Andrew Yee[_1_]
June 25th 08, 09:10 PM
Public Information Office
University of California-Santa Cruz
Contact:
Tim Stephens, (831) 459-2495
June 25, 2008
Evidence of massive asteroid impact on Mars supported by computer
simulations
The dramatic differences between the northern and southern hemispheres of
Mars have puzzled scientists for 30 years. One of the proposed explanations
-- a massive asteroid impact -- now has strong support from computer
simulations carried out by two groups of researchers. Planetary scientists
at the University of California, Santa Cruz, were involved in both studies,
which appear in the June 26 issue of Nature.
"It's a very old idea, but nobody had done the numerical calculations to see
what would happen when a big asteroid hits Mars," said Francis Nimmo,
associate professor of Earth and planetary sciences at UCSC and first author
of one of the papers.
Nimmo's group found that such an impact could indeed produce the observed
differences between the Martian hemispheres. The other study used a
different approach and reached the same conclusion. Nimmo's paper also
suggests testable predictions about the consequences of the impact.
The so-called hemispheric dichotomy was first observed by NASA's Viking
missions to Mars in the 1970s. The Viking spacecraft revealed that the two
halves of the planet look very different, with relatively young, low-lying
plains in the north and relatively old, cratered highlands in the south.
Some 20 years later, the Mars Global Surveyor mission showed that the crust
of the planet is much thicker in the south and also revealed magnetic
anomalies present in the southern hemisphere and not in the north.
"Two main explanations have been proposed for the hemispheric dichotomy --
either some kind of internal process that changed one half of the planet, or
a big impact hitting one side of it," Nimmo said. "The impact would have to
be big enough to blast the crust off half of the planet, but not so big that
it melts everything. We showed that you really can form the dichotomy that
way."
Nimmo's group includes UCSC graduate student Shawn Hart, associate
researcher Don Korycansky, and Craig Agnor of Queen Mary University, London.
The other paper is by Margarita Marinova and Oded Aharonson of the
California Institute of Technology and Erik Asphaug, professor of Earth and
planetary sciences at UCSC.
The quantitative model used by Nimmo's group calculated the effects of an
impact in two dimensions. Asphaug's group used a different model to
calculate impacts in three dimensions, but with lower resolution (i.e., less
detail in the simulation).
"The two approaches are very complementary; putting them together gives you
a complete picture," Nimmo said. "The two-dimensional model provides high
resolution, but you can only look at vertical impacts. The three-dimensional
model allows nonvertical impacts, but the resolution is lower so you can't
track what happens to the crust."
Most planetary impacts are not head-on, Asphaug said. His group found a
"sweet spot" of impact conditions that result in a hemispheric dichotomy
matching the observations. Those conditions include an impactor about
one-half to two-thirds the size of the Moon, striking at an angle of 30 to
60 degrees.
"This is how planets finish their business of formation," Asphaug said.
"They collide with other bodies of comparable size in gargantuan collisions.
The last of those big collisions defines the planet."
According to Nimmo's analysis, shock waves from the impact would travel
through the planet and disrupt the crust on the other side, causing changes
in the magnetic field recorded there. The predicted changes are consistent
with observations of magnetic anomalies in the southern hemisphere, he said.
In addition, new crust that formed in the northern lowlands would be derived
from deep mantle rock melted by the impact and should have significantly
different characteristics from the southern hemisphere crust. Certain
Martian meteorites may have originated from the northern crust, Nimmo said.
The study also suggests that the impact occurred around the same time as the
impact on Earth that created the Moon.
This research was funded by NASA.
Note to reporters: You may contact Nimmo at (831) 459-1783
IMAGE CAPTIONS:
[Image 1:
http://www.ucsc.edu/news_events/img/2008/06/mars-349.jpg (87KB)]
The northern hemisphere of Mars is low (blue) and smooth, while the southern
hemisphere is high (red) and heavily cratered. Image courtesy of NASA.
[Image 2:
http://www.ucsc.edu/news_events/img/2008/06/impact-349.jpg (48KB)]
This image from a computer simulation shows the type of impact that could
have created the Mars hemispheric dichotomy. Credit: M. Marinova, O.
Aharonson, E. Asphaug.
University of California-Santa Cruz
Contact:
Tim Stephens, (831) 459-2495
June 25, 2008
Evidence of massive asteroid impact on Mars supported by computer
simulations
The dramatic differences between the northern and southern hemispheres of
Mars have puzzled scientists for 30 years. One of the proposed explanations
-- a massive asteroid impact -- now has strong support from computer
simulations carried out by two groups of researchers. Planetary scientists
at the University of California, Santa Cruz, were involved in both studies,
which appear in the June 26 issue of Nature.
"It's a very old idea, but nobody had done the numerical calculations to see
what would happen when a big asteroid hits Mars," said Francis Nimmo,
associate professor of Earth and planetary sciences at UCSC and first author
of one of the papers.
Nimmo's group found that such an impact could indeed produce the observed
differences between the Martian hemispheres. The other study used a
different approach and reached the same conclusion. Nimmo's paper also
suggests testable predictions about the consequences of the impact.
The so-called hemispheric dichotomy was first observed by NASA's Viking
missions to Mars in the 1970s. The Viking spacecraft revealed that the two
halves of the planet look very different, with relatively young, low-lying
plains in the north and relatively old, cratered highlands in the south.
Some 20 years later, the Mars Global Surveyor mission showed that the crust
of the planet is much thicker in the south and also revealed magnetic
anomalies present in the southern hemisphere and not in the north.
"Two main explanations have been proposed for the hemispheric dichotomy --
either some kind of internal process that changed one half of the planet, or
a big impact hitting one side of it," Nimmo said. "The impact would have to
be big enough to blast the crust off half of the planet, but not so big that
it melts everything. We showed that you really can form the dichotomy that
way."
Nimmo's group includes UCSC graduate student Shawn Hart, associate
researcher Don Korycansky, and Craig Agnor of Queen Mary University, London.
The other paper is by Margarita Marinova and Oded Aharonson of the
California Institute of Technology and Erik Asphaug, professor of Earth and
planetary sciences at UCSC.
The quantitative model used by Nimmo's group calculated the effects of an
impact in two dimensions. Asphaug's group used a different model to
calculate impacts in three dimensions, but with lower resolution (i.e., less
detail in the simulation).
"The two approaches are very complementary; putting them together gives you
a complete picture," Nimmo said. "The two-dimensional model provides high
resolution, but you can only look at vertical impacts. The three-dimensional
model allows nonvertical impacts, but the resolution is lower so you can't
track what happens to the crust."
Most planetary impacts are not head-on, Asphaug said. His group found a
"sweet spot" of impact conditions that result in a hemispheric dichotomy
matching the observations. Those conditions include an impactor about
one-half to two-thirds the size of the Moon, striking at an angle of 30 to
60 degrees.
"This is how planets finish their business of formation," Asphaug said.
"They collide with other bodies of comparable size in gargantuan collisions.
The last of those big collisions defines the planet."
According to Nimmo's analysis, shock waves from the impact would travel
through the planet and disrupt the crust on the other side, causing changes
in the magnetic field recorded there. The predicted changes are consistent
with observations of magnetic anomalies in the southern hemisphere, he said.
In addition, new crust that formed in the northern lowlands would be derived
from deep mantle rock melted by the impact and should have significantly
different characteristics from the southern hemisphere crust. Certain
Martian meteorites may have originated from the northern crust, Nimmo said.
The study also suggests that the impact occurred around the same time as the
impact on Earth that created the Moon.
This research was funded by NASA.
Note to reporters: You may contact Nimmo at (831) 459-1783
IMAGE CAPTIONS:
[Image 1:
http://www.ucsc.edu/news_events/img/2008/06/mars-349.jpg (87KB)]
The northern hemisphere of Mars is low (blue) and smooth, while the southern
hemisphere is high (red) and heavily cratered. Image courtesy of NASA.
[Image 2:
http://www.ucsc.edu/news_events/img/2008/06/impact-349.jpg (48KB)]
This image from a computer simulation shows the type of impact that could
have created the Mars hemispheric dichotomy. Credit: M. Marinova, O.
Aharonson, E. Asphaug.