Mercury has been called a planet of extremes for many reasons. Of all the planets,
- it’s the closest to the Sun.
- it’s the smallest; in fact, it’s just a little larger than the Moon.
- it’s the fastest; it only takes 88 Earth days to go around the Sun once.
- it has the longest day: it takes 58.6 Earth days for Mercury to rotate once.
- it has the smallest axis tilt, less than a degree; so it’s upright.
- it has the most elliptical orbit. (See the table and diagram in Extra Information at the end.)
- it has the highest orbital inclination. (See the table and diagram in Extra Information at the end.)
The planet Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie
Mercury doesn’t have an atmosphere per se. There are atmospheric gases—oxygen, sodium, hydrogen, helium, and other gases—but they form a thin, inconsistent exosphere above the planet. An exosphere is the layer of atmosphere farthest from the surface of a planet. The space between the exosphere and Mercury’s surface is a vacuum.
Mercury is not volcanically active, there is no vegetation, no life, so how does Mercury keep adding gases to its exosphere? And Mercury’s gravity is low, so how can Mercury hold its exosphere in place?
Some impacts from meteoroids vaporize some of materials from the meteoroids themselves, as well some of Mercury’s surface rocks. The vaporized material goes off into the exosphere or off the planet. However, the major source of gases is through the interaction of Mercury’s magnetic field and the solar wind; the magnetic field also helps keep the tenuous exosphere in place. See below.
I first became interested in Mercury when I learned about its gravitational field. The strength of Mercury’s magnetic field is about 1% the strength of Earth’s magnetic field, so not strong at all. In addition, Mercury’s magnetic field is really offset: it’s three times stronger in the northern hemisphere than in the southern hemisphere. Earth’s magnetic field is more or less consistent in both hemispheres.
The solar wind is a thin plasma (charged particles), and it is more intense near the Sun than farther out. So, because Mercury is so close to the Sun, Mercury’s magnetic field interacts a fair amount with the fast-moving solar wind. Sometimes, as a result of this interaction, magnetic tornadoes form. These tornadoes aren’t like tornadoes on Earth: On Mercury, there is no dust, no air, no clouds, no weather. Magnetic tornadoes are invisible. They carry the solar wind plasma to the surface of the planet. When the charged particles in the plasma hit the surface, they cause atoms to fly off the surface into space and into the exosphere.
Double Sunrise/Sunset at Perihelion and Retrograde Motion of the Sun
As I read more about this planet, I became more intrigued. At one point in Mercury’s orbit around the Sun, the Sun undergoes retrograde motion: for an observer on Mercury’s equator about to experience daylight, the Sun rises, sets, and rises again. That point is when Mercury is closest to the Sun, or perihelion. (The point at which a planet is farthest from the Sun is called aphelion. See the diagram showing perihelion and aphelion at the end.) For an observer about to experience nighttime, the opposite happens. Check out these animations of this phenomenon:
https://www.youtube.com/watch?v=xA87d1l-N7I (double sunrise)
https://www.youtube.com/watch?v=kEerzCUnnjo (double sunset)
An observer at noon, at perihelion, would see the Sun stop, move eastward, then westward.
If you could be on Mercury’s surface, at the equator, at noon, at perihelion, then you could see the Sun in retrograde motion. The Sun’s real motion does not change. This is just a phenomenon that we would see if we were in the right place at the right time.
This phenomenon only happens on Mercury. Why does this happen?
To start, here’s a reminder of how planets move, thanks to Johannes Kepler (1571–1630). Kepler identified three laws of planetary motion. We just need the first two, but I have included the third law for completeness:
Law 1: Planets move in ellipses (not perfect circles), with the Sun at one focus; that is, the Sun is not the centre of the ellipse. (See the table and diagram at the end.)
Law 2 (Summarized): Planets move fastest at perihelion and slowest at aphelion.
Law 3: The time, T, it takes a planet to complete one orbit, squared, is proportional to the mean distance, r, from the Sun to the planet, cubed: T 2 µ r3.
Now, let’s look at Mercury: As per law 1, when a planet is at perihelion, it is moving faster than at any other time in its orbit. At perihelion, Mercury is moving about 1.5 times faster than it moves at aphelion. Note: Mercury’s speed around the Sun changes, but its rate of rotation does not change. Mercury’s rotation is pretty slow, about 58 Earth days to rotate once.
Also, since it only takes Mercury 88 Earth days to go around the Sun once, and since Mercury’s rotation is so slow, the Sun is in the Mercury sky for 176 Earth days every year (Mercury’s solar day). The 176 days is about three times longer than the time it takes for Mercury to rotate once on its axis.
Next, let’s look at the Sun from Mercury: As Mercury goes around the Sun, the Sun is in the sky for 176 days. During that time, the Sun moves, on average (remember how Mercury changes speed at different points), 2° west every Earth day. At aphelion, Mercury has slowed down, so the Sun’s movement appears to speed up a bit, to about 3° west every day.
Now as Mercury gets closer to perihelion, it starts to speed up, but its slow rotation doesn’t change. The opposite to what happens at aphelion happens at perihelion: Mercury has sped up, so the Sun’s movement appears to slow. As Mercury moves close to perihelion, arrives at perihelion, and moves past perihelion, the Sun stops, then moves eastward instead of westward, and starts moving westward again. As Mercury continues to slow down after perihelion, the Sun’s apparent movement returns to about 2° west until Mercury reaches aphelion again, when the Sun speeds up to about 3° west every day.
So over a period of about 8 days before and after perihelion, an observer on the equator, at the terminator on Mercury, would see the Sun rise in the east, set, then rise again.
And throughout the entire orbit, the size of the Sun from Mercury appears to change: On average, the Sun appears to be over three times larger than what we see on Earth. So at perihelion, it would appear larger. At aphelion, it would appear smaller. Of course, the size of the Sun does not actually change.
There is some sodium in Mercury’s exosphere, but not that much. What makes the sodium mentionable is the fact that it absorbs and emits light quite easily, so it’s more visible than other elements. The solar wind is constantly interacting with Mercury’s magnetic field. The wind pushes gases, including sodium, from Mercury’s surface and exosphere away, and they form a tail behind the speeding planet. The ultraviolet light in sunlight causes the sodium to glow, or fluoresce, a yellowish colour, so under favourable conditions we can see a yellowish tail trailing behind Mercury.
Mercury’s tail. The sodium particles in the tail fluoresce, causing a yellowish glow. Credit: Andrea Alessandrini
Why Don’t We Send Probes to Mercury More Often?
Mercury is truly an intriguing planet. But so far, only two spacecraft have visited Mercury, both American: Mariner 10 (1973–1975) was the first spacecraft to study Mercury, and it only took 147 days to get there. However, Mariner 10 did not go into orbit around Mercury; it just did flybys. Messenger (2004–2015) was an outstanding mission. It took seven years to get to Mercury. The difference here is that Messenger didn’t just do flybys; it spent four years in orbit around Mercury.
The next mission to Mercury is the ESA spacecraft called BepiColombo. It was launched in October 2018 and will begin orbiting Mercury in December 2025, another seven-year mission. I’ll be following BepiColombo for sure.
An engineering model of BepiColombo, seen in the Science Museum, London. Photo by Randy Attwood
So why haven’t there been more spacecraft sent to Mercury, especially compared with missions to planets beyond Earth?
Mercury is tough to get to. Imagine that you are at the top of a high hill and you start running down the hill. Halfway there you decide to stop running. You will likely be unsuccessful and go tumbling head over heel down the rest of the hill. This is a rough comparison to what it’s like to send a spacecraft to Mercury. Mercury is so close to the Sun that a spacecraft en route to this tiny planet has to more or less head straight for the Sun. The closer it gets, the more the Sun’s gravity pulls it in. So spacecraft heading to Mercury with the purpose of going into orbit around it need to use gravity effects from Earth, Venus, and Mercury to slow down. This help is called a gravity assist. To do that takes longer.
And once the spacecraft gets there, it has to be able to endure the extreme heat and cold: Mercury’s daytime side can reach a temperature of 430 °C; the nighttime side can reach –180 °C. So spacecraft designers face design challenges to protect the spacecraft as well as the electronics in the instruments.
Eccentricity, e, is a measure of how elliptical a shape is. The closer e is to 0, then the closer the shape is to being a circle. In the table below, we can see that Venus’s orbit is the closest one to a circle, and Mercury’s orbit is the most elliptical.
Left: a circle with 0 eccentricity; middle: an ellipse with an eccentricity of 0.2; right: an ellipse with an eccentricity of 0.3. Not to scale.
Source: RASC Observer’s Handbook 2021
Perihelion and Aphelion
At perihelion, a planet is closest to the Sun. At aphelion, a planet is farthest from the Sun. Not to scale. Exaggerated to highlight differences.
If we could view the planet orbits from the side, we could see how much they are tilted.
The orbital inclination is how much the orbit is tilted. Earth’s orbit is almost completely flat. But Mercury’s orbit is tilted a fair amount.
Source: RASC Observer’s Handbook 2021