Mercuryisoneoffourso-calledterrestrialplanetsinoursolarsystem.Mercuryisessentiallyadeadrock orbitingtheSun.ItisaverydifferenttypeofplanetthanEarth.Afteraperiodofcratering,theinterior cooledratherquickly.Itisheavilycrateredanddoesnothavethelarge,flatmariawhicharecommononthe Moon'sNearside. ThefeaturesonMercuryarenamedafterartists,poetsandmusicians.Ithasnothingtospeak ofinthewayofatmosphere,itappearstobegeologicallyinactive,andbecauseofitsproximitytotheSun,itssurface temperatureisalmosthighenoughtomeltsomemetals.Mercury'sironcoreseemstoberatherlargeforsuchasmall sizedplanet.Thisgivestheplanetarelativelyhighoveralldensity.Thespin-orbitcouplingofthisplanetisinteresting. This causes the same side of Mercury to face the Sun on every other orbit.Observing Mercury
Mercury has a radius of 2439 km, which is approximately one third of Earth.
Mercury rotates in 58.6 days at the sidereal rate, referenced to the stars. Its synodic day, from noon to noon, is 176 days. There is a 3 to 2 resonance to the rotation and revolution.
Mercury revolves around the Sun in just 88 days.
The atmosphere of Mercury is practically non-existent. There are some captured helium atoms from the Sun, and a very thin cloud of sodium atoms which have been ripped from the surface.
Hey! No atmosphere means no clouds!.
Mercury has no moons
Silly Earthling! Terrestrial planets have no rings!.