(click image for larger view) American Flag and mini-DVD attached to deck of Phoenix Mars Lander.

NASA researchers this week said Mars appears to have been more hospitable to life in the past than previously believed.

Using the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, on NASA's Mars Reconnaissance Orbiter, scientists have found carbonate minerals on the planet. Because carbonates dissolve rapidly in acid, their presence has forced researchers to revise conclusions based on previous evidence that the Martian environment was primarily acidic.

NASA says that its findings, published in the Dec. 19 issue of Science magazine, indicate that different types of watery environments existed, raising the chance that at least one may have supported life.

"Although we have not found the types of carbonate deposits which might have trapped an ancient atmosphere," said Bethany Ehlmann, lead author of the article and a spectrometer team member from Brown University, in a statement, "we have found evidence that not all of Mars experienced an intense, acidic weathering environment 3.5 billion years ago, as has been proposed. We've found at least one region that was potentially more hospitable to life."

While the likelihood that there was once life on Mars has improved, no conclusive evidence of Martian life has yet been found.

NASA has only gone so far as to say that there's "strong evidence pointing to primitive bacterial life on Mars." This statement is based on a meteorite found on Earth that is believed to have come from Mars. The minerals found in the meteorite "bear strong resemblance to mineral alterations caused by primitive bacteria on Earth," according to NASA.

NASA's next mission to Mars, the Mars Science Laboratory rover, could help determine once and for all whether life existed on Mars, or continues to do so. Its launch had been planned for 2009, but earlier this month the agency said that it has postponed the mission until 2011 because of problems with the rover's design.