One of the most common types of asteroid zipping through our solar system may have originated as giant chunks of space mud, argues a new paper out today in Science Advances.
Yes, mud. When I first saw this study, I assumed that I was taking the name of the hypothesis it describes—the "mudball model"—too literally. So I phoned author Philip Bland, a planetary scientist at Curtin University in Perth, Australia. "I'm sure you don't mean mudballs in space," I said. He cleared it up for me.
"When you envisage mud," he said, "that's pretty much what we're thinking about."
Bland, who wrote the paper with B.J. Travis of the Planetary Science Institute in Arizona, studies the evolution of the solar system, and how bodies like rocky asteroids and planets came to be. To do this, he looks at meteorites (asteroids that have crashed to Earth).
"A lot of people, including me, have assumed that the rocky meteorites [we see today] came from rocky asteroids, and it's a natural assumption to think they've always been rock," he said. Other studies of how water and rock interact with each other have suggested these objects lithified—a process that describes when sediments compact under pressure.
GIF: Ralph Damman
Bland's research told him something different. When water moves very slowly through a rock, "it strips out the elements and moves them somewhere else," he explained. "That didn't happen in these objects," he continued, referring to the meteorite samples they examined. "Although they saw a lot of water, no soluble elements got moved."
There's more, though. When Bland looked at samples on a finer scale, he saw "grains right next to each other that seem to have a completely different history." Taken together, the meteorite pieces didn't show signs of coming from an object with a "thermal gradient" from hot in the middle, to cold on the outside, as he'd expected. All these factors are hard to explain with existing models of asteroid formation, he told me.
To Bland and his colleagues, these facts taken together suggest that dark-coloured carbonaceous asteroids, which make up about 75 percent of those in our solar system, started out as a kind of sludgy space mudball. They formed not too long, relatively speaking, after our solar system was born: Fine particles suspended in water, kept warm by radioactive elements within. Eventually, the mudballs solidified and were locked into their current shape.
While it seems strange to think that there's enough water in space to make space sludge, it's important to remember that one of water's building blocks, hydrogen, is the most abundant element in the universe that we know of. In space, water is everywhere.
So why does it matter? "The thing I enjoy about planetary science is how much is still up for grabs," Bland told me. "In geology we've got a grand unifying theory, plate tectonics, that we can plug everything into. There's nothing really like that in planetary science," and processes like asteroid formation, for example, are still imperfectly understood, he said. Bland is hoping to apply this new "mudball model" to other kinds of asteroids and see how it holds up.
Carbonaceous asteroids hold some of the basic ingredients for planet-building. Unravelling how they're born is important for understanding our solar system, and places beyond it, too.
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