Mercury is a little planet with big implications. Appearing much like Earth’s moon, this small world is barren, cratered, rocky, and lifeless. It is a world of extremes, with temperatures on the day side reaching 800 degrees Fahrenheit, and temperatures on the night side dropping to 280 degrees below zero. The smallest of the eight classical planets, Mercury frustrates secular thinking, but confirms the creativity of the Lord.
Ancient Observations
Since Mercury is quite bright, it was well known to the ancient world. Many ancient cultures considered the planets to be gods since planets move relative to background stars in ways that were difficult to predict at that time, as if planets had their own will. In most cultures, Mercury was considered to be the messenger of the gods because it is the fastest moving of all the planets. The name given to the planet depended on the culture. Today, we use the Roman name Mercury. The Greeks celled it Hermes. And some cultures had two names for this little planet – one for when it appeared in the evening sky, and another for when it appeared in the morning sky. This planet is even mentioned in Scripture in Acts 14:12 where people mistakenly claimed that the Apostle Paul was Mercury/Hermes, and that Barnabas was Jupiter/Zeus.
The reason that Mercury moves so fast is because it is the closest planet to the sun. The creation scientist Johannes Kepler discovered that the period of a planet around the sun depends on its distance, such that the period squared is proportional to the distance cubed. Another creation scientist discovered the cause of this relationship; Isaac Newton realized that gravity is what keep planets orbiting the sun. The closer a planet is to the sun the stronger the sun’s gravity pulls on the planet. Hence, greater speed is needed to stay in orbit. Mercury takes only 88 days to orbit the sun.
An Unusual Planet
Mercury has some unique properties. It has the shortest year of any planet, but the longest (solar) day. It is the smallest of the planets, being only one third the size of Earth. And it has the most eccentric orbit – meaning its orbit is the most elliptical.[1] The eccentricity of an orbit is a mathematical parameter which can be anything between zero and up to (but not including) one. An eccentricity of zero means the orbit is a perfect circle; the planet remains exactly the same distance from the sun at all times. The eccentricity of Mercury’s orbit is 0.205. For comparison, the Earth’s eccentricity is only 0.0167.
Consequently, Mercury’s distance to the sun changes quite drastically throughout its orbit. Its point of closest approach, called the perihelion, is only 28.6 million miles. That’s more than three times closer than Earth orbits. At such a time, the sun would appear more than three times larger in Mercury’s sky and would be over 10 times brighter than it appears from Earth! Only 44 days later, Mercury reaches its farthest distance from the sun called the aphelion, at a distance of 43.4 million miles. At aphelion, the sun would still appear more than twice the size as seen from Earth, and would shine over four times as bright.
Such powerful sunlight causes the day side of Mercury to reach temperatures exceeding 800 degrees Fahrenheit. But Mercury has a very slow rotation. It takes 58.6 days for Mercury to rotate once. This slow rotation allows heat to build up on the day side. But it also allows the night side to cool off by radiating that heat away into space. This is why the temperature on the night side can drop to 280 degrees below zero. Furthermore, Mercury has almost no atmosphere; therefore, it cannot redistribute its thermal energy. The mass of Mercury is only 5.5% that of Earth. The gravity at its surface is just over one-third that of Earth. Mercury has no moons.
An Unusual Day
Mercury’s rotational period of 58.6 days is precisely two-thirds of its orbital period of 88 days. This came as quite a surprise to astronomers who previously thought that Mercury’s day matched its year. Small objects that orbit very close to large ones tend to be tidally locked, meaning that they rotate at the same rate they orbit. Earth’s moon is this way, rotating exactly once each orbit, which is why we always see the same side. This is a very energetically favorable configuration because it minimizes tidal torqueing. Before1965, most astronomers assumed that Mercury too was tidally locked. But radar observations revealed that this is not so. The Mariner 10 spacecraft flew past Mercury in 1974, and confirmed a rotational period of 58.6 days.
When two periods, such as an orbital period and a rotational period, form a simple ratio like 2:3, this is called a resonance. There is a physical reason why Mercury has this resonance, and it has to do with its elliptical orbit. As Johannes Kepler discovered, a planet orbits faster when it is near perihelion, and slower when near aphelion. But a planet must rotate at a constant speed. So, when Mercury is closest to the sun at perihelion it is orbiting faster than its average speed, and so its rotation rate and orbital rate temporally match. You can think of it as being temporarily tidally locked at the point in its orbit where the sun’s gravity is strongest. In other words, if you were standing on the surface of Mercury at a time near perihelion, the sun would appear to be essentially stationary for a period of time. After Mercury moves away from perihelion, its orbital speed slows, but its rotation rate remains constant, and is therefore now faster than the orbital speed. So, the sun would then appear to slowly drift westward across Mercury’s sky.
Mercury’s rotation period of 58.6 (Earth) days is called its sidereal day. Sidereal means “in relation to stars.” That is, if we were to watch Mercury rotate from a distant star, we would see it rotate once every 58.6 days. But if we could somehow watch Mercury from the sun, we would see it rotate once in 176 days. This is called a solar day. The difference between the two periods is because Mercury orbits the sun.[2] By contrast, Earth’s sidereal rotation rate is 23 hours, and 56 minutes. But one sunrise to the next is 24 hours on average because the sun’s position relative to the background stars changes slowly as the Earth orbits around it. For a given location on Mercury, you would only see a sunrise every 176 Earth days. So, Mercury’s solar day is actually twice as long as its year.
Earth-Based Observations
Although it appears relatively bright in Earth’s sky, Mercury is the most difficult of the five naked-eye planets to observe. This is because it orbits so close to the sun. Mercury is never farther than 28 degrees away from the sun as seen from Earth, and the angle is usually far less. So, your best opportunity to see Mercury is in the western sky just after sunset, or in the eastern sky just before sunrise. It is visible to the unaided eye within a couple of weeks of greatest elongation (the dates where Mercury appears farthest in angle from the sun). The timing window is rather narrow. Minutes after sunset, the sky is still too bright to see Mercury. But if you wait too long, Mercury will have set.
There are three other ways to see Mercury. It is possible to see Mercury in broad daylight using a telescope. But this only works well for dates near greatest elongation, otherwise the telescope would be pointed dangerously close to the sun. Another way to see Mercury is during a total solar eclipse. When the moon completely covers the sun, Mercury is easily visible, as is Venus and some of the brighter stars.
A third way to see Mercury is to observe its shadow during a solar transit. A solar transit happens when a planet crosses directly in front of the sun as seen from Earth. Using a telescope (with appropriate solar filters!) you can see Mercury as a tiny black dot slowly moving across the sun over the course of a few hours. Mercury transits happen roughly 13 times per century and always in either May or November. The last one was in November 2019. The next two will occur in 2032 and 2039 and will be visible from Europe. The next Mercury transit visible from the United States will occur on May 7th, 2049.
Visits by Spacecraft
Since Mercury is small and never departs far in angle from the sun, detailed images of its surface are difficult from ground-based telescope observations. Our best images of Mercury were provided by two unmanned spacecraft. The Mariner 10 probe was launched on November 3, 1973 and reached Mercury on March 29, 1974. This was the first spacecraft to harness the gravity of one planet (Venus) to slingshot the craft past another planet (Mercury). Consequently, Mariner 10 was the first spacecraft to visit two planets besides Earth. The probe detected a dipole magnetic field around Mercury, about 1% the strength of Earth’s magnetic field.[3] This observation challenges the secular timescale because magnetic fields decay with time and a planet as small as Mercury should have exhausted all its electrical energy if it were billions of years old.
Mariner 10 was directed into an elliptical orbit around the sun that caused it to fly past Mercury every 176 days. That is, every time Mercury orbited the sun twice, Mariner 10 would fly very close to Mercury and thereby obtain detailed images of the surface. But since Mercury rotates once every 58.6 days, it would rotate exactly three times every time Mariner 10 flew past. This means the spacecraft photographed the same side of the planet on every pass. For three decades, we had detailed images of only one side of Mercury.
The Messenger Spacecraft was launched in 2004, and harnessed the gravity of both Earth and Venus to eventually achieve orbit around the planet Mercury in 2011. Since it was in orbit around Mercury, Messenger was able to image the entire surface, and fill in the details from the hemisphere missing from the Mariner 10 mission. Messenger confirmed that Mercury indeed has a magnetic field. However, its measurements suggest that the magnetic field has weakened slightly even since the Mariner 10 visit in the 1970s. This again is what biblical creationists would expect.
Planetary magnetic fields are caused by electrical current in the core. Since electricity naturally encounters resistance, magnetic fields spontaneously weaken over time. We have been able to measure this in detail for Earth’s magnetic field for nearly two centuries. The timescale for such a decay is on the order of several thousand years. So, secularists have speculated that there must be a recharging mechanism to reenergize planetary magnetic fields over time. Such dynamo models suppose that mechanical energy from a planet’s rapid rotation can induce new electrical current. But such models require rapid rotation. Recall that Mercury rotates only once every 58.6 days.
Physical Properties
Thanks to the teams responsible for the Mariner 10 and Messenger spacecraft, we now have spectacular images and other data for this tiny world. In appearance, Mercury resembles the moon more than any other planet. It is rather colorless and covered everywhere with craters along with plains called maria. The moon also has maria which appear darker than the rest of the surface. However, the maria on Mercury are far less distinctive than those on the moon. Like the moon, Mercury has almost no atmosphere. It is rocky in composition. The surface is mainly composed of silicates, and based on measurements of density, Mercury must have an enormous iron core, perhaps comprising more than 50% of the planet’s volume.
The overabundance of iron on Mercury relative to the rest of the solar system is puzzling to secularists. In their view, all the planets accreted from a nebula, and therefore should have the same composition. Some people have suggested that Mercury started with the same relative abundances as the rest of the solar system, but that a giant impact vaporized the original crust and mantle, leaving primarily the heavier elements. But the Messenger spacecraft detected lighter elements such as potassium on Mercury’s surface, which would have been stripped away by such an impact. Due to its abundance of iron, Mercury has the second highest density of all the planets and moons in the solar system.[4]
The heavily cratered surface of Mercury, like the moon, prompts us to ask what caused such craters. Most of the craters on Mercury and the moon are thought to have resulted from impacts. A small asteroid occasionally crosses the path of a planet, and impacts on its surface. The rate at which this happens today is rather infrequent. Both creationists and secularists believe that the rate of impacts was much higher in the past. Perhaps the Lord used process in creating the planets on day 4 of the creation week. Craters may be due to the last bits of material that the Lord drew together to make these planets.
The Earth has relatively few impact craters compared to the other terrestrial planets. Secularists believe that Earth once had abundant craters, but that these have been erased by geologic activity over time. However, it may be that the Earth never had many craters because the Lord used a different process to create it than the other planets. The Earth was created on day 1, whereas the other planets were made on day 4 (Genesis 1:1-5, 14-19).[5]
A Test of Physics
Mercury experiences a phenomenon called orbital precession. This means that the angular location of the perihelion of the orbit drifts over time. Although the perihelion distance and aphelion distance do not change very much, their location drifts counterclockwise very slightly. In fact, this is true for all the planets, and is predicted on the basis of the physics discovered by Isaac Newton. The gravity of each planet slightly affects the position of all the others, and drives orbital precession. The effect is very slight, but is measurable with modern instruments. Mercury’s perihelion drifts by 5600 arcseconds per century. One arcsecond is 1 /3600 of a degree.
The apparent problem is that the physics of Newton predicts that the perihelion of Mercury’s orbit should precess by 5557 arcseconds per century. So, by the beginning of the 20th century, there was a small but measurable discrepancy between the predictions and observations: a difference of 43 arcseconds per century. What could cause this? Many explanations were proposed.
Albert Einstein discovered the solution. In 1915 Einstein published his General Theory of Relativity, which deals with the nature of space and time, how objects move in response to gravity, and how mass, lengths, and durations are affected by gravity and velocity. When objects are moving relatively slowly or in response to very weak gravitational fields, the predictions of Einstein match the predictions of Newton. And since Newtonian physics is much, much easier than Einstein’s, we still use Newton’s laws of motion and gravity in situations where the gravity is very meager and when velocities are low compared to the speed of light.
But Mercury orbits closer to the sun than any other planet. And the gravitational forces are strong enough that the differences between the predictions based on Newtonian physics and those of Einstein become measurable. And what is that difference? General relativity predicts a precession of Mercury’s perihelion that is 43 arcseconds per century greater than the Newtonian estimate. In other words, the predictions of Einstein exactly match observations. Mercury’s orbit therefore provides a test for modern theories of physics. It confirms general relativity. Some of the things that the Lord created are perhaps not necessary for life; but they help us make other discoveries in science. Mercury may be one such example.
[1] These latter two titles were regained by Mercury when Pluto was demoted from planethood in 2006. Pluto is both smaller than Mercury and has a more eccentric orbit, but is no longer classified as a planet.
[2] Suppose for example that Mercury’s day and year were both 88 days, as was previously assumed. Then the sun would only see one side of Mercury, and its solar day would be infinite. For this reason, the solar day is always longer than the sidereal day if the planet rotates in the same direction that it orbits.
[3] A dipole magnetic field is a field with exactly two poles, a north pole and a south pole.
[4] Earth is the densest planet in our solar system. However, some of that density is due to the compressional effect of gravity.
[5] The Hebrew word for “stars” used in Genesis 1:16 also includes planets.