Crescent Mercury

Crescent Mercury. Image courtesy of Flowers Gallery. © NASA/APL/Michael Benson, Kinetikon Pictures

Factfile: Mercury

Don't be taken in by its dull appearance - Mercury's eccentric orbit and wide-ranging temperatures make it a world of extremes.

Skirting the Sun at a fraction of Earth's orbit, Mercury is our solar system's smallest and innermost planet. Named for the swift-footed Roman messenger god, it takes just 88 Earth days to complete one circuit around the Sun, travelling an average of 47 kilometres every second - almost 60% faster than the Earth.

Its cratered surface is visually similar that of our Moon and, like the Moon, there are indications that some of Mercury's permanently shaded areas may hold deposits of ice - a surprising finding, given the planet's proximity to the Sun.

This image, one of 77 composite photographs appearing in Otherworlds: Visions of our Solar System, shows these craters being thrown into sharp relief by Mercury's terminator line, which divides the planet's day and night sides.

It was captured by NASA's MESSENGER spacecraft, one of just two probes to have visited the planet.

Slow cooker

Contrast can also be found in Mercury's unusually elongated orbit. Over the course of its year, the planet ventures to a distance of 46 million kilometres from the Sun, before retreating to a comparatively distant 70 million kilometres.

In comparison, Earth orbits from a distance of 147 to 152 million kilometres.

Being so close to the Sun has strange effects on Mercury's rotation. It takes 59 Earth days it to spin once on its axis, meaning that the planet experiences just three days for every two of its years.

Without a substantial atmosphere to distribute heat away from the areas facing the Sun, this slow rotation makes for stark temperature differences between Mercury's dark and light sides.

At its hottest, the planet's surface can reach 427°C - hot enough to melt lead - but at its coldest, it can fall to a frigid -173°C.

Extraterrestrial materials

Discover more about Museum research on the Moon, Sun, Mars, comets and asteroids.