The smallest planet in our solar system and nearest to the Sun, Mercury is only slightly larger than Earth's Moon. From the surface of Mercury, the Sun would appear more than three times as large as it does when viewed from Earth, and the sunlight would be as much as seven times brighter. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system – that title belongs to nearby Venus, thanks to its dense atmosphere.Because of Mercury's elliptical – egg-shaped – orbit, and sluggish rotation, the Sun appears to rise briefly, set, and rise again from some parts of the planet's surface. The same thing happens in reverse at sunset.
Mercury's surface is dominated by impact craters, basaltic rock and smooth plains, many of them a result of flood volcanism, similar in some respects to the lunar maria, and locally by pyroclastic deposits.
As noted previously, the density of Mercury and the magnetic field suggest geological differentiation and a large iron core. In the case of the Earth the metallic core occupies about 16% of the interior by volume and in the case of the Moon the core occupies about 4% by volume. In the case of Mercury the core is thought to occupy about 50% of the interior by volume (and a whopping 70% by mass). Thus, Mercury is a planet with a very large iron core and a comparatively thin mantle compared with the Earth.