Mercury, the smallest and innermost planet of the solar system, has been fascinating scientists and explorers for centuries. Despite being the closest planet to the sun, its interior has been cooling down as internal heat leaks away. This means that the rock and metal of which it is composed must have contracted slightly in volume, resulting in the formation of spectacular scarps and hollows on its surface.
Mercury’s scarps reveal its history
One of the most striking features of Mercury’s surface are the lobate scarps, which are ramp-like slopes that snake their way for hundreds of kilometers across the terrain. These scarps are evidence of thrust faults, where one tract of terrain gets pushed over the adjacent terrain. They are a response to Mercury having shrunk in radius by a total of about 7 km over billions of years.
The first evidence of Mercury’s shrinkage came in 1974 when the Mariner 10 mission transmitted pictures of kilometers-high scarps. The Messenger mission, which orbited Mercury from 2011 to 2015, showed many more scarps in all parts of the globe. By counting the density of impact craters on the scarps, scientists can estimate their age and the rate of Mercury’s contraction. The consensus view is that most of the scarps are about 3 billion years old, but some may be younger and still active today.
Mercury’s hollows pose a mystery
Another intriguing feature of Mercury’s surface are the hollows, which are shallow depressions that often have bright rims and floors. They are found mostly on crater walls and peaks, where the sun’s intense heat can vaporize the volatile elements in the rocks. The hollows are thought to be a result of a process called sublimation, where a solid turns directly into a gas without passing through a liquid phase. This process may be unique to Mercury in the solar system, as no other planet has such a combination of high temperatures, low gravity, and low atmospheric pressure.
The hollows were discovered by the Messenger mission, which also measured their depth, size, shape, and composition. Some of the hollows are hundreds of meters deep and several kilometers across, while others are smaller and shallower. They are mostly composed of sulfides, halides, and oxides, which are minerals that contain elements such as sulfur, chlorine, and oxygen. These elements are likely to have originated from the interior of Mercury, and their presence on the surface suggests that the planet has a complex and dynamic history of volcanism and tectonism.
Mercury’s future is bright
Mercury is not only a shrinking planet, but also a growing interest for the scientific community and the public. The planet offers a unique opportunity to study the formation and evolution of rocky worlds, as well as the effects of extreme environments on geology and chemistry. Mercury is also a potential target for future exploration and exploitation, as it may harbor valuable resources such as metals, water ice, and solar energy.
The next mission to Mercury is BepiColombo, a joint project of the European Space Agency and the Japan Aerospace Exploration Agency. The mission consists of two orbiters, one to study the planet’s surface and interior, and the other to study its magnetosphere and atmosphere. BepiColombo was launched in 2018 and is expected to arrive at Mercury in 2025, after a series of gravity assists from Earth, Venus, and Mercury itself. The mission aims to provide new insights into the origin and evolution of Mercury, as well as its current state and potential for future exploration.