A couple of months ago the NASA Space probe Messenger made another fly-by of its target planet Mercury. Messenger is the only mission after the Mariner probe in the late seventies to visit Mercury. However Messenger is different. It is designed to go into orbit around Mercury, and run a complete mapping of the planet. Whereas the Mariner probe simply flew past taking some quick snaps as it went. The mapping aspect of Messenger’s mission should begin in 2011, but until then Messenger must make several fly-bys of Mercury, so it will be capable of getting into the correct orbit. And it is one of these build up fly-bys that happened a couple of months ago. During this Fly-by Messenger revealed previously unseen parts of the Mercury surface and, made a few surprised discoveries. But first here is a brief run down of are present knowledge of Mercury.
Mercury is the closest planet to the Sun that much is certain. There was a brief stint in the early 20th Century, when people believed there was a planet closer in called Vulcan, but that that was later disproved. Mercury is officially the closest planet to the Sun.
Mercury is also known to be very very hot, with the dayside of the planet reaching temperature as high as 167 Degrees Celsius. Mercury as no atmosphere to speak of, except a very tenuous cloud of trace gases known as an exosphere. The exosphere contains trace amounts of elements such as Calcium, Oxygen and Potassium. Know doubt the remains of countless Comet impacts billions of years ago. Added to this there is a large amount of Hydrogen and Solar particles, certainly the by products of the constant stream of Solar wind hitting the planet.
The surface of Mercury is best described as ancient, with no current geological activity occurring today. Dead in other words .The main surface feature seen is impact craters. Most billions of years old, caused by the impact of Asteroids and Comets at the end of the late heavy bombardment, (Final stage of main solar system formation). And it is one of these impact craters that holds the title of largest crater in the solar system. It is called the Caloris Basin and has as a diameter of over 1500Km! So large was this impact that it forced up the surrounding are to form a mountain range 2 Km high. Other crater features on Mercury are called chain cratering. Chain cratering is formed when a planets gravity rips apart an object say an Asteroid or comet, and the subsequent fragments impact the planet or it's moon, like a chain of bullets from a machine gun. As seen right. This is seen on many planets and moons including mars and Jupiter's moon Calisto.
Mercury is very special terrestrial planet, because besides Earth it is the only other with a magnetic field. The magnetic field is small at only 1.1% that of the earth's, but even that is too much as Mercury is predicted to be completely solid. And therefore incapable of having a molten magnetic field generating core, like the earth.
It is at this point that we return to messenger, after messenger made it's fly-by scientists analysing the data made tantalising discoveries, that may challenge are knowledge of Mercury. And perhaps more intriguingly pose new questions. Messenger imaged previously unseen parts of Mercury with it's cameras and sensors and, found near the poles and around other deep craters large amounts of Water present in the exosphere. Now this was completely unexpected. And scientists now believe this might mean that at the bottom of these craters, some of which are in permanent darkness there might be water ice mixed in with the soil. Not a ice cap worth but more than a trace! This Ice may have arrived from impacting comets, the like of which created the Caloris Basin. As well as this Messenger provided visual evidence on the previously unseen far side of Mercury, of supposed volcanic activity! Which may indicate recent geological activity and, perhaps a still molten core which could finally put to the rest the source of the unexplained magnetic field.
Of course Messenger is an on going mission and it's discoveries are far from over, so if your interested check out there homepage below. I'm sure however that after Messenger is finished Mercury might prove to be not so dead after all!