Once upon a time, earthlings dreamed of exploring a planet so close to the Sun that, like Icarus, they would fail attempting to uncover its secrets. Devoid of facts, the scientists created an image of that planet in their minds and in their textbooks. How wrong they were.
A NASA mission launched in mid 2004 is bursting bubbles so long held by scientists. MESSENGER is a spacecraft designed to reach the planet closest to the fiery star at the center of our solar system. The discovery-class spacecraft?s acronym stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. Yes, the planet Mercury was its goal and it reached its destination in March of this year. The planet, subject of much curiosity, is finally letting scientists in on its surprising secrets.
Dr. Sean Solomon of the Carnegie Institution of Washington, the Principal Investigator for MESSENGER, says: "Mercury is not the planet described in the textbooks. Although a true sibling of Venus, Mars, and Earth, the innermost planet has had a much more exciting life than anyone predicted."
Theories are flying out the window as MESSENGER flies around Mercury. It’s not like other planets we have come to know. For example, the magnetic field isn’t like others in the Solar System. Its weak field differs from those on more familiar planets and Mercury’s magnetic equator is far north of the planet’s geographic equator.
Artistic rendering of ionized sodium and helium near Mercury
Charged sodium (Na) particles near the planet?s polar regions are likely the result of neutral atoms liberated from the planet?s surface by solar wind ion bombardment. Thomas Zurbuchen of the University of Michigan, one of the authors of several related papers in Science says: "We were able to observe the formation process of these ions, one that is comparable to the manner by which auroras are generated in the Earth atmosphere near polar regions." In addition to the sodium ions, helium ions are also found in Mercury’s magnetosphere. Zurbuchen thinks: "Helium must be generated through surface interactions with the solar wind."
Smooth lava plains cover more than six percent of Mercury?s surface as a result of volcanic episodes. Another author of the Science papers, James Head of Brown University, thinks the Columbia River Basalt Group formed over several million years on Earth have a lot in common with the flood plains seen on Mercury. Although the lava flood area of the US is up to six miles deep, those on Mercury reach only 1.2 miles.
Apparently, new landforms are still being produced. Images of what have been termed hollows show bright interiors with halos. "Analysis of the images and estimates of the rate at which the hollows may be growing led to the conclusion that they could be actively forming today," David Blewett of Johns Hopkins University Applied Physics Laboratory (APL) said.
Mercury?s surface material is less oxidized than other planets. It has more sulfur and potassium (K) than expected. Potassium and thorium (Th) on the surface of Mercury is measured by the Gamma-Ray Spectrometer (GRS). Sulfur (S), however, is detected by the XRay Spectrometer. Their amounts indicate, because the elements vaporize at relatively low temperatures, that Mercury did not experience events of extreme heat during its development. Larry Nittler of Carnegie admits: "Most previous ideas about Mercury’s chemistry are inconsistent with what we have actually measured on the planet’s surface."
Mercury has a large metal-to-silicate ratio compared to Earth, Mars and Venus. There are several theories why. Gamma-Ray Spectrometer readings rule out the giant impact, vaporization, and refractory condensation models and point instead to formation from primitive material, similar to some forms of chondritic meteorites.
Mercury is the smallest, most dense, terrestrial planet with the oldest surface and largest daily variations in surface temperature, and until now, the least explored of our solar systsem. Mission controllers at APL made a few trajectory adjustments and MESSENGER is scheduled to circle Mercury for one year observing the planet.
The devices aboard the spacecraft will be returning high-resolution images, making continuous observations of the exosphere and magnetosphere, and will have time to take measurements of surface composition. There should be more surprises to come.
Several more chapters in Mercury?s story are soon to be written. A Look at Mercury written in 2003 however, right before MESSENGER launched covers popular wisdom of the day in a well-researched children?s book by Spangenberg and Moser.
If you want to take a look at the planet Mercury yourself, you can see it with the naked eye only when the planet is above the horizon while the sky is fully dark – at least 40 minutes or so before sunrise, or after sunset, because the planet orbits so close to the sun.