Soon after Mars was formed, it was bombarded by numerous large meteorites and asteroids. Scientists have discovered an unexpectedly large grouping of impact basins buried under Mars' northern plains that resulted from this pounding. They used Mars Orbiter Laser Altimeter (MOLA) topographic data to find them, because they can't be seen in images of the Martian surface.
Above these basins are thin young plains, but the lowland crust beneath them is actually extremely old and was formed very, very early. According to Herbert Frey of the Geodynamics Branch of NASA's Goddard Space Flight Center, this is a radical departure from the popular belief that the northern lowlands were formed later in Martian history, perhaps by plate tectonic style processes.
Frey discussed these findings November 8, at the Geological Society of America's annual meeting in Boston, Massachusetts.
This discovery is a crucial piece to one of the greatest unsolved puzzles about Mars-why does its surface have two distinct hemispheres: one that is high and heavily cratered and one that is low and sparsely cratered?
The origin of this fundamental "crustal dichotomy" is uncertain both in terms of how and when it formed. But this recent discovery of the numerous buried craters may pin down the answer to when the lowlands first formed.
"The ancient age of the lowlands means whatever process produced them occurred both early and relatively quickly," explained Frey. "Things like plate tectonics may not work.
"Another ramification is that there have been lowlands in the northern parts of Mars for essentially all of Martian history. That means that at whatever early time conditions permitted liquid water to exist on Mars, there was a northern lowland into which that water could drain.
"So it is quite possible that a shallow ocean may have existed on Mars very early in its history, as some have suggested based on completely different data."
"The origin of the crustal dichotomy on Mars has been one of the main areas of my own research for a long time, so anything that could tell us how old the lowlands really were naturally was of interest," Frey said.
"And of course, the discovery aspects of 'seeing' (in elevation data) things that no one else had ever seen or even guessed might be there is intrinsically intoxicating. Not only has this work turned out to be very important, but it's also been fun!"
Mars MOLA Team
Geological Society of America
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