Lesson 2) Mercury
4.2: Mercury
Are we all here? Good! We will start our imaginary tour of the solar system with Mercury. As we get close, I’ll project on the screen images of Mercury as seen by Mariner 10 and other Muggle space probes. Counting down: ten, nine, eight, seven, six, five, four, three, two, one, liftoff! While we’re approaching Mercury, I’ll fill you in with some information about the planet.
Mercury is unique in the solar system as it is the smallest and fastest planetary object. Its radius of 1,516 miles (2,240 kilometres) makes this planet almost 3 times smaller in width than Earth. Its name comes from the Roman messenger god, Mercury, who was also the patron of travellers and thieves. Mercury travels so fast that ancient observers in the Americas considered it to be four different planets, and represented it as four owls.
Indeed, most representations of this planet relate to the ideas of messages and death. Why messages? Well, Mercury is related to communication, a fact that was discussed in Year Three Divination. The reason for this is because Mercury is so very close to the Sun, meaning that it can only be seen in our sky in the morning and evening. Thus, ancient peoples saw a bright object in the sky, well before they had telescopes, and associated it with the death of the day in the evening and the return to life in the morning.
As stated before, observing Mercury can be quite difficult because the Sun is always so close. In fact, it never gets more than 28 degrees apart from the Sun, and it’s usually much closer, so some amateur astronomers have never been able to see Mercury at all! That doesn’t make the experience any less rewarding, however. Because Mercury orbits between the Earth and the Sun, the planet has visible phases like the Moon.
Planetary Motions
While astronomers could observe Mercury’s progress through the night sky, they always seemed to be looking at the same side of the planet. For that reason, they used to believe that Mercury was tidally locked to the Sun, just like the Moon is locked to Earth. In 1965, radar observations showed that in fact, Mercury rotates around its axis three times during two revolutions around the Sun. Its rotation period lasts for about 59 days, whereas it takes 88 days to orbit the Sun.
A day on Mercury is actually much longer than that, though. After completing one revolution around the Sun, the planet has rotated one and a half times around its axis in the same direction; so the Sun is now on the opposite side. It’s not until Mercury has completed a second revolution that the Sun is now on the same side as it was at the beginning, meaning that the length of a solar day is twice the orbital period, or about 176 Earth days. In the image below, we see a point on the surface of Mercury as a little red line; the numbers indicate successive positions of the planet with respect to the Sun and of that point on the surface. As you can see from the image, Mercury’s orbit is highly eccentric. Its perihelion is about 0.3 Astronomical Units and its aphelion is a little more than one and a half times as great.
Mercury rotates three times while it revolves twice around the Sun.
Source: here
Viewing Mercury, from Ancient Times to the Present
Having exhausted you all with a technical discussion of Mercury’s revolution and rotation, we turn to an easier topic: viewing the planet. Mercury belongs to a special group of planets, the visible planets, which can be observed without a telescope. Ancient wizards and Muggles alike were able to look up at the night sky and observe Mercury’s path through the heavens alongside the four other planets in this category. These strange moving lights were given the names of gods and mythological figures and ideas; they were studied and worshipped for centuries.
All the planets that can be seen with the naked eye are displayed in this diagram.
Source: here
Galileo made it possible to study these planets in more detail when he built his own telescope and looked skyward. Since then, people have used increasingly complex versions of this telescope, both magical and mundane, to look into space. A telescope by itself, though, has some limitations as an instrument for studying the details of Mercury’s surface, because the planet is so close to the Sun that sunlight obscures them.
As computers and radio waves became more advanced these were also turned skyward. Radio waves were sent to objects in the solar system, and the echoes that returned were translated by computers into even more detailed images of the planets.
By far the most detailed and accurate observations of the planets have come from space probes, which are spacecraft equipped with cameras, scientific measuring equipment, and radios to transmit their data back to Earth. While some missions have visited many planets, others have been dedicated to the study of a single planet or other object.
Sending space probes to Mercury is a very delicate task; being so close to the Sun means that the spacecraft has to stand up to strong solar radiation. Mariner 10 was sent to Mercury in 1974 to take pictures of the planet’s surface, flying by it three times between 1974 and 1975. Its closest approach to the surface of the planet was about 327 kilometres. Even though Mercury had been studied for centuries, these pictures revolutionised our understanding of the planet. Mariner 10 also discovered a magnetic field around Mercury, a feature which helps to deflect the solar winds and radiation away from the planet. Another important discovery made by the probe was that the atmosphere of Mercury is primarily made of helium, and the core of the planet is rich in iron.
Ah! Here we are! Now you can all see Mercury as it was seen by Mariner 10.
Mercury as seen from Mariner 10
Source: here
The MESSENGER mission (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was launched by NASA in August 2004. It flew by Mercury three times between 2008 and 2009, then entered into orbit on March 18, 2011, where it took pictures with a much higher resolution than Mariner could, until it crashed into Mercury’s surface on April 30, 2015. As the name suggests, this mission studied Mercury much more in depth, looking to understand the formation of the planet as well as its composition. Some of the most important discoveries made by this spacecraft involved uncovering evidence of past volcanic activity and finding a lot of water in Mercury’s exosphere. The MESSENGER mission also gave us a better understanding of this planet’s magnetic field, which, in turn, had a huge impact on the magical understanding of Mercury. Below are the first and last pictures taken of Mercury’s surface by that spacecraft while in orbit.
And now we’re close enough for you to see Mercury as it appeared to MESSENGER at its closest. That’s the end of this leg of our imaginary tour. Next week, when we start the next leg, we will be orbiting Venus.
First (March 29, 2011) Last (April 30, 2015)
This is not humanity’s last mission to Mercury, however. The European Space Agency and the Japan Aerospace Exploration Agency combined to launch a joint mission, called BepiColombo, sending two probes on October 20, 2018. They are expected to reach Mercury in November 2026, where they will expand on MESSENGER’s work.
What Is It Like On Mercury?
Having discussed exploring the planet from the ground and from satellites, we turn to the possibility of exploring it from its surface. Colonising other worlds has fascinated people, both Muggle and magical alike, for ages. Now, you can’t land on any of the outer planets because there is no surface to land on, and you wouldn’t want to go to such a hot and toxic planet as Venus; however, Mercury, Mars, and some of the moons of the outer planets are possible destinations for adventurous people some time in the future.
If prospective colonists are to avoid carrying too much weight during a voyage to Mercury, they will need to find as many resources as they can locally. MESSENGER detected some water ice at the bottom of some deep craters at the poles, where the Sun never shines; so that would be a good place for a manned mission to land.
Mercury’s close proximity to the Sun makes it a very inhospitable place for life. The planet experiences wild fluctuations in temperature: during the day temperatures may reach up to 427°C (800°F); however, since Mercury lacks a substantial atmosphere, it cannot keep the heat in, and its temperature can drop to as low as -173°C (-280°F) at night. Any colonies on Mercury would have to be underground to avoid the extreme temperatures, but the colonists would want to be on the surface at least some of the time, since the whole point of voyaging to a distant world is to explore it. You’d have to wear a space suit, of course, which would add to your weight, but it wouldn’t make you all that heavy. The surface gravity on Mercury is only 0.38 times what it is on Earth, so if you weighed 100 kilograms in your space suit on Earth, you’d weigh only 38 kilograms on Mercury.
What would colonists expect to see on the surface of the planet? From the pictures of Mercury above, you can see that it looks very much like the Moon, due to its rocky and heavily cratered surface. The biggest crater is the Caloris Basin (see the image below), which is 1550 kilometres in diameter and is surrounded by a two-kilometre high rim.
Mercury’s Caloris Basin
Source: here
Aside from craters, the planet also has mountains, valleys, and plains (some darker than others) like the Moon. There are fewer maria and fewer large impact craters, and there is also a chain of great cliffs, which can be seen in the image below.
Chain of great cliffs on Mercury.
Source: here
Magical Mercury
And now we come to the most interesting part of the class – the magic. As you learned in Year One, Mercury’s magic improves your creativity, problem solving ability, and inspiration. As a consequence, it also increases your success in brewing potions. Now, the latest version of the von Rheticus telescope will tell you what Mercury’s A.M.E. is at the time you look through it, but if you want to plan ahead, you’ll need to know when it’s going to be at or near its maximum.
Mercury is farthest from the Earth when it’s in the full phase and nearest when it’s in the new phase. The difference in phase more than compensates for the difference in distance, so its A.M.E. Quotient is at its highest when it’s in its full phase (unless it is occulted by either the Sun or Venus). Of course, you can’t actually see the planet at this time, because it appears close to the Sun in the sky and sunlight scattered by the atmosphere washes it out. But the Sun’s magic reflected by Mercury still reaches Earth, and it’s almost doubled by constructive interference from the Sun in this location, which is why it is at its most powerful. Since you won’t be able to see the planet by sight or with a telescope, there are plenty of sources you can reference that will tell you when Mercury will next be in its full phase. Since it will be in conjunction with the Sun, it will be above the horizon at daytime, and that’s when its magic will reach you.
Some wizards are more sensitive to planetary magic than others. A certain potioneer named Carl Brewer was particularly sensitive to Mercury’s magic. He often wondered why he brewed his best potions about every four months and only during the day. He figured that it had something to do with magic and astronomy, so he did some research and learned that Mercury contributes most to potion brewing when its A.M.E. is the greatest, which occurs when it’s full. He looked up when Mercury would next be full and that’s when he brewed his most difficult potions – with more success than he had previously achieved. Studying magical astronomy is recommended for all wizards and witches!
In Year Two you learned about elemental magic, and in particular, that the Moon has an impact on water magic, which enhances the water charms. Well, all the rocky planets, including Mercury, have an impact on earth magic, which enhances earth-related charms: the Gouging Charm, the Tunnelling Charm, the Earth Extraction Charm, and the Shaping Charm. You’ll learn more about these charms in Year Four, Lesson Eight of your Charms class.
If you’re on Mercury, its magic will be much more powerful than it is from Earth, but it will also be unpredictable because Mercury’s atmosphere contains almost no water vapour to filter it. There is a solution to this problem, though: Dr. Mansour did study other things besides the A.M.E. Can you figure out what that solution is?
With that, we end today’s lesson. There will be the usual ten-question quiz. Class dismissed.
Original lesson written by Professor Gagarina
Parts of this lesson written by Professor Plumb
- ASTR-301
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