Mercury's surface features formations that indicate its crust may have shrunk. They are "lobate scarps," which are lengthy, sinuous cliffs. These scarps appear to be the surface manifestations of thrust faults, in which the crust is fractured along an inclined plane and forced upward. The fractures close up as the rock re-grows, leaving a steep-sided mountain range.
The best examples of lobate scars can be found on Mercury's western hemisphere. They are more prominent there than elsewhere on the planet because these areas were exposed to the solar wind for many millions of years, which has weathered much of the crust away. The most extensive system of lobate scars is called the Ares Range after one of Mercury's characters in Greek mythology. Scientists think that many of the rocks inside the scarps are recycled material from deeper within the planet's mantle. As the crust moves over them, they harden into rock again.
Lobate scars provide evidence that some planets may be dynamically active, which means they probably change shape frequently due to seismic activity. On Earth, tectonic plates move around the planet like pieces on a board, causing mountains to rise up and then fall down again. But some planets may have more violent surfaces than others. Ganymede, for example, is thought to have large ice caps at its poles due to tidal forces caused by Jupiter's gravity.
To the naked eye, the majority of Mercury's surface appears grayish-brown. The brilliant streaks are referred to as "crater rays." They are generated when an asteroid or comet collides with the Earth's surface. The immense amount of energy generated in such a collision creates a large hole in the earth and smashes a large amount of rock beneath the point of contact. This rock is then projected into space where it becomes orbit debris.
These collisions are very rare but when they do occur they can have serious consequences for life on Earth. If an object of sufficient size hits the planet, then the impact would be felt worldwide. The energy released by this collision could cause widespread damage and loss of life.
The asteroid that caused the K-T extinction event was approximately 10 km in diameter. It is estimated to have hit Mexico about 65 million years ago. This was one of the most devastating events in history causing the death of nearly all species except for those that managed to survive in isolated places.
After an impact, the sky above the damaged area will darken because much of the sunlight is blocked out by the remaining crater. Rainbows also form after impacts because light from the sun reflects off droplets in the atmosphere and returns to our eyes. However, within hours or days there will be another phenomenon visible in the night sky: flares as far as the eye can see.
"Then you notice lovely light-blue lines over Mercury's surface. Those are crater rays, which are generated by impacts when fresh, ground-up rock is strewn across the planet's surface "According to the mission scientist, Robert Pappalardo, "the surface of Mercury is very dark in color, but with peaks and valleys. The blue color comes from weathered rocks that have been scoured away by the solar wind."
Mercury's surface is similar to that of Earth's moon, with many impact craters caused by impacts with meteoroids and comets. The most prominent feature on Mercury's surface is Marius Hills, a group of large hills approximately 50 miles (80 km) across. These features are thought to be the results of asteroid impacts.
Like the Moon, Mercury has a thin veneer of earth-like materials called regolith that was once covered by water. The thickness of this crust varies from place to place, but is generally less than 15 feet (5 m). Beneath the surface lies a dense layer of metal almost all made up of gold and silver. This is why the planet is named after the god of medicine and commerce - it is believed that these elements were first released when Uranus broke up about 585 million years ago.
In addition to these two worlds, astronomers have also found evidence for at least three other extraterrestrial planets using the technique known as "transit spectroscopy". Transits occur when one of these planets passes directly between its star and the planet Mercury/Earth/Venus. By measuring the decrease in brightness of the star as a body crosses it, scientists can calculate the size of the planet which orbits around it.