Mercury is the innermost, smallest and densest and has the most elliptical and highest orbital inclination of the major planets. This statement needs some qualification, it presumes that Pluto is not counted as a planet and the density is the uncompressed density. It is presumed that Mercury has a relatively large iron/nickel core.

The large eccentricity e=0.206 means the solar insolation changes quite a bit between perihelion a(1-e) and aphelion a(1+e), ratio=(1+e)2/(1-e)2=2.31 or almost a stellar magnitude. At perihelion the insolation is 10.6 times that received at earth and at aphelion it is 4.6 times. So spacecraft rendezvous at aphelion, perihelion is too perilously hot! Viewed from Mercury, the Sun would look almost three times as large as it does from Earth.

Mercury is an inferior planet and so exhibits the phases of the moon through a synodic period. Since it never is seen more than arcsine(a[1+e])=27.8 degrees from the sun, it is very difficult to observe from earth (it is too close to the sun for the Hubble telescope). Our best views have come from the intrepid Mariner 10 which exploited a Venus gravity assist to put it into a 2:1 resonant orbit with Mercury giving us three encounters with the elusive planet. Unfortunately, due to Mercury's 3:2 spin-orbit resonance, Mariner always saw the same hemisphere so we have mapped only half of Mercury.

Mercury also has the lowest albedo of the planets, reflecting about 6%, about the same as the moon. Of course if mercury and the moon are seen together in a telescope at the same phase (a possibility when mercury is at crescent phase near inferior conjunction) Mercury's surface will be rather brighter. When Mercury is gibbous its surface brightness is even greater since the reflectivity, like that of the moon, depends on phase angle and peaks strongly near the full phase. Always consider the distance to the sun when thinking about surface brightness.

Finally, Mercury has essentially zero obliquity, it's rotation axis is perpendicular to the orbital plane so no seasons. Solar tides have done their work well.

Like our Moon, Mercury has almost no atmosphere , mostly burned off millions of years ago by the planet's close proximity to the Sun. What little atmosphere exists is made up of atoms blasted off its surface by the solar wind and has lessthan a million- billionths the pressure of the Earth's atmosphere at sea level. It is composed chiefly of argon, neon and helium. Because of Mercury's extreme surface temperature, these atoms quickly escape into space and are constantly replenished. With no atmosphere to protect the surface, there has been no erosion from wind or water, and meteorites do not burn up due to friction as they do in other planetary atmospheres.

Mercury's surface is pocked by many deep craters much like the moon, but has none of the maria, no obvious faults or straight rills. Unlike the moon there are broad flat cratered plains and ridges, and steep scarps up to 3km high possibly caused by the core shrinking after the mantle solidified.

Mercury is the only body in the solar system I know of that is tidally locked into a three to two spin-orbit resonance. every time Mercury revolves around the sun it rotates one and one-half times. Note that prograde revolution causes the sun to move through the sky from west to east and prograde rotation drives the sun east to west. Since the rotation is faster than the revolution the sun, on average, moves from east to west just like here on earth. The combined effect of these motions means that in one Mercury year only a half a day elapses! Stranger still, near perihelion the angular revolution rate exceeds the (constant) rotation rate and so the sun appears to move backwards not quite a full sun apparent diameter. At longitude 90 you might see the sun rise in the east, pause, set, then rise a second time.

One large feature, the Caloris basin, probably a large impact feature, was so named since at perihelion the sun is directly overhead at perihelion. Longitude zero is defined here. Of course a "year" later it is midnight here so Caloris probably can be said to have the greatest extremes in surface temperature in the solar system.

Well, perhaps longitude 180 can claim that too. The terrain around here has no name but is really jumbled terrain. Perhaps the impact that formed the Caloris basin sent shock waves through Mercury that rattled the surface here?

The Caloris Basin