Tantalizing mars

No solar-system world excites both the general public and the planetary scientist as the fabled Red Planet. Positioned near the outer edge of the Sun's habitable zone or ecosphere, Mars is the only solar-system world (excluding Earth) on which solar heating alone can result in some non-frozen water. With visible seasonal changes, polar caps that swell in winter and shrink in summer, and a surface gravity about 40% that of the Earth, telescopic astronomers eagerly turned their sights on this enigmatic world.

Noted nineteenth-century astronomers, such as Giovanni Schiaparelli and Percival Lowell, detected linear markings on the planet's surface, at the very limit of visual observation. These observations spawned the legend of the Martian canals, as popularized by early science fiction authors such as H.G. Wells and Edgar Rice Burroughs. Not only might primitive life exist on Mars, but possibly even the remnants of a planet-wide civilization.

And so as the Space Age dawned, mission planners turned their attention toward this fascinating world. At an average distance of 1.52 AU

from the Sun, this world presents some challenges to mission planners. Energy-optimized trajectories to Mars are possible about every two years, when Earth and Mars are at their closest. But even during very close planet alignments, such missions would take 6-9 months.

This is a long time for a robotic spacecraft to function in the interplanetary environment. It is interesting to note that less than half of the robots targeting Mars have successfully completed their missions.

First to survive the rigors of launch, trans-Mars injection, the long interplanetary cruise, and the appetite of the "Great Galactic Ghoul'' that seems to claim so many Mars-bound craft, was America's Mariner 4. Launched in 1964, this craft flew by the Red Planet and photographed less than 1% of its surface. Alas, there were craters aplenty—but no canals! In 1969, Mariners 6 and 7 photographed Mars during their flybys and conducted atmospheric studies, confirming the findings of their earlier sistership. The canals were an optical illusion; technological extraterrestrials had not evolved on Mars.

But flybys could survey only a fraction of the planet's surface and it was necessary to conduct a long-term mapping survey from Mars-orbit. Mariner 9, the first successful Mars orbiting probe, was launched in 1971. Not only did this craft photograph essentially the entire surface of the Red Planet; it also provided high-resolution photos of Mars' tiny satellites Deimos and Phobos.

Mariner 9's orbit around Mars was quite eccentric, with a low point of about 500 kilometers and a high point of about 16,000 kilometers above Mars' surface. This ninth (and final) Mars Mariner is one of the most successful interplanetary missions to date. During its one-year operational lifetime, this 570-kilogram robot snapped more than 7,000 photos of Mars and its satellites. Planetary scientists studied Martian dust storms at close range and learned that wind-blown dust was a major conributor to seasonal changes on the Red Planet.

But some of Mariner 9's photos revealed a great diversity in Martian terrain. There were vast dormant volcanoes that dwarf Mount Everest, sinuous markings that resemble extinct river valleys. One enormous rift valley was charted—Valle Marineris is far larger than the Grand Canyon and Olduvai Gorge—it may have been visible to nineteenth-century terrestrial observers and been mistaken as a canal.

Visionaries planning future human habitation of Mars were especially inspired by at least one finding of Mariner 9. Clouds were observed over the planet's polar caps, and it was suggested that these were perhaps composed of water. Combined with moderately clement equatorial temperature readings during daylight hours in Martian summer, this data

FIGURE 5.2 The contributions made by the two Viking Landers are often overlooked during our present-day renaissance of Mars planetary exploration. (Courtesy NASA)

indicated that Mars might not be totally inhospitable to properly protected terrestrial life.

Mariner 9's observations of the thin Martian atmosphere were instrumental in planning the next US missions to the planet—probes that would touch down on the planet's surface and search for signs of life.

Suddenly, Mars was no longer a dull, moonlike world. For a while it seemed not impossible that at least simple life may be found there.

In an attempt to settle this issue, Vikings 1 and 2 arrived in Mars orbit in 1976. These craft each consisted of an orbiter plus a lander. For years, planetary scientists were treated to a plethora of photographs from both surface and orbital locations (Figure 5.2).

A series of photographs from the Viking orbiters revealed a mysterious structure in a region called Cydonia that resembled a sculpted human head. This "Great Face of Cydonia'' excited tremendous public interest in Mars exploration, even after higher resolution images from a later probe revealed that it is almost certainly a dust-covered dune in the Martian desert.

Although the life-detection experiments were inconclusive, the Viking landers surveyed Martian geology and operated as weather stations. The north polar cap contained frozen water, not only dry ice (frozen carbon dioxide) as many planetary scientists had suspected.

The Viking landers discovered that organic compounds—molecules based on carbon—were absent from the planet's surface layers. Perhaps Mars' atmosphere was the culprit. Much thinner than Earth's atmosphere, Mars' atmosphere admitted high-energy solar electromagnetic radiation that might destroy complex molecules like those required for known types of life to exist.

But what about the subsurface layers? Viking revealed the presence of permanently frozen subsurface water—or permafrost. Perhaps there were regions where underground lakes existed. Perhaps ancient Martian life had evolved on the planet's surface during the early evolution of the solar system and migrated undergound as conditions worsened.

Interest in Martian life reached near-fever pitch in the late 1990s after analysis ofa meteorite found in the Allen Hills ofAntarctica. The meteorite, called ALH84001 (Allen Hills, 1984 #001) was determined to have originated on Mars and revealed microscopic forms suggestive to many of fossilized life. Since the science of life detection (on another world!) is not well developed, NASA adopted an exploration strategy best described, as "follow the water.'' All known forms of life require some sort of water to survive. If life exists or existed on Mars, it is more likely to be found near water. To this end, a series of orbital missions and landed rovers began their exploration of Mars, searching for the elixir of life. These next-generation planetary explorers would land in a new fashion, with cushioned beach-ball-like balloons, and deploy rovers to explore regions distant from the relatively smooth landing sites—regions with very interesting geology where, just possibly, extinct or existing life might be found.

A NASA probe, the Mars Global Surveyor, was launched toward the Red Planet in 1996. As well as continuing the surface and atmosphere reconnaissance from orbit, this craft had the capacity to communicate with landed probes and could serve as a data relay to Earth.

The first of the new rovers was a NASA technology demonstrator called Pathfinder that carried the experimental rover Sojourner. This highly successful test of a new landing technique was flown in 1997, and not only demonstrated the utility ofairbag-cushioned Martian descent but also showed that rovers on the Martian surface could operate far from their mother ships. This mission also demonstrated the public's interest in further Mars exploration. The Pathfinder Website was extremely popular and gathered a record number of hits (for a government website!) during the surface phase of the mission.

Following the failed 1998 Mars Climate Orbiter and Mars Polar Lander missions, and the successful Mars Odyssey (2001) orbiter, NASA launched two more sophisticated rovers—Spirit and Opportunity. Arriving on Mars in early 2004, these rovers have ventured kilometers from their landing sites. They have returned thousands of images of Martian terrain features and have conducted microscopic analysis of geological features, some of which have been gathered by subsurface drilling. Some ofthe geological findings point to the almost certain ancient existence of widespread Martian oceans, and lakes.

While suffering more than their fair share of lost missions, the Russians (formerly the Soviet Union) also contributed a great deal to humanity's knowledge of Mars. The Mars 2 and Mars 3 orbiters (1971) returned numerous images of the planet's surface, allowing the construction of early topographic maps. A follow-on series of orbiters and landers, Mars 4, 5, 6 and 7 were largely unsuccessful and returned only a very limited data set. Two missions were launched to Mars' moon Phobos in 1998. The first was lost in transit, but the second successfully orbited Phobos before communications were lost.

The japanese Mars orbiter mission, Nozomi (1998), japan's first attempt to send a probe to Mars, also failed to reach the planet. And the Europeans' Mars Express (2003) is busily sending data back from Mars orbit, though its Beagle 2 Lander was lost. One significant bit of data from Mars Express was its discovery of atmospheric methane—which greatly increases the possibility of there being surviving, subsurface life somewhere on the planet.

Exploring another world is challenging indeed!

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