On the assumption that Mercury's craters were formed by the same populations of remnant planetary building blocks (planetesimals), asteroids, and comets that struck the Moon, the majority of the craters would have formed before and during an especially intense period of bombardment in the inner solar system, which on the other hand, would have formed before and during an especially intense period of bombardment in the outer solar system. The most likely time for this to have occurred is around 4 billion years ago.
The source region for these impacts was primarily within 20 degrees of the ecliptic, but with some contributions from outside this range. Most of the impacts came from beyond Earth's orbit, but some came from as close as 5 million km (3 million miles).
At first glance, the distribution of these impacts might lead one to believe that they were caused by two different populations of bodies - one responsible for most of the events between 4 and 1.5 billion years ago, and another population responsible for the remaining events up until today. However, closer examination of the data reveals that there is actually only one population of impacting body that changed over time. It's just that at different times in the past, when conditions were different, different parts of this population hit Mercury.
For example, if we look at the ages of the largest Mercury reservoirs, which are called maria, we find that they are mostly less than 500 million years old, with a few exceptions.
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. They are formed from a mixture of rock types, including some that appear to be basalt under the surface. There are several smaller groups of hills on Mercury. Some of them may be volcanic in origin.
Like Earth, Mercury has a thin atmosphere composed of gases such as sulfur dioxide, oxygen, and carbon dioxide. However, because Mercury has no magnetic field, any particles thrown up into its atmosphere are likely to be lost forever. Thus, its atmosphere is very dense and almost completely covers the planet's surface.
The rest of Mercury remains invisible to radar because it too is made of iron and nickel, which are similar in composition to those of the earth's outer core. Although we can't see it, this solid inner core accounts for more than 95% of Mercury's mass.
In addition to these features, data from NASA's Messenger spacecraft indicate that there may be shallow layers of water ice beneath the surface in certain areas.
The Moon and Mercury Both bodies are devoid of liquid water, which would erode impact craters over time. They also lack an atmosphere, which might destroy meteoroids before they reach the surface on worlds like Earth and Venus. Old craters, on the other hand, can be degraded by fresh impact events. The most dramatic example is the North American continent, where millions of years of weathering has smoothed out much of the original evidence of ancient collisions.
On Earth, craters come in two main types: young and old. Young craters are formed by high-speed projectiles that hit with little erosion of their own. Old craters are formed by slower-moving objects that gouge out deep valleys and scour the sides of the crater floor.
The majority of craters on Earth are young. They're typically less than 10 million years old, a very short time in the scale of planetary geology. Because they were formed by sudden bursts of energy, they often contain bright material called spherules inside their walls. Spherules look like small, solid spheres that form when particles of dust or glass are vaporized by the heat of the impact and then condensed again at low temperature. They can be found inside some craters on Earth today.
But there are also many older craters on Earth's surface. They usually have flat floors covered in debris from the collapsed surrounding wall.
Mercury resembles Earth's moon in appearance. Mercury's surface, like our moon's, is riddled with craters created by space rock strikes. But because Mercury is so much closer to the sun, it experiences much higher temperatures than Earth's satellite.
The planet Mercury has a very different composition from that of Earth. It is mostly iron with some silicon and oxygen. The outer layer of Mercury is called a crust. It is made up of a mixture of metals and minerals that came from the planets core when it was still forming about 590 million years ago. As the core cooled down, the surrounding material condensed into shells around the core. Over time these shells of metal and mineral were weathered away by wind and water, leaving only the inner core behind.
The average distance of Mercury from the Sun is 59 million km (37 million miles). Because it is so close to the Sun, all of Mercury is either in full or partial sunlight for most of its orbit around the Solar System. Only around half of Mercury's orbit is night-time though, so there are regions where parts might be experiencing darkness now, but others will later wake up to discover a new sunrise.
Its mass is about 58 percent gold, 31 percent iron, and 9 percent silicon and oxygen.
Mercury's lithosphere is too thick for magma to enter, hence there is no more volcanic activity. What accounts for the significant discrepancies between craters on the Moon and those on Mercury (ejecta blanket extent and secondary cratering)? The majority of these differences can be explained by the different evolutionary histories of the two bodies.
The impact that created the Moon removed most of the Earth's mantle and crust, including all of its volcanic material. Thus, if Mercury had been formed along with the rest of the Solar System out of a common disk of dust and gas, it would not have retained any evidence of a volcanic past. However, research has shown that the early Earth may have been largely oceanic with only small landmasses at its surface. If this was the case for Mercury, then we would not expect to find much evidence that it used to be more earthlike before it lost most of its atmosphere and ice caps in a giant collision with Venus.
However, even though most of Mercury is cold and dark today, scientists believe it used to be much more hospitable to life. Studies have shown that it used to have many more large lakes and perhaps even oceans. Also, recent findings suggest that some parts of Mercury might still retain a thin veneer of ice that could be as deep as 30 miles (50 km).
It has numerous impact craters. Mercury is virtually completely devoid of atmosphere. Mercury's dark side is that it is extremely cold since it has practically no atmosphere to hold in heat and keep the surface warm. Surface temperatures range from -300 to +450 degrees F.
Did you know that there is water on Mercury? Scientists discovered this by accident in 2008. They were looking at images taken by the MESSENGER spacecraft as it flew over the planet's surface. They noticed bright spots where the image showed rocks to be stained brown or red. Then, when they analyzed these images more closely, they found that some of the features looked like lakes. These are called "maria". The scientists also found that many of the maria are connected by narrow strips of land called "rivers".
Has any one been to Mercury? No, it is impossible to go there. Astronauts from outside Earth's environment have never been to Mercury because it is too far away from Earth or other planets for us to travel there. However, we can learn about it from data collected by astronauts who have flown by or landed on it.
Do people live on Mercury? Yes, there are several communities on Mercury that contain buildings and vehicles. In fact, the first human being to walk on another planet was American astronaut John Glenn.