This week, NASA broke incredible news: It had found strong evidence that water still flows on the surface of Mars. This means, technically, Mars contains the ingredients to support life, at least according to our experience here on Earth.
But we're not going to be going anywhere near the hypothesized water streams, at least not with the Mars rovers currently rolling around collecting data.
The reasons for this are twofold. The first is access: Curiosity is physically unable to climb the steep slopes where the darkish streaks might indicate water flow.
The second? Even if it could, it's not allowed to, according to a United Nations treaty written in 1967. The Outer Space Treaty, which lays out the rules for space exploration along the principles of mutual cooperation, states that exploration will be conducted in such a way as to avoid contamination from Earth life, Quartz reports.
"Because liquid water appears to be present, these regions are considered special regions where we have to take extra precautions to prevent contamination by earth life," explained Rich Zurek, a scientist on the Mars Reconnaissance Orbiter team, in a Reddit AMA session.
"Our current rovers have not been sterilised to the degree needed to go to an area where liquid water may be present."
A "special region" on Mars is described by the Mars Exploration Program Analysis Group as places where terrestrial organisms might survive and breed under Mars conditions. Because Curiosity may harbour microbes from Earth, it is not allowed near these special regions.
We already know that bacteria can survive in space. A 2014 study found that microorganisms could very easily hitch a ride on Earth's spacecraft. In particular, spore-forming bacteria, which can survive harsh sterilisation procedures and have shown to be resistant to spacecraft-cleaning techniques, could survive on Mars. An experiment that placed spores of Bacillus pumilus on a facility on the outside of the International Space Station also saw the bacteria survive for 18 months.
But completely sterilising a spacecraft is impossible. The only surefire method involves intense heat, which would damage delicate instrumentation, so different techniques need to be used. Solar panels are cleaned with chemicals, for example, while microelectronics are placed in a vacuum chamber and treated with gas that oxidises biological material, which renders it harmless.
These techniques are able to reduce the number of microorganisms to 300 per square metre, according to the European Space Agency, compared to the several billion that can be found in a square metre in a clean kitchen.
That doesn't mean a rover can't get to the water in the future; there are ways to work with the UN's restrictions.
"The rovers have been sterilized for their particular landing sites where there's been no evidence of present day liquid water," Zurek explained. "To go to the [water flows] rovers will be required to be sterilized to a higher level. We also take samples of microbes that might be on the spacecraft before they're launched, so we can compare with any future discoveries."
NASA's next Mars mission launches in 2020. Given that it's being designed to the same basic specifications of Curiosity, though, it won't be able to get to the water flows either. So our curiosity about Mars microbes is just going to have to continue unslaked... for now.