Mercury

Mercury in enhanced colour, imaged by NASA’s MESSENGER orbiter.

The innermost planet. A world of extremes: surface temperatures swing from −180°C at night to 430°C during the day — a wider range than any other planet — because it has almost no atmosphere to retain heat.

A Mercurian day (sunrise to sunrise) is longer than its year. It rotates exactly 3 times for every 2 orbits — a 3:2 spin-orbit resonance locked in by solar tidal forces billions of years ago. This was only confirmed in 1965 by radar; before that, astronomers assumed it was tidally locked with one face always toward the Sun.

Surface

MESSENGER (2011–2015) mapped the entire surface. Findings:

• Caloris Basin — one of the largest impact craters in the solar system, 1,550 km across. The shock wave was so powerful it created chaotic terrain on the exact opposite side of the planet.
• Hollows — shallow, irregular depressions unique to Mercury. Still unexplained; possibly formed by volatile sublimation.
• Ice at the poles — permanently shadowed craters near the poles hold water ice. Confirmed by MESSENGER’s neutron spectrometer. On the closest planet to the Sun.

Why so dense?

Mercury is the second densest planet after Earth, despite being the smallest. Current theory: a giant impact early in solar system history stripped away most of its rocky mantle, leaving an oversized iron core that makes up ~85% of the planet’s radius.