After spending six years gazing at the Red Planet’s atmosphere and recording its water signatures, scientists have gathered strong evidence to suggest that not only was early Mars a much wetter world than previously believed, but it also possessed a vast northern ocean covering as much as a fifth of its surface. This expanse of water would have been around the size of our Arctic Ocean and lasted for millions of years; long enough, perhaps, for life to have had a chance to evolve. The findings have been published in Science.
As observational techniques have evolved over the years, so too has our picture of the Red Planet’s history. Not so long ago, ancient Mars was regarded as a mostly barren place, with flowing water appearing only sporadically and never hanging around long enough to form significant long-lasting pools. But that portrait of a parched, inhospitable young Mars is slowly being replaced with a much wetter one, adorned with meandering rivers and enduring lakes. For example, NASA’s Curiosity rover recently gathered evidence that suggests the Gale Crater was filled with water that lingered for millions of years.
But what kept these lakes topped up? There must have been a vigorous hydrological cycle to keep the atmosphere moist enough, but without a vast body of water to keep everything humid, water would have rapidly evaporated or frozen out. It is for these reasons that scientists hypothesized that oceans must have existed early on in Mars’ history.
Although these lakes are dehydrated now, by working out how much water was lost to space, scientists can estimate the amount of water Mars held in its early life. And that is precisely what NASA and ESO scientists have been doing for the past six years, with the help of a trio of ground-based observatories.
To do this, they measured the amounts of two different types of water in the Martian atmosphere. One is the form we are all familiar with, H2O, whereas the other is a naturally occurring heavier version called HDO in which one of the hydrogen atoms is replaced by a slightly different form known as deuterium. Whereas the vast majority of hydrogen atoms consist of one proton and one electron, deuterium also contains a neutron.
Since HDO is heavier than H2O, when water is lost to space, the latter is preferentially lost, meaning that the concentration of deuterium in water left behind goes up. So by looking at the ratio of HDO to H2O in water on Mars today and comparing this with the ratio found in Martian meteorites dating back some 4.5 billion years, scientists can work out how much water has escaped into space over time.
From these data, the scientists estimate that early Mars would have had enough water to cover its entire surface with a layer around 137 meters (450 ft) deep. But a more likely scenario is that it would have formed a vast ocean covering almost half of the northern hemisphere, or 19% of the planet’s surface, which could have been a mile deep in some areas.
“With Mars losing that much water,” study author Michael Mumma said in a statement, “the planet was very likely wet for a longer period of time than was previously thought, suggesting it might have been habitable for longer.”