New tests on what's considered the best-preserved meteorite in the world have shed light on the way asteroids may have served up a varying menu of the building blocks of life.
In January, 2000, a large meteoroid exploded in the atmosphere over northern British Columbia, Canada, scattering fragments across the frozen surface of Tagish Lake. Because many people collected pieces within days and kept them preserved in their frozen state, there was very little contamination from terrestrial life.
"The first Tagish Lake samples - the ones we used in our study that were collected within days of the fall - are the closest we have to an asteroid sample return mission in terms of cleanliness," says Dr Michael Callahan of NASA's Goddard Space Flight Center.
As expected, the team discovered the fragments contained an assortment of organic matter including amino acids. Surprisingly, though, they found that different pieces had greatly differing amounts of amino acids.
"We see that some pieces have 10 to 100 times the amount of specific amino acids than other pieces," says Dr Daniel Glavin of NASA Goddard.
"We've never seen this kind of variability from a single parent asteroid before. Only one other meteorite fall, called Almahata Sitta, matches Tagish Lake in terms of diversity, but it came from an asteroid that appears to be a mash-up of many different asteroids."
Analysis of the different minerals present in each fragment indicated that they'd been exposed to different amounts of water, suggesting that water alteration could account for the diversity in amino acid production.
If the variability in the Tagish Lake meteorite turns out to be common, researchers will have to be cautious about deciding whether meteorites delivered enough bio-molecules to help jump-start life.
"Biochemical reactions are concentration dependent," says Callahan. "If you're below the limit, you're toast, but if you're above it, you're OK. One meteorite might have levels below the limit, but the diversity in Tagish Lake shows that collecting just one fragment might not be enough to get the whole story."