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The Planet Mercury

© Copyright 1997, Jim Loy

Back when I was a child, I memorized a lot of facts about astronomy. And, some of the facts were about Mercury. It was the smallest planet (about 3000 miles in diameter). It had no moon. It was the planet closest to the sun, orbiting the sun in 88 days. It rotated (turned on its axis) in 88 days, as well, keeping the same face toward the sun, just as the moon keeps its same face toward earth.

Well, a couple of those facts are incorrect. Mercury is not the smallest planet. Pluto is much smaller. And Mercury rotates in a little less than 59 days (not keeping the same face toward the sun). Are you as upset as I am?

Actually, I am not upset. Science progresses by correcting old misconceptions. Every once in a while, the "facts" that we took for granted are shot down. And this is an opportunity to see actual science at work. Those facts in the first paragraph are kind of dry. But, the reason some were wrong is fairly interesting.

Pluto was thought to be bigger than Mercury, because it is fairly bright, for a small planet way out there away from the sun. No one ever saw a visible disk, in any photo of Pluto. It was just a star-like dot on all photos. It was assumed to have a dark, rocky surface, like Mercury or the Moon. So, it's diameter was deduced from its brightness. Well, better photos have been made, and spectra of the surface have been taken. And it turns out to be very tiny indeed, with a very bright icy surface. It is about 1300 miles in diameter, roughly half the size of our moon.

So Mercury is now bigger than Pluto. But, how did astronomers make the mistake in Mercury's rotation period (length of its day)? There's a big difference between 88 days and 59 days.

Well, Mercury is very difficult to see. The sky is never dark when you can see Mercury, because Mercury is fairly close to the sun. In fact, before space flight, the best photos of Mercury have all been taken during the daytime, because that is the only time that Mercury is far above the horizon. There is too much atmospheric distortion near our horizon when Mercury is visible at dawn or dusk.

So, originally, Mercury's rotation period was deduced from theory. A rotation period that has synchronized itself to be the same as the orbital period is fairly common in the solar system. And, a planet that had been that close to the sun for the entire history of the solar system should have become synchronized by now. But, there are other forms of sychronization, and Mercury has stumbled onto a second kind. It's rotation period is 2/3 of it's orbital period. This seems to be a stable situation.

mercury's rotationIf Mercury's orbit were nearly circular, then this 2/3 ratio would not be stable. Mercury's spin would continue to slow down until the rotation period synchronized at 88 days. But, Mercury has a fairly eccentric orbit (noticeably elliptical). Because of this, Mercury's distance from the sun varies from about 29 million miles to about 43 million miles. That is quite a big difference. With the 58.7 day rotation period, Mercury's long axis is always lined up with the sun, when Mercury is closest to the sun (at perihelion). This is apparently enough synchronization to remain stable forever.

See the diagram, above left. I have drawn a man standing on Mercury, to show how Mercury is oriented throughout one orbit, and from one orbit to the next. And I have drawn Mercury as an exaggerated ellipse, to show that its long axis is lined up with the sun at perihelion.

I think that this situation (the 2/3 ratio) could have been predicted, through computer simulation, long before the true rotation period was discovered. Too bad no one thought of it beforehand.

Getting back to the difficulty of observing Mercury, many of the earth-based photos of Mercury seemed to show the same face when Mercury was on the right side of the sun, just as you would expect with an 88 day rotation period. And it often seemed to show the same face when it was on the left side of the sun. This conformed to the current theory, so everyone remained happy. But, because of the 2/3 synchronization, Mercury often does show the same face to us, but not always. It happened just often enough to fool the astronomers. And, of course, the surface features were very faint on the photographs.

Question: How long does it take a person on Mercury to experience a solar day (noon to noon, for example)?

Answer: The diagram should help you with that one. It takes 176 earth days. That is three Mercury days (with respect to the stars). So, the sun appears to move very slowly in Mercury's sky.

Note: One rotation with respect to the stars is a siderial day. A solar day is the time that it seems to take the sun to go around the sky once. On earth, time is measured in solar days (midnight to midnight); one solar day is 24 hours. The siderial day on earth is 23 hr. 56 min.

Addendum:

craters on MercuryThe surface of Mercury looks quite a bit like that of the Moon. Here is a typical crater filled photo from NASA (photo PIA02937.17449.gif), near the south pole. Mercury shows some variety of surface features. But there are no seas (lava planes) as there are on the Moon.

Here are some data about Mercury (AU=astronomical units, Earth=1):

Less than half of Mercury's surface has been mapped. Mercury has a weak magnetic field (about 1/1000 of Earth's). No magnetic field was predicted, as it has a solid core, and rotates slowly. The core is about 42% of the size of the planet. Mercury has a very thin trace of an atmosphere.

Calaoris Basin (edge)mosaicThe largest surface feature on Mercury is the Caloris Basin, a huge crater surrounded by concentric mountain ranges, at the western edge of the mapped portion of Mercury. The edge of it can be seen on the left side of this picture (left). On the right is a mosaic of photos of Mercury. Some of the other large surface features are craters: Tolstoj, Beethoven, Michelangelo, Renoir, Homer, Shakespeare, as well as Sobkou Planitia, Tir Planitia, Caloris Montes, and Discovery Rupes (scarp).

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