• Wrong info last lecture: Homework #4 due next week!  But it is half as long.
• Curve for the exam:
• Drop date 3/31 (next Friday)

• Mercury overview
• Observing Mercury from Earth
• 3-2 spin-orbit coupling of Mercury
• Mercury's surface
• Evidence for iron core
• formation theories
• Magnetic field

• Mercury has no atmosphere
• Too hot
• Not enough gravity

• Mercury orbits close to the Sun
• visible to naked eye only within 2 hours of sunrise or sunset
• brightest object in the sky
• orbit is eccentric, some elongations are greater than others
• telescopes see Mercury best during the day (with filters)
• best view through atmosphere is straight up
• Mercury sometimes transits the sun (analogy solar eclipse)
• like the moon, not everything lines up every time

• Mercury rotates 3 times every 2 orbits around the sun
• Most planets/satellites rotate independently or in 1-to-1 spin-orbit coupling (synchronous rotation, e.g. our moon)
• Reason: Mercury is lopsided and has an eccentric orbit
• Earth-based optical telescopes can't see that well
• Discovered using radio telescopes
• Measured temperature of nighttime side to be too warm
• used RADAR to measure rate of rotation
• If you stood on Mercury, the sun would move backwards sometimes!  Also very cold or very hot

• Mariner 10 (unmanned) mapped 45% of surface
• Surface looks a lot like our moon
• Cratering period probably same as our moon
• Intercrater plains probably lava flows
• Long lines of cliffs call "scarps" evidence of crust contraction
• analogy: skin of dried apple
• Like our moon, Mercury had at least one very large impact toward the end of the bombardment period
• Mercury: Caloris basin
• Moon: Orientale Basin
• Impact so large, seismic waves focused on other side of moon/mercury and made jumbled hills

Mercury:

Mercury left, moon right

Jumbled hills on Mercury

• Mercury's average density is 5430 kg/m³
• Earth's is 5520 kg/m³
• Surface rock density on Earth is 3000 kg/m³
• Obviously something dense inside Earth (iron core)
• Some density enhancement due to compression from upper layers
• paper filled plastic bag analogy
• Uncompressed densities:
• Earth = 4400 kg/m³
• Mercury = 5300 kg/m³
• Therefore Mercury has more iron

• During formation, heat from sun vaporized other elements with lower boiling points
• Why didn't this happen to the Earth?
• Brief period of intense solar wind (sun burp) may have blown away outer layers
• Mercury hit by large planetesimal

• Mariner 10 measured Mercury's magnetic field
• 1% of Earth's
• By analogy with Earth's core, there must be a liquid and solid part of iron core
• Liquid allows electric charges to move
• Solid forms from cooled liquid

• Mercury overview
• Observing Mercury from Earth
• 3-2 spin-orbit coupling of Mercury
• Mercury's surface
• Evidence for iron core
• formation theories
• Magnetic field

• Chapter 18 Question 18
• average density = total volume / total mass
• volume of a sphere = 4/3pR³
• Chapter 18 Question 24
• Figure 3-5 might help you think about this
• Chapter 18 Question 25
• Use E = mc²
• Chapter 10 Question 21
• See solutions to homework #3, chapter 4 question 28