Mercury is the closest planet from the Sun and it orbits the Sun once every 88 Earth days. Due to its eccentric orbit, Mercury has a minimum distance of 46.0 million kilometers from the Sun and a maximum distance of 69.8 million kilometers from the Sun. This causes the intensity of sunlight on Mercury to vary by over a factor of two as Mercury orbits the Sun. Mercury is also locked in a spin-orbit resonance where the ratio of orbital period to spin period is precisely 3:2. This means that Mercury completes three rotations about its axis for every two orbits around the Sun.
With a diameter of 4880 kilometers, Mercury is the smallest of the terrestrial planets and despite its size; it is the only other terrestrial planet besides the Earth that has a global magnetic field. Mercury also has a large and dense iron core which makes up well over half the planet’s mass. This makes Mercury the second densest planet in the Solar System with Earth being the densest due to gravitational compression effects. Had it not been for Earth’s gravitational compression effects, Mercury would have been the densest planet in the Solar System.
Mercury has a large surface temperature range which goes from a maximum of 700 degrees Kelvin at the subsolar point when Mercury is closest to the Sun to a minimum of below 100 degrees Kelvin at the bottoms of craters located around the poles. The subsolar point on a planet such as Mercury is where the Sun is directly overhead and hence, the Sun’s rays strike the surface perpendicularly at the subsolar point.
Permanently shadowed craters are known to exist around the north and south poles of Mercury. Since Mercury has an axial tilt that is almost zero, the rims of many of these craters are able to shield the Sun and keep the floors of the craters in permanent darkness. The temperatures within these permanently shadowed craters can go below 100 degrees Kelvin as the Sun never rises above the crater rims to warm the frigid interiors of these craters. Within these permanently shadowed craters, water can exist in the form of ice and remain stable over billions of years. In fact, radar observations have revealed the presence of ice deposits within permanently shadowed craters in Mercury’s polar regions.
These permanently shadowed regions within craters around the poles of Mercury receive only scattered sunlight and thermal emissions from the surrounding topography. The temperatures within these permanently shadowed regions are therefore sensitive to the orientations of the surface and surrounding topography. Finally, burial under a thin regolith layer can enable the ice deposits to remain stable at higher temperatures and can extend the presence of ice deposits to lower latitudes.
Currently, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft is on its way to Mercury and MESSENGER is expected to enter orbit around Mercury on 18 March 2011. Since its launch on 3 August 2004, MESSENGER has made one Earth flyby, two Venus flybys and three Mercury flybys. These flybys are a form of gravity assist maneuvers which greatly reduce the amount of fuel required to fly MESSENGER on the right trajectory that will allow it to eventually enter orbit around Mercury. The three flybys of Mercury by MESSENGER have already generated an astonishing amount of interesting science that is poised to greatly change and increase our understanding of the elusive closest planet from the Sun. If you want to find out more, visit the mission homepage at http://messenger.jhuapl.edu/.