A type of meteorite made mostly of iron can contain small amounts of amino acids, suggesting that the rocks could have been a source of the organic molecules for the early Earth and perhaps provided some of the ingredients from which life originated.
The examination of several iron meteorites offers the first analysis of amino acids in this kind of space rock, scientists reported March 25 in Meteoritics & Planetary Science.
"We weren't really sure what we were going to find, or if we would find any detectable amino acids in the meteorites, so just detecting the amino acids was a bit surprising," said Jamie Elsila, a research scientist at the NASA Goddard Space Flight Center and first author of the study.
About 4% of the meteorites that fall to Earth are iron meteorites. However, this group makes up nearly half of the mass of recorded meteorite falls. This means, Elsila said, "that they might be a significant source for delivery of extraterrestrial materials to this planet."
Understanding the composition of meteorites can also reveal insights about the chemistry of the early solar system, she and her colleagues noted.
They and other researchers have found a wide range of organic compounds such as amino acids — the building blocks that lifeforms use to make proteins — in stony meteorites, particularly a class called carbonaceous chondrites.
"We wondered whether amino acids could be formed at metal surfaces, leading us to want to examine these iron meteorites," Elsila said.
She and her team examined three iron meteorites known as Campo del Cielo, Canyon Diablo and Cape York that were found in Argentina, Arizona and Greenland, respectively. They also examined a type of meteorite called a pallasite, which contains silicate materials as well as iron and nickel.
The researchers first crushed their meteorite samples, which were about 1 centimeter in size.
"We then took that powder and heated it with water at 100 [degrees Celsius] for 24 hours, basically making 'meteorite tea' out of it," Elsila said.
This allowed them to extract any water-soluble materials, which they then analyzed for the presence of amino acids.
Compared with carbonaceous chondrites, the amount of amino acids in the iron meteorites was pretty low, Elsila says. Additionally, the researchers did identify some amino acids that they suspected came from earthly lifeforms and had contaminated the meteorites after they struck the ground.
"However, we also noticed amino acids that are uncommon in life on Earth and which also looked distinct from patterns we've seen in other meteorites," Elsila said.
The differing amino acids in stony and iron meteorites suggest that the compounds formed or were preserved through different processes in the two kinds of space rocks.
In particular, the researchers focused on a set of amino acids containing five carbon atoms, which can be arranged in 23 possible structures. The researchers noticed that forms with a straight "backbone" of carbon were particularly common in the iron meteorite samples. They suspect these compounds are leftovers from chemical reactions that created the amino acids on metal surfaces in asteroids, which eventually broke into fragments that entered Earth's atmosphere and became meteorites.
The researchers also examined iron granules that had originated on Earth for the sake of comparison. Here, as well, they detected low amounts of amino acids that aren't commonly found in living organisms.
"We aren't fully sure of the origin of these amino acids," Elsila said. "One possibility is that there was a formation mechanism that occurred in the terrestrial iron similar to what we saw in the meteorites."
Alternatively, she says, both the meteorite and the terrestrial iron might contain molecules that reacted during the grinding and extraction processes to create amino acids.
"If it's true that the meteorites contained precursors rather than amino acids, it's still possible to imagine their release into the Earth's environment and their potential contribution to the prebiotic chemistry of the early Earth," Elsila said.
She and her colleagues will continue exploring the organic contents of meteorites and asteroids. They're particularly excited to examine the giant asteroid 16 Psyche, to which NASA will launch a mission in 2022. This unusual metal-rich asteroid may be the exposed iron and nickel core of an early planet.
"It's possible that we'll learn more about the presence of carbon and organics in metal asteroids, which could help us better understand these iron meteorites," Elsila said.
The study, "Amino acid abundances and compositions in iron and stony‐iron meteorites," published March 25 in Meteoritics & Planetary Science, was authored by Jamie E. Elsila, Natasha M. Johnson, Daniel P. Glavin and Jason P. Dworkin, NASA Goddard Space Flight Center; and Jose C. Aponte, NASA Goddard Space Flight Center and Catholic University of America.