Since 1991,
planetary scientists have floated the idea that Mars once harbored vast oceans
that covered roughly one-third of the planet. Two long shore-like lips of rock
in the planet's northern hemisphere were thought to be the best evidence, but
experts argued that they were too "hilly" to describe the smooth edges of
ancient oceans.
The view
just changed dramatically with a surprisingly simple breakthrough.
The
once-flat shorelines were disfigured by a massive toppling over of the planet,
scientists announced today. The warping of the Martian rock has hidden clear
evidence of the
oceans, which in any case have been gone for at least 2 billion years.
"This
really confirms that there was an ocean on Mars," said Mark Richards, a
planetary scientist at the University of California at Berkeley and co-author
of the study, which is detailed in the June 14 issue of the journal Nature.
Twin
shores
Two major
shorelines exist on Mars, each thousands of miles long--one remaining from the
older Arabia Ocean, and another from the younger Deuteronilus Ocean, said study co-author Taylor Perron of UC Berkeley.
"The Arabia would have contained two
to three times the volume of water than in the ice that covers Antarctica," Perron told SPACE.com.
Somewhere
along the way to toppling over 50 degrees to the north, Mars probably lost some
of its
water, leaving the Deuteronilus Ocean's shoreline exposed. "The volume of
water was too large to simply evaporate into space, so we think there is still
some subterranean reservoirs on Mars," Perron said.
The
remaining sea would have been located in the same lowland plain as the Arabia Ocean, but almost 40 degrees to the north.
Unstable
spin
As a planet
spins, the heaviest things tend to shift towards the equator, where they are
most stable. Earth, too, has a
bulge at its equator. The volcanic Tharsis region of Mars, a vast raised
area along Mars' equator, is evidence for how this works.
"This is
the reason why this discovery packs extra punch," Perron said. More than a
billion years ago, he explained, something happened in the way mass was
distributed on Mars to cause the imbalanced portion to shift toward the
equator—and allow the vast shores of the Martian oceans to warp.
"We found
evidence of the path the shift would have to have occurred, and it matches with
the deformation of the shorelines," Perron said.
Elastic
surface
Near the
equator, the surface of a planet stays in a relatively flattened bulge under
the pressure of centripetal forces. But outside of the equator, the rock
behaves elastically and often bunches up, like the surface of a deflating
balloon. Perron and his team reasoned that the oceanic shorelines were once
near the equator, but warped into hilly up-and-down elevations
of rock as they move towards the north with the tilting planet.
"On
planets like Mars and Earth that have an outer shell ... that behaves
elastically, the solid surface will deform," Richards said.
By
calculating the deformation, which occurs in a predictable way, the planetary
research team found the ridges had to have once been flat, like ocean
shorelines.
"This
is a beautiful result that Taylor [Perron] got," Richards said. "The
mere fact that you can explain a good fraction of the information about the
shorelines with such a simple model is just amazing. It's something I never
would have guessed at the outset."
Perron and
his colleagues aren't certain what caused the toppling of the planet, but they
think forces beneath the surface are to blame. "There could have been a massive
change in the distribution of mantle," Perron said, "which would have caused
the planet to shift into its current position."
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