Since spacecraft sent back the first close-up images of Mars more than 30 years ago, some experts have insisted that oceans once existed on the now dry, cold planet. Critics have maintained for decades that such an idea is the product of unrestrained imaginations. Now, a study published in today’s (June 14) issue of the British journal Nature reports new evidence that our neighbor in space once boasted an ocean or oceans as big, relative to planet size, as the Atlantic on Earth.
“We were able to lay to rest one of the main objections to the idea that there once were oceans on Mars,” says Taylor Perron, a postdoctoral fellow in Harvard’s Department of Earth and Planetary Sciences.
To those looking for life on other planets, that conclusion is a wave in the right direction. “Follow the water,” is the mantra chanted by the National Aeronautics and Space Administration in its search for extraterrestrial life, because wherever there’s water on Earth, there’s life.
When geologists started making maps of Mars, using data broadcast by spacecraft that landed there in the 1970s, they were struck by the sight of a huge basin covering a third of the planet. Located in the northern plains, the flat, sediment-covered area reminded them of the ocean floors on Earth.
The basin is bordered by shorelines, which are cut by the downstream ends of huge channels eroded into the face of Mars. It is easy to imagine these channels as places where water flowed from the higher southern regions into the basin.
Two shorelines run continuously for thousands of miles. Planetary scientists named them Arabia and Deuteronilus, and they estimate their ages at between 2 billion and 4 billion years.
But are they really shorelines? Shorelines are flat. Arabia and Deuteronilus rise and fall. Measurements made later by instruments aboard spacecraft that orbited as low as 3,000 feet above the Red Planet show that these features rise and fall by more than a mile in height.
Nothing like that exists on Earth, so for a while such ups and downs dried up enthusiasm for Martian oceans and possible life. After a close analysis, however, a group at the University of California, Berkeley, saw the wavy shorelines as evidence for, not against, oceans. On planets like Mars and Earth, which have a hard outer shell underlain by hot, soft rock, shifts in the location of the rotational pole can cause deformations of the surfaces as large as the variations in height seen on the Martian shorelines, explains Perron, who was part of the Berkeley group at the time.
The researchers believe that a redistribution of mass on the surface or in the interior of Mars produced a change in the orientation of the planet relative to the axis on which it spins. The process, called true polar wandering,” has occurred on Earth when thick slabs of oceanic crust, riding on interior currents of molten material, slammed into the continents and sank below the surface. Such shifts, which still take place, occur gradually, over millions of years, and could account for the rise of Martian shorelines, Perron believes.
“The mere fact that you can explain a good fraction of the information about the shorelines with such a simple model is just amazing,” comments Mark Richards, a Berkeley professor of Earth and planetary science who participated in the research.
Billions of years ago, when Mars was still a hot young planet, immense volcanic eruptions built what is now known as the Tharsis rise, a mountain range that boasts the largest known volcano in the solar system. Olympus Mons soars to about three times the 29,000-foot height of Mt. Everest, a rock cathedral extending 16 miles into the Martian sky.
A load like that would have a strong influence on the planet’s rotation. When such a rise happens, physics mandates that the mountain should move its weight toward the equator, and that’s where it sits.
Jerry Mitovica, a professor of physics at the University of Toronto and one of the team members, is now trying to determine how long such a remolding of Mars’ surface would take. Results so far suggest that it required millions, maybe tens of millions, of years.
Source of the water
Where did enough water to cover a third of Mars come from, and where did it go? The most likely answer is that it welled up from below the surface through broken slabs of crust and later drained away the same way.
Today, the Martian air is too cold and thin for liquid water to flow. But billions of years ago, lots of volcanic activity heated the surface. The place shows signs of ancient channels that might have carried water emerging from the subsurface into the ocean basin.
Frozen water exists today on Mars as ice caps at both poles. However, there’s not enough water trapped in the ice to account for a whole ocean. “We estimate that enough water existed in the Arabia and Deuteronulis oceans to cover the entire surface of the plant to a depth of hundreds of yards,” Perron says. “Water in the visible ice capping the poles today would only be tens of yards deep if it were spread uniformly over the Martian surface.”
If all this modeling is correct, there might still be a substantial amount of subsurface water on Mars. If you go that far, there might even be life down there; bacteria and other microscopic organisms live in the deep, wet subterranean basements of Earth.
“Yes,” agrees Perron. “Our study raises the possibility that living things, if they ever existed on Mars, could have endured for much of the planet’s history. I wouldn’t rule that out.”