Rediscovering Mars After 20 years of pondering the results of the first wave of Mars Exploration, researchers are using some of the latest remote sensing techniques to bring the Red Planet into sharper focus. --Raymond Ladbury

For Mars researchers who depend on complicated, expensive interplanetary probes for their investigations, data seem to come either in trickles or in floods. From 1977 to 1996, only two probes were launched toward Mars --- and both were unsuccessful. The delay was frustrating to researchers, because previous missions had revealed Mars to be a very strange planet. Indeed, at times, parts of Mars almost seemed to be from two different planets. Although most terrain in the south was rough, heavily cratered, and therefore ancient, most of the Northern Hemisphere had been resurfaced to a nearly billiard ball smoothness. This hemispheric dichotomy was accentuated by an average altitude difference of several kilometers between north and south. Superimposed on this background were some of the Solar System's highest volcanoes and some of its deepest craters.

Researchers waited a long time for another close-up look at the Red Planet. Now, they have had over two years to digest the data from the Pathfinder mission. Recently, the Mars Global Surveyor (MGS) also celebrated an anniversary --- one Martian year (687 days) in orbit. So, when researchers gathered in July at Caltech for the Fifth International Conference on Mars (informally, Mars V), they had a lot to discuss. Not surprisingly, although topics at the conference ranged from pole to pole and from core to ionosphere, much discussion was devoted to integrating the three most recent results from MGS into the body of previous work.

The most surprising of these new results came in March, when researchers published MGS magnetometer data revealing that although Mars now has little global magnetic field, some of the planet's oldest terrain exhibits nearly linear bands of highly magnetized material. (See PHYSICS TODAY, June, page 17.) The magnetometer data suggest that, in the past, Mars was much more active magnetically --- and probably geologically as well.

In May the team responsible for the Mars Orbiter Laser Altimeter (MOLA) on MGS released maps of Martian terrain¹ with vertical resolutions of 13 meters and horizontal resolutions of about 1°, (or 59 km at the planet's equator). In contrast, pre-MOLA data sets had vertical errors of several kilometers. The MOLA data's accuracy will improve until the mission ends in early 2001.

Then, at Mars V, researchers revealed a map of Mars's gravitational field made by inverting all of the tiny deviations in MGS's position and velocity, as monitored by the Deep Space Network. The gravity data complement those from the laser altimeter, not only revealing large hidden features (such as those buried under dust or lava flows), but also showing how Mars's lithosphere responds to large stresses from huge mountains and craters.

CLOUD-SHROUDED VOLCANOES in this portion of Mars's Tharsis region are among the Solar System's largest. Topographic data from the Mars Orbiter Laser Altimeter (MOLA) show that Tharsis consists of two broad rises. The massive lava flows associated with the volcano Alba Patera dominate the northern rise (not visible in this image). The southern rise includes most of the volcanoes in this image, along with an uplifted, ancient, cratered plateau that may have been raised partially by crustal buckling. Olympus Mons (upper left), at 22 km high and nearly 550 km wide, is a separate but probably related feature. (Courtesy of Mailin Space Science Systems/NASA.)

These three results have implications for nearly every aspect of Martian evolution and dynamics. Bruce Jakosky (University of Colorado at Boulder) is emphatic: "Between the magnetic and gravity data and the satellite altimetry, we may have the constraints we need in hand to characterize the physical evolution of Mars." Already, the gravity and altimetry data sets are casting many of Mars's topographic features in a subtly new light, while all three data sets have researchers rethinking the planet's early evolution.

Looking for answers
Even prior to the MGS and Pathfinder missions, researchers had not neglected Mars. In addition to over a century of astronomical observation, Mars, from 1965 to 1977, was the target of three flyby missions (Mariners 4, 6, and 7), three orbiters (Mariner 9 and Vikings 1, and 2) and two landers (Vikings 1 and 2). Researchers even have samples of Martian rocks --- meteorites whose trace element and isotopic compositions closely match those of samples measured by the Viking landers. Probably, these meteorites were blown off of Mars in large impacts and, by either serendipity or sheer numbers, found their way to Earth. Because of all this attention, researchers knew what questions they wanted answered in the current wave of exploration.

Of particular interest to them was the nature and origin --- or origins --- of Mars's hemispheric discontinuity. Past theories on the origins of these hemispheric differences included massive impacts, plate tectonics, and preferential recycling of Mars's northern crust by asymmetric convection in the mantle.

The nature of the Tharsis region, which contains some of the highest features on Mars, was also uncertain. Although volcanism was clearly evident in the peaks and lava flows that make up much of the province, some researchers speculated that uplift and crustal buckling could also have played a role in the region's formation. (See the figure on page 34.)

Perhaps the most significant controversies concerned the role of water in shaping the surface of Mars. Images from previous missions clearly showed channels and even huge canyons. However, conspicuously absent in most images were tributaries to these larger channels or any other evidence that the channels were fed by precipitation. Many researchers hoped that the additional resolution afforded by MOLA and the Mars Orbital Camera (MOC) might better characterize the role of water in the planet's distant past, and perhaps even what happened to that water.

Highs and lows
According to the MOLA team leader, David Smith (NASA's Goddard Space Flight Center in Greenbelt, Maryland), currently the MOLA data's horizontal resolution is about 0.25° and its vertical accuracy is about 5 m. (See the figure on page 35.) At this resolution, the northern plains, which previously looked flat and featureless over hundreds of kilometers, are just beginning to reveal some structure. The finer resolution is also assisting researchers in determining the nature and origin of previously known features.

For example, MOLA has confirmed that, on Mars, downhill generally means north, with the South Pole lying about six km higher than the North Pole --- equivalent to an average, pole-to-pole slope of 0.036°. As previously predicted by Smith and fellow MOLA team member Maria Zuber (MIT), most of this altitude asymmetry is attributable less to the planet's topography than to its shape. That is, one can characterize the asymmetry by a displacement of the planet's center of mass 2.986 km north of its geometric center. Although such a characterization says nothing about what could have caused such a bizarre shift, the MOLA team concludes that the hemispheric dichotomy most likely resulted from internal processes (such as plate tectonics or mantle convection). They reach this conclusion because neither MOLA's Northern Hemisphere topography nor the gravity data reveal evidence of impacts large enough to account for such differences.

MOLA's improved resolution also sheds light on the origin of the continent-sized Tharsis region, revealing evidence of uplift and lithospheric buckling in the south of Tharsis, as well as volcanism on a massive scale. Overall, the morphology of Tharsis superficially resembles the sorts of terrestrial features seen above large plumes from Earth's mantle.

In addition to looking for anomalies buried under the resurfaced Martian terrain, researchers are using the gravity data to derive information about the planet's crust and lithosphere by seeing how they respond to massive positive and negative loads. In general, when a stress is placed on a planet's surface, the crust and supporting lithosphere compensate to lessen the load --- thinning under positive loads like mountains and thickening (and perhaps pulling up heavier material from the mantle) under negative loads. The MGS gravity data show that the Martian crust is thicker in the south than in the north. They also show that although the southern lithosphere has almost completely compensated for its loads, the gravity data in the north are considerably rougher, possibly indicating that the northern lithosphere is colder, and hence more rigid. Indeed, there are several gravitational signals in the north that do not correspond to any identifiable topographic feature, perhaps indicating that several large impact basins lie buried under the now-smooth terrain.

Paleohydrology?
Even with the additional topographic and gravity data, questions about Mars's paleohydrology remain both puzzling and controversial. The MOLA topography and MOC images still reveal no evidence that most of the flood channels on Mars are fed by smaller tributaries as on Earth. Because this pattern is so different from that seen on Earth, the question of whether Mars was ever warmer, wetter, and more Earth-like remains one of the most significant unresolved issues in the planet's geologic history.

The MOLA team has identified three main basins that could have once been Martian oceans. Of these, the northern lowlands are by far the largest, and would have drained three-quarters of the planet. One tempting explanation for the flatness of the northern basin is that it could represent an ancient ocean bed, with any original relief filled in by sedimentation. The other two basins --- the 2000 km wide and 7 km deep Hellas, and the shallow basin in Argyre-Solis Planum --- would each have drained about an eighth of the planet.

Still, the question of whether there was ever any substantial volume of liquid water on Mars remains controversial. The MOLA team estimates that the only known reservoirs currently on the planet, its poles, contain a maximum 4.7 million km³ of water --- only about 5% of the amount in the Pacific Ocean alone. So, even with the additional precision of MOLA and MOC, the question of whether Mars was ever warmer and wetter and hence more hospitable to life remains for future missions to resolve.

MORE THAN MOUNTAINS. As the Mars Orbiter Laser Altimeter (MOLA) continues to fill in the topographic map of Mars, researchers' understanding of Mars may change qualitatively as well as quantitatively. This image of Tharsis and its environs was constructed from the latest data set, which has a horizontal resolution of 0.25° and a vertical resolution of 5 m. One of the most striking newly visible features in the image is the dendritic structure at about 30° north and 300° east, which resembles a terrestrial river delta. Moreover, the northern plains, which looked flat and featureless in previous data sets, are now beginning to reveal some fine structure. The new data set is also revealing interesting structure in other areas including the buried Utopia impact feature, leaving researchers to wonder what Mars will look like by the end of the MGS mission. (Courtesy of Goddard Space Flight Center/NASA.)

Making Mars make sense
The new wave of Martian exploration is also causing many researchers to rethink some aspects of the planet's origins. Because Mars is believed to have gradually coalesced from many much smaller bodies, it probably started out relatively homogeneous. However, the ratios of isotopes in the Martian meteorites suggest that early in its history, Mars, like Earth, became a differentiated body, with a mostly iron core, a rocky mantle, and a thin crust. Past studies have also determined several other global properties of Mars, including its rough composition, mass, size, and shape. The arrival of Pathfinder provided researchers with an opportunity to nail down another fundamental property of the planet --- its moment of inertia.² Using the shift in the planet's axis of rotation in the 20 years between the Viking and Pathfinder missions, researchers measured the rate of precession in the planet's rotation --- a value that then yielded the moment of inertia. For the probable range of compositions of the planet's core, the Pathfinder result implies a core radius of between 1300 and 2400 km. Even with this uncertainty, the added constraint of Mars's moment of inertia may imply that the planet formed from a greater diversity of materials than previously thought. Researchers have had difficulty developing a model for Mars's interior that both matches the Pathfinder moment of inertia and assumes an initial composition similar to those of the most common chondrites --- remnants from the beginning of the Solar System.³

In the absence of additional information, it will be difficult to model Mars's interior. The dimensions and compositions of Mars's core and mantle remain unknown. Nor do we know whether Mars has a core that is purely solid, purely liquid, or a solid inner core and a liquid outer core (like Earth's core). On Earth, such additional information could be derived from seismology. On Mars, which was found by the Viking 2 lander not to be very seismically active, resolving the planet's internal structure may be more problematic.

According to Caltech's David Stevenson, careful and prolonged observation of the behavior of Mars's rotation may reveal some information, such as whether the planet's core is solid or liquid.

One puzzling aspect of Mars's early history concerns the short life of its dynamo. The MGS magnetometer observations show that during the planet's first half billion years or so, the dynamo was sufficiently vigorous to strongly magnetize the planet's crust down to several tens of kilometers. Yet, there is little evidence of magnetization in terrain that formed later, suggesting that by that time the dynamo had largely shut down. What could have caused such a radical transformation? Stevenson speculates that a leading possibility could be the transformation of Mars's lithosphere from a regime of plate tectonics to a thick, insulating solid lid. In plate tectonics, cold lithosphere mixes with the mantle and promotes heat transfer from and convection in the core. A solid, unbroken lithosphere would inhibit heat transfer from the interior, and possibly bring an end to the dynamo. Thus, Mars's core and mantle could still be quite hot, even though a significant fraction of the planet's heat-producing radioactive elements would reside in the crust, where their heat can be readily conducted to the surface.

Next steps
Although the MGS mission will continue until 2001, attention is now turning toward the next two probes, the Mars Polar Lander (MPL), scheduled to land near Mars's South Pole in December, and the Mars Climate Observer (MCO), which was scheduled to arrive in Mars orbit on 23 September. MCO will relay back to Earth data about Mars's current climate, including temperatures, pressures, and the planet's water, dust, and carbon-dioxide cycles. It will also investigate how topography affects weather and climate and examine surface features for evidence of past climates.

Using the MOLA topography and MOC images of the South Pole (see the cover of this issue), researchers in late August targeted the MPL for a landing at 76° south latitude and 195° east longitude. MPL will image the terrain around Mars's South Pole from orbit and during its descent. Once on Mars, the lander will monitor local weather and look for evidence of water. The probe's camera will image its surroundings and minutely examine trenches dug by a robotic arm to look for evidence of annual layers. MPL will be accompanied in its descent by two separate experimental probes, dubbed Deep Space 2 (DS2). Slamming into the planet at 200 m/s and with the instrument portion of each probe penetrating up to 2 m into the polar soil, the two probes will relay 50 hours of data on soil temperatures, water content, and local weather. MIT's Zuber, for one, is hopeful that MPL and its DS2 companion probes will indeed find evidence of water, citing MOLA data and MOC images that seem to show features in the polar region exhibiting viscous creep and other characteristics of icy terrain.

NASA has an ambitious program of missions to Mars stretching well into the next decade. A lander and an orbiter are slated for 2001, and a series of missions beginning in 2003 or 2005 in collaboration with the European Space Agency are intended to return Martian rock samples to Earth by 2008. Other planned ESA projects include the Mars Express in 2003 and the Dynamo Micro-orbiter, a very low orbit probe intended to gather improved magnetic and gravity data. The Japanese spacecraft Nozomi is also scheduled to arrive at Mars in 2003.

Always in the background as the data rolls in is the possibility of an ultimate manned mission to the Red Planet. The nearly two-year mission, which might include stays on Mars of up to 90 days, would stretch technology --- and perhaps also human endurance for low-gravity and high-radiation environments --- to its limits. Launch windows for such a mission exist in 2007, 2009, 2011, and 2014. So, if the technologies now being evaluated prove viable and, most important, if the political will for sucha mission holds, the first decade or so of the new millennium could see humans standing in the shadow of Olympus Mons.

References
1. D. E. Smith et al., Science 284, 1495 (1999).
2. W. M. Folkner, C. F. Yoder, D. N. Yuan, E. M. Standish, R. A. Preston, Science 278, 1749 (1997).
3. C. M. Bertka, Y. Fei, Earth Planet Sci. Lett. 157, 79 (1998). C. M. Bertka, Y. Fei, Science 281, 1838 (1998).

RAYMOND LADBURY is a contributing editor to Physics Today.

© 1999 American Institute of Physics