Part II: Solar System Mercury Draft: Nov 06, 2010

Mercury Orbital Stuff The Surface Composition and Interior Formation 2 Mercury Orbital Stuff The Surface Composition and Interior Formation Notes

A. Orbital Stuff Transits of Mercury Greatest Elongation Rotation Rate 3 A. Orbital Stuff Transits of Mercury Greatest Elongation Rotation Rate

4 Solar Transit Transits occur about twelve times a century when the sun, Earth and Mercury are aligned There was a transit on November 8, 2006

Best time to view Mercury? 5 88 days At Greatest Elongation

Greatest Elongations 6

Earth-based optical observations of Mercury are difficult 7 At its greatest eastern and western elongation, Mercury is never more than 28° from the sun It can be seen for only brief periods just after sunset or before sunrise

Best Earth-based Views of Mercury It is difficult to see any features at all from earth 8 Note phases like the moon

Rotation of Mercury 9 Difficult to see features on Mercury, but early observations 1882-3 led Schiaperilli to propose Mercury keeps same side to sun.

Theory: Tidal Locking 10 The moon keeps same face to earth due to tidal forces It was generally accepted Mercury kept same face to sun

Rotation of Mercury 11 1962 (Radar) Observations of dark side show that its too hot to be always facing away from sun.

1965 Radio telescope observations from sites such as Arecibo gave evidence of a non-synchronous orbit 12

Measuring Rotation Rate 13

Mercury rotates slowly and has an unusual spin-orbiting coupling 14

Mercury spins 1 ½ times on its axis for every complete orbit 15 Mercury spins 1 ½ times on its axis for every complete orbit Mercury spins three times during every two orbits

B. Surface of Mercury Craters & Lava Plains Scarps Evidence of Ice 16 B. Surface of Mercury Craters & Lava Plains Scarps Evidence of Ice Caloris Basin Rachmaninoff Basin (new evidence)

First Probe to Mercury: Mariner 10 17 Most of our detailed information about Mercury’s surface is from this fly-by mission in 1974/1975. Mariner only saw one side of the planet. Measured weak magnetic field!

Mariner 10 (1973-1975) 18 First probe to use “gravity assist” to reach planets

Mariner 10 (1973-1975) 19 Probe only sees about half of surface

Mariner 10 (1973-1975) 20

Mercury has craters like the moon! 21 The Moon and Mercury are comparable in size Initially it appears that unlike the moon, Mercury has no “marias” (volcanic plains)

Mariner 10 Map of Mercury: Many Plains 22

Mercury Mariner 10 23

Crater Density Less than Moon! 24 Lots of rolling plains (lava?) between craters Implies lava covered up the earliest craters Most craters we see came later (4 by ?)

Scarps 25 Wrinkles are evidence Mercury has shrunk as it cooled.

Timeline of Scarps 26

Evidence of Ice on Mercury 27 Using VLA (large radio array) 1991 evidence of ice at South Pole, 1994 at North pole (red spot) [surprising as at equator its hot enough to melt lead!]

Evidence of Ice 28

Messenger Probe 2008 29

Messenger Mission 30

Caloris Basin (Mariner 10) 31 The Caloris Basin is evidence of a large impact

Caloris Basin (Mercury Messenger) 32 [Enhanced color] 1500 Km across.

33 The seismic waves from the impact that caused the Caloris Basin caused this deformation on the opposite side of Mercury

Jumbled Terrain opposite It has been suggested that the immense shock waves produced by the impact of the body that produced the Caloris Basin were focused around the planet at the “antipode” resulting in seismic disturbances which broke up the surface.

Rachmaninoff Basin (250 km) 35 Recent evidence upsets ideas as to timeline of volcanism Lack of craters in inner basin implies volcanism after impact, as recent as 1 or 2 billion years ago Messenger

C. Composition and Formation 36 C. Composition and Formation Big Iron Core Has magnetic field Formation Theory

Mercury Is more dense than moon 37 The Moon and Mercury are comparable in size But they are very different in density Moon: 3.3 g/cm3 Mercury: 5.4 g/cm3

Has BIG iron core (little on surface) 38

Mariner 10 measures a weak magnetic field 39 1% of earth’s, tilted 7° Implies interior is still partially molten.

Formation Ideas 40

Formation Ideas 41

Some ideas about Mercury’s early history 42 Some ideas about Mercury’s early history Its original composition may have been quite similar to Earth’s (a mix of rocky materials and metals) After Mercury differentiated, much of its rocky crust and mantle were blasted off by continued impacts This left a disproportionately large iron core, and the overall high density The core is about 75% of the diameter of the whole planet (Earth’s core is 55% of its diameter)

References http://www.daviddarling.info/encyclopedia/M/Mercuryrot.html http://history.nasa.gov/SP-424/contents.htm http://solarsystem.nasa.gov/missions/profile.cfm?MCode=MESSENGER http://en.wikipedia.org/wiki/Geology_of_Mercury http://adsabs.harvard.edu/full/1988JBAA...98..146D (Schiaparelli map)