Tues day, October 22 Mercury

1. Tuesday, October 22 Mercury

2. Guest lecturer: Professor Barbara Ryden ryden.1@osu.edu

3. Mercury is hard to see from Earth. It’s always within 28° of the Sun as seen from Earth; it’s seen low in the east at sunrise or low in the west at sunset.

4. Mercury is best seen from Earth at maximum elongation (maximum angular distance from the Sun). Mercury is much more difficult to see than Venus; it’s smaller, lower in albedo, and closer to the Sun’s glare.

5. Mercury’s next maximum elongation? In mid-November, when it will be visible low in the ESE before sunrise. If we’re lucky, comet ISON will be visible, too!

6. Little was known about Mercury before the 1st spacecraft visit. Orbital period (Mercury’s “year”) = 88.0 days Semi-major axis of orbit = 0.39 AU (smallest orbit of the 8 planets) Eccentricity of Mercury’s orbit = 0.206 (most eccentric orbit of the 8 planets)

7. Solar heating of Mercury varies greatly during an (eccentric) orbit. Mercury’s aphelion distance (farthest from Sun) is 1.52 times its perihelion distance (closest to Sun). Thus, solar brightness at perihelion is (1.52)2 = 2.3 times its value at aphelion. Sun seen from Mercury (aphelion) Sun seen from Mercury (perihelion) Sun seen from Earth

8. The rotation speed of Mercury was determined by Doppler radar.

9. Sending a radio signal to Mercury (and back!) demands a large radio dish (such as the Arecibo radio telescope in Puerto Rico). diameter = 1000 feet

10. Rotation period of Mercury = 58.6 days Orbital period of Mercury = 88.0 days Rotation period = 2/3 × orbital period! This 3-to-2 spin-orbit coupling is caused by tides due to the Sun. If Mercury were like the Moon As Mercury actually is

11. In half an orbit (44 days) an observer on Mercury goes from noon to sunset. In one orbit (88 days) an observer goes from noon to midnight. In two orbits (176 days) an observer goes from noon to noon.

12. Two kinds of “day”: sidereal and solar A planet’s sidereal day (sideros = star in Greek) is the time it takes the planet to rotate once, seen from a distant star. Mercury’s sidereal day is 58.6 Earth days, 2/3 of its year. A planet’s solar day is the time from one noon to the next, as seen by an observer on the planet. Mercury’s solar day is 176 Earth days, twice its year.

13. At successive perihelion passages, the red point & blue point are alternately the places on Mercury pointing straight toward the Sun. The red point & blue point are the two hot poles of Mercury. They experience noon when Mercury is closest to the Sun. Hot, hot, hot!

14. At Mercury’s hot poles, the afternoon high temperature reaches 370°C = 700°F. hot enough to melt lead! During the long, long nights, the temperature drops as low as -180°C = -300°F. cold enough to liquify oxygen! Huge temperature swings can occur because Mercury lacks an atmosphere (which acts as a fluffy blanket).

15. The 1st close look at Mercury came when the Mariner 10 spacecraft did 3 flybys of Mercury in 1974 and 1975. Limited technology, limited time; but it did map 45% of Mercury’s surface.

16. The hemisphere of Mercury mapped by Mariner 10: Mercury is a cratered world, like a larger version of Earth’s Moon.

17. Mercury Moon

18. A Mariner 10 closeup of the cratered highlands of Mercury:

19. Mercury isn’t a big planet: although larger than dwarf planets like Pluto, it’s smaller in radius than the moons Ganymede & Triton.

20. Mariner 10 flybys enabled an accurate determination of the mass of Mercury. Deflection of the spacecraft’s trajectory by Mercury’s gravity led to the deduction that Mercury is surprisingly massive.

21. Mercury volume = 2.8 times Moon volume Mercury mass = 4.5 times Moon mass Mercury density = 1.6 times Moon density The Moon’s density (3.3 grams/cm3) is near that of rock; Mercury’s density (5.5 grams/cm3) is near that of iron.

22. Mercury must have a huge iron core (relative to its size), & only a skimpy rock mantle.

23. The 2nd spacecraft to visit Mercury: MESSENGER (MErcury Surface, Space Environment, GEochemistry, and Ranging) Launched August 2004, reached Mercury orbit March 2011.

24. Interplanetary photobomb! MESSENGER took as long to reach Mercury (61 million miles away) as Cassini took to reach Saturn (900 million miles away). Why so slow, MESSENGER?

25. MESSENGER took the “scenic route”. It had to, in order to lose energy & angular momentum.

26. Mercury enigmas investigated by MESSENGER: Mercury has a weak, but measurable, magnetic field. (Earth’s magnetic field is 31,000 nT) (Venus & Mars have no magnetic field)

27. Why is Mercury’s magnetic field unexpected? Other planets with magnetic fields (Earth, Jupiter, etc.) have a layer of liquid metal where the magnetic field originates. Mercury is small, and has cooled thoroughly; its interior should be solid.

28. Mercury’s magnetic field is still puzzling. Perhaps a thin outer layer of the iron core is still molten. Perhaps contamination with lighter elements has lowered the melting point of the iron. There are many “perhapses”.

29. Mercury enigmas investigated by MESSENGER: Mercury has a huge impact crater called Caloris Basin. Diameter of crater = 1500 km Diameter of impacting asteroid = 100 km

30. At the antipodes to Caloris Basin is a jumbled, hilly landscape known as “Weird Terrain” or “Chaotic Terrain”. closeup

31. Focusing of seismic waves from the huge Caloris impact may have caused the weird terrain at the antipodes.

32. Mercury enigmas investigated by MESSENGER: Mercury, where the temperature can rise as high as 700° F, has water ice on its surface! Northern regions of Mercury: Yellow regions are ice patches.

33. The presence of northern ice is explained by the fact that some deep polar craters are in shadow all year round. Red regions are shadowed in every image taken by MESSENGER. No sunlight, no ice melt.

34. Mercury, once regarded as a simple, slightly oversized version of our Moon, has enigmas that reveal themselves on closer viewing.

35. Thursday’s lecture: Venus Next Tuesday: Quiz #3