A main component of NASA's search for evidence of life on Mars is the identification of biosignatures. The most common of these indicative markers of extant or extinct life are carbonate minerals, which are formed when carbon dioxide in the atmosphere reacts with other minerals and liquid water. A widely held belief among astrobiologists is that proof of running or standing liquid water can be construed as diagnostic evidence of the existence of life. It has been established that a large portion of the red planet's surface contains areas of frozen water, leading some scientists to theorize that the climate of ancient Mars was a greenhouse-like atmosphere heavy with carbon dioxide. In this type of atmosphere, the existence of vast oceans similar to those of Earth would have been a very real possibility. Images of the landscape of Mars have lent support to these theories. Massive surface erosions resembling the Grand Canyon and land features that appear to be dried-up sea floor suggest that liquid water was indeed present on Mars at one point in the planet's history. But recent findings on the planet paint a different picture. Though trace carbonates have been identified, the amounts are not commensurate with the prolonged existence of large bodies of flowing liquid water.