On the whole, there are two types of people in the world – those who love Marmite (or Vegemite) and those who hate the spread. Hardly anyone falls in between the camps. In astronomy, there’s a similar scenario – most meteorites that have landed on Earth are fragments of either a completely melted planetesimal (a small rocky body formed during the creation of planets) or an unmelted one.
But just like the ambivalent Marmite consumer, there is an intermediate family of meteorites that bucks the trend, originating from a chimera body that was both melted and unmelted.
“These IIE irons are oddball meteorites,” Professor Benjamin Weiss, from the Massachusetts Institute of Technology (MIT), said in a statement. “They show both evidence of being from primordial objects that never melted, and also evidence for coming from a body that's completely or at least substantially melted. We haven't known where to put them, and that's what made us zero in on them.”
Previous studies had found this ancient parent planetesimal, formed from collisions during the first few million years of the solar system’s existence, to have a solid crust overlying a liquid mantle, similar to Earth. But Clara Maurel, a graduate student at MIT and lead author of the study published in Science Advances, was curious as to whether this was where the similarities ended.
“Did this object melt enough that material sank to the center and formed a metallic core like that of the Earth?” Maurel wondered. “That was the missing piece to the story of these meteorites.”
If this planetesimal had a metallic core, it would be reasonable to assume it also generated a magnetic field. Over time, this ancient field could have left its mark on minerals within the celestial body, causing them to align in the direction of the field, like a needle in a compass. Fast forward a few billion years, and in some cases this property could still be detected.
Maurel and her colleagues searched IIE meteorite samples for a particular magnetism-maintaining substance – a type of iron-nickel mineral. Under the probing power of X-rays, the team found that electrons within a number of these grains were aligned in a similar direction, providing evidence for a magnetic field long ago. In fact, its strength was probably about the same as the Earth’s magnetic field of up to several tens of microtesla.
But a question still remained – whereabouts in this complex planetesimal’s cross-section did the meteorites come from? High-velocity simulations by the University of Chicago revealed that collisions with another object possibly dislodged material from the magnetic core, which then migrated to pockets near the body’s surface.
“As the body cools, the meteorites in these pockets will imprint this magnetic field in their minerals,” Maurel explained. “At some point, the magnetic field will decay, but the imprint will remain. Later on, this body is going to undergo a lot of other collisions until the ultimate collisions that will place these meteorites on Earth's trajectory.”
This puzzling cosmic chimera has certainly opened up the spectrum of planetesimals, but there could still be similar celestial structures yet to be found.
“Most bodies in the asteroid belt appear unmelted on their surface,” Weiss said. “If we're eventually able to see inside asteroids, we might test this idea. Maybe some asteroids are melted inside, and bodies like this planetesimal are actually common.”