The shocking discovery that Mercury has a strong magnetic field led to three working hypotheses. First, Mercury might have once been a bigger planet that somehow got its outer layers stripped off. Earth, like the other terrestrial planets, consists of about one-third iron core by mass; its outer two-thirds is made of a lighter, nonmetallic shell of silicates. If Mercury started out the same way, somehow it would have had to get rid of about three-quarters of its silicate outer shell to arrive at the planet’s current density. What event could cause such a downsizing? Did the sun have a temper tantrum, throwing off enough heat and radiation to make today’s sunspots and coronal mass ejections look like hiccups by comparison? Such a radiation bath would have vaporized most of Mercury’s crust, and over the eons the solar wind would have blown the remnants back out into the solar system.
Another possibility is that Mercury was once a bigger planet and lost its outer shell not through an encounter with the sun but in a collision with an asteroid or another planet. Back in the early years of the solar system, planetoids formed by the accretion of dust and gas, and increasingly these objects began bumping into one another. Big collisions were fairly commonplace; a similar collision almost surely created our moon. Eventually the collisions led to a war of relative giants, like sumo wrestlers knocking each other out of the orbital ring. The destruction of Mercury’s outer shell would have meant a collision with another planet of similar size, the remains of which would have plunged into the sun.
There’s a third, more intriguing possibility. Back when the solar system was a swirling disk of dust and gas (the solar nebula), the material closer to the sun—about Mercury’s distance—was composed of stuff that was different from the material of which Venus, Earth, and Mars were made. Perhaps this stuff was richer in iron and other metals. As the nebula cooled, silicate material and metallic material tended to condense into small particles. Because the rocky silicate particles would have been much lighter than the metal ones, they would have slowed down more as they moved through the dust and gas of the still-forming solar system, forming two separate rings. The lighter, innermost ring may have been consumed by the sun, leaving behind the ring of metallic material, which had more time to cool into larger bodies that resisted the slowing effect of the gas and dust. Eventually, that ring formed into Mercury.
This last scenario puts Mercury squarely in the center of some of the most important questions in astronomy. If correct, it upsets current thinking on how our solar system formed and, by extension, how other life-supporting solar systems might have formed. One way to address the issue would be to observe the process occurring in other, younger planetary systems, but that would require making observations that planetary scientists haven’t yet been able to manage. Another way is to look in our own backyard, as it were, and get to the bottom, once and for all, of the riddle of Mercury.
“Even though we didn’t set out to link anything we’re doing at Mercury to other planetary systems, that field has been advancing so quickly that Mercury has achieved another interesting importance,” says Sean Solomon, principal investigator for the Messenger mission. “Mercury is our outpost on the innermost remnant of the solar nebula, the material out of which all the planets form.”
Shooting Mariner 10 past Mercury was one thing. Getting the orbiter to slow down enough to orbit the planet involves a challenge of a much higher order. The Messenger mission calls for a Delta 2 rocket to send the probe off in the same direction as Earth’s orbit and have it spiral inward toward the sun. Like a car merging onto a highway, Messenger must first accelerate in order to get in sync with Mercury as it speeds around the sun. But before it reaches Mercury, the probe will overtake Venus three times, killing some momentum each time. Messenger will then do two flybys of Mercury and on the third will fire its rockets and slow virtually to a halt relative to the planet, allowing the planet’s gravity to grab it.