The Terrestrial Worlds Physics 112 Star Systems Lecture 12 The Terrestrial Worlds Mercury Kevin H Knuth University at Albany Fall 2013

"It remains for us to speak of the five stars which many have called wandering, and which the Greeks call Planeta ... This fifth star is Mercurius’ [Hermes], named Stilbon. It is small and bright. It is attributed to Mercurius because he first established the months and perceived the courses of the constellations." Hyginus, Astronomica 2.42 (ca. 64 BC – AD 17)

Venus, Earth, Mars and Mercury

Mercury Mass: 3.3 x 1023 kg Diameter: 4,880 km Density: 5.43 gm/cm3 0.38 AU from the Sun Rotation: 58 days Revolution: 88 days Surface Temp Day: 765 °F (407 °C) Night: -300 °F (-180 °C)

Mercury Geology Mercury has escarpments (cliffs) that are several kilometers high! These formed by compression, suggesting that Mercury shrank in size as it cooled. Discovery Rupes (a thrust fault) This escarpment is 2 km high

Crumpling of the Surface During Cooling As Mercury cooled, it shrank in size causing the surface to Buckle and crumple. This formed the 2km high cliffs in Discovery Rupes.

Caloris Basin and Weird Terrain Mercury has a large impact basin called the Caloris Basin. On the opposite side of the planet one can find “weird terrain”. Caloris Basin Weird Terrain This was probably also caused by the huge impact that created the basin.

Mercury’s Orbit Mercury has the most elliptical orbit of all the planets. Its perhelion advances by 5557 seconds of arc per century, which is not predicted by Newton’s theory of gravity, but is explained by Einstein’s Theory of General Relativity It is tidal-locked to the Sun in a 3:2 ratio (spin-orbit resonance) so that it has three days (58 Earth days) every two years (88 Earth days).

Mercury’s Orbit Mercury has the most elliptical orbit of all the planets. Its perhelion advances by 5557 seconds of arc per century, which is not predicted by Newton’s theory of gravity, but is explained by Einstein’s Theory of General Relativity It is tidal-locked to the Sun in a 3:2 ratio (spin-orbit resonance) so that it has three days (58 Earth days) every two years (88 Earth days).

Mercury’s Internal Structure 1. Crust—100–300 km thick 2. Mantle—600 km thick 3. Core—1,800 km radius Mercury’s highly eccentric orbit and 3:2 spin-orbit resonance imposes tidal stresses, which Are 17 times stronger than the Moon’s effect on the Earth. This keeps the core molten. http://en.wikipedia.org/wiki/Mercury_%28planet%29

Mercury’s Magnetic Field Mercury has a magnetic field about 1.1% as strong as Earth’s. The molten iron core probably produces a dynamo effect, which produces Mercury’s magnetic field. The solar wind distorts this field and forms a magnetosphere.

Exosphere and Space Weathering Mercury’s gravity is too weak to maintain an atmosphere. However, it has what is called a tenuous exosphere. This is an unstable layer of gas that is constantly replenished From various sources. Hydrogen and Helium come from the solar wind blowing off The Sun. Helium, Sodium and Potassium come from the radioactive decay of elements in Mercury’s crust. Comets Striking the surface release oxygen from the surface rock and create water. Like the Moon, the surface is weathered by micro-meteor impacts and the solar wind. This is called space-weathering.

Messenger Mission The Messenger probe, launched in 2004 has made three fly-bys of Mercury and will be going into Mercury orbit in 2011

Rembrandt Impact Basin

Volcanic West The smoothed crater floors implies that they were once flooded with molten lava indicating a history of volcanism.

Extensive Rays Kuiper Crater is in the Center of the image. Note the extensive rays from the impact crater at the top of the image.

Transit of Mercury Here is an image of Mercury transiting in front of the Sun. Events like this are used to detect planets around other stars by detecting the small amount of light blocked by the planet.

Venus, Earth, Mars and Mercury