Short answers, pulled from the story.
One solar day on Mercury, from sunrise to sunrise, lasts about 176 Earth days, which is twice the length of the planet's 88 Earth-day year. This results from Mercury's 3:2 spin-orbit resonance, where it rotates three times for every two orbits of the Sun.
Mercury is the smallest planet in the Solar System, with an equatorial radius of 2439.7 km. It is even smaller than the largest natural satellites Ganymede and Titan, though it is more massive than they are.
Mercury's core occupies about 57 percent of its volume, compared with 17 percent for Earth, and is estimated to have a radius of 2020 km. Competing theories explain the high iron content, including a giant impact that stripped away crust and mantle, vaporization of surface rock near the young Sun, and drag in the solar nebula that removed lighter particles.
Yes, observations strongly suggest water ice exists in deep craters at Mercury's poles, which never receive direct sunlight and stay below 102 K. Radar observations in the early 1990s from the Goldstone Solar System Radar and the VLA found reflective patches near the poles, and MESSENGER images of north pole craters later confirmed the ice.
In 1859, Urbain Le Verrier found that the precession of Mercury's orbit could not be fully explained by Newtonian mechanics. Albert Einstein's general theory of relativity accounted for the remaining 42.980 arcseconds per century by describing gravity as the curvature of spacetime.
Three space probes have visited Mercury. Mariner 10 flew by in 1974 and 1975, MESSENGER orbited from 2011 and crashed into the surface on the 30th of April 2015, and the joint European and Japanese mission BepiColombo, launched on the 20th of October 2018, will enter Mercury's orbit in 2026.