In our tour of the solar system, we should begin by exploring the innermost planet. A brief distance from the inferno of our sun we find it, hurtling through space at 40 to 60 km per second in its 88 day cycle around the sun. It too is heavily cratered, with no atmosphere (or only a hint of one) to protect its surface or erode indications of previous collisions. The sun as seen from this distance is approximately twice 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’ve 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 skies dominated by the fiery globe of a sun almost three times bigger than we are familiar with. There’s only the faintest trace of an air, and the only way we 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, as well as the extreme temperature differentials implemented from a hellishly close sun. The processes which shaped this world across the eons were much different than the ones that shaped our home planet.
Around us are the pockmarks of craters, some mountain ranges, and a horizon considerably closer than that which we know at home.
Contrary to what astronomers once thought, there is truly a cycle of days here, but it is much different than anything we are accustomed to. Mercury isn’t tidally locked with the sun, but tidal forces have slowed it to the point where its day is 58 of our times (really it will one day many years from now become locked one side to its enormous 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 really backtrack a brief distance before continuing its slow march across the sky.
We take one more look around this world. No, Mercury isn’t an enticing place for humans to visit. Maybe one day we’ll have reason to be here, possibly for the rich deposits of iron and other metals or for observational purposes, but this isn’t the day.
Mercury has been know and observed at least since the days of the Sumerians in the 3rd millennium BC. After the Sumerians there’s better documentation from the Babylonian Empire. Records in the 7th century BC refer back to much earlier documents which described observations of Earth. 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 entire world, Apollo observable in the morning and Hermes visible in the evening. They eventually came to understand that these were exactly the identical object, and it was even suggested this early in history that Mercury and Venus orbited the sun.
Early modern astronomy started making observations of Mercury in the early 17th century when Galileo turned into an early telescope to the inner world. A sketchy vision of the planet developed from there, 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 lately 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 issues. An application of Newton’s Laws of Motion shows that it takes more rocket fuel to reach an orbit around Mercury than necessary to escape the solar system. Due to this only one spacecraft has visited the world thus far, the Mariner 10.
The Mariner 10 was launched on November 3, 1973 to research 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 world’s surface. The Mariner 10 circles the sun in its lone orbit around the sun to this day, although its digital devices 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. It is carrying high resolution imaging devices, spectrometers to determine the composition of the crust, and magnetometers to study charged particles around Earth.
There is an astronomical event which is of interest to amateurs and professionals alike, but not necessarily for any scientific value. After every couple of centuries there’s an occultation of Mercury and Venus. This occurs when Venus really passes directly in front of Mercury for a couple of minutes. The last one occurred on May 28,1737 and another will occur in 2133.
Structure of Mercury
Much of Mercury’s mass is composed of an iron rich core. Recent theories suggest that this core comprises the majority 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. Due to Mercury’s slow rotation period, there’s very little tectonic or volcanic action.
There have been a number of theories presented to explain why Mercury is so metal rich, and why its core features such a massive part of the world’s structure. One popular theory is that the world 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 portion of the planet, providing the young Mercury a dense atmosphere of gaseous stone, which was carried away by the massive solar winds of a much more volatile sun. The competing theories to explain the unusually heavy composition of Mercury will be tested by the approaching Messenger mission.
The Future of Mercury and Its Potential Role in Our Own Future
Mercury’s most dominant feature makes it an attractive part of our future: its huge stores of heavy metals, particularly iron. Though its proximity to the sun makes it hard to envision humans living and working there, I can envision automated factories mining its surface and freighters picking up the ore to bring it back for our use.
Mercury will stay pretty much as it is for billions of years to come. As the sun slowly gains in intensity during the next 4 to 5 billion years, the planet’s surface temperature will gradually rise with it. Then a singular moment will arrive as the entire solar system varies with the beginning of the sun’s passage into its next stage. After the sun consumes a critical amount of its hydrogen fuel it will, within a matter of only a few days, enter its red giant phase.
At the point Mercury will be the first of the inner planets to be entirely consumed.