New measurements of water in the Martian atmosphere by Geronimo Villanueva and Michael Mumma of NASA’s Goddard Space Flight Center and their colleagues indicates that early Mars had at least seven times as much water as it does today, enough to cover about 20 percent of the planet’s surface.
Although more and more evidence has been accumulating in recent years for the existence of early Martian oceans, the conclusion had remained contentious. However, the authors’ new paper in Science may just seal the case. A heavy isotope of hydrogen called deuterium is enriched on Mars, because normal hydrogen is lighter and escapes more easily to space due to the planet’s lower gravity. Fortunately for scientists, the precise amount of deuterium enrichment allows us to quantify the amount of water that Mars held in its past.
Villanueva and colleagues used high-resolution spectrometers from spacecraft orbiting Mars, as well as observations from Earth, to determine the deuterium enrichment. They arrived at an enrichment value of seven relative to Earth’s oceans, and calculated that early Mars would have been covered by about 400 feet of water if the planet’s surface were perfectly smooth and the water was evenly distributed. In reality, Mars has mountains and lowlands just like Earth does, so only about 20 percent of the surface would have been covered by water.
And this is only a minimum value. According to an earlier study by Emily Pope and colleagues published in the Proceedings of the National Academy of Sciences, even Earth’s oceans held about 25 percent more water in ancient times than they do today. That suggests that even more than 20 percent of the Martian surface was covered with water between 4 billion and 4.5 billion years ago.
The new results strongly support the argument for early oceans on Mars made by Tyler Perron in 2007, based on observations of ancient shorelines. And with such a huge amount of water on the surface, the early Martian environment was most likely a favorable place for the origin and persistence of life. If the oceans on Mars existed for hundreds of millions of years, as suggested by Alberto Fairén and colleagues, it is feasible that early Earth life was transferred from Earth to Mars via asteroids, and would have found suitable habitats to thrive on Mars. In this case, we should be able to find remnants of that life today. Of course, the alternative scenario—life hitchhiking on an asteroid from Mars to Earth—is also an option, in which case we would all have Martian ancestors!