Mercury, like Earth, still possesses a molten core. Mars, our solar system's nearest neighbor to another habitable planet, is a rocky planet composed of the same material as Mercury, Venus, and Earth. However, it appears to have a partly molten core. It does not have an active tectonic plate system.
In November 2016, scientists reported that they had found evidence of past water on Mercury thanks to data from the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft. The discovery was made when NuSTAR detected high levels of hydrogen activity in regions where we would expect to find minerals formed by liquid water, such as iron sulfides. This suggests that there may have been water on Mercury in the past.
However, this does not mean that it is currently flooded. The presence of hydrogen indicates that there could be water under the surface but it does not prove it. There are other ways for hydrogen to be released into the atmosphere without being due to water. For example, if the interior of Mercury were heating up, more hydrogen would be released than if it were at the average temperature of -180 degrees Celsius. Also, there are some processes on Earth that can produce large amounts of hydrogen gas that might be able to do the same thing on Mercury.
The finding has important implications for understanding how and when life may have emerged on Mercury.
Mercury appears to be tectonically active at the moment. Aside from Earth, it is the only rocky planet in our solar system that is still gently pushing up sections of its crust and altering its surface over time. This implies we now have something to compare Earth's dynamic geology to. We know from space missions that much of this activity takes place beneath the ocean, where tidal forces are even stronger than on land.
Scientists think that perhaps every 100,000 years or so, increased pressure from a rising body of water above creates enough tension in the ocean floor to cause it to burst open, spreading out like a sheet over the surface of the planet. The resulting seismic activity generates powerful waves that travel through the planet's interior, causing volcanic eruptions and triggering an increase in the temperature and pressure deep within the mantle, which in turn leads to more earthquakes. It's thought that this cycle may have been responsible for creating large ice sheets over Greenland and Antarctica many times in the past.
So yes, Mercury seems to show clear signs of active tectonics, just as Earth does.
Mercury, along with Venus, Earth, and Mars, is a rocky planet. It, like our Moon, has a solid surface covered with craters. It has no moons and has a scant atmosphere. Mercury prefers to keep things as simple as possible. There are two large regions: a volcanic center and a younger, more rigid crust formed from the flow of lava.
The center of Mercury is dominated by Marius Crater, which is actually a small world that was created when something (most likely an asteroid) hit the planet about 700 million years ago. This impact sent out a massive cloud of gas that expanded rapidly, forming a large bubble around Marius Crater. The shell of gas eventually collapsed, creating a new world inside the original one. Today, this inner world is called Mercury, and it spins around its axis in only 58 days.
You can see the crater itself for about 250 miles in any direction, but because of severe weather patterns most of the surface remains unseen.
The age of the core of Mercury is not known with certainty, but scientists think it is probably between 4500 and 7500 million years old. Its surface is severely damaged by strong radiation and particle impacts from space. The magnetic field is also weak, making this planet unsuitable for any sort of life as we know it.
However, because Mercury is so much closer to the Sun than we are, it experiences much more intense heat and pressure, which causes its surface to change quickly. The impact of meteoroids and other small objects continually reshapes the surface.
Like the Moon, Mercury has many old craters that are rich in material that can be recovered by orbiting spacecraft. Scientists use data from these missions to learn about the history of the solar system and how planets were formed over time.
In addition to old craters, most of Mercury's surface is young. The only area of older surface (3 billion years or more) is the very center of the planet. Here, there is an enormous mountain called Marius Hills that rises about 3 miles (5 km) above the surrounding terrain.
Marius was named after the Roman god of war. The mountain is also known as Chaumonot Mons because it was discovered by Jean-Louis Michel and Gilles Chaumont during 1667 to 1669 French observations from two different locations nearly 100 miles (160 km) apart.