The spiral troughs of Mars’ polar ice caps have been called the most
enigmatic landforms in the solar system. The deep canyons spiraling out
from the Red Planet’s North and South poles cover hundreds of miles. No
other planet has such structures.
A new model of trough formation suggests that heating and cooling alone
are sufficient to form the unusual patterns. Previous explanations had
focuse on alternate melting and refreezing cycles but also required wind
or shifting ice caps.
“I applied specific parameters that were appropriate to Mars and out of
that
came spirals that were not just spirals, but spirals that had exactly the
shape we see on Mars.” said Jon Pelletier, an assistant professor of
geosciences at the University of Arizona in Tucson. “They had the right
spacing, they had the right curvature, they had the right relationship to
one another.”
His report, “How do spiral troughs form on Mars?,” is published in the
April issue of the journal Geology. One of his computer simulations of
the troughs graces the cover.
How the icy canyons formed in a spiral has puzzled scientists since the
pattern was first spotted by the Viking spacecraft in 1976.
Pelletier, a geomorphologist who studies landforms on Earth such as sand
dunes and river channels, has a fondness for natural patterns that are
regularly spaced.
Spirals fit the bill, and while perusing a book on mathematical patterns
in biology, he was struck by the spiral shape formed by slime molds. He
wondered whether the mathematical equation that described how the slime
mold grew could also be applied to geological processes.
“There’s a recipe for getting spirals to form,” he said. So he tried it
out, using information that described the situation on Mars.
Temperatures on Mars are below freezing most of the year. During very
brief
periods during the summer, temperatures on the polar ice caps get just
high enough to let the ice melt a bit, Pelletier said.
He proposes that during that time, cracks or nicks in the ice’s surface
that
present a steep side toward the sun might melt a bit, deepening and
widening the crack. Heat from the sun also diffuses through the ice.
Much as ice cubes evaporate inside a freezer, on Mars, the melting ice
vaporizes rather than becoming liquid water.
The water vapor, when it hits the cold, shady side of the little canyon,
condenses and refreezes. So the canyon expands and deepens because one
side is heated occasionally while the other side always remains cold.
“The ambient temperatures on Mars are just right to create this form. And
that’s not true anywhere else in the solar system,” he said. “The spirals
are created because melting is focused in a particular place.”
Pelletier said the differential melting and refreezing is the key to the
formation of Mars’ spiral troughs.
So he put mathematical descriptions of the heating and cooling cycles
into
the spiral-generating equation and ran computer simulations to predict
what
would occur over thousands of such cycles. He did not include wind or
movement of polar ice caps in his model.
The computer made patterns that match what’s seen on Mars, even down to
the imperfections in the spirals.
“The model I have predicts the spacing between these things, how they’re
curved, and how they evolve over time to create spiral feature,” he said.
“A lot of planetary sciences is about making educated guesses about the
imagery that we see. We can’t go there, we can’t do do field
experiments,” he said. “The development of numerical models provides
strong suggestions as to what’s essential to create the form that we
see,” and allows scientists to test their assumptions, he said.
Related Web site
http://geomorphology.geo.arizona.edu//personal.html
