Researchers say equal elevation of dry valleys and river deltas points to an ancient Martian ocean

Mars was once a water world, concludes a new study that dives into the controversy over whether an ocean covered much of the Red Planet’s northern hemisphere early in its history.

Ever since researchers found hints in the late 1980s that Mars’ northern lowlands are ringed by what appears to be a dried-up shoreline, planetary scientists have debated whether the region was covered by water about 3.5 billion years ago. Now, Gaetano Di Achille and Brian Hynek of the University of Colorado in Boulder have analyzed spacecraft data to find that 29 of 52 dry river deltas and thousands of river valleys within and surrounding the northern lowlands all lie at about the same elevation. That’s just what would be expected if a sea once blanketed the region, leaving behind a level coastline, the researchers report online June 13 in Nature Geoscience.

The scientists relied on altitude data recorded by the now-defunct Mars Global Surveyor spacecraft and on images collected by an array of missions going back to 2001. Di Achille and Hynek estimate that the amount of water in the ancient lowlands would have been enough to fill a 550-meter-deep ocean if spread across the entire planet. That’s about one-tenth the volume of water currently in Earth’s oceans.

Researchers believe any ocean in the lowlands would have to date to at least 3.5 billion years ago, based on the age of the terrain there. The hypothesized ocean would have vanished during an abrupt change in climate, its water either evaporating into the atmosphere, sinking beneath the surface or becoming incorporated into the planet’s polar ice caps.

In a separate study in press at the Journal of Geophysical Research–Planets, Hynek and two University of Colorado colleagues, Michael Beach and Monica Hoke, find some 40,000 dry river valleys on Mars — about four times the number previously identified. Hynek says the additional valleys add further support to the notion that one-third of ancient Mars was covered by an ocean. A study published last year by other researchers arrived at a similar conclusion, but that team missed many of the smaller valleys because they were working with lower-resolution data, Hynek says.

Di Achille and Hynek “do a credible job of integrating a variety of observational lines of evidence suggesting an integrated hydrosphere and large bodies of water” 3.5 billion years ago on Mars, comments planetary scientist James Head of Brown University in Providence, R.I. However, “this sort of analysis is unlikely to ever give the final word on the ocean debate, since it is such indirect reasoning.” 

Jim Bell of Cornell University raises the possibility that some of the features characterized as dry deltas by the Colorado researchers may be due to landslides or gravitational slumps rather than sculpting by water.

In addition, a decade of spacecraft observations have revealed only small amounts of clay, sulfate and other water-modified minerals on Mars, says Bell. “The relatively minor degree of alteration of Martian surface minerals argues against there ever having been a global, long-lived ocean on the Red Planet,” he says.

“Ultimately, however, the authors are careful to point out that their hypothesis is just that, a hypothesis — a suggestion — that will need to be tested with further observations and models,” adds Bell. “In that respect, the result is intriguing and certainly timely.”