Yet Mercury is a dense planet, with a large iron core and a relatively thin overlay of silicon-based rock. This may be because the planet formed so close to the Sun that most of the silicate material in the region was too hot to condense and solidify; it remained gaseous and was blown away on the solar wind, leaving little material for the planet to build on except iron and other metals.

Another possibility, though, is that most of Mercury’s rocky crust was blasted away into space by the impact of a mammoth asteroid early in the solar system’s history. Mercury’s battered, airless surface looks much like the Moon’s, testimony to the pitiless barrage of asteroids and larger planetesimals that hurtled through the solar system more than three billion years ago. Caloris Basin is a huge bull’s-eye of circular mountain ridges some 1,300 kilometers in diameter. This gigantic impact crater is the center of fault lines that run for hundreds of kilometers across the planet’s rocky surface.

An asteroid roughly one hundred kilometers wide smashed into Mercury nearly four billion years ago, gouging out Caloris Basin and perhaps blasting away most of the planet’s rocky crust.

Despite the blazing heat from the nearby Sun, water ice exists at Mercury’s polar regions. Ice from comets that crashed into the planet has been cached in deep craters near the poles, where sunlight never reaches. Just as on the Moon, ice is an invaluable resource for humans and their machines.

DANTE’S INFERNO

Yamagata rode the small shuttle down to the planet’s airless surface in his shirtsleeves, strapped into an ergonomically cushioned chair directly behind the pilot and copilot. Both the humans were redundancies: the shuttle could have flown perfectly well on its internal computer guidance, but Himawari’s captain had insisted that not merely one but two humans should accompany their illustrious employer.