Andrew Yee[_1_]
June 25th 08, 09:17 PM
Media Relations
Caltech
Contact:
Elisabeth Nadin, (626) 395-3631
June 25, 2008
Giant Impact Explains Mars Dichotomy
PASADENA, Calif. -- The surface landscape of Mars, divided into lowlands in
the north and highlands in the south, has long perplexed planetary
scientists. Was it sculpted by several small impacts, via mantle convection
in the planet's interior, or by one giant impact? Now scientists at the
California Institute of Technology have shown through computer modeling that
the Mars dichotomy, as the divided terrain has been termed, can indeed be
explained by one giant impact early in the planet's history.
"The dichotomy is arguably the oldest feature on Mars," notes Oded
Aharonson, associate professor of planetary science at Caltech and an author
of the study. The feature arose more than four billion years ago, before the
rest of the planet's complex geologic history was superimposed.
Scientists had previously discounted the idea that a single, giant impactor
could have created the lower elevations and thinner crust of Mars's northern
region, says Margarita Marinova, a graduate student in Caltech's Division of
Geological and Planetary Sciences (GPS) and lead author of the study, which
appears June 26 in the journal Nature. This special issue of the journal
features a trio of papers on the Mars dichotomy.
For one thing, Marinova explains, it was thought that a single impact would
leave a circular footprint, but the outline of the northern lowlands region
is elliptical. There is also a distinct lack of a crater rim: topography
increases smoothly from the lowlands to the highlands without a lip of
concentrated material in between, as is the case in small craters. Finally,
it was believed that a giant impactor would obliterate the record of its own
occurrence by melting a large fraction of the planet and forming a magma
ocean.
"We set out to show that it's possible to make a big hole without melting
the majority of the surface of Mars," Aharonson says. The team modeled a
range of projectile parameters that could yield a cavity the size and
ellipticity of the Mars lowlands without melting the whole planet or making
a crater rim.
After cranking 500 simulations combining various energies, velocities, and
impact angles through the GPS division's Beowulf-class computer cluster
CITerra, the researchers narrowed in on a "sweet spot" -- a range of
single-impact parameters that would make exactly the type of crater found on
Mars. Although a large impact had been suggested (and discounted) in the
past, Aharonson says, computers weren't fast enough to run the models. "The
ability to search for parameters that allow an impact compatible with
observations is enabled by the dedicated machine at Caltech," he adds.
The favored simulation conditions outlined by the sweet spot suggest an
impact energy of around 10**29 joules, which is equivalent to 100 billion
gigatons of TNT. The impactor would have hit Mars at an angle between 30 and
60 degrees while traveling at 6 to 10 kilometers per second. By combining
these factors, Marinova calculated that the projectile was roughly 1,600 to
2,700 kilometers across.
Estimates of the energy of the Mars impact place it squarely between the
impact that is thought to have led to the extinction of dinosaurs on Earth
65 million years ago and the one believed to have extruded our planet's moon
four billion years ago.
Indeed, the timing of formation of our moon and the Mars dichotomy is not
coincidental, Marinova notes. "This size range of impacts only occurred
early in solar system history," she says. The results of this study are also
applicable to understanding large impact events on other heavenly bodies,
like the Aitken Basin on the moon and the Caloris Basin on Mercury.
The Caltech study comes at a time of renewed interest in the ancient crustal
feature on Mars, Aharonson notes. Also in this issue of Nature, Jeffrey
Andrews-Hanna and Maria Zuber of MIT and Bruce Banerdt of JPL examine the
gravitational and topographic signature of the dichotomy with information
from the Mars orbiters. Another accompanying report, from a group at UC
Santa Cruz led by Francis Nimmo, explores the expected consequences of
mega-impacts.
The other author on this study is Erik Asphaug, a professor of earth and
planetary sciences at UC Santa Cruz.
Caltech
Contact:
Elisabeth Nadin, (626) 395-3631
June 25, 2008
Giant Impact Explains Mars Dichotomy
PASADENA, Calif. -- The surface landscape of Mars, divided into lowlands in
the north and highlands in the south, has long perplexed planetary
scientists. Was it sculpted by several small impacts, via mantle convection
in the planet's interior, or by one giant impact? Now scientists at the
California Institute of Technology have shown through computer modeling that
the Mars dichotomy, as the divided terrain has been termed, can indeed be
explained by one giant impact early in the planet's history.
"The dichotomy is arguably the oldest feature on Mars," notes Oded
Aharonson, associate professor of planetary science at Caltech and an author
of the study. The feature arose more than four billion years ago, before the
rest of the planet's complex geologic history was superimposed.
Scientists had previously discounted the idea that a single, giant impactor
could have created the lower elevations and thinner crust of Mars's northern
region, says Margarita Marinova, a graduate student in Caltech's Division of
Geological and Planetary Sciences (GPS) and lead author of the study, which
appears June 26 in the journal Nature. This special issue of the journal
features a trio of papers on the Mars dichotomy.
For one thing, Marinova explains, it was thought that a single impact would
leave a circular footprint, but the outline of the northern lowlands region
is elliptical. There is also a distinct lack of a crater rim: topography
increases smoothly from the lowlands to the highlands without a lip of
concentrated material in between, as is the case in small craters. Finally,
it was believed that a giant impactor would obliterate the record of its own
occurrence by melting a large fraction of the planet and forming a magma
ocean.
"We set out to show that it's possible to make a big hole without melting
the majority of the surface of Mars," Aharonson says. The team modeled a
range of projectile parameters that could yield a cavity the size and
ellipticity of the Mars lowlands without melting the whole planet or making
a crater rim.
After cranking 500 simulations combining various energies, velocities, and
impact angles through the GPS division's Beowulf-class computer cluster
CITerra, the researchers narrowed in on a "sweet spot" -- a range of
single-impact parameters that would make exactly the type of crater found on
Mars. Although a large impact had been suggested (and discounted) in the
past, Aharonson says, computers weren't fast enough to run the models. "The
ability to search for parameters that allow an impact compatible with
observations is enabled by the dedicated machine at Caltech," he adds.
The favored simulation conditions outlined by the sweet spot suggest an
impact energy of around 10**29 joules, which is equivalent to 100 billion
gigatons of TNT. The impactor would have hit Mars at an angle between 30 and
60 degrees while traveling at 6 to 10 kilometers per second. By combining
these factors, Marinova calculated that the projectile was roughly 1,600 to
2,700 kilometers across.
Estimates of the energy of the Mars impact place it squarely between the
impact that is thought to have led to the extinction of dinosaurs on Earth
65 million years ago and the one believed to have extruded our planet's moon
four billion years ago.
Indeed, the timing of formation of our moon and the Mars dichotomy is not
coincidental, Marinova notes. "This size range of impacts only occurred
early in solar system history," she says. The results of this study are also
applicable to understanding large impact events on other heavenly bodies,
like the Aitken Basin on the moon and the Caloris Basin on Mercury.
The Caltech study comes at a time of renewed interest in the ancient crustal
feature on Mars, Aharonson notes. Also in this issue of Nature, Jeffrey
Andrews-Hanna and Maria Zuber of MIT and Bruce Banerdt of JPL examine the
gravitational and topographic signature of the dichotomy with information
from the Mars orbiters. Another accompanying report, from a group at UC
Santa Cruz led by Francis Nimmo, explores the expected consequences of
mega-impacts.
The other author on this study is Erik Asphaug, a professor of earth and
planetary sciences at UC Santa Cruz.