—Sir William Jones, "Caissa"

© NASA/STScI

Around the holidays each year, my radio has an irritating tendency to shower me with more advertisements about buying stars for my "loved ones." (It probably says something about my sense of humor that that phrase always brings to mind the wicked book by Evelyn Waugh.) What better way is there to show your affection and regard, these ads challenge me, than to buy a star for the object of that affection and that regard, named personally by you? This name will be recorded for posterity, we are assured, in book form—whatever that means—in the U.S. Copyright Office.

The most irresponsible of these ads manage to omit the fact that these stars are generally too dim to be seen by the unaided eye or even binoculars, and even under the darkest of skies; that any book that is published may be similarly recorded in the U.S. Copyright Office; that the certificate printed in honor of the star has no more official standing than any certificate you could print out yourself; and that no astronomer is compelled to use any of the star names so purchased, or is even likely to have heard of any of them. The companies involved view the stars chiefly in one light: a way to make money.

(Where is this all going, and what does it have to do with Mars? Patience, have patience.)

The sole body with any authority to name celestial objects is the International Astronomical Union, or IAU. When astronomers, whether professional or amateur, discover a new object, they may be permitted to propose a name, depending on the type of object, but the final decision rests with the IAU. Thus, for example, someone who discovers a new asteroid can suggest a name for it, and the suggestion is usually adopted after a firm orbit is established for the asteroid; comets, however, are named after their discoverer or discoverers.

These names are not legally binding in any way. You can, if you like, write an article in which you refer to the red star marking the right shoulder of Orion as Mikey, but no journal will accept it for publication unless you instead use Betelgeuse, or alpha Orionis, or even 58 Orionis. The IAU bases its naming decisions not on fleeting concerns of money, but on making it easier to conduct the scientific enterprise of astronomy.

That is not to say that confusions don't happen from time to time. The name Betelgeuse ultimately comes, via an error-riddled line of progression, from Arabic words meaning "the hand of al-jauza," where al-jauza is a female figure of uncertain identity. When the German astronomer Johann Bayer (1572-1625) assembled his beautiful star atlas, Uranometria (from Greek words meaning "the measure of the sky"), he assigned Greek letters to the brightest stars in each constellation. By and large, the brightest stars got the letters near the beginning of the alphabet. For instance, the seven most prominent stars in the constellation of Ursa Major, or the Great Bear, form the familiar pattern of the Big Dipper, and Bayer named these alpha Ursae Majoris through eta Ursae Majoris, after the first seven letters of the Greek alphabet:

alpha, beta, gamma, delta, epsilon, zeta, eta

Because of this, people often think that the stars are always given Bayer names in strict order of brightness, but that isn't so. Bayer named them somewhat haphazardly, with an eye toward prominent patterns rather than slavishly following brightness. Besides, many stars are variable for one reason or another, which makes it difficult to come up with a strict order of brightness. As it so happens, Betelgeuse is a variable star, and on rare occasions it is indeed the brightest star in Orion; but it usually isn't, and its brightness isn't the reason it gets the first Greek letter. More likely, it is called alpha Orionis because it is a notably bright red star in a constellation largely made up of bright blue ones.

In 1725, Britain's first Astronomer Royal, John Flamsteed (1646-1719), published his Historia Coelestis Britannica, or British History of the Heavens. Part of it is a catalogue of some 3,000 stars, with their celestial positions and brightnesses given to a greater precision and accuracy than anyone else had managed up to that time. Those stars are arranged in order of increasing right ascension, a celestial coordinate corresponding to the terrestrial coordinate of longitude. In this system, Betelgeuse was the fifty-eighth star in the constellation of Orion, so that it is also called 58 Orionis. Such numbers are called Flamsteed numbers.

But Flamsteed numbers were not assigned by Flamsteed at all. He had done all the work, in terms of cataloguing the stars in order of right ascension, but he did not refer to them by number. It was the French astronomer Joseph-Jerome Lefrancais de Lalande (1732-1807), who actually assigned the numbers. But we don't call them Lalande numbers; we call them Flamsteed numbers, and as a result, many people think they were assigned by Flamsteed. Probably even many astronomers do, too.

There are other examples of multiple naming. The fourth planet in our solar system is often referred to as the Red Planet, and I suspect that many observers looking up for it in the sky do so in search of a blood-red beacon in the night. If so, they won't find it. That's because it isn't red, the popular name notwithstanding. If others refer to the Red Planet, I will understand, and I will probably have to refer to that color, too, if I am to be understood myself. But to me, privately, Mars will always be. . .the Orange Planet.

(See, I got there.)

Mars is big news these days. Last August, Mars passed closer to the Earth than it had in nearly five dozen millennia—a fact reported by the media with predictable results. A friend of mine asked if it was all right to wait a couple of days to observe Mars, as she had business to attend to on the special night, and she didn't want to miss it. (I wasn't sure if she was referring to the business or the planet.)

Well, the fact is that Mars does not rush by us with the urgency of a too-full commuter train. Although it's true that Mars passed closer to us in August than it had at any other point in recorded history, it was hardly any further away a week later, or a week earlier. The planets move at speeds that are fast by ordinary human standards—the Earth revolves around the Sun at 30 km/s, Mars at 24 km/s—but their orbits are so large that if you could get high enough above the plane of the solar system, so that you could see both orbits at once, the planets would hardly appear to be moving. After all, it takes the Earth one full year to go once around the Sun, while it takes Mars about a month and a half shy of two years.

The fact that both planets travel in orbits also means that Mars is not bobbing toward us and away from us as though it were attached to us like a yo-yo at the end of a string. Rather, we are passing Mars on the inside, on a celestial race track, as it were. When the Earth is most nearly on a straight line connecting the Sun and Mars, Mars is then said to be "in opposition," and it is then more or less closest to the Earth.

It isn't precisely closest to the Earth at that time, either, because the orbits of the Earth and Mars are not perfectly circular, but are ellipses. (See "The Music of the Ellipses.") The Earth's orbit is reasonably round, but Mars' orbit is not. Its orbit has an eccentricity of about 0.09, meaning that Mars is sometimes 9 percent closer to the Sun than it is on average, and at other times, it is 9 percent further from the Sun than it is on average.

If Mars reaches opposition at a time when it is getting closer to the Sun, then its closest approach to the Earth will come after opposition, because although the Earth is passing by and pulling away from Mars, the orbit of Mars is carrying it closer to the Sun, and therefore closer to the orbit of the Earth. If, on the other hand, Mars reaches opposition at a time when it is receding from the Sun, its closest approach to the Earth will come before opposition.

The eccentricity of Mars' orbit also means that not all oppositions are created equal. When Mars reaches opposition at aphelion—that is, when it is furthest from the Sun, it will then be further from the Earth, roughly speaking, than it is at any other time. (It isn't exactly, because the Earth's orbit is also slightly eccentric.) And when Mars reaches opposition at perihelion, when it is closest to the Sun, it will then also be closer to the Earth than at any other time. And the nearer to perihelion the opposition occurs, the closer Mars will be to the Earth.

This makes a big difference to Earthbound observers, including any telescopes orbiting the Earth, such as the Hubble Space Telescope. Mars is never very big as seen from the Earth—even last August, Mars looked to be only about 1/70 the width of the Moon. But in other opposition years, Mars never looks to be much more than about 1/120 the width of the Moon. Details that are never easy to see through a telescope become that much harder to discern, that much more distorted by the Earth's moving atmosphere.

As it so happens, the August 2003 opposition of Mars took place very close to perihelion, and also at a time when the Earth, with its lower eccentricity, was close to its aphelion. Those two factors helped to make the close passage of Mars the closest in tens of thousands of years.

But that's not the only reason. When Kepler deduced his laws of planetary motion from Tycho Brahe's observations of Mars, he assumed that although the Earth and Mars were both moving in their orbits around the Sun, the orbits themselves were not changing. Whatever path Mars followed this time around, Kepler reasoned, it would also follow the next time around, regardless of where the Earth happened to be at the time, and vice versa.

And yet its orbit does change. (E pur si muove, Galileo might have said.) The Sun is so massive in comparison to the planets that it is not a great departure from reality to assume the planets exert no gravitational influence on each other. And yet, a departure it is. Mars' orbit is being perturbed by the Earth and by the other planets, notably Jupiter. It is slowly becoming more elliptical and more eccentric, and Mars will gradually come even closer to the Sun and the Earth. So although the last time Mars was this close to the Earth was almost 60,000 years ago, it will come even closer less than 300 years from now. That's little comfort to anyone who's upset they missed this one, but in astronomical terms, it's little more than a blink of an eye.

Might Mars' orbit become so elliptical as to be ejected altogether from the solar system—or worse yet, to collide with the Earth? Nothing is impossible, but unless some other as yet unknown body enters the solar system, the same forces that are now making Mars' orbit more elliptical will sometime in the future make it more circular again. Then, the cycle will repeat. The solar system may not be entirely stable, but as far as astronomers can see, if Mars were going to be thrown entirely out of its current place, it would have happened long ago, and we would now be talking about the eight planets of the solar system.

Mars must have been one of the earliest planets to be discovered in pre-historic times, partly because it is bright, partly because being close to the Earth, it moves very noticeably from night to night when it is visible, and partly because it has that distinctive color.

As a matter of fact, Mars is the only planet in the night sky visible to the unaided eye to have a distinctive color at all. Venus, enshrouded in its eternal clouds, is fairly white. The other planets—Mercury, Jupiter, and Saturn—have perhaps a bit more color, but those colors can only be seen through the telescope or binoculars. To the unaided eye, these planets look essentially white.

Not so Mars. The colors of Mars and the other planets can be compared in an interesting way to those of stars. As I described in "The Color Green," stars are roughly black-body emitters and therefore have a characteristic spectral signature that mostly varies only by temperature.

Classifying stars based on their spectral signature, however, is a time-consuming process. Fortunately, stars with similar spectra will also have similar colors, and color is easier to measure. The star's brightness, or magnitude, is computed at two different bands of wavelengths, called B (for blue) and V (for visual), which is in the yellow-green portion of the spectrum. If a star looks bluish, it will be brighter at the B wavelengths than at the V wavelengths; if it looks reddish, it will be brighter at V than at B.

For historical reasons, lower magnitudes mean brighter stars, so bluish stars will have lower B magnitudes than V; it's the other way around for reddish stars. What's more, stars of similar colors but different brightnesses will have different B and V magnitudes, but the difference between the B and V magnitudes will be approximately the same for the two stars. Consequently, we can summarize the color of a star by subtracting its V magnitude from its B magnitude, yielding its B-V value. Thus, the Sun has a B-V value of about 0.64, indicating that its V magnitude is 0.64 smaller—that is, brighter—than its B magnitude. Bluish Rigel has a B-V of -0.03, and reddish Betelgeuse one of 1.85.

Planets don't shine by their own light; they simply reflect the light of the stars they orbit. Their colors are chiefly the result of absorption of certain wavelengths of light by various substances on their surfaces or in their atmospheres. For example, Neptune looks greenish-blue because methane in its atmosphere absorbs most of the wavelengths except for those of green and blue. These colors are reflected back into space, where they account for the planet's distinctive hue. In particular, planets are not black-body emitters.

However, their colors can be compared by pretending that they're just like stars, and by measuring their B-V values. We can even go so far as to give the spectral class (see "One Little Star") of a star with the same B-V value as the planet. Such a table has been prepared (with the B-V values given by C. W. Allen in his Astrophysical Quantities, and the corresponding spectral classes given by amateur astronomer Paul Schlyter):

Spectral classes run—from hot and blue to cool and red—in the order O, B, A, F, G, K, M. Within each class there are up to 10 subdivisions, from 0 to 9, with 0 being hotter than 9. The Sun is a G2 star. Rigel, which is bluish-white, is a B8 star, while Betelgeuse is an M2 star. As you can see, Mars is the reddest planet in the solar system by a significant margin, and it might surprise you to see that the Earth is the bluest. (It must be all the ocean water.) Still, as red as Mars is, it's still only spectral class K8—orange, not red. But I digress.

That redness must have reminded ancient humans of another distinctively red fluid—blood—because the Babylonians named the planet after their god of war, Nergal. The Greeks named it after theirs, Ares. And the Romans, who appropriated many of the Greek gods, named it after their version of Ares, Mars. As with all other planet names, we've kept the Roman name.

Why should Mars be so red? To put it in short, Mars is rusty. The dust that covers the Martian surface is apparently filled with iron oxides. This made sense when some astronomers (but by no means most of them) believed the surface of Mars was criss-crossed by the canal network envisioned by Percival Lowell, but the Mariner spacecraft showed conclusively that the surface of Mars is dry.

Just because Mars is dry today, however, doesn't mean that it was always dry. Indeed, there is some evidence that Mars once had substantial bodies of water. Many trenches—not visible from the Earth and in any event not corresponding to any of Lowell's canals—show signs of having been carved by running water. The reason that Mars doesn't have any water today is that its atmosphere is so thin that any water on the surface would simply evaporate, despite the frigid temperatures.

There's a catch, though. Recent images gathered by orbiters sent by the U.S. indicate that the surface of Mars is poor in carbonates, such as chalk and limestone. Mars' atmosphere today is composed largely of carbon dioxide, and as thin as it is, if the Martian crust had come into prolonged contact with the atmosphere and with water, those carbonates should have formed.

Those same images show significant amounts of magnesium and iron silicates, a collection of minerals that come under the general heading of olivine. Olivine is common throughout the Earth, but not in very wet areas. That's because when it comes into contact with water, it breaks down into clays and iron oxides. The widespread presence of olivine on the Martian surface and the lack of carbonates both suggest that it's been some time since Mars was wet.

There's still some doubt, since we really don't know how long the olivine has been exposed at the surface, and we can't tell if there are carbonates that are simply covered by a relatively thin layer of Martian dust. (Huge dust storms can cover substantial portions of the Martian globe, and as recently as in 2001, one covered the entire surface between the polar ice caps.) All the same, astronomers would like some mechanism to explain how rust could be generated in the absence of large amounts of water.

Quite recently, Albert Yen, of the Jet Propulsion Laboratory in Pasadena, California, has suggested another theory. Photons of the ultraviolet light radiated by the Sun can liberate an electron from an iron atom, ionizing it. Left to its own, the iron atom will recapture an electron, returning it to its original state, but even in the thin Martian air, iron is frequently not left to its own. What little oxygen exists in the air is strongly attracted to the ionized iron and can oxidize or rust the iron first.

Such a mechanism wouldn't work on the Earth, where the surface is protected from the Sun's ultraviolet light by the Earth's ozone layer, but Mars lacks an ozone layer. This still won't work if the iron is already locked into a mineral, where the electrons are more strongly bound to the atoms; the iron must be elemental. But where would this elemental iron come from? Yen has an idea: he thinks that enough meteorites—of which about 10 percent are nickel-iron; the rest are stony—strike Mars to supply the iron.

© NASA

This idea can be tested. For one thing, the metallic iron that does not get oxidized by ultraviolet light should remain in elemental form and exhibit magnetism. As it turns out, some Martian dust is ever so slightly magnetic. Furthermore, the meteorites contributing the iron aren't pure iron, but a mixture of nickel and iron. If Yen's idea is correct, some of the dust should consist of nickel as well as iron.

At the beginning of 2004, the first of two Mars Exploration Rovers, called Spirit and Opportunity, [1] respectively, landed on the Martian surface to examine, among other things, this very possibility. Like the Viking landers before them, these two mobile explorers will scoop up samples of Martian dust and run chemical tests for nickel content. Yen predicts that the dust should contain about a tenth of a percent nickel, if the Martian rust is due to ionized iron. We should know in the near future.

When I bought my 5-inch telescope in 1998, the first planet I looked at was Jupiter, with its colorful bands and belts. The second planet was Saturn, of course, with is beautiful set of rings. I didn't observe Mars until almost a year later, during its 1999 opposition, but I was hooked. Since then, Mars has been my favorite planet to observe through the telescope. Maybe it's because Mars is the only planet with a solid surface that we can see with any detail on it. Maybe it's because Jupiter is detailed no matter when you look at it, and Saturn has its rings, but Mars is furtive, yielding its secrets only with resistance. For whatever reason, I look forward eagerly to any opposition with Mars. It doesn't hurt that the three so far have been good ones, except for the dust storm that obscured Mars for the second half of 2001.

So I've been itching for some time to write an Astronomical Games on Mars. Something always came up to interfere with that plan, though. Most recently, I started this essay in December 2003, but the holiday season got in the way.

But the Martian landers have kept Mars in the news, lasting much longer than originally planned. (Admittedly, those plans are often scripted to be conservative.) The findings they've already begun sending back to Earth finally motivated me to finish this essay—something they probably didn't specifically include in their budget!

Adapted from Astronomical Games, December 2003.

[1] So named by nine-year-old Sofi Collis in an essay submitted to a "Name the Rovers" contest run by NASA. I'm not a big fan of abstract ideas for robot names, and these two are no exception, but I suppose anything that gets the public excited about space exploration can't be all bad.