Our sun’s beginnings are a mystery. It burst into being 4.6 billion years ago, about 50 million years before the Earth formed. Since the sun is older than the Earth, it’s hard to find physical objects that were around in the sun’s earliest days that bear chemical records. But in a new study in Nature Astronomy by scientists with the University of Chicago, the Field Museum and ETH Zurich, ancient blue crystals trapped in meteorites reveal what the early sun was like.
And apparently, it had a pretty rowdy start.
“The sun was very active in its early life—it had more eruptions and gave off a more intense stream of charged particles. I think of my son: He’s three; he’s very active too,” said study co-author Philipp Heck, associate curator at the Field Museum and part-time associate professor at the University of Chicago. “Almost nothing in the solar system is old enough to really confirm the early sun’s activity, but these minerals from meteorites in the Field Museum’s collections are old enough. They’re probably the first minerals that formed in the solar system.”
The study examined microscopic ice-blue crystals called hibonite. Their composition bears earmarks of nuclear reactions that only would have occurred if the early sun was spitting lots of energetic particles.
“These crystals formed over 4.5 billion years ago and preserve a record of some of the first events that took place in our solar system,” said first author Levke Kööp, a UChicago postdoctoral scholar. “And even though they are so small—many are less than 100 microns across—they are able to retain highly volatile noble gases that were produced through irradiation from the young sun such a long time ago.”
Glimpse into early solar system
In its early days, before the planets formed, the solar system was made up of the sun and a massive disk of gas and dust spiraling around it. The region close to the sun was really hot—more than 2,700 degrees Fahrenheit (1,500 degrees Celsius). For comparison, Venus, the hottest planet in the solar system, with surface temperatures high enough to melt lead, is a measly 872 degrees Fahrenheit.
As the disk cooled down, the earliest minerals began to form—blue hibonite crystals.
“The larger mineral grains from ancient meteorites are only a few times the diameter of a human hair. When we look at a pile of these grains under a microscope, the hibonite grains stand out as little light blue crystals—they’re quite beautiful,” said co-author Andrew Davis, professor of geophysical sciences at the University of Chicago.
When the crystals were newly formed, the young sun continued to flare, shooting protons and other subatomic particles out into space. Some of these particles hit the blue hibonite crystals. When the protons struck the calcium and aluminum atoms in the crystals, the atoms split apart into smaller atoms—neon and helium—which remained trapped inside the crystals for billions of years. These crystals got incorporated into space rocks that eventually fell to Earth.
Researchers have looked at meteorites for evidence of an early active sun before, scientists said, but the evidence remained ambiguous. “We were finally able to establish a clear record because we studied the right samples—hibonites, which are very hard to obtain because they are so rare in meteorites—with the right instrument,” said Kööp.
Lasers melt through meteorite
This time, the team examined the crystals with a unique state-of-the-art mass spectrometer in Switzerland—a garage-sized machine that can determine objects’ chemical makeup. Attached to the mass spectrometer, a laser melted a tiny grain of hibonite crystal from a meteorite, releasing the helium and neon trapped inside so they could be detected. “We got a surprisingly large signal, clearly showing the presence of helium and neon—it was amazing,” said Kööp.
The bits of helium and neon provide the first concrete evidence of the sun’s long-suspected early activity.
“It’d be like if you only knew someone as a calm adult—you’d have reason to believe they were once an active child, but no proof. But if you could go up into their attic and find their old broken toys and books with the pages torn out, it’d be evidence that the person was once a high-energy toddler,” said Heck.
Unlike other hints that the early sun was more active than it is today, there’s no other good explanation for the crystals’ makeup. “It’s always good to see a result that can be clearly interpreted,” said Heck. “The simpler an explanation is, the more confidence we have in it.”
UChicago graduate student Jennika Greer was also an author on the paper, as well as ETH Zurich researchers Henner Busemann, Colin Maden, Matthias Meier and Rainer Wieler.
Citation: “High early solar activity inferred from helium and neon excesses in the oldest meteorite inclusions.” Kööp et al, Nature Astronomy, July 30, 2018. DOI: 10.1038/s41550-018-0527-8