WASHINGTON -- Scientists see several ramifications arising from new observations that Mars may not be a dried-up and dead world after all. The most immediate issue confronting NASA is a decision to send future spacecraft to Mars to investigate what could be "watering holes" for life.

A Thursday morning NASA press conference is set to unveil the recent discovery of evidence for liquid water on Mars. Imagery taken by the Mars Global Surveyor is to be shown, suggestive of present-day sources of liquid water on Mars.

The Valles Marineri on Mars

Christopher Chyba, the Carl Sagan Chair for the Study of Life in the Universe at the SETI Institute Mountain View, California, said Martian springs, if they existed, "would be one of the most exciting discoveries in the history of solar-system exploration."

Chyba said that images of dried-up rivers on Mars indicate the presence of liquid water at the surface in the planet's past.

"One of the most exciting prospects, but until now little more than a hope, has been that there might still be places on Mars where liquid water reaches the surface. These Martian springs, if they existed on the surface, would immediately become the new focus of Mars exploration. They would be the first place to go to look for signs of life," Chyba told SPACE.com.

The Viking Lander exploring the Martian surface.

Chyba said, however, that lander missions to such locales must be carefully planned. "There is a need to avoid any possibility of introducing viable Earth microbes that might be able to live in Martian spring environments," he said.

Ron Greeley, a space geologist at Arizona State University, Tempe, said water is the crosscutting element for NASA's Mars exploration goals. Looking for signs of life, understanding the planet's climate and the use of local resources on the Red Planet are all part of a "follow the water" strategy, he said.

"But just because water is present, that doesn't mean there's life there," Greeley said. "Conversely, life as we understand it, requires waterthat's how it works," he said.

Promising worlds where liquid water might be close to the surface, Greeley said, include both Mars and Europa, a moon of Jupiter. Easy access to water makes a difference in sorting out exploration strategies and types of equipment to send via spacecraft.

Greeley is chair of a Mars Exploration Program Assessment Group, a group of about 50 scientists and engineers that provides help to NASA on future Mars planning.

In mid July, NASA is expected to pick between a Mars orbiter and a Mars lander craft to be launched in 2003. How new revelations about possible water being found on the planet might favor one spacecraft over the other remains unknown.

"I think everybody agrees that the next lander is going to work, period. So given that, it's also a fact that geologically interesting places are also difficult to get to," Greeley said.

Greeley warned that there "may be a little disconnect" between places that scientists would like to go versus sites where spacecraft can safely land," he said.

Should NASA dispatch a lander to a site found to contain water in a search for life?

"We should go there directly and look for living organisms," said Carol Stoker, a space scientist at NASA's Ames Research Center, Moffett Field, California.

The results of this type of search would be profound, whether they are positive or negative, Stoker said.

"It is difficult to imagine a cataclysm so extreme as to extinguish all life, even when liquid water is present. It would be important to know that Mars never developed life, even though Earth and Mars were climatically similar at the time of the origin of life on Earth. It would mean that life may be an extremely unique occurrence," Stoker said.