Once upon a time, a deep ocean covered one-third of Mars. Then, billions of years ago, it dried up, leaving the arid, rocky planet we see today. It's a provocative idea, but is it true? In a new study, researchers argue that it is, pointing to river deltas that, they say, mark the ancient sea level. But experts say the new evidence still isn't enough to carry the day.
Proving that 100 million cubic kilometers of water once filled the martian lowlands is a lot harder than it might seem. The putative sea floor was long ago covered by lava, and faulting and erosion have muddled any lingering shoreline along the lowland-highland boundary. So Gaetano Di Achille and Brian Hynek of the University of Colorado, Boulder, considered 52 martian deltas—piles of river-borne sediment—that formed at the level of some body of standing water, as the Mississippi delta is forming at the level of the Gulf of Mexico.
As the pair reports today in Nature Geoscience, 17 of the 52 deltas formed either on the edge of the lowlands or on basins or huge channels that open into the lowlands. And although the 17 are spread around the planet, they are at the same elevation plus or minus 177 meters (one standard deviation). The simplest explanation, they say, is that all of these deltas formed around the same ocean about 3.5 billion years ago, when Mars is thought to have been at least somewhat wetter and warmer than it is now. The level of the deltas is also generally consistent with "large portions" of a previously claimed shoreline as well as with the locations and terminations of ancient martian valley networks.
"I doubt there's a Q.E.D. in the offing," says planetary fluvial geologist Alan Howard of the University of Virginia, Charlottesville, but the deltas' strong clustering around a single elevation "does suggest we had to have something in the way of a large body of water." Planetary fluvial geologist Rossman Irwin of the Planetary Science Institute in Tucson, Arizona, is more skeptical of the delta evidence. "I don't think the [elevation] relationship is quite as clear as these folks would have it," he says. Irwin points to some deltas and valley networks that lie well below the team's favored sea level, a physical impossibility if an ocean that deep existed at the time they formed.
Even so, both Irwin and Howard think it's likely that some sort of northern ocean once existed. The martian highlands hold strong evidence for past abundant water—evidence such as paleolakes with obvious river channels leading in and out. "If there was so much water in the highlands," Irwin asks, "how would you have kept it out of the lowlands?" But proving the existence of an ancient ocean, he acknowledges, "is going to be hard to do."