Red Planet shaped by familiar forces
Norman Ness, retired professor at UD’s Bartol Research Institute, and a team of NASA scientists have discovered additional evidence that Mars once underwent plate tectonics, the slow movement of the planet’s crust, like the present-day Earth.
A new map of the distant planet’s magnetic field made by the Mars Global Surveyor spacecraft reveals a world whose history was shaped by great crustal plates being pulled apart and smashed together.
Scientists first found evidence of plate tectonics on Mars in 1999. Those initial observations, also done with the Mars Global Surveyor’s magnetometer, covered only one region in the Southern Hemisphere. The data were taken from differing heights above the crust while the spacecraft performed an aerobraking maneuver.
The new high-resolution magnetic field map, the first of its kind, covers the entire surface of Mars and is based on four years of data taken in a constant orbit. Each region on the surface has been sampled many times.
“The more measurements we obtain, the more accuracy and spatial resolution we achieve,” Jack Connerney of NASA’s Goddard Space Flight Center in Greenbelt, Md., says.
The new map supports and expands on the 1999 results, Ness, who retired from UD in June, says. “Where the earlier data showed a ‘striping’ of the magnetic field in one region, the new map finds striping elsewhere,” he says. “More importantly, the new map shows evidence of features—transform faults—that are a ‘telltale’ of plate tectonics on Earth.”
Each stripe represents a magnetic field pointed in one direction, either positive or negative, and the alternating stripes indicate a “flipping” of the direction of the magnetic field from one stripe to another. Scientists have seen similar stripes in the crustal magnetic field on Earth.
Stripes form whenever two plates are being pushed apart by molten rock rising up from the mantle, such as along Earth’s Mid-Atlantic Ridge. As the plate spreads and cools, it becomes magnetized in the direction of the Earth’s strong global field. Since that field changes direction a few times every million years, on average, a flow that cools in one period will be magnetized in a different direction than a later flow.
As the new crust is pushed out and away from the ridge, stripes of alternating magnetic fields aligned with the ridge axis then develop. Transform faults, identified by shifts in the magnetic pattern, occur only in association with spreading centers.
To see this characteristic magnetic imprint on Mars indicates that it, too, had regions where new crust came up from the mantle and spread out across the surface. When new crust comes up, old crust plunges back down—the exact mechanism for plate tectonics.
Connerney says the concept of plate tectonics provides a unifying framework to explain several Martian features, including the magnetic pattern itself. Also, the Tharsis volcanoes lie along a straight line. These formations could have formed from the motion of a crustal plate over a fixed “hotspot” in the mantle below, just as the Hawaiian islands on Earth are thought to have formed.
The results were published in a recent edition of the Proceedings of the National Academy of Science.