1. The Terrestrial Worlds
Physics 112
Star Systems
Lecture 12
The Terrestrial Worlds
Mercury
Kevin H Knuth
University at Albany
Fall 2012

2. "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 (ca. 64 BC – AD 17)

3. Venus, Earth, Mars and Mercury

4. 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)

5. 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

6. 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.

7. 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.

8. 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).

9. 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).

10. 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.

11. 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.

12. 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.

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

14. Rembrandt Impact Basin

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

16. 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.

17. 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.

18. Venus, Earth, Mars and Mercury