The MESSENGER probe to Mercury has been sending back interesting information lately. Presently swinging back and forth as it tries to loose energy and settle into a Mercury orbit in 2011, the probe has nevertheless used its visits to the innermost planet to perform science operations.
The BBC has reported on two new discoveries made during the latest visit. Firstly, there are signs of volcanic activity in the relatively recent impact basin Rachmaninoff. By looking at how many impacts there have been over the volcanic outflow since it last burst into life, an estimate can be got as to the time that has passed. Surprisingly, the answer comes out as two billion years younger than when the majority of volcanic activity on the planet is believed to have stopped. The question that this brings up is was that due to the massive impact that formed the basin releasing a final flurry of activity or was there a steady stream of volcanoes on the planet at this time? Observations taken once a settled orbit is established should answer that.
The second big thing seen was a magnetic substorm. Mercury, like Earth, has a magnetic field. The gas that expands from the surface of the Sun due to the heat of the thing also carries with it a magnetic field. This gas is known as the solar wind and the region that a planet’s magnetic field dominates over the solar wind’s field is called the magnetosphere. These tend to be drawn out windsock like shapes due to the pressure of the incoming wind on the sunward side compressing the magnetosphere, and the wake drawing it out on the night side. During a magnetic substorm, the tail of the magnetosphere gets loaded with charged particles and eventually a portion breaks off to form a sort of plasma island called a plasmoid. The bit of the tail still attached to the rest of the magnetosphere convects back towards the planet, supplying the energy used by charged particles to accelerate into the polar regions to become the aurora.
The spacecraft flew through the magnetotail of Mercury and saw the loading and unloading occurring, noting the process taking just two minutes for the inner planet as opposed to an hour long process on Earth.