When we found what seemed to be liquid water flowing across the surface of Mars in 2015, scientists around the world were itching to test it. There was just one problem, and it was a biggie: the United Nations' Outer Space Treaty of 1967 mandates that space exploration must be conducted in such a way as to avoid contamination.
Since we have no way of sterilising our equipment completely of Earth's microbes, that meant no touchy on the water.
According to new research, however, we needn't have worried - although there could be briny liquid water on Mars, the surface conditions otherwise really are inhospitable to terrestrial life.
"Life on Earth, even extreme life, has certain environmental limits that it can withstand," explained planetary scientist Edgard G. Rivera-Valentín of the Universities Space Research Association (USRA) and the Lunar and Planetary Institute (LPI).
"We investigated the distribution and chemistry of stable liquids on Mars to understand whether these environments would be suitable to at least extreme life on Earth."
While seeking to understand how life might exist elsewhere, we often look at extremophiles - organisms that live in some of Earth's most extreme environments. These include the arid Atacama Desert in Chile, the salty, acidic Dallol Geothermal Area in Ethiopia, and even near-Earth space aboard the ISS.
But while these environments have things in common with Mars, they are distinctly not Mars. Liquid water seems to be a requirement for life, but on Mars, liquid fresh water can't hang around on the surface. It's so dry and cold there, the water will either freeze or evaporate.
Of course, water doesn't have to be fresh to support life. Earth's salty oceans are teeming with it. And we know that salts of sodium, magnesium, and calcium are abundant on Mars; if these salts mixed with the water to create a high-salt solution called brine, it would lower the freezing point and slow the evaporation rate of the liquid, potentially allowing it to linger on the surface.
And if there was enough moisture in the Martian atmosphere, some of the salts could undergo a process called deliquescence, whereby they absorb the moisture to form a liquid solution.
But questions remain: Can this liquid brine form and remain on the Martian surface long enough for terrestrial life to thrive?
"Our team looked at specific regions on Mars - areas where liquid water temperature and accessibility limits could possibly allow known terrestrial organisms to replicate - to understand if they could be habitable," said planetary scientist Alejandro Soto of the Southwest Research Institute.
"We used Martian climate information from both atmospheric models and spacecraft measurements. We developed a model to predict where, when and for how long brines are stable on the surface and shallow subsurface of Mars."
Based on years of experimental data on chemical reactions in simulated Mars conditions in the laboratory, as well as the climate data, the team put together a picture of when and where liquid brines might be present on the surface of Mars, and a few centimetres below.
They found that liquid brines could persist for up to six hours from the equator to high latitudes, over 40 percent of the Martian surface. And this could only occur seasonally, for around 2 percent of the year.
It may not sound like a lot, but it's a broader range than scientists previously thought. But that still doesn't mean Earth's life could survive in it.
"The highest temperature a stable brine will experience on Mars is -48 degrees Celsius (-55 degrees Fahrenheit)," Rivera-Valentín said. "This is well below the lowest temperature we know life can tolerate."
This means, the team concluded, that Martian brines don't meet the Special Region requirements laid out by the Committee on Space Research (COSPAR) of the International Council for Science, and should therefore prove no hindrance to a crewed Mars exploration mission.
It's also important to note that these results don't have any bearing on native Martian life, if there is or was any throughout the planet's history - they're based entirely on our understanding of terrestrial life. And that could be a limitation, too.
"We have shown that on a planetary scale the Martian surface and shallow subsurface would not be suitable for terrestrial organisms because liquids can only form at rare times, and even then, they form under harsh conditions," Rivera-Valentín said.
"However, there might be unexplored life on Earth that would be happy under these conditions."
The research has been published in Nature Astronomy.