Location and Orbit
Missions to Mercury
References & Links
Mercury presents a battered and ancient face to the observer. The Mariner 10 spacecraft reveals huge multi-ring basins and lava flows. Similar to the moon, the terrain is pocked with craters and divided by canyons. Fault-cliffs several kilometers high extend for hundreds of kilometers. Some of these features are new, as shown by their sharp rims or unbroken crests. Others are worn down, the bombardment of meteorites leveling once dramatic features of the terrain. Meteorite penetration is far more common in a planet like Mercury, which has an inconsiderable atmosphere. Mercury's great curved cliffs, or lobate scarps, were formed as the planet cooled and shrank a few kilometers in size. This shrinkage produced the wrinkled crust with scarps kilometers high and long, features not found on lunar terrain.
The majority of Mercury's surface is covered by plains. Much of it is old and heavily cratered, but some of the plains are less heavily cratered. Scientists have classified these plains as intercrater plains and smooth plains. Intercrater plains are less saturated with craters and the craters are less than 15 kilometers in diameter. These plains were probably formed as lava flows buried the older terrain. The smooth plains are younger still with fewer craters. The largest craters are 3 or 4 billion years old. Their existence is proof that Mercury has no tectonic plates nor any tectonic activity similar to that we have on Earth. Smooth plains , called maria, can be found around the Caloris basin. In some areas patches of smooth lava can be seen filling craters.
Similar to Earth and the other planets, Mercury was formed around 4.5 billion years ago. All of the embryo planets scooped up the matter and debris left from the nebula that formed them. It is thought that, early during this stage, Mercury seperated into a dense metallic core and a silicate crust. After the period of intense meteorite bombardment, lava flowed across the surface and covered the older crust. During this period the intercrater plains formed. When Mercury cooled its core contracted, which cracked the crust and produced the prominent lobate scarps. During this third stage, lava flooded the lowlands and produced the smooth plains. During the fourth and final stage micrometeorite bombardment created the regolith, a dusty surface. A few later meteorites impacted the surface and left fresh, sharp rimmed craters. Other than occasional meteorite collisions, however, Mercury's surface is no longer active and remains the same as it has for millions of years.
So close to the sun and with so little atmosphere, it seems impossible that Mercury could support any form of water on its surface. In 1991, however, Caltech scientists bounced radio waves off Mercury, finding an unusually "bright" return from the north pole. This brightening could be explained by surface or subsurface ice. Feasible? As Mercury's rotation is almost perpendicular to its orbital plain, the north pole only sees the sun just above the horizon. Thus the insides of craters would never be exposed to the Sun. One theory holds that the interiors of these craters would remain colder than -161 degrees Celsius. These freezing temperatures could trap water exuded from the planet or ice brought in from comets. Though covered with a layer of dust, this ice would still give off a "bright" radar return.
The Caloris basin is 1,300 kilometers in diameter, and was probably caused by a projectile larger than 100 kilometers in size. The impact produced concentric mountain rings three kilometers high and sent ejecta 600 to 800 kilometers across the planet. The seismic waves produced from the Caloris impact focused onto the other side of the planet and produced a region of chaotic terrain. After the impact the crater was partially filled with lava flows.