This question has been on the front burner of discussion with Mars scientists who have the arduous task of selecting a site where it is safe to land and yet is rich in rocks, layered terrain and other geologic features that will beckon a host of scientific inquiries and discoveries for the Mars Exploration Rover mission scheduled to launch in 2003.

Mars scientists all agree on one thing: the search is on for landing sites where water was once present on the surface of Mars. The science instruments on the rovers are all geared toward understaning if the planet was warmer and wetter in the past, and for how long. Answering these questions is important to understanding how Earth and Mars have differed in climate and geology throughout their development. Since water is key to living organisms, they also address the potential that life may have developed on Mars long ago.

Leading the Charge

As more than a hundred scientists gathered in study teams and burned the midnight oil over six months of intense calculations, Dr. Matt Golombek has overseen a lively but collegial process that has taken place.

As JPL's Mars Exploration Landing Site scientist, he looks after the selection process, carefully weighing the choices at hand. Scientists and engineers working with him have painstakingly narrowed the best places to land from 185 to four, and are now focused on selecting the final two.

"We want to go to sites with terrains that will challenge our minds but not the safety of the rovers," said Golombek, who was also project scientist on the Mars Pathfinder mission and selected its landing site.

Plainly speaking, he said, the science group has ruled out areas that are flat and safe but boring, and have homed in on sites that appear flat, safe and interesting. The site selection process is a convergence between engineers who know the capabilities and limitations of the machines they are sending to Mars, and scientists who can determine the scientific worth of the areas accessible to the spacecraft. Everyone, he said, is working toward that goal.

Narrowing the Options

Major constraints dramatically narrowed down the territory on Mars that could even be considered. The candidate regions chosen, each comprising an area about the size of Southern California, exist below a certain elevation to provide enough atmosphere for the lander's parachute to descend properly. The sites also sit in a largely equatorial latitudinal band where enough sunlight shines to keep the solar-powered rovers supplied with electricity. Areas dominated by steep slopes, such as ravines or crater walls, are ruled out as hazardous to the lander and rover.

Reducing the Risk to the Airbags

Next to be eliminated were areas with large rocks. A rock larger than about one-half meter high, or knee-high to most people, is too tall for safety reasons. If the landing airbag system bounced hard on a rock that size, the rock might protrude high enough inside the airbags to damage the lander. Shorter boulders are considered acceptable, because even in the event of a direct bounce on top of one, the rock would not be tall enough to impinge on the lander inside.

But using even the highest-resolution images available to search for sites dominated by right-size rocks, said Golombek, "you can't guarantee there won't be bigger rocks. You can't eliminate them." With vigilant study and deduction, however, "you can try to make smaller the probability of landing on one."

Beware of Stealthy Terrain and 'Foo-foo Dust'

Laser altimeters will gauge the lander's altitude during descent in order to fire the solid rockets and deploy the parachutes and airbags at the right time. For those measurements to be made, the landers must be targeted to areas where the altimeter's radar will bounce back from the surface. Ruled out as landing sites are so-called "stealth regions".

"Stealth regions" are locales on Mars where the radar penetrates the surface but doesn't bounce back - a characteristic these regions share with the military's radar-avoiding stealth technology. In the case of Stealth fighters and bombers, the aircraft surfaces are made of a high-tech, radar-absorbing material. In the case of Mars' "stealth regions," however, the answer isn't known, said Golombek. They may be covered with a meter or more of "foo-foo dust," a Dr. Seuss-like term that Golombek uses to describe possibly fluffy accumulations of Mars' fine iron-oxide dust particles that can pile up in drifts like red snow.

In addition, "sending a solar-powered spacecraft to a dusty spot isn't a good idea. The stuff gets on the solar panels and reduces the power, gets stuck in the wheels and gears and generally gunks up the works" Golombek said.

Rocks: Too Much of a Good Thing?

Sites with too many rocks of any size are not desirable either, because a densely populated rock field can create a treacherous obstacle course for a rover. "Too many rocks inhibit mobility, but then again, you're going there to look at the rocks," said Golombek, pointing out another area where safety and scientific appeal must compromise.

The site evaluation process started in September 2000 when Golombek and fellow scientist Tim Parker (also at JPL) identified nearly 200 possible landing sites that met the basic engineering constraints. Subsequent work and meetings have reduced that to four prime candidates and two backups. By May of 2002, a region measuring 600 by 900 kilometers will be selected - one for each rover. At that time, targeting data will be hardwired into the launch vehicles that will carry each rover . After launch, the two spacecraft will be more finely targeted during their cruises to Mars based on detailed navigation measurements taken on the way. At that time, the final landing boundary will be narrowed to a football-shaped ellipse of about 100 to 200 kilometers long by 20 kilometers wide.

Mars Global Surveyor, an orbiter currently at Mars, has provided global elevation data through its laser altimeter, surface temperature and mineralogical readings from the thermal emission spectrometer, and images from the camera. New data collected by these instruments will be used to better characterize the sites in coming months. In addition, the recently arrived 2001 Mars Odyssey orbiter will start taking routine scientific data in early 2002, which will also be used in determining the final two sites selected.

The Four Finalists and their Runners-Up

HEMATITE

"Hematite is a special place. It's one of three sites on Mars with detectable mineral signatures for coarse grained hematite." This type of Hematite generally forms in water, so "finding hematite is like finding a sign that says 'Water Was Here!'"

Not only does it rank high in scientific interest; Hematite measures high on the safety scale as well. Of the four sites, Golombek said, Hematite is very unique: "it's one of the smoothest, flattest, safest place in the equatorial region. All the other sites have good things about them and not-so-good things about them."

MELAS

The Melas region is a canyon with 10-kilometer high walls (6 miles high) that "make the Grand Canyon look insignificant," said Golombek. "There is a area at its very center that has interior deposits that look like some type of sedimentary rock. Did these rocks form in water, was there a lake there? Were the layers deposited by water? Are they due to wind erosion or some other process? It's a prime place to address very important questions."

Attractive though it is, said Golombek, Melas is surrounded by sand dunes. A bullseye in targeting would put the lander in fascinating terrain, but anything short of that could be disappointing.

GUSEV CRATER

"Gusev is perhaps the classic crater that looks like it was a crater lake," said Golombek.. "For all the world, it looks like a crater that filled with water, which at some point breached the crater wall and the water escaped. If this occurred, the crater should be filled with sediments deposited in the lake." And if the sediments are there, they were laid down in watery solutions that will provide valuable clues in the search for water's past on Mars.

The original landing ellipse considered for Gusev was found to contain some rough-looking terrain in Mars Global Surveyor data, so the ellipse was moved to gentler terrain slightly to the west.

ATHABASCA VALLES

Finally, there's Athabasca Valles in the Elysium Planitia, or the "Plains of Elysium." "It is one of the youngest outflow channels on Mars," said Golombek. "It's hundreds of kilometers long with a catastrophic outflow channel, kind of like Ares Valles where Pathfinder landed. Geologically, it's very young, just tens to hundreds of millions of years old." The channel has been worn by water and has young volcanics as well, making it a prime location to look for hydrothermal deposits.

Runners-Up

Two backup sites wait in the wings in case there are problems found with the other sites: Isidis Planitia and Eos Chasma [image link]. The former sits close to some of the oldest material exposed on Mars, near the rim of a giant impact basin. The area is expected to be rich in very old rocks and so may provide clues to the early environment and whether it was watery or not.

Telecommunications Constraints

Telecommunications constraints will bear on the selection of the final two sites. The two rovers will communicate via the same Deep Space Network and Mars orbiter spacecraft antennas, so the rovers must be separated by at least 36 degrees in latitude so there will be no telecommunications overlap between the two. If Hematite is chosen as one of the sites, it is located far enough away from the other sites that there would be no overlap, said Golombek.

Choosing the Right Targets

In April 2002, the third landing site workshop will meet in Pasadena to share any new scientific information gained about the top sites, and to discuss and evaluate the safety of the sites with mission engineers. From the discussions, two sites will be selected for landing the two Mars Exploration Rover spacecraft.

A Little Help from Orbiter Friends

"This is a unique period where we have orbital missions that can help us make the selection," he said. Mars Global Surveyor's continuing presence at Mars, now coupled with Mars Odyssey, provides unprecedented tools to gather targeted information down to 3-meter resolution - about the length of a small sedan -- to help scientists make the landing site selection.

Golombek compares today's comparative wealth of detailed data with the relative paucity of information he had in selecting Pathfinder's landing site in the mid-1990s. Studying images from the 1970s-era Viking mission, "we had a hundred meter resolution for the Pathfinder landing site. That's about the size of a football field. Now, we're directing the Mars orbiter camera on Surveyor to get pictures of landing sites at 3-meters resolution. Our data sets for Mars are so new and growing so quickly. It's a very dynamic, exciting time for Mars exploration."

Suitable for Human Landing?

Though no human exploration missions are planned for Mars yet, Golombek says the landing site selections could be driven by different constraints. "For future astronauts, water would be a prime resource," he said, noting that the hydrogen and oxygen in water could be a source for rocket fuel for a return trip to Earth. "There could be a completely different suite of constraints that could take you to completely different sites than we're considering right now," he said.

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