[meteorite-list] Curiosity Rover Confirms Origins of Martian Meteorites

[Ron Baalke]

http://news.agu.org/press-release/curiosity-confirms-origins-of-martian-meteorites/

Curiosity confirms origins of Martian meteorites
American Geophysical Union
16 October 2013

WASHINGTON, DC - Earth's most eminent emissary to Mars has just proven 
that those rare Martian visitors that sometimes drop in on Earth - a.k.a. 
Martian meteorites - really are from the Red Planet. A key new measurement 
of Mars' atmosphere by NASA's Curiosity rover provides the most definitive 
evidence yet of the origins of Mars meteorites while at the same time 
providing a way to rule out Martian origins of other meteorites.

[Image]
Scientists identified meteorites, such as this one nicknamed "Black Beauty," 
as Martian in origin. NASA

The new measurement is a high-precision count of two forms of argon gas 
- Argon-36 and Argon-38 - accomplished by the Sample Analysis at Mars 
(SAM) instrument on Curiosity. These lighter and heavier forms, or isotopes, 
of argon exist naturally throughout the solar system. But on Mars the 
ratio of light to heavy argon is skewed because a lot of that planet's 
original atmosphere was lost to space, with the lighter form of argon 
being taken away more readily because it rises to the top of the atmosphere 
more easily and requires less energy to escape. That's left the Martian 
atmosphere relatively enriched in the heavier Argon-38.

Years of past analyses by Earth-bound scientists of gas bubbles trapped 
inside Martian meteorites had already narrowed the Martian argon ratio 
to between 3.6 and 4.5 (that is 3.6 to 4.5 atoms of Argon-36 to every 
one Argon-38) with the supposed Martian "atmospheric" value near four. 
Measurements by NASA's Viking landers in the 1970's put the Martian atmospheric 
ratio in the range of four to seven. The new SAM direct measurement on 
Mars now pins down the correct argon ratio at 4.2.

"We really nailed it," said Sushil Atreya of the University of Michigan, 
Ann Arbor, the lead author of a paper reporting the finding today in Geophysical 
Research Letters, a journal of the American Geophysical Union. "This direct 
reading from Mars settles the case with all Martian meteorites," he said.

One of the reasons scientists have been so interested in the argon ratio 
in Martian meteorites is that it was - before Curiosity - the best measure 
of how much atmosphere Mars has lost since the planet's earlier, wetter, 
warmer days billions of years ago. Figuring out the planet's atmospheric 
loss would enable scientists to better understand how Mars transformed 
from a once water-rich planet more like our own to the today's drier, 
colder and less hospitable world.

Had Mars held onto its entire atmosphere and its original argon, Atreya 
explained, its ratio of the gas would be the same as that of the Sun and 
Jupiter. They have so much gravity that isotopes can't preferentially 
escape, so their argon ratio - which is 5.5 - represents that of the primordial 
solar system.

While argon comprises only a tiny fraction of the gases lost to space 
from Mars, it is special because it's a noble gas. That means the gas 
is inert, not reacting with other elements or compounds, and therefore 
a more straightforward tracer of the history of the Martian atmosphere.

"Other isotopes measured by SAM on Curiosity also support the loss of 
atmosphere, but none so directly as argon," said Atreya. "Argon is the 
clearest signature of atmospheric loss because it's chemically inert and 
does not interact or exchange with the Martian surface or the interior. 
This was a key measurement that we wanted to carry out on SAM."

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Curiosity 
mission for NASA's Science Mission Directorate, Washington. The SAM investigation 
on the rover is managed by NASA Goddard Space Flight Center, Greenbelt, 
Md.

*****

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scientific institutions who have registered with AGU can download a PDF 
copy of this early view article by clicking on this link: 
http://onlinelibrary.wiley.com/doi/10.1002/2013GL057763/abstract

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Neither the paper nor this press release is under embargo.

Title

"Primordial argon isotope fractionation in the atmosphere of Mars measured 
by the SAM instrument on Curiosity, and implications for atmospheric loss"

Sushil K. Atreya and Michael H. Wong
Department of Atmospheric, Oceanic and Space Sciences, The University 
of Michigan, Ann Arbor, Mich.;

Melissa G. Trainer, Heather B. Franz, Charles A. Malespin, Paul R. Mahaffy, 
Pamela G. Conrad and Anna E.  Brunner
Goddard Space Flight Center, Greenbelt, Md.;

K. Manning
Concordia College, Moorhead, Minn.;

Laurie A. Leshin
School of Science, Rensselaer Polytechnic Institute, Troy, N.Y.;

John H. Jones
NASA Johnson Space Center, Houston, Tx.;

Christopher R. Webster
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, 
Calif.;

Tobias C. Owen
University of Hawaii, Honolulu, Hawaii;

Robert O. Pepin
University of Minnesota, Minneapolis, Minn.;

R. Navarro-González
Universidad Nacional Autónoma de México, Ciudad Universitaria, Apartado, 
México.

Contact information for the authors:

Sushil Atreya, Phone: +1 (734) 936-0489, Email: atreya at umich.edu