[meteorite-list] NPA 06-25-1981 Meteorites...Martian fragments, McSween

[MARK BOSTICK]

Paper: Syracuse Herald Journal
City: Syracuse, New York
Date: Thursday, June 25, 1981
Page: A-11, National

Meteorites indicate Martian fragments

By PATRICK YOUNG
Newhouse News Service

     WASHINGTON - Scientists studying four rare meteorites believe they may 
be looking at stony fragments from one of Earth's neighboring planets - 
probably Mars.
     Dating techniques show rocks in three of the meteorites formed between 
600 to 1.2 billion years ago when molten lava cooled.  This is the youngest 
rock ever found in such visitors from outer space.  Dating efforts are 
continuing on the fourth, an 18-pounder found in the Antarctic less than two 
years ago.
     The four meteorites are classified as Shergottites, and they are the 
only ones of their kind scientists know about.  Dating them has proved 
difficult because of their history.

Shock effects

     "They suffered a lot of shock effects from being ripped from their 
parent body," says Harold McSween of the University of Tennessee in 
Knoxville, who has studied all four meteorites.  How they were blasted free 
remains unknown.
    New evidence indicates the meteorites originated on some sizable body, a 
finding that has surprised scientists.  "Before this we thought all 
meteorites came from the Asteroid Belt," McSween says.
     The Asteroid Belt is a ring of material ranging from dust grains to 
planetoids orbiting the sun between Earth and Mars.  Meteorites from there 
date back 4.6 billion years, the time of the solar system's formation.
     Researchers say it is unlikely the type of conditions that formed the 
Shergottite meteorites existed in the Asteroid Belt within the last 1.2 
billion years.
     "We think we understand how melting could occur on a small body 4.6 
billion years ago, but not 1 billion years ago," McSween says.  "That leaves 
two alternatives.  One they all came from a planet; two, we don't understand 
melting on small bodies."
     Rocks returned by the Apollo astronauts rule it out as the meteorite's 
source, and other factors seem to rule out Mercury and Venus.

Volcanic acitivity

     But the right conditions may well have existed on Mars.  Volcanic 
activity still occurred on the planet at least 500 million years aog, and 
some scientists think Mars may still be active.
     Researchers have also compared the chemical composition of the 
Shergottite meteorites to soil studies curried out by the two Viking 
spacecraft that landed on Mars in 1976.  The Martian surface is rich in 
sulfur and chlorine, which may be left from volcanic eruptions.  If these 
two chemicals are subtracted, the composition of Mars and the meteorites is 
remarkably alike.

(end)

Meteorites referenced include: Shegotty, Zagami, ALH 77005, and Yamato 
793605.