General Information
Mercury is the closest planet to the sun, at a mean distance of 58 million km. If one could stand on the surface of Mercury when it is at its closest point to the Sun, the Sun would appear almost three times as large as it does from Earth. Scientists used to think that the same side of Mercury always faces the sun, but in 1965, astronomers discovered that the planet rotates three times during every two orbits. Mercury speeds around the sun every 88 days traveling through space at nearly 50 km per second — faster than any other planet. One Mercury day equals 176 Earth days.
Because Mercury is so close to the Sun, it is hard to observe directly from Earth except during twilight. Mercury makes an appearance indirectly, however — 13 times each century, Earth observers can watch Mercury pass across the face of the Sun, an event called a transit.
With a diameter of about 4,900 km, Mercury is the smallest planet in the solar system, only slightly larger than the Moon. It is also the second densest planet after Earth, with a large iron core having a radius of 1,850 km, about 75% of the planet’s radius. Its outer shell, comparable to Earth’s outer shell (called the mantle), is only 500 km thick.
Temperatures on Mercury’s surface can reach 430°C. Because the planet has no atmosphere to retain that heat, night time temperatures on the surface can drop to –70°C. Rather than an atmosphere, Mercury possesses a thin “exosphere” made up of atoms blasted off its surface by the solar wind and micrometeorites.
Craters
Mercury’s surface resembles that of Earth’s Moon, scarred by many impact craters (materialized by sequins on the tactile image- point2) resulting from collisions with meteorites and comets.
Caloris Basin
The Caloris Basin (materialized by the largest sequin on the tactile image- point 3), one of the largest features on Mercury, is about 1,300 km in diameter. It was the result of an asteroid impact on the planet’s surface early in the solar system’s history. Over the next half-billion years, the outer crust contracted and grew strong enough to prevent magma from reaching the surface, ending the period of geologic activity.
Scarps or cliffs
While there are areas of smooth terrain, there are also lobe-shaped scarps or cliffs (materialized by deep strips on the thick fabric on the tactile image- point 4), some hundreds of kilometres long and soaring up to a kilometre high, formed by early contraction of the crust.