Mercury’s strange anatomy could be the result of intense violence in the very young solar system.
Unlike Earth, the solar system’s shrinking, smallest planet has an iron core that occupies 65 percent of its mass – Earth’s core, in contrast, is a measly 32 percent. The origin of Mercury’s massive metal heart has been a long-standing conundrum.
Now, scientists think they’ve solved the mystery, and the story goes like this.
For about 100 million years, little Mercury grew up as rocky planets normally do. Dust swirling near the infant sun coalesced into clumps that collided and formed bigger clumps, which continued to collide until they eventually became something planet-like.
Proto-Mercury, scientists now say, was originally a much larger planet – something more like the Earth in size and composition, and in an orbit much more similar to Earth’s. Its innards differentiated into a heavy metal core and a thick, light, silicate mantle, just as a planet’s innards should.
But then something happened. A massive, catastrophic collision with one of Mercury’s planetary siblings – young Earth or young Venus – shattered the small planet. Much of Mercury’s mantle fractured and peeled off, then re-congealed around its bigger sibling. Over the next tens of thousands of years, a series of subsequent collisions may have continued to shave off bits of young Mercury, and the planet was booted from its orbit and flung toward the inhospitable sun.
“Each of these collisions would blast off about one-third of the planet’s mantle,” says planetary scientist Erik Asphaug, of Arizona State University, who described the findings July 6 in Nature Geoscience. “The most likely scenario of course is for Mercury itself to be gobbled up by Venus (or the Earth). But this is not a story of the most likely — it is the story of the unlikely survivor.”
By the time the chaos subsided, that unlikely survivor was the dense, tough little roasted world we recognize today.
The story explains both the planet’s large core and its retention of volatiles – elements such as potassium and sulfur, as well as water ice and organics – which aren’t explained by older, discarded theories (among them, the idea that the sun boiled off much of Mercury’s mantle).
It also suggests that Mercury – and, indeed, Mars – are leftover scraps from the early ages of planet formation in our solar system. Planets like Earth are thought to form when planetary embryos, or planetesimals, collide and merge to form ever-larger proto-planets. But sometimes, the team’s work suggests, those collisions don’t result in total mergers. Sometimes, the smaller, bruised embryos are left behind and continue growing into planets on their own.
“To not be accreted requires rare good luck,” Asphaug says.
Rather than poking at planetary scars, reconstructing Mercury’s bruised infancy meant turning toward computers. Asphaug and Andreas Reufer, of the University of Bern, ran a number of computer simulations of early planet formation; in them, they varied the sizes and speeds of the impactors and the angles at which the bodies collide. They found that if you start with 20 Mars-size bodies, you often end up with most of them colliding and combining to form two large, rocky planets (Earth and Venus); among the surviving proto-planetary relics are one bruised and beaten up small planet (Mercury) and one rocky, more-or-less unscathed small planet (Mars).
“The statistics of being lucky suggests that we should have one of each kind of survivor in our pair of next-largest planets,” Asphaug says. “The remnants of planet formation are exotic, hit-and-run survivors.”
This isn’t the first time that impact scenarios have been proposed to explain Mercury; in fact, Asphaug and his colleagues have been batting around the possibility for years. And, at a meeting of earlier this year, a different group of scientists proposed a similar idea. But Mercury’s mantle being stolen by the larger planet is new.
“The idea that Mercury is the surviving portion of the projectile, rather than the target, and it might be a survivor of multiple such events” is intriguing, says planetary scientist Steve Hauck, of Case Western Reserve University. Hauck studies Mercury’s interior and is a member of the MESSENGER team, which has been studying the planet from a spacecraft in orbit there since 2011. “It would be interesting to know how often a Mercury-like body, in terms of the relative amounts of rock and metal, survives,” Hauck says.
It’s not an unusual proposition, that a maturing planetary system might be fundamentally reshaped by these sorts of spasms. “Catastrophic disruption is part and parcel with planet formation, not window dressing that happens after the planets have finished forming,” Asphaug notes. Similar origins have been suggested for large asteroids, like Vesta and Psyche, and scientists think they’ve detected the same kinds of collisions in exoplanetary systems.
The trick and the trouble with these kinds of modeling studies is verifying the scenarios empirically. Though asteroids bear ample evidence for these kinds of hit-and-run collisions, Asphaug says it will be hard to chemically link the mantle of Mercury with either Venus or Earth. “If we can’t even agree on whether or not we have Mercury meteorites in our collection, I doubt we can agree on whether these are related to the mantle rocks of Earth,” Asphaug says. “The missing mantle of Mercury is maybe right beneath our feet.”