Mercury’s physical characteristics
Because the planet is so close to the sun, Mercury’s surface temperature can reach a scorching 840 degrees F (450 C). However, since this world doesn’t have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 F (minus 170 C), a temperature swing of more than 1,100 degrees F (600 degree C), the greatest in the solar system.
Mercury is the smallest planet — it is only slightly larger than Earth’s moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. About 4 billion years ago, an asteroid roughly 60 miles wide (100 kilometers) struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 km) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas.
As close to the sun as Mercury is, ice may exist in its craters. In 1991, astronomers using radar observations discovered that water ice may lurk at Mercury’s north and south poles inside deep craters that are perpetually shadowed and cold. Comets or meteorites might have delivered ice there, or water vapor might have outgassed from the planet’s interior and frozen out at the poles.
Mercury apparently shrank about 0.6 to 1.2 miles (1 to 2 km) as it cooled in the billions of years after its birth. This caused its surface to crumple, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high.
Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 km) wide, or about 75 percent of the planet’s diameter. In comparison, Mercury’s outer shell is only 300 to 400 miles (500 to 600 km) thick.
A completely unexpected discovery Mariner 10made was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth’s size — that its core should have cooled off long ago. The recent discovery from 2007 Earth-based radar observations that Mercury’s core may still be molten could help explain its magnetism.
Although Mercury’s magnetic field is just 1 percent the strength of Earth’s, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury’s field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet’s surface.
Instead of a substantial atmosphere, Mercury possesses an ultra-thin “exosphere” made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space,forming a tail of particles.