The passage of time, from the formation of the planet and other bodies in the solar system over four thousand six hundred million years ago, has left evident traces on the surface of Mercury. Mariner 10 has shown such evidence and has made it possible to reconstruct the geological history which can be broken down into four principle stages. The first stage involves the period immediately following Mercury's aggregation, starting from the primordial cloud when the entire planet was heated until it began to melt. At that time, the heavier materials, especially iron, were concentrated due to the effect of gravity from the core; instead, the lighter materials remained on the surface and cooled gradually to form rocks which are not very different from those which constitute the Earth's crust under the oceans: they probably had the appearance of a smooth surface like what can be seen in some points of the planet between craters. However, the smooth plains did not last very long. In that period, massive showers of small and large rocky fragments passed through the entire inner part of the solar system: irresistibly attracted to the Sun, they were destined to fall into and dissolve in our star. But first, before they arrived, many of them collided with other bodies that happened to get in their way. These meteorites, that were probably also present in remote epochs, left traces of their passage only when they encountered a solid crust, and Mercury was no exception. So, the planet began to acquire its distinctive appearance. The cooling process continued during the same stage and the core probably got smaller: so the young crust broke and folded together like the skin of a withered apple. Steep slopes, several thousand meters tall and some more than two thousand kilometers long, formed over the entire surface of the planet. The second stage, which represented what was truly a cataclysmic period, separated the history of the planet into two parts: the time before and after the event. Mercury was continually "bombed" by meteorites until one day something much larger appeared on the horizon. Maybe it was a fragment of a planetoid that was shattered by the attraction from the Earth or Venus. But it certainly was enormous: a gigantic mountain that obscured the Sun while falling to the surface of the planet. The collision was tremendous: the friction and the impact of the meteorite heated the rock until it melted, while bits of dust and lava gushed everywhere like a red-hot shower. Similar to an egg shell, the surface of the planet broke while huge earthquakes shook the entire planet. However, this stage was rather brief. Calm soon returned, but Mercury now has an additional structure that is christened the Caloris Basin: with a diameter of 1 400 kilometers it is surrounded by mountains that are 2 000 meters high; its interior is devoid of any relief formations except for a certain number of craters and some fractures and wrinkles. On the opposite side of the planet, the earthquakes even disturbed the rocks of the antipodes, where the ground became hilly and striated. The third stage, that probably lasted one billion years, was a period marked by an incredible amount of volcanic activity. Large quantities of lava were spread over wide areas of Mercury's surface covering much of the pre-existing structures, a process very similar to the filling of the so-called lunar "seas". The fourth and final stage of Mercury's history, that began three billion years ago and has continued to the present, is still characterized by a shower of meteorites, although much less than the primordial beginnings. It is during this period that craters were formed and marred by light furrows in a widening radius resulting from material hurled by the impact.
The passage of time, from the formation of the planet and other bodies in the solar system over four thousand six hundred million years ago, has left evident traces on the surface of Mercury. Mariner 10 has shown such evidence and has made it possible to reconstruct the geological history which can be broken down into four principle stages. The first stage involves the period immediately following Mercury's aggregation, starting from the primordial cloud when the entire planet was heated until it began to melt. At that time, the heavier materials, especially iron, were concentrated due to the effect of gravity from the core; instead, the lighter materials remained on the surface and cooled gradually to form rocks which are not very different from those which constitute the Earth's crust under the oceans: they probably had the appearance of a smooth surface like what can be seen in some points of the planet between craters. However, the smooth plains did not last very long. In that period, massive showers of small and large rocky fragments passed through the entire inner part of the solar system: irresistibly attracted to the Sun, they were destined to fall into and dissolve in our star. But first, before they arrived, many of them collided with other bodies that happened to get in their way. These meteorites, that were probably also present in remote epochs, left traces of their passage only when they encountered a solid crust, and Mercury was no exception. So, the planet began to acquire its distinctive appearance. The cooling process continued during the same stage and the core probably got smaller: so the young crust broke and folded together like the skin of a withered apple. Steep slopes, several thousand meters tall and some more than two thousand kilometers long, formed over the entire surface of the planet. The second stage, which represented what was truly a cataclysmic period, separated the history of the planet into two parts: the time before and after the event. Mercury was continually "bombed" by meteorites until one day something much larger appeared on the horizon. Maybe it was a fragment of a planetoid that was shattered by the attraction from the Earth or Venus. But it certainly was enormous: a gigantic mountain that obscured the Sun while falling to the surface of the planet. The collision was tremendous: the friction and the impact of the meteorite heated the rock until it melted, while bits of dust and lava gushed everywhere like a red-hot shower. Similar to an egg shell, the surface of the planet broke while huge earthquakes shook the entire planet. However, this stage was rather brief. Calm soon returned, but Mercury now has an additional structure that is christened the Caloris Basin: with a diameter of 1 400 kilometers it is surrounded by mountains that are 2 000 meters high; its interior is devoid of any relief formations except for a certain number of craters and some fractures and wrinkles. On the opposite side of the planet, the earthquakes even disturbed the rocks of the antipodes, where the ground became hilly and striated. The third stage, that probably lasted one billion years, was a period marked by an incredible amount of volcanic activity. Large quantities of lava were spread over wide areas of Mercury's surface covering much of the pre-existing structures, a process very similar to the filling of the so-called lunar "seas". The fourth and final stage of Mercury's history, that began three billion years ago and has continued to the present, is still characterized by a shower of meteorites, although much less than the primordial beginnings. It is during this period that craters were formed and marred by light furrows in a widening radius resulting from material hurled by the impact.
