An even more compelling question is whether indigenous life ever took root on Mars, as many suspect but cannot prove.
"If you look at the surface of Mars today, it's a desolate place. It's dry. It's cold. It's barren," said Cornell University astronomer Steven Squyres, who heads the science teams for two NASA rovers scheduled to land on Mars beginning next month. "It's not an inviting environment for life, and yet we see these tantalizing clues," he said.
The unprecedented barrage of spacecraft is due partly to the relatively close distance between Earth and Mars, an alignment that occurs every two years. This year's lineup is particularly favorable, with Earth and Mars about 35 million miles apart -- as close as they have been in more than 60,000 years. At that distance, spacecraft can be dispatched from Earth using less powerful and less expensive boosters.
No one expects everything to go perfectly, however.
"Mars has been a most daunting destination," said NASA's associate administrator for space science Ed Weiler. "Some -- including me -- have called it 'The Death Planet.'"
NASA would know. The twin Mars Exploration Rovers, named Spirit and Opportunity, follow the loss of the Mars Polar Lander and the Mars Climate Orbiter. Two other probes, however -- the Global Surveyor and the Mars Odyssey -- have been returning compelling pictures and data about geologic features and other attributes of Mars.
Europe defied the odds this week, successfully putting its Mars Express spacecraft in orbit for a planned two-year mission to look for underground water. The spacecraft, which arrived Dec. 24, is designed to use radar to probe 2-1/2 miles beneath the planet's surface. In addition, Mars Express has a high-resolution stereo camera to map the planet's surface.
The fate of Beagle 2, the small companion rover, dispatched for an independent mission and scheduled to make contact on Christmas day, was unknown.
Japan had hoped to join the international science fest at Mars, but could not fix a problem with its spacecraft's power system to slip into orbit. Nozomi, which means hope, joins a long list of failed Mars probes that represent two-thirds of all spacecraft launched to the planet.
Even more difficult than settling into orbit is landing on the planet, with only three successful missions to date: two Viking landers sent by NASA in July and September 1976 and the Mars Pathfinder mission, which included the pint-sized Sojourner rover, in 1997.
NASA's next attempt comes Jan. 3, when Spirit is scheduled to make its high-speed descent to the planet's surface. If it survives the risky parachute descent and airbag-cushioned landing, Spirit is expected to spend about three months combing the dry lakebeds in Gusev Crater, a huge impact basin that once might have held a lake formed by water that flowed down from the highlands. The rover will search for sediments that could hold geologic evidence of past water.
The rover's twin, Opportunity, is slated to arrive Jan. 24, landing at an unusual mineral concentration on the equatorial Meridiani Plain. The area contains large quantities of a mineral that on Earth forms almost always in the presence of liquid water. The iron oxide, called hematite, was detected by the Mars Global Surveyor.
The goal of the missions, Weiler said, is to answer questions about whether the planet's surface was ever suitable for life.
"If successful," he said, "Spirit and Opportunity will help humans take a giant leap forward in our understanding of Mars' potential for past or current life."
Irene Mona Klotz covers aviation and space for UPI Science News. E-mail firstname.lastname@example.org