The moon is one of the most fascinating objects in the sky. It is one of just two celestial objects (along with the Sun) which are resolvable by the human eye. Through the course of a month, different parts of it are illuminated depending on its position in the sky relative to the Sun. Occasionally, a full moon turns blood red, and even more rarely a new moon will block out the Sun at midday. Respectively, these phenomena are called lunar eclipses and solar eclipses, and they result from perfect alignments between the Sun, Earth, and Moon.
The Moon is not featureless: patches of dark and light run across the only face it ever shows the Earth. Sorting out what the Moon is was not a difficult task. The first views with a telescope showed that it is covered with mountains, craters, and relatively very smooth dark “seas” which we call maria. It’s another world with a variable landscape much like Earth’s.
Even on the scale of Solar System objects, the Moon is close. It orbits the Earth at a distance roughly equal to ten times the Earth’s circumference. If you’ve ever flown halfway around the world, multiply the time it took by 20 and that’s how long it would take to get to the Moon travelling at the same speed. The Moon is the only celestial body other than Earth that humans have visited. For comparison, space observatories such as the James Webb Space Telescope now operate more than a million kilometres from Earth, well beyond the Moon’s orbit.
Mercury is a different object entirely. It is a planet. It orbits the Sun far from Earth: at the nearest point in its orbit, it is 240 times further from Earth than the Moon is; at its furthest distance, on the opposite side of its orbit, Mercury is 540 times further from us than the Moon is. Therefore, in contrast to the big, bright phase-changing Moon we earthlings are so familiar with, Mercury has been a mystery to us until quite recently.
Mercury is one of the seven “planets” visible to the naked eye (along with the Sun, Moon, Venus, Mars, Jupiter, and Saturn). Yet because of its distance from us, its small size, and its proximity to the Sun,
- it is the most difficult to resolve of the seven naked-eye “planets”,
- it never gets further away from the Sun than about 28°, so it is never very far from the horizon and either sets shortly after or rises shortly before the Sun,
- it is deep in the Sun’s gravitational well, which actually makes getting there very tricky, and
- it is hot, therefore anything we do send to study it must be able to handle the temperature.
For these reasons, our ability to learn anything at all about Mercury has come quite slowly. For instance, due to its proximity to the Sun we can’t even image Mercury through the Hubble Space Telescope. Even so, the more we explore it, the more we’ve found that Mercury bears a striking resemblance to the Moon.
The first thing we ever learned about Mercury, for instance, is that like the Moon and Venus it goes through a full range of phases. Despite the fact that Galileo had observed Mercury with his telescope in 1610, as had a number of others who followed him, the discovery was only made in 1639 by another Italian astronomer, Giovanni Zupi. The fact that this discovery, which was a key piece of evidence that would further support heliocentrism, took another thirty years to be made, should indicate to you just how much more difficult Mercury is to observe than the rest of the naked-eye planets.
In addition to the phases it exhibits, occasional alignments between the Sun, Earth and Mercury result in a type of eclipse called a transit. These two similarities—the phases and “eclipses” of the Moon and Mercury—were both discovered in the early seventeenth century, and they appear as they do because of our own observational perspective from Earth. However, through technological advances we have discovered a number of physical characteristics of the two bodies themselves that are similar.
In 1965, radar observations from Arecibo revealed Mercury’s unusual spin–orbit resonance: the planet rotates exactly three times for every two orbits around the Sun. This means the same hemisphere faces the Sun at every second perihelion, not continuously as in true tidal locking. The planet is actually shaped kind of like a football, and the ends that face towards and away from the Sun flip from one perihelion to the next. If you think about it for a moment, this means Mercury goes through exactly 1.5 rotations every orbit. Similarly, the Moon is tidally locked to the Earth so that we always see exactly the same side. If you think about that for a moment, it means the Moon goes through exactly one rotation every orbit. This is called synchronous rotation.
When NASA scientists finally managed to fly the Mariner 10 space probe past Mercury in 1974 and 1975, a crater-ridden surface was discovered. However, Mercury’s surface is without the prominent maria observed on the Moon. Neither the Moon nor Mercury show signs of wind erosion due to their lack of atmospheres. In fact, the density of particles surrounding either body, which are kicked up by solar radiation, the solar wind, or meteorites in a process known as sputtering, or else released through radioactive decay, is only about 105 particles per cubic centimetre—less than a hundred trillionth the density of Earth’s atmosphere at sea level. This density is too low to absorb an appreciable amount of radiation, so the temperature differences between the day and night sides—and even in the shadows of their craters—are very great. As a result, both the Moon and Mercury—the closest planet to the Sun, whose surface temperature rises to 700 K in the daytime—have significant deposits of water ice locked away in craters near their poles that never see sunlight, as confirmed decades later by the MESSENGER mission.
All these similarities and more are the reasons why we are studying the Moon and Mercury together in this module. We will begin by explaining the properties that we observe from Earth—i.e. what gives rise to their phases and eclipses. After that, we will explore in more detail the similarities and differences in their physical characteristics. Recent missions continue to refine our understanding, showing that both worlds preserve records of early solar-system history and processes still shaping airless bodies today.
cosmiCave.org 