Distinctive
fractures in ancient lava flows on Mars suggest that water occasionally flooded
portions of the planet's surface. The research piles onto previous findings that
suggest the same.
The
fractures, called columnar joints, are the first that have been observed on a
planet other than Earth.
"Columnar
joints form as cooling lava contracts," said Moses Milazzo, a geophysicist
with the U.S. Geological Survey in Flagstaff, Ariz.
The
characteristics of the column-like fractures can help scientists understand the
role of
water in geologic processes on Mars. Milazzo worked with the NASA Mars Reconnaissance
Orbiter HiRISE camera team to make the finding, which is detailed in the
journal Geology.
The Martian
columns, about 3 feet (1 meter) in diameter and about 100 feet (30 meters)
high, were identified in the tilted inner walls of an impact crater.
"The
HiRISE instrument just barely has the resolution to pick out the columns if
they're facing the camera with the perfect orientation," said Milazzo.
When the impact crater formed, the rocks were tilted backward, toward the sky,
which is what allowed the identification.
The impact
crater where the columns were discovered is in a region that has a history of
extensive volcanic activity. Milazzo suspects that the columnar joints formed
as lava flows were episodically flooded by liquid water, which quickly cooled
the lava. Flooding cycles may have lasted from a few months to a few years,
they estimate.
On Earth,
columnar joints are common in the rocks of the Colorado Plateau, which provide
a study site for comparisons to Mars.
Other
geological evidence on Mars points to periods when floods washed across the
surface, including similarities of certain canyons to Idaho's
Box Canyon and modeling
studies that suggest some areas were inundated for at least 10,000 years.
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