In our tour of the solar system, we should begin by exploring the innermost planet. A short distance from the inferno of our sun we find it, hurtling through space at 40 to 60 kilometers per second in its 88 day cycle around the sun. As we approach it we notice its similarity to a body much closer to Earth, our own moon. It too is heavily cratered, with no atmosphere (or only a trace of one) to protect its surface or erode signs of previous collisions. The sun as seen from this distance is approximately three times as large as from Earth, and we know this is going to be the most dominant force affecting our trip to this planet. Now we have touched down and we take a quick look around. We know that the surface gravity of Mercury is 38% of Earth’s, so if we weighed 200 pounds before, here we weigh only 76 pounds. Looking up at the sky we see a black, star-speckled sky dominated by the fiery globe of a sun almost three times larger than we are familiar with. There is only the faintest trace of an atmosphere, and the only way we even know it is there is the instrumentation we have with us. It will be totally useless to us. The sun beats down unabated in all its fury, and the temperature hovers around 800 degrees Fahrenheit. If we were on the night side the temperature would drop to -270 degrees. We would never survive here without space suits.
Bending down to inspect the surface we find a layer of dust similar to what astronauts found on the moon. This is the result of billions of years of meteor impacts, and also the extreme temperature differentials applied from a hellishly close sun. The processes which shaped this world across the eons were much different than the ones which shaped our home planet.
Around us are the pockmarks of craters, some mountain ranges, and a horizon much nearer than what we know at home.
Contrary to what astronomers once thought, there is actually a cycle of days here, but it is much different than anything we are used to. Mercury is not tidally locked with the sun, but tidal forces have slowed it to the point where its day is 58 of our days (indeed it will one day many years from now become locked one side to its huge neighbor). This means that Mercury’s day is about three quarters as long as its year. This results in some very peculiar observations if we were to stay that long. We would observe the sun snake across the sky very slowly. In certain places on this world we would then see the sun slow and actually backtrack a short distance before continuing its slow march across the sky. Then the very long night would follow.
We take one more look around this world. No, Mercury is not an enticing place for humans to visit. Maybe one day we will have reason to be here, either for the rich deposits of iron and other metals or for observational purposes, but this is not the day.
Studies of Mercury Through History Mercury has been know and observed at least since the times of the Sumerians in the 3rd millennium BC. Following the Sumerians there is better documentation from the Babylonian Empire. Records from the 7th century BC refer back to much earlier records which described observations of the planet. The Babylonians called the planet Nebu, after the messenger of the gods in their mythology. This is an interesting parallel to the later Greek mythology and reference to the innermost planet. The ancient Greeks had two names for the planet, Apollo visible in the morning and Hermes visible in the evening. They eventually came to understand that these were the same object, and it was even proposed this early in history that Mercury and Venus orbited the sun.
Early modern astronomy began making observations of Mercury in the early 17th century when Galileo turned an early telescope to the inner world. A sketchy vision of the planet developed from that point, but its proximity to the sun has always made this one of the most difficult members of our solar system to study. It has only been recently with the advent of new imaging techniques that ground-based observation has drastically improved. And what of research with space probes? This is also laced with technical difficulties. An application of Newton’s Laws of Motion shows that it takes more rocket fuel to achieve an orbit around Mercury than it takes to escape the solar system. Because of this only one spacecraft has visited the planet to date, the Mariner 10. The Mariner 10 was launched on November 3, 1973 to study both Venus and Mercury. After its study of Venus, the probe made three flybys of Mercury in 1974 and 1975 when it mapped about 45% of the planet’s surface. Its closest approach was 203 miles on March 16,1975. The Mariner 10 circles the sun in its own lone orbit around the sun to this day, although its electronic instruments have long since been destroyed by the sun’s intense radiation. Our most ambitious Mercury research project to date was begun August 3,2004 with the launch of the Messenger by NASA from Cape Canaveral. After two quick flybys of Venus in 2006 and 2007 the Messenger will finally work its way inward and make three flybys of Mercury in 2008 and 2009. Then in March of 2011, if all goes well, it will finally become the first orbiting spacecraft around Mercury. It is carrying high resolution imaging devices, spectrometers to determine the composition of the crust, and magnetometers to study charged particles around the planet. There is an astronomical event which is of interest to amateurs and professionals alike, but not necessarily for any scientific value. Once every couple of centuries there is an occultation of Mercury and Venus. This occurs when Venus actually passes directly in front of Mercury for a few minutes. The last one occurred on May 28,1737 and the next will occur in 2133.
Structure of Mercury Much of Mercury’s mass is composed of an iron rich core. Current theories suggest that this core comprises most of the 4879 mile diameter of the planet. This is surrounded by a 350-400 mile thick mantle and finally by a crust some 100 miles thick. Because of Mercury’s slow rotation period, there is very little tectonic or volcanic activity. There have been several theories presented to explain why Mercury is so metal rich, and why its core comprises such a huge part of the planet’s structure. One popular theory is that the planet was struck by a large body early in its history and lost most of its outer, “lighter", mantle. Another theory is that the extreme heat of the early sun vaporized the outer part of the planet, giving the young Mercury a dense atmosphere of gaseous rock, which was carried away by the huge solar winds of a much more volatile sun. The competing theories to explain the unusually heavy composition of Mercury will be tested by the upcoming Messenger mission.
The Future of Mercury and Its Possible Role in Our Own Future Mercury’s most dominant feature makes it an attractive part of our future: its huge stores of heavy metals, especially iron. Although its proximity to the sun makes it hard to imagine humans working and living there, I can envision automated factories mining its surface and freighters picking up the ore to bring it back for our use. Mercury will remain pretty much as it is for billions of years to come. As the sun slowly gains in intensity over the next 4 to 5 billion years, the planet’s surface temperature will slowly rise with it. Then a singular moment will arrive as the entire solar system changes with the beginning of the sun’s passage into its next phase. When the sun consumes a critical amount of its hydrogen fuel it will, within a matter of just a few days, enter its red giant phase. At that time Mercury will be the first of the inner planets to be entirely consumed. (Look for more articles soon in the Planetary Series. )
Planetary Statistics of Mercury
closest distance to the sun(perihelion). . . . .46,001,272km(28,583,865 miles)
farthest distance from the sun (aphelion). . . . .69,817,079km(43,382,322 miles)
density. . . . .5.427 g/cubic cm
diameter. . . . .4879.4km (3031.9 miles)
orbital period (year). . . . .87.96934 days
rotation period (day). . . . .58.6462 days
max surface temperature. . . . .700K (800F)
satellites. . . . . None
atmospheric pressure. . . . . trace
surface gravity. . . . .38% Earth’s
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Michael D. Watson is a free-lance science writer and editor/webmaster of http://future-seek.com , a futurist/science site featuring science forums, a science fiction bookstore, articles, and science links.