Northern Exposure

We’ve already seen water ice on Mars. NASA’s Phoenix lander will reach out and touch it.

The Phoenix lander (artist's conception) will use its robotic arm to dig into the Martian permafrost. (Corby Waste/JPL)
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All previous Mars landing missions have been dusty affairs. This could be the first one to make mud. After the arm collects the frozen samples, they’ll be placed in miniature ovens and heated for study. A suite of instruments (see “Land, Look, Dig, Cook,” p. 55) will inspect the soil and meltwater for organic molecules and other signs of biochemical activity. Ratios of hydrogen and deuterium (an isotope of hydrogen) should tell scientists whether the ice in the permafrost came from ancient groundwater or fell as rain. Meanwhile, a meteorology package provided by Canada will take weather readings; the pressure gauge comes from Finland, the wind sensor from Denmark. Phoenix’s cameras will inspect the shallow trench dug by the arm, looking for layering or variations in chemistry that would indicate whether liquid water existed at the site. The planet’s orbit and axial tilt change in cycles lasting tens of thousands to millions of years. That means there may have been epochs with warmer summers during which water persisted on or near the surface within the past 100,000 years. Phoenix will help scientists piece together that story.

The “nominal” mission—the length of time needed to achieve the major scientific goals—is three months. That’s how long the sun will stay high enough for the spacecraft to produce sufficient electrical power to run its robotic arm and shovel. Plans are to go through seven digging cycles, each lasting about eight Martian days, or sols (a Martian day is 37 minutes longer than an Earth day). By December, as the sun drops too low to keep the batteries charged, the spacecraft should begin dying. By the time the sun rises again in the Martian spring, the craft “may be buried up to its deck in carbon dioxide snow,” or perhaps frost, Smith says.

At the tucson operations center last November, things were fairly quiet. The spacecraft itself was still in a clean room at the Lockheed Martin Space Systems plant in Littleton, Colorado, where it was built. Here in Tucson a young engineer, Lori Harrison, was attaching a set of instruments called TEGA, for Thermal Evolved Gas Analyzer, to a full-size engineering test version of the lander sitting on a simulated Martian landscape. Better to discover any glitches with the instruments’ operation now instead of next year on the surface of Mars.

Smith showed me into a room equipped with computer consoles where data from the mission will be analyzed. Spread on a large table were glossy photos, blown up to the size of hall carpets, showing the Phoenix team’s first choice for a landing zone. They came courtesy of another NASA spacecraft, the Mars Reconnaissance Orbiter, whose most powerful camera, called HiRISE, was also built at the University of Arizona.

Smith is a Tucson native and has spent most of his career at this school, which has one of the best planetary science departments in the world. He led the team that built the camera for the Mars Pathfinder lander, which, with its little rover Sojourner, kicked off the modern era of Martian exploration in 1997. Since then Smith has had a hand in HiRISE and other Mars cameras developed at Arizona. He also was a co-investigator for the descent camera on the European Huygens probe, which in January 2005 returned broad panoramas of the surface of Saturn’s haze-shrouded moon Titan (see “219 Minutes on Titan,” Oct./Nov. 2005).

Not all his memories are happy. In 1999, Smith sat tensely watching monitors at JPL as the Mars Polar Lander, whose stereo lander camera his group had built, entered the atmosphere in preparation for a touchdown near the planet’s south polar icecap. It was never heard from again. “We just sat and sat, and it got quieter and quieter,” Smith recalled. Engineers later discovered a flaw in the spacecraft’s software that shut off the craft’s landing rocket, causing it to go into a free-fall high above the surface. Four years later, a British lander named Beagle also vanished on arrival—one of Smith’s devices was on that one too. “Getting to Mars is difficult,” he says slowly, leaning forward in his chair. “About 50 percent of the missions fail.”

That’s one reason the Phoenix team spent so much time scouting landing sites. The HiRISE pictures on the table show an essentially flat landscape with a pattern of cracks resembling polygons—in many places, polygons within polygons. It’s the kind of terrain seen in Earth’s polar permafrost, which is saturated with (frozen) water. Smith explained that the pattern, which repeats itself for thousands of miles at the northern latitudes where Phoenix will touch down, results from the expansion and contraction of ice.

There was something else in the pictures. Speckled on the polygons were irregular blobs. They looked pretty, like pebbles with a bluish sheen. Those, Smith explained, were boulders. How big? He compared them to the size of SUVs, like the ones in the parking lot outside. The boulders weren’t packed in; the density was more like a stadium parking lot an hour after the game ends. But there were still enough to pose a danger. “You land on one of those, it’s over,” Smith says.

That’s why, after much discussion, the Phoenix team abandoned their first-choice landing site and looked in other places, including a region north of a collapsed volcano called Alba Patera, the broadest mountain on the planet. One promising site—the current top pick for a landing zone—was dubbed Green Valley because the computerized maps were coded by boulder density, and green has the fewest boulders. By comparing the detailed HiRISE images to wider-angle infrared images taken from another orbiter, the scientists found that rocky terrain appeared warmer in infrared images taken in the morning (a boulder’s surface holds heat longer than sandy soil does). That helped speed up the process of scouting landing sites, since the infrared images cover larger areas of ground. The target landing zone is about 100 by 30 miles—the smallest footprint for which the scientists can accurately predict the spacecraft’s aim.

The name Phoenix comes from the mythical bird that periodically dies in fire, then arises reborn from the ashes. It’s an appropriate metaphor for this mission, some of whose parts originated with another spacecraft that died before reaching its goal. Phoenix’s Surface Stereoscopic Imager and the ovens for soil analysis are close copies of gadgets on the Mars Polar Lander, the spacecraft that crashed in 1999. The loss of that lander, which came during a nightmarish stretch of Mars program failures, led NASA to cancel another mission, the 2001 Mars Surveyor Lander, and stash its hardware, which had already been built, in a cold-storage clean room in Colorado. The basic structure of Phoenix, including its robotic arm, the camera on the arm, and the chemistry lab on the main deck, was recycled from the 2001 lander. That spacecraft was to have touched down near the equator in dusty soil. To accommodate the switch to hard permafrost, the Phoenix team put stronger bearings in the robotic arm joints, added the ice-cutting rasp, and beefed up the drive motors.

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