After years of data gathering and mineral-mapping from over 350 locations on the planet Mars, scientists have not proven that life currently exists on the planet. It does not mean, however, that no life form has ever existed on Martian environments with abundant liquid water. In fact, a new NASA study hints that if there had been living creatures and organisms on Mars, then the longest lasting habitats may have been under the red planet's surface.

Scientists agree that liquid water on Mars' surface existed only for short episodes. These episodes occurred toward the end of hundreds of millions of years during which warm water interacted with subsurface rocks. This implies life may have existed on Mars, and further studies would also be done to determine how the planet's atmosphere has changed over millions of years.

"The types of clay minerals that formed in the shallow subsurface are all over Mars," said John Mustard, professor at Brown University in Providence, R.I.

"The types that formed on the surface are found at very limited locations and are quite rare," Mustard added.

The report's lead author, Bethany Ehlmann, assistant professor at the California Institute of Technology and scientist at NASA's Jet Propulsion Laboratory in Pasadena, noted the most stable Mars habitats over long durations appear to have been underground. Considering Earth's underground geothermal environments have active ecosystems, it will not be impossible for Mars to have experienced the same situation.

Mustard, co-author of the study in the journal Nature, said the discovery of clay minerals on Mars in 2005 indicated the planet once hosted warm, wet conditions, during which clay formed from the interaction of water with rock. Those conditions required a much thicker atmosphere than Mars has now. What could cause a thick atmosphere to be lost over time? That is what the scientists are looking into as studies on Mars continue to progress.

Mars has been closely observed for life forms because the seasons on this planet are notably Earth-like, due to the similar tilts of the two planets' rotational axes, a feature not shared by the rest of the planets in the Solar System.

The discovery of water on Mars gave scientists hope that they were on to something. In July 2005, radar data from Mars Express and the Mars Reconnaissance Orbiter showed large quantities of water ice at Martian poles. In 2008, water was also found at mid-latitudes of Mars. That same year, the Phoenix lander directly sampled water ice in shallow Martian soil.

BBC News reported that in 2018, the European Space Agency (ESA) plans to launch its first Rover to Mars -- the ExoMars rover. It will be capable of drilling 2 m into the soil in search of organic molecules, an experiment that will be linked to water, clay minerals and other findings on Mars, so far.