How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 15. The planet Mercury

15.1 Characteristics of Mercury

Mercury is the closest planet to the Sun. It’s the smallest and least massive planet in the Solar System. It’s also the fastest, with an orbital velocity of over 47 km/s. Mercury has the shortest year, completing one orbit in 88 Earth-days, but it has very long days, with one day on Mercury taking about 176 Earth-days. This makes it the only planet in the Solar System with days that are longer than years.

A photograph of Mercury.

Figure 15.1
Image credit

Mercury, an image taken by MESSENGER.

The length of the day is the time from sunrise to sunrise. For the inner planets, the length of the day is different from the rotation period. This is because the planet needs to rotate an extra distance to be at the same angle relative to the Sun (as shown in Figure 15.2). The day is generally longer, relative to the rotation period, the closer the planet is to the Sun.

A diagram showing how Mercury’s day is longer than the rotation of the planet due to the angle between the planet and the Sun.

Figure 15.2
Image credit

The length of the day (1-3) is longer than the rotation period of the planet (1-2).

Mercury also has the most eccentric orbit of all the planets in the Solar System.[1] This means that its orbit is more elliptical than the orbits of the other planets, which are very close to being circular. This is due to the effects of general relativity (discussed in Chapter 8).

Mercury is a rocky planet, similar to the Moon. It has at least as many craters and is covered with dust. It has no satellites of its own and, like the Moon, it’s not massive enough to sustain an atmosphere. Unlike the Moon, however, it has a massive iron core that generates a magnetic field about 1% as strong as the Earth’s.[2]

Radar observations conducted by the Goldstone antenna in California and the Very Large Array in New Mexico in the early 1990s suggested that frozen water exists at the bottom of deep craters on Mercury’s poles.[3]

Mercury is bright when viewed from Earth but it’s difficult to observe as it’s usually hidden in the light of the Sun. It’s best viewed early in the morning or at twilight. The ancient Greeks gave Mercury two names: Apollo, which was visible at sunrise, and Hermes, which was visible at sunset. Hermes was later named Mercury by the Romans and was considered the messenger of the gods.[4]

15.2 Missions to Mercury

Only two spacecraft have been to Mercury. These are NASA’s Mariner 10 and MESSENGER spacecraft. Mariner 10 launched in 1973 and mapped almost half of Mercury’s surface by 1975.[2] MESSENGER launched in 2004 and orbited Mercury between 2011 and 2015. It mapped the whole surface[5] and found further evidence of frozen water.[6]

The next mission to Mercury, BepiColombo, is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It launched in 2018 and should arrive in 2025.

Close up photograph of Mercury.

Figure 15.3
Image credit

Mercury, an image taken by MESSENGER.

Mercury Fact Sheet[1]

Designation = Terrestrial (rocky) planet
Mass = 3.3×1023 kg (5.5% mass of Earth)
Radius = 2440 km (38.2% radius of Earth)
Density = 5427 kg/m3 (98.4% density of Earth)
Length of Day = 4222.6 hours (175.9 Earth-days)
Length of year = 88.0 Earth-days
Days per year = 0.50 days on Mercury per year on Mercury
Distance from the Sun = 5.8×107 km (0.39 AU)
Orbital Velocity = 47.4 km/s
Orbital Eccentricity = 0.205
Obliquity (tilt) = 0.01°
Mean Temperature = 167 °C
Moons = None
Ring System = None

15.3 References

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