Unfortunately these naturally-made hard drives don't contain alien music or video games. But they do carry records of the way their parent asteroids formed -- and it turns out that the asteroids of yore may give us a glimpse into Earth's distant future.
The new method of reading the magnetic messages in space rocks is reported this week in Nature.
"Until we developed this method, it was always thought that the metal in meteorites made a very poor magnetic recorder," said study author Richard Harrison, a geologist from the University of Cambridge.
"Imagine writing information onto a computer hard drive and burying it for 4.5 billion years, then digging it up and trying to pull the data off of it. Unless you've got a very particular kind of hard drive, you're going to have some trouble," he said.
Harrison and his colleagues had good reasons to keep trying. Meteorites are fallen fragments of asteroids that formed in the very early days of the solar system. Some of those asteroids had liquid outer cores (like Earth's) that gave them their own magnetic fields. And any time a space object is sort-of-but-not-quite-like our own planet, researchers get excited: Understanding what makes Earth different can help us understand how our planet came to be.
They accomplished the feat by shining high-intensity X-rays on the meteorite samples to detect their magnetic signals, which they were able to decode -- even though the space magnets carrying asteroid information are 1/1000th the width of a human hair.
"We treat it as a kind of cosmic archaeology," Harrison said. "As if we found an ancient scroll with text written in tiny, tiny letters. And this method enables the meteorite to tell us its story."
And what they found is that the asteroids behaved very much like miniature versions of Earth: They had liquid iron inner cores that swirled around inside them to generate a magnetic field.
Just as Earth will in a few billion years, the asteroids then cooled down until their cores were solid -- a process that killed their magnetic fields. But the new study suggests that the asteroids kept their fields much longer than previously believed -- and they have their similarity to Earth to thank for it.
As the asteroids cooled, the segregation of their cores into liquid metal and rock actually drove the swirling motions of the inner core. That generated a long-lasting magnetic field just like the one on Earth.
But because the asteroids were so much smaller, their cooling still occurred on a relatively short timescale.
"It's funny that we study other bodies in order to learn more about the Earth," first author James Bryson, a Cambridge PhD student, said in a statement. "Since asteroids are much smaller than the Earth, they cooled much more quickly, so these processes occur on shorter timescales, enabling us to study the whole process of core solidification."
The new method opens up a new observational tool for all sorts of asteroids. "There's a whole range of different types of meteorites out there, and now we can apply this method to lots of them," Harrison said.