Sunday, March 20, 2011
The Messenger has arrived! As of 9:10 p.m. EDT on Thursday March 17th, NASA's Messenger spacecraft entered orbit around the planet Mercury and as of 9:45 p.m. rotated towards Earth and started transmitting data. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. The spacecraft was launched August 3, 2004 from Cape Canaveral, Florida with the mission to "unravel the mysteries of planet Mercury." Messenger has spent the last years circling through the inner solar system, performing flybys and gravity assists of Earth, Venus, and even Mercury itself until finally settling into orbit around the innermost planet. The spacecraft carries several instruments that will take images and measurements of the planet:
The Mercury Duel Imaging System (MDIS) consists of wide-angle and narrow-angle imagers that will map landforms and topographic features. The Gamma-Ray and Neutron Spectrometer (GRNS) will detect gamma rays and neutrons that are emitted by radioactive elements on the surface and use that to map relative abundances of different elements. The Magnetometer (MAG) will map Mercury's magnetic field and magnetized rocks in the crust. The Mercury Laser Altimeter (MLA) will beam light to the planet's surface and a sensor will collect any reflected light in order to gather further information about topography. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) is an instrument that is sensitive to light in the infrared to ultraviolet range in the spectrum and so can measure the abundances of various atmospheric gases and minerals on the surface. The Energetic Particle and Plasma Spectrometer (EPPS) will measure the composition, distribution, and energy of charged particles in the magnetosphere. And finally, the Radio Science (RS) instrument will use the Doppler effect to measure changes in spacecraft's velocity and use that to study Mercury's mass distribution, including variations in the thickness of the crust.
Mercury is the planet closest to the Sun. It is small and extremely dense with a lot of metals, a thin crust, and a thin atmosphere. It is thought to have formed out of the solar nebula, the disk of gas and dust around the newly formed Sun, but it is not known whether or not the planet formed in the location that it is today. It is odd that one of the densest objects in the solar system formed so close to the Sun. There are several ideas out there to explain why. Because Mercury resembles a planet core rather than a whole planet some think it was originally a larger object that suffered an impact that knocked part of the surface off and/or required the planet to reform. Others think that the very active early Sun blasted off the crust early in Mercury's history. Despite this possibly violent possible history, the planet has formed a thin atmosphere. Now we're not talking rain, clouds, wind etc. like we are used to thinking of on Earth. Mercury's atmosphere is more like scattered particles that only occasionally come in to contact with one another, it is more like a gas layer close to the surface, or an "exosphere." That surface has a cracked, scorched look to it. It is thought that the planet's iron core froze (or solidified) and contracted. To visualize this think about a balloon tightly covered by a piece of cellophane, if you let air out of the balloon it creates space between itself and the cellophane, but the cellophane still wants to cling to the surface of the balloon and so develops cracks or folds in order to stay in place. This theory says that Mercury's crust did something similar when the core solidified (note: the presence of a magnetic field suggests that at least part of the core may be liquid). Add to that the impact craters similar to the ones you see on our Moon and you start to get a picture of the planet. Oh yeah, and did I mention the possibility of ice? Uh-huh, the planet closest to the Sun may have ice at its poles. The rotational axis is straight, not tilted like Earth's, and so the poles never see sunlight. Barring other factors, no sunlight equals very cold and that means there could be ice. Neat.
As you can tell, there's a lot of ideas and theories out there about Mercury. Our lack of information mainly comes from the fact that Mercury is very difficult to observe. Think about it. It is really close to the Sun, and the massive amount of light coming from the Sun tends to obscure a lot of the planet's features. If you try to observe it when the Sun is below the horizon, at night, you will find that the planet never gets more than 28 degrees above the horizon. Then add the fact that the planet is tidally locked to the Sun. Tidal locking is when an object takes as long to rotate around its axis as it takes to complete its orbit, just like our Moon to us (a 1:1 resonance). In the case of Mercury, the planet does not have an exactly circular orbit and actually rotates three times for every two orbits around the Sun, in a 2:3 resonance (a year is only one and a half days long!). That tidal locking means that Mercury is always presenting the same side to us. From the two high-speed flyby's by Mariner 10 spacecraft in 1974 and 1975 we have some images, but only of one side of the planet.
It is likely that Mercury is the key to us unlocking the mechanism of terrestrial planet evolution. Messenger will gather data that will help answer the questions of why is the planet so dense, what is its geologic history, what is the nature of the magnetic field, what is the structure of the core, what volatiles (gases etc.) are present and important, and if there is ice at the poles.
But Messenger will not be the last spacecraft to visit Mercury. The European Space Agency (ESA) in a joint mission with the Japanese Aerospace Exploration Agency (JAXA) will be launching the BepiColombo in 2014 that will arrive at Mercury in 2020. This mission will further study the planet's evolution, form, interior, structure, geology, composition, atmosphere, magnetosphere, and polar regions. Find out more about this mission at their website: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=30
Visit these Messenger websites to learn more about the spacecraft, the mission, and up to the minute information and images:
A few news stories about the newly attained orbit:
Learn more about Mercury:
and the Mariner Program:
(image from nasa.gov)