[meteorite-list] Giant Asteroid Collision May Have Radically Transformed Mars

[Ron Baalke]

http://www.scientificamerican.com/article/giant-asteroid-collision-may-have-radically-transformed-mars/

Giant Asteroid Collision May Have Radically Transformed Mars

An ancient, global-scale impact could explain the Red Planet's mysterious "two-faced" appearance

By Robin Wylie
Scientific American
February 25, 2016
 
The planet Mars has been associated with its namesake god of war for millennia, 
but its own past may have been more violent than was previously imagined. 
A new study suggests that Mars was once hit by an asteroid so large that 
it melted nearly half of the planet's surface.

Researchers came to this conclusion while studying a strange feature known 
as the Martian hemispheric dichotomy - a dramatic drop in surface elevation 
and crustal thickness that occurs near Mars' equator. In the northern 
hemisphere the land's elevation is on average about 5.5 kilometers lower 
and the crust is around 26 kilometers thinner.

The dichotomy was discovered in the early 1970s when NASA's Mariner 9 
probe made the first detailed map of the Martian surface. The feature 
has perplexed astronomers ever since. Previous studies hinted that the 
dichotomy was formed by a glancing asteroid strike near the Martian north 
pole. But the new work, published in Geophysical Research Letters in December, 
suggests that a far more violent impact, at the opposite end of the planet, 
may have been the actual cause.

In the study astronomers at the Swiss Federal Institute of Technology 
(E.T.H. Zurich) used an advanced 3-D computer model to simulate the effect 
of an asteroid impact on Mars 4.5 billion years ago, when experts think 
the dichotomy formed. They tested a rival hypothesis for its origin—that 
it had been formed by an impact at Mars' south pole.

When the team simulated a collision with an asteroid about 4,000 kilometers 
across (slightly larger than Earth's moon) they found that it caused the 
crust of the "virtual" Mars to reform into two distinct zones: a thicker 
one in the southern hemisphere and a thinner one in the north, similar 
to what we see on the real planet. What's more, the predicted thicknesses 
of the two crustal segments matched the real values observed on Mars almost 
exactly. Taken together these predictions provide compelling evidence 
that a south polar impact was the cause of the dichotomy. "This study 
advances an alternate impact origin for the Martian dichotomy," says Craig 
Agnor, an astronomer at Queen Mary, University of London, who was not 
involved in the work.

The simulation predicted that the impact would have generated so much 
heat that large swathes of Mars' crust would have melted, forming a "magma 
ocean" across most of the planet's southern hemisphere. It also predicted 
that, as the molten rock subsequently cooled and solidified, it would 
leave a thicker, higher-elevation crust over part of that hemisphere.

These findings do not conclusively solve the mystery of the dichotomy's 
origin. The Swiss team's model is not perfect; for example, it cannot 
explain the dichotomy's exact size. And in any case, it is not possible 
to prove a hypothesis using only a computer model. But there is another 
reason to think that the southern impact hypothesis might be right: It 
sheds light on another oddity of Mars' surface - the locations of its volcanoes.

When large asteroids hit rocky planets they tend to induce volcanic activity 
by causing "plumes" of hot rock to rise up within the planet's mantle, 
many years later. A drawback of the previous, "northern" impact scenario 
was that the high northern latitudes of Mars contain relatively few volcanoes, 
which occur mostly in equatorial and southern latitudes. But the southern 
impact simulation predicted that a few million years after the asteroid 
struck, volcanic plumes would slowly begin to rise toward the surface, 
at first near the equator and then gradually migrating toward the south 
pole. This prediction agrees well with the actual locations of the Red 
Planet's volcanoes.

Asteroid impacts of the scope suggested by this study are extremely unlikely 
to happen today. They were probably more common in the early days of the 
solar system, when it was still littered with the rocky debris left over 
from planet-building. But even then such events would have been extremely 
uncommon. "This result has the potential to significantly change our understanding 
of Mars' past," says Giovanni Leone, a planetary scientist at E.T.H. 
Zurich and the study's lead author. "A rare event may have occurred early 
in its history that shaped the planet as we see it today."