Ridges on the red planet might have been shorelines
More than 30 years after Viking spacecraft images originally suggested the red planet might once have been blue, U of T physics professor Jerry Mitrovica and a team of U.S. researchers from Berkeley and Harvard have found evidence that Mars long ago supported two huge oceans. “What the rovers are seeing today, with isolated evidence of water here and there, is just the tip of it,” says Mitrovica.
The group’s findings were recently published in the journal Nature and have reignited interest in a theory that had fallen out of favour in the 1980s. Although the Viking images revealed long, linear landforms stretching across the Martian landscape, few accepted the idea that they were the remains of ancient shorelines. This was because the landforms varied in height, ranging between one and three kilometres instead of being mostly flat, as one might expect shorelines to be.
That wasn’t a sticking point for Mitrovica and his colleagues, though. They proposed that the height variation was the result of a gradual shift in the planet’s north-south axis within the past two to three billion years. This shifting axis deformed the land, causing it to buckle. “[The axis] changed on Earth, so there’s no reason to think it couldn’t have changed on Mars,” says Mitrovica.
Armed with that theory, the researchers studied how the poles could have moved to create the shoreline deformation visible today. They discovered that the poles could have moved along only one path because of the location of Tharsis, a volcano so massive that it could not be budged from its position on the planet’s equator – even by as great a force as a shifting planetary axis. That one path matches the pattern of height variation in the shorelines today. “This all just hangs together so elegantly that there’s simply no way around it,” says Mitrovica. “It fits what we know about the dynamics of Mars, it fits what we know about the age of the shorelines and it fits what we know about Tharsis.”
He says his team’s findings have provocative implications for future Mars exploration missions. “Water isn’t stable in the current Martian atmosphere, so most conclude that the water just went downward into the subsurface and later froze. So, we still have oceans there, they just aren’t visible to the eye.”