Mercury: What do we know about our solar system’s smallest planet?

Although we’ve been observing Mercury from Earth for thousands of years, its close proximity to the Sun – about 58 million kilometres, on average – has made it difficult for astronomers to learn much about the planet. The Hubble Space Telescope cannot observe it, because turning that close towards the Sun would damage the telescope’s instruments. Most of what we know came from the 1975 Mariner 10 space probe’s fly-by.

With the naked eye, Mercury can only be seen at dawn or dusk, depending on the time of year (unless there is a solar eclipse). This is due to the Sun’s glare. Mercury can also be seen as a small black spot moving across the Sun at intervals of seven, 13 and 33 years. This is known as a transit of Mercury across the Sun and occurs when the planet comes between the Earth and the Sun.

Mercury has the shortest year of any planet at 88 Earth days. It also orbits around the Sun faster than any other planet, which is why it was named after the speedy Roman messenger god. Conversely, Mercury has the longest day of any planet due to its slow rotation. Because it revolves so quickly around the Sun, yet only rotates on its axis once every 59 Earth days, the time between sunrises on Mercury lasts 176 Earth days. Mercury also has the most eccentric, or stretched-out, elliptical orbit. Like our Moon, Mercury can be observed going through apparent changes in its shape and size called phases.

Mercury’s atmosphere

Mercury has a very thin, almost airless atmosphere. At one time it was believed that the planet didn’t have an atmosphere at all, but it does contain small concentrations of the gases helium, hydrogen and oxygen as well as calcium, potassium and sodium. Because of Mercury’s size, it does not have a strong enough gravitational pull to keep a stable atmosphere. It is constantly being lost and replenished via solar wind, impacts and radioactive decay of elements in the crust.

Temperature extremes

While Mercury has an average surface temperature of around 179°C, temperatures on the planet fluctuate wildly depending on the location on the planet, the time of day and how close it is to the Sun in its orbit. At night, surface temperatures can go down to -170°C. During the day, they can reach 450°C. Some scientists believe that ice may exist under the surface of deep craters at Mercury’s poles. Here temperatures are below average because sunlight cannot penetrate.

Terrestrial planet

Mercury's surface
Mercury’s surface

Like Earth, Mercury is a rocky planet. It comprises about 70 per cent metal and 30 per cent silicate materials. Because Mercury is so dense – almost as dense as Earth, although it’s much smaller – it probably has a very large, iron-rich core. Scientists believe that Mercury’s core makes up almost half of the planet’s total volume and three-fourths of its total radius. It also contains more molten iron than any other major planet in the solar system. The core is estimated to have a radius of about 1,800 kilometres, with a mantle about 600 kilometres thick and a crust about 300 kilometres thick. There are a few potential explanations for this large core. Mercury may have had a more substantial crust and mantle that were stripped away by high temperatures and solar wind from the Sun, or it could have been hit by a still-forming planet called a planetesimal.

Moon-like surface

The surface of Mercury looks much like the surface of our moon. The largest crater on Mercury is the Caloris Basin at 1,300 kilometres across. The impact caused lava eruptions and shockwaves that formed hills and furrows around the basin. Mercury also has two different types of plains. The smooth plains were likely formed by lava flows, while inter-crater plains may have been formed by lava or by impacts. The most unusual features are the wrinkles and folds across its plains and craters, caused by the cooling and contraction of the planet’s core.

Mercury's surface
Mercury’s surface

Mercury’s orbit

Of all the Solar System’s planets, Mercury has the most eccentric orbit. Moving in an ellipse its distance from the Sun varies from 46 million kilometres (28.6 million miles) to 70 million kilometres (43.5 million miles) across its orbital cycle.

Not only does Mercury travel in an ellipse, but the planet’s closest approach to the Sun is not always in the same place. Mercury’s orbit drifts, with each ellipse around the Sun seeing it move along slightly, tracing a shape similar to the petals of a daisy (see picture).

This drifting is partially caused by the gravitational pull of local bodies; the Sun, of course, has the most influence, but other planets and asteroid belts also have an effect, dictating its path. However only part of the drift is accounted for by other objects’ gravity near Mercury. The orbit can only be fully explained by Einstein’s general theory of relativity. The Sun’s gravitational field distorts the fabric of space and time, forming a curvature. This distorted space geometry also affects the route Mercury takes around the Sun.

Facts about Mercury

1. Heavily cratered surface

Although telescopes had revealed that Mercury looked much like our moon, the nearly 10,000 images recorded by Mariner 10 confirmed that it had a heavily cratered surface.

2. Lobate scarps

Mariner 10’s images showed that Mercury was also covered in curved cliffs called lobate scarps, which formed when the planet’s core cooled and shrank.

3. Ultraviolet radiation

Mariner 10 recorded large amounts of ultraviolet radiation near Mercury. It was eventually determined to come from a nearby star called 31 Crateris.

4. Magnetic field

The Mariner 10 space probe’s instruments picked up a magnetic field on Mercury, which is rather similar to Earth’s own magnetic field.

5. Exosphere

Mercury has an atmosphere like the exosphere on Earth – the upper layer of our planet’s atmosphere. Its lightness and low density allows molecules to escape into space.

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