Ancient stardust in the meteorite

presolar dust

A few micrometers small granules from a meteorite (detail) turned out to be presolar star dust. (Image: NASA, W. Sparks (STScI) / R. Sahai (JPL), Janaina N. Avila)

Our solar system and almost all of its components were created around 4.6 billion years ago. But there are cosmic “time capsules” that have preserved material from before the sun was formed – meteorites. In one of them, the Murchison meteorite that struck Australia in 1969, researchers have now discovered the oldest known dust grains on earth. According to isotope dating, some of these small micrometer-sized granules are five to seven billion years old. According to the researchers, they are probably from an episode of increased star formation at that time in our cosmic neighborhood.

We all consist of star dust – in the truest sense of the word. Because most chemical elements in the cosmos were once created by nuclear fusion inside stars and by star explosions. The atoms that make up the planets of the solar system ultimately also stem from these processes. Most of this cosmic material in our solar system has been changed so much by chemical reactions and weathering that its origin is no longer recognizable. Original star dust is only preserved in the interstellar medium and partly inside meteorites. However, dating these tiny granules is extremely difficult because they are largely too small for common dating methods and often have exotic isotope compositions that falsify the measurements, as Philipp Heck from the Field Museum of Natural History in Chicago and his colleagues explain.

Interstellar dust from the Murchison meteorite

However, the researchers have now found a way to determine the age of stellar dust trapped in meteorites despite these difficulties. The starting material for this was a sample from the Murchison meteorite, a very original rock meteorite that struck 1969 near the Australian town of Murchison. Since then, a large part of the meteorite fragments has been stored in special insulated containers in the Chicago Field Museum. The researchers isolated a few granules of interstellar dust from samples of these fragments. “It starts by grinding the rock fragments into a powder,” explains Heck’s colleague Jennika Greer. “Then when all the pieces have been broken up, you have a kind of paste, which gives off a characteristic smell of rotten peanut butter.” This paste is then dissolved with acids until only the presumably granular grains remain. “It’s a bit like burning the haystack to find the needle,” says Heck.

In total, this procedure gave the researchers 40 granules of presolar material that were only a few micrometers in size. “They are real samples of star material – stardust,” says Heck. In order to determine their age, the researchers analyzed the granules for their content as cosmogenic neon isotopes – atomic variants that result from the impact of cosmic rays on the material. “The longer the grains are exposed to this radiation, the more these isotopes are formed,” explains Heck. “By measuring how many of these elements generated by cosmic rays are contained in a presolar granule, we can determine how long they have been exposed to the radiation and thus how old they are.” About falsifications by the strong radiation of the young sun To avoid this, the scientists used a model that took these and other disturbances into account when calculating the age.

Up to seven billion years old

The analyzes showed that many of these grains are older than the solar system. Most date from 4.6 to 4.9 billion years ago, but some are up to seven billion years old. “These are the oldest solid materials that have ever been found – and they tell us something about how the stars formed in our galaxy,” says Heck. Because the researchers suspect that the majority of these presolar granules once emerged in a phase of increased star formation around seven billion years ago. “We estimate that the stars that came together in this phase reached their dust-generating phase as giant stars between 4.9 and 4.6 billion years ago,” report the scientists. At this time, the dust particles were released and floated around in the interstellar medium until they were enclosed and preserved by the meteorites that formed in the young solar system.

As the researchers explain, this also sheds new light on the history of star formation in our galaxy: “Some people think that the star formation rate of the galaxy is constant,” says Heck. “But thanks to these granules from the meteorite samples, we now have direct evidence of a phase of increased star formation around seven billion years ago. That is one of the key results of our study. ”But the isotope analyzes revealed something new: at least some of these presolar granules must have drifted through the interstellar medium in the form of clumps that had stuck together. These aggregates were around 30 times larger than the granules themselves, as the researchers report. “Nobody thought that this would be possible on this scale,” said Heck. “It’s really exciting to decode the history of our galaxy this way.”

Source: Philipp Heck (Field Museum of Natural History, Chicago) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.1904573117

Recent Articles

Related Stories