[meteorite-list] Why a Mars Rock Hits Earth Every Month

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:14 2004
Message-ID: <200211072040.MAA16323_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/solarsystem/mars_knocks_021107.html

Why a Mars Rock Hits Earth Every Month
By Robert Roy Britt
space.com
07 November 2002

Every month, on average, a rock from Mars lands on Earth. Most are never
found, but those that have been picked up suggest that the theory for how
they get here - having been booted from the Red Planet by very large
asteroid impacts - is not fully accurate.

Now a new computer simulation appears to solve the puzzle by showing that
relatively small collisions can do the trick.

Scientists know that space rocks ranging from the size of a car to that of a
city have hit Mars many times throughout history. In some of these
collisions, chunks of Mars are flung into space and never return. Some go on
journeys that can last millions of years before being captured by our own
planet's gravity.

Meteorite hunters have found about 26 rocks on Earth that have been
identified as having come from Mars (some of these broke apart upon entering
the atmosphere, so the 26 rocks were found as about 40 separate pieces).

Scientists had thought it took a serious wallop to instigate these
interplanetary exchanges. Yet the new research finds that craters as small
as 1.9 miles (3 kilometers) wide on Mars could have been the starting points
for rocky odysseys.

This minimum crater diameter is at least four times smaller than previous
estimates, the scientists write in an account published today in the online
version of the journal Science.

The study was done by James Head and Jay Melosh of the University of
Arizona, with Boris Ivanov of the Russian Academy of Sciences.

The scientists said terrain covered by weaker material, which might be
created in previous impacts, requires larger events to scoot stuff all the
way to Earth. That means, they say, that Martian meteorites found on Earth
should tend be from a young Mars, a projection that fits with the dating
done on actual rocks that have been collected.

In an interview with SPACE.com, Head, who also works for Raytheon Missile
Systems, explained what the new simulation reveals.

An asteroid one-and-a-half times the size of a football field slams into
Mars at 22,370 mph (10 kilometers per second). The energy of the impact is
equal to about 60 megatons of TNT, comparable to the largest nuclear devices
ever tested.

A strong shock wave begins to form. The leading edge of the shock wave
reflects off the surface from below and interferes destructively with the
rest of the incoming shock wave, canceling out the high pressure near the
surface. At the surface, the pressure is zero, according to the simulation.
Just below the surface, however, the pressure is great.

"The pressure difference accelerates the material to high speed," Head said.
"About 10 million fragments averaging 5 centimeters across [2 inches] are
accelerated to speeds in excess of 5 kilometers per second [11,180 miles per
hour]."

That is the escape velocity of Mars, the speed needed to leave the planet
without going into orbit around it.

"According to the celestial mechanics people, about 7.5 percent of this
material is destined to land on the Earth," Head says. "More than half of
that lands in the first 10 million years after the impact."

Impacts of this size and larger occur every 200,000 years or so on Mars.
About once every 2 million years, an impact of this size occurs on terrain
suited to the scenario Head and his colleagues lay out. This means fragments
from several impacts are in transit all the time.

"This works out to about one Martian meteorite landing on Earth each month,"
Head said.

These are not the only space rocks that hit Earth, Head points out. While
only a few dozen Mars meteorites have been discovered, the total number of
space rocks collected on our planet is about 20,000.
Received on Thu 07 Nov 2002 03:40:02 PM PST