator for the GRS. "The search for water is really just one part of the investigation...we're sensitive to a wide range of elements, including the major rock-forming elements like iron, magnesium, aluminum, and silicon, as well as trace elements like potassium, thorium, and chlorine."
The same cosmic radiation that excites Martian atoms to emit gamma rays will also bathe the Odyssey. Hence the GRS itself has been mounted on the end of a 6-meter-long boom to prevent it reacting to the elements that compose the spacecraft. The heart of the spectrometer is a gamma ray detector consisting of a high-purity 1.2-kg germanium single crystal [see illustration]. About 3000 V is placed across the crystal, but no charge flows through it until a gamma ray strikes it, inducing a current through the crystal proportional to the energy of the incoming gamma ray. To reduce noise, the crystal will be cooled to about 90 K by passively radiating heat to black space (space without a radiating celestial body, such as the Sun, in it). Because the GRS will have to travel for several months through space before it reaches Mars orbit, the device may be damaged by radiation on the way. If so, the gamma ray detector can be heated to 100 ºC in an attempt to anneal any flaws in the crystal. "It will depend on how much of a radiation dose we receive and how resistant to damage the detector is. It's a risky proposition to heat up the crystal. We won't do it unless [the crystal] has deteriorated quite badly," said Boynton.
Of particular importance is GRS's ability to detect chlorine, because if there was ever an ocean on Mars, it would have left a strong chlorine signature. Chlorine is easily leached from rocks and would accumulate in any ocean, as on Earth. Many scientists think they may already know the location of an ancient seafloor in the Red Planet's northern hemisphere. This part of the Martian globe is mostly a vast, generally uniform, lowland plain, in sharp contrast to the rugged highlands of the southern hemisphere. It is reminiscent of how Earth would appear if its water were removed--vast smooth ocean basins, separated by rough continental highlands. Tantalizing evidence, in photographs taken by the Mars Global Surveyor, for a shoreline at the edge of the northern plains has bolstered this theory.
The inclusion of the GRS is fortuitous, given the current heightened interest in locating present-day Martian water. A similar experiment was flown on the Mars Observer, which disappeared on its way to Mars in 1993. NASA committed itself to completing the science of that mission, albeit in a piecemeal fashion better suited to the space agency's new "better, faster, cheaper" mantra. "The GRS was selected long before we decided that life and climate and resources and water were the main themes of Mars exploration," explained Saunders.
Life on Mars
But it is the search for water that may be present on Mars today that has become the key driver of exploration. The evidence for recent flows of water from the Mars Orbiter Camera is startling. Explained Malcolm Walter of Australia's Macquarie University in Sydney, New South Wales, and author of The Search for Life on Mars: "Liquid water is not stable on the surface of Mars at the present because of low temperatures and very low atmospheric pressure--it would simply evaporate." The evidence suggests to Walter that aquifers exist under the surface of Mars, trapped below a layer of permafrost and kept liquid by the internal heat of the planet. Ultimately, the newly discovered outflow gullies must be examined directly, along with their sediments, "for some evidence of life, life that might be quite widespread in the underground aquifers and might still come to the surface from time to time," he said.
Measuring radiation, near and far
The third experiment on board the Odyssey is the Mars Radiation Environment Experiment, or Marie. A sensitive solid-state ionizing radiation detector, it will provide critical information for any human mission to Mars.
"Marie is the highest-priority experiment for NASA's Human Exploration and Development of Space (HEDS) group," explained Saunders. "We haven't really measured this kind of radiation outside the Earth-Moon environment. If you're near Earth, you're still protected by the Earth's [magnetic field], so we want to measure the radiation environment on the way to Mars, [both] in deep space and in Mars orbit."
The goal is to determine if this is another problem to solve before sending humans to Mars. Astronauts traveling to Mars will have to bring adequate radiation shielding. However, shielding, by its very nature, is massive. With current rocket technologies, every additional kilogram brought to Mars multiplies itself many times over in the initial vehicle launch mass from Earth, because of the need for more fuel and engine and tank mass.
Originally several HEDS-related experiments were to have been flown to Mars on a companion mission to the Odyssey. This was to have been a lander equipped with a Sojourner-style rover [see "The Mission That Wasn't"] and included a second Marie experiment for simultaneous measurement of the radiation environment on the ground and in orbit.
"We really wanted to measure at the surface and in orbit at the same time, say during a solar storm," noted Saunders, but "a lot of people believe you can model the radiation on the surface and do it just about as well as if you measured it."
He regrets the cancelation of the lander following the loss of the Mars Climate Observer (MCO) in 1999 and the Mars Polar Lander (MPL) in 2000. "I think people just lost faith in our ability to land. I'm a firm believer that we fixed all the problems that could have occurred with the lander...I think it was a loss of will...the only thing that changed was a feeling that we had to be utterly safe."
Illustration: William Boynton
A Fishing Rod for Gamma Rays: The detector head of the Gamma Ray Spectrometer contains a 1.2-kg germanium crystal [not shown], which is cooled to 90 K in operation. To prevent it from reacting to the elements in the Odyssey spacecraft itself, the head will be extended on a 6-meter boom.
Building a better mission
George Pace is the Odyssey project manager at JPL, and he, too, talked about the effect of the twin failures on the Odyssey mission. "Right after the MCO failure, we tried to get out in front of the issues. Frankly, a couple of us were real shook up that one number--one misplaced number--could lose a whole mission." A mismatch between imperial and SI units caused a navigation error that resulted in the MCO burning up in the atmosphere when it arrived at Mars [see"Why the Mars Probe went off course," IEEE Spectrum, December, 1999, pp. 3439].
Accordingly, Pace and his team went through the entire mission plan, which has about 200 000 parameters in it. They identified 10 percent of these as mission critical, where an error would seriously jeopardize the mission. "We've spent a lot of time looking at those as well as doing the units and subsystem handovers correctly."
A headache for Pace is that much of the aerospace industry still uses imperial units instead of the SI units NASA uses. To guard against a repeat disaster, "in all of our documentation where we have an interface between systems, we've put both sets of units on there, so there's no mistake."
While Pace's team was performing an internal review, NASA assembled a so-called red team composed of about 50 industry experts to perform its own series of evaluations. "They generated some 144 action items for us, many of which we were already working on, but now it was formalized, we had to answer them." The action items covered every aspect of the spacecraft, its instruments, and of mission control.
The biggest design change made was in the Odyssey's main propulsion system, used to perform course corrections and bring the probe into orbit around Mars. The Odyssey uses a hypergolic system, which means that once the fuel and the oxidizer combine, combustion occurs spontaneously, without the need for an additional source of ignition. "The [original] design didn't have isolation between the fuel and oxidizer pressurant line once [a valve at the combustion chamber] opened. Once you opened those valves right at Mars orbit insertion, there was a potential for the fuel and oxidizer mixing in the lines back above the tanks," worried Pace. If this occurred, at the very least, the fuel line would rupture and prevent the engine from firing, killing the mission. "The system was all welded up, but we went back to put in some check valves," said Pace. In addition, a lot of software and flight sequence changes were made that allow for a much larger margin to recover from a fault.
Mars or bust
Although there has been much greater scrutiny of the mission by NASA, and all concerned have had to cope with numerous additional reviews and other administrative hurdles, they feel there have been benefits. Christensen explained: "In many ways, things got better because we went from a very cost-constrained way of doing things, where people would say 'It would be nice to do, but we can't afford that' to 'Tell us how much it costs and just go do it'...It's not a blank check, but it's a different environment, where instead of cost being the most important thing, success is now the most important thing."
Boynton has had a similar experience. "With the MPL, we were always squeezed for funding and so forth. Here, in the last year or so, if we needed something to fix a problem, they were there with the money. We still ended up building the instrument for pretty much the same price we said we could build it for, though."
Canceling the lander did little to reduce the cost of the overall mission. "The lander was essentially all finished...all the boxes were built and integrated and, in fact, it's sitting in the bay at Lockheed Martin. We've been asked to store it for a couple of years [for possible future use]," said Pace.
One vestige of the lander mission remains aboard Odyssey. It is equipped with a UHF radio, which was intended to act as a relay station for the lander to send data back to Earth [see illustration]. Pace explains why the relay capability has been retained. "There are two future missions we're interested in--the European Beagle lander, and the NASA rovers in 2003." Although these landers can transmit directly to Earth, "we can really enhance their data return...with a short link up to an orbiter, you can pipe up a lot more data." The Odyssey will be able to relay "about 5 megabits per sol."
Future missions are far from foremost in Pace's mind: he is concerned about getting the Odyssey into its mapping orbit in one piece. The probe will burn its engine to drop into a highly elliptical orbit around Mars and will then use aerobraking--skimming through the top of the Martian atmosphere to reduce speed--to bring it into its final orbit over a period of 70 to 80 days.
An earlier design relied on aerocapture, where a protective housing is used so that the orbiter can go much deeper in the atmosphere. It has the advantage of bringing the spacecraft into the desired orbit faster. It was too expensive, however, "so we went back to the MCO design," said Pace. This required adding thermal blankets to shield spacecraft components because "many of the spacecraft elements are out in the [atmospheric] flow. We even blanket the high-gain antenna to protect it [unlike most spacecraft]."
Pace also noted a concern about the GRS boom, "If the boom doesn't deploy, that could kill the mission. You'd have this floppy thing that isn't where it's supposed to be, making it very hard to point the spacecraft in the right direction." However, he points out there has been a long history of successful operation of similar booms.
The basic science mission will allow the GRS to look at the whole planet passing through one complete cycle of seasons. A Mars year is 687 Earth days, but the GRS will not be able to map for that entire time. "We have some sun angle constraints in some portions of the mission, where the sun comes in the GRS's radiative cooler, so we can't operate the instrument," said Pace. For GRS to get a full Martian year of data will take 917 days, but it is hoped there will be enough propellant remaining afterwards to extend the mission and use Odyssey in conjunction with other simultaneous missions to Mars.
Although the shadow of the recent failed NASA missions to Mars looms over the Odyssey, the scientists and engineers have faith in the spacecraft and its mission. "We can't promise success because things can happen that even the best of people have overlooked," said Pace. "But I can say that all the people working on it have done their best to find everything they could and challenge every assumption and look at every number...a lot of people want this to work. It's another step in the exploration of Mars."
To Probe Further
For more about this mission and to follow its progress, visit the NASA Web site at http://mars.jpl.nasa.gov/2001/ It has links to the research teams responsible for individual instruments as well as to many mission documents. Results from the Mars Global Surveyor are available at http://mars.jpl.nasa.gov/mgs/