@Gordon Barret
Mercury does get VERY hot in the midday Sun, but it also gets VERY cold in the middle of the night. There's no atmosphere to balance temperatures, so the range is extraordinary.
Mercury also has probably not had any significant tectonic activity for a few billion years. It's a small planet so it has a high surface to volume ratio and radiated its internal store of heat into space a long time ago. There's some conflicting evidence whether any of Mercury's interior is molten or in a plastic condition, but if it ever had plate tectonics they grated to a halt in the early days of the Solar System.
With no tectonics and no atmosphere, Mercury's surface preserves features for millions, if not billions of years. Many of these craters are probably a similar age to those on the Moon and date from around 4 billion years ago. The very bright craters are relatively recent - say in the last billion years; they're bright because of the freshly-broken ejected material splashed over the surrounding terrain. They're gradually eroded by micrometeorite bombardment and the impact of solar particles.
Now what would be interesting is if we found ANY signs of recent vulcanism. Many experienced telescopic observers have seen temporary patches of brightness and light on the Moon which have been called transient lunar phenomena and might be dust or gas erupting from the deep interior. Mercury, being more massive, should be hotter inside and might even have patches of molten material finding their way to the surface.
Biting the hand that feeds IT
