Up close with the Tagish meteorite, thanks to atom probe tomography CREDIT: Chi Ma
As meteorites go, the one that landed on Tagish Lake just over 20 years ago might be dubbed a falling superstar, since it offered a rare opportunity to conduct chemical analysis of material delivered directly from farthest reaches of our solar system. Researchers at the Royal Ontario Museum (ROM) have taken advantage of that opportunity to examine one of the most intriguing questions in chemical biology: why are some of the fundamental molecules of life on Earth almost exclusively left-handed?
This could be the short answer: those were the more common molecules that originated in space and arrived on Earth more often, so life here simply took advantage of its most abundant feedstock.
Meteorites are the pieces of asteroids that are not vaporized as they plummet into our planet’s atmosphere, something that occurs more often than most of us would probably be comfortable knowing. Those who do know represent the dedicated community of meteorite chasers who eagerly travel to sites where asteroid falls have been noted, either by local observers or radar stations. In most cases these samples become quickly contaminated through extensive contact with the terrestrial environment, such as landing in water or soil, which would interact with local organic material in ways that make it all but impossible determine the chemical fingerprint of the original asteroid.
However, it was the dead of winter when an asteroid that was estimated to weigh about 56 tonnes exploded in the sky over Tagish Lake in northern British Columbia. The resulting shower of meteorites landed on the frozen lake’s surface and stayed frozen until they were collected over the next few days and then kept in that state while they were transported to various research facilities.
Among those destinations was the ROM, which has kept its set of Tagish Lake specimens stored at around -50C ever since they arrived. In this state the delicate organic molecule complexes they contain are among the best examples to be found anywhere of material that would have been found in the asteroid and others like it.
“It’s very pristine to us and allows us to do a lot of nice studies,” says Elizabeth Lymer, a doctoral student in earth and space sciences at York University who has been part of a ROM team working on the meteorites. “If we do find anything organic in it, we can pretty much prove that it’s from the meteorite and not contamination from Earth.”
Another outstanding feature of the Tagish meteorites are their carbonaceous make-up, which includes amino acids. While this family of molecules are well known as the building blocks of biological proteins and enzymes, they can be formed abiotically in the harsh environment of deep space. Although the details are still not fully understood, a growing body of research suggests that ultraviolet radiation can interact with ice in space to form a wide range of amino acids with no discrimination toward left- or right-handed chirality.
That prospect has led to speculation that life on Earth was originally seeded with amino acids that were formed this way in space and subsequently arrived here via meteorite. If so, however, that leaves the further mystery of why only a select group of amino acids are found in biological systems, and those are invariably left-handed.
The ROM team therefore referred back to their Tagish specimens, which reveal a bias toward left-handed amino acids. Upon nano-scale examination with atom probe tomography at McMaster University’s Canadian Centre for Electron Microscopy, these molecules were shown to be embedded in interlocking crystals of sodium-rich fluid.
Meteorite analysis often assigns a neutral pH to such fluids in the original asteroid, simply because there is seldom any other reference data. In this case, however, the atom probe technique made it possible to create a computer simulation of the observed structure that suggests the fluid in the asteroid would have been more alkaline, with a pH between 7 and 10.
“If you change the pH even a little bit, things racemize a lot faster,” she explains, noting that an alkaline environment would have accelerated the evolution of left-handed amino acids from thousands of years to a matter of days. If this process were occurring on a significant portion of asteroids heading to Earth, then these amino acids would have been the more common ones that landed, which could explain why they are now the only biological ones found here
These findings were reported in the Proceedings of the National Academies of Science, which published a paper from lead authors Lee White and Kimberly Tait of the ROM group. And while these insights into pH do not put an end to speculations around amino acid chirality, they add a new dimension to what had been a much simpler conception of the conditions that created these molecules.
“It’s like we’re putting together pieces of a puzzle where we don’t know what the shapes look like, and we’re just finding out shapes now,” concludes Lymer. “The more pieces we can add to the picture, the clearer it will become in the future.”