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For decades, scientists have hypothesized that some meteorites striking Earth originated on Mars, after being blasted into space when other space objects slammed into the Red Planet or by explosive volcanic eruptions. In the 1990's those surmises took on particular poignancy when researchers set off a scientific firestorm by announcing that they had discovered submicroscopic structures that appeared to be fossilized lifeforms and found biochemical signatures in those structures. See this video about a presumably Martian meteorite designated ALH84001:
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(The bandwidth impaired will find a transcript of the video out in the tall grass.)

Interestingly, there was little controversy about where the rock came from. Scientists inferred the Martian origin of ALH84001 by analyzing the composition of atmospheric gas found within it, comparing it to measurements from the first NASA Martian landers, and finding the gases matched the atmosphere of Mars:  

The strongest link between Mars and the martian meteorites is the discovery of martian atmosphere gas inside the meteorites. But even before martian atmosphere gas was discovered in the meteorites by Bogard and Johnson (1983), many scientists thought that the meteorites were from Mars because of their young crystallization ages and their complex chemical compositions. Even then, it was certain that the meteorites were not from the Earth because their oxygen isotope compositions are utterly distinct from those of Earth rocks (Clayton and Mayeda, 1996).
However, no matter how good a scientific inference may be, measurable and testable observations are always preferred, the more precise, the better. Thanks to the Mars Science Laboratory on the Curiosity Rover, we now have those observations. According to NASA:  
A key new measurement of the inert gas argon in Mars' atmosphere by Curiosity's laboratory provides the most definitive evidence yet of the origin of Mars meteorites while at the same time providing a way to rule out Martian origin of other meteorites.

The new measurement is a high-precision count of two forms of argon -- argon-36 and argon-38 -- accomplished by the Sample Analysis at Mars (SAM) instrument inside the rover. These lighter and heavier forms, or isotopes, of argon exist naturally throughout the solar system. On Mars the ratio of light to heavy argon is skewed because much of that planet's original atmosphere was lost to space. The lighter form of argon was taken away more readily because it rises to the top of the atmosphere more easily and requires less energy to escape. That left the Martian atmosphere relatively enriched in the heavier isotope, 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 of argon-38). Measurements by NASA's Viking landers in the 1970s 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, lead author of an Oct. 16 paper reporting the finding in Geophysical Research Letters. "This direct reading from Mars settles the case with all Martian meteorites."

So while controversy may continue about what Martian meteorites tell us about Mars, the question of their origin is now "nailed" shut.

 

Transcript of video:

That wasn't why ALH84001 was so important. This rock would become the center of a media firestorm that would polarize the scientific community.

[President Bill Clinton] "If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered."

[Planetary Scientist Dr. David McKay] "The simplest explanation to us is that they are the remains of Martian life."  

In 1996, scientists concluded that this ancient rock appeared to contain evidence of Martians.

[Unidentified Scientist/Technician] "Not living organisms, but the fossilized and chemical remains of ancient Martian life.  And that's why this meteorite has become the center of research over the years."

Their evidence included chemicals that form from the breakdown of bacteria. Iron oxide crystals that are formed on Earth by bacteria and, most compelling, what appeared to be fossils.

[Unidentified Scientist/Technician] "There are tiny little objects within this meteorite, that can only be seen by a high powered electron microscope, that look like fossilized, single cell organisms, fossil bacteria."

The scientists studying the rock determined that with such dramatic news as fossils from another planet, they needed to go public with their results.

(Gary R. Huss, Arizona State University] "Ideas have been tested now for over a decade and a lot of the original ideas didn't hold up. Some of them may still have a little bit of life left in them and, depending upon who you are, that's either enough to make you sure that the theory is right or, basically, the proof that says it's not right"

For all my Mars diaries and all things Mars on Daily Kos, go to Kossacks on Mars.

Originally posted to Kossacks on Mars on Sun Oct 20, 2013 at 11:36 AM PDT.

Also republished by SciTech.

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