We think Mars once had a vast amount of liquid water on its surface, possibly a large ocean in its northern hemisphere. We also think it once had a thicker atmosphere, with temperatures and an atmosphere more conducive to life.
At some point in the last few billion years, Mars lost its magnetic field and thus its atmosphere for reasons unknown, boiling away the water on its surface. But underground and even on the surface, there are large reservoirs of water ice that could contain remnants of that habitable past. Trickles of water also suggest Mars is not quite dead just yet.
“My guess is that there are places on Mars that could very well be contaminated by Earth organisms,” said Rummel. “But so far they are very hard to get to.”
If such regions do exist, then issues will concern how we clean our spacecraft before we send them there, to eradicate Earth microbes. We can sterilize spacecraft pretty well, but there’s always a chance some microbes might sneak along for the ride.
"There are some places where our data are not sufficient to show they are non-Special. They may exceed the Special conditions during some parts of the year," David Beaty, Chief Scientist for the Mars Exploration Directorate at NASA's Jet Propulsion Laboratory in Pasadena, told IFLScience.
In 2006, a map of the potential Special Regions on Mars was drawn up. This basically included regions known to have water ice within 5 meters (16 feet) of the surface. This is deemed important because if a lander crashed on Mars, it would be expected to reach up to 5 meters in depth. As more ice was found near the equator, the non-special zone began to shrink
However, in that re-evaluation in 2016, it was decided that you couldn’t really predict where Special Regions would be. Instead, it was better to look at each region on a case by case basis. So if you want to go somewhere that looks good for past or present life, then sure, go for it. But it’ll need to be properly studied first.
“If we found a Special Region on Mars, certainly you’d want to have kind of a hands-off zone until we fully understood it,” said Rummel.
Spacecraft could still visit these regions, but they’d need to go through some pretty stringent sterilization procedures first. Under the COSPAR Planetary Protection Policy, there are five relatively broad categories of sterilization that spacecraft must adhere to.
At the lower end of this scale is Category 1, for bodies that that are not of interest in the search for life. These require minimal sterilization. At the upper end, we’ve got Category IV. This concerns landers hoping to land in biologically interesting locations on Mars, Europa, and other important places in the search for life. These spacecraft must be sterilized to Viking levels.