Mercury is heavily cratered, like the Moon, but it is different from the Moon in many ways

1. Mercury is heavily cratered, like the Moon, but it is different from the Moon in many ways

2. Announcements • Reading Assignment • Finish Chapter 11 • Chapter 28 • Homework 4 now posted on class website (due March 22, 2 weeks from today)

3. Mercury: Basic Facts • The smallest planet (now that Pluto is no longer a planet) • Avg. distance from the sun: 0.387 AU • Orbital Period: 88 days • Rotational period: 58.6 days 3:2 spin-orbit coupling • Eccentricity: 0.206 • hot. airless, Moon-like • Daytime temp: +662 oF • Nightime temp: -274 oF • 1/3 size of Earth • heavily cratered, like the Moon, but with many differences • Magnetic field and magnetosphere (like Earth, but weaker)

4. Viewing Mercury from Earth • It is difficult to observe from Earth, despite the fact that it is bright • 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 Mercury and Venus 3/08/12

5. Mercury has a slow rotation period (long days and nights) Long believed (incorrectly) to be 88 days Difficult to tell because it is so hard to observe from Earth Ground based radar found it to be about 59 days! Used Doppler shift in radar signals sent from Earth Arecibo radio telescope, Puerto Rico PTYS/ASTR 206 Mercury and Venus 3/08/12 Moon and Mercury 3/6/08

6. Mercury’s 3:2 spin/orbit ratio Incorrect for Mercury A 1:1 spin-orbit resonance (like the Moon) A 3:2 spin-orbit resonance Mercury turns on its axis 1 ½ times in one complete orbit about the Sun This is a 3:2 spin-orbit resonance (i.e. it spins 3 times on its axis during 2 orbits about the Sun) Mercury and Venus 3/08/12

7. Spacecraft missions to MercuryMariner 10 (1974/75) • Mariner 10 (1974/75) • Only observed one side of the planet due to Mercury’s 3:2 spin-orbit resonance. • Mercury Messenger was launched in 2004, had flybys of Mercury in 2008 (January and October), another on 9/29/09 and is now in orbit about the planet Mercury and Venus 3/08/12

8. Photo of Mercury from Messenger flyby in 2008

9. Mercury has many scarps – like cliffs This one is more than a km high They probably formed as the planet cooled and shrank Mariner 10 image Mercury and Venus 3/08/12

10. Mariner 10 image of Mercury • Mercury has a heavily cratered surface, like the moon • The cratering is less “dense” than moon • Intercrater plains (not seen on the moon) • Other surface features: • Gently rolling plains • Scarps • “wrinkled ridges” This is a photo of the Moon: Notice how much more densely packed the craters are Image showing “wrinkled ridges”: terrain resulting from cooling (and shrinking) of surface

11. Caloris Basin • Mercury has only 1 mare – the Caloris Basin • Thought to have formed from a huge impact Mercury and Venus 3/08/12

12. Caloris Impact • Created seismic waves that traveled through the planet • Created hilly terrain on the opposite side → On the side of the planet opposite the impact

13. Pantheon Fossae: strange fault-like features near a crater within Caloris basin

14. A few key findings from the NASA’s ongoing Mercury Messenger mission • The discovery of “hollows” • Pit-like depressions primarily within larger craters • The presence of volcanic vents • Rich in volatiles • The ratio of certain volatile elements are much more like that seen in Earth’s surface (and not like the moon). • This has ruled out some theories of Mercury’s origin. The surface is not so dead! Also, it seems Mercury formed someplace other than its present orbit.

15. Mercury MESSENGER discovered unusual reflective material seen in some large craters

16. Mercury’s “Hollows”

18. A “Ghost” crater: One that was later filled with lava

19. Ice at Mercury’s poles • Some craters provide permanent or near permanent shade from the Sun • Reflective features within some of these craters • Water ice • Messenger is close to finding proof

20. The Interior of Mercury • Metallic iron-nickel core • About 75 % of the planet’s diameter • About 42% of the planets volume • Earth’s core occupies only 17% of its volume • Mean density is similar to (but less than) Earth • Smaller, so iron core takes up more volume • The core is likely to be hot and at least partially molten. • Solid rocky layer topped with a thin crust about 100 km thick.

21. Why does Mercury have such a High Iron Content? • Maybe Mercury hit something that tore off a lot of its early mantle. • Maybe a very powerful solar wind from the early Sun blew away some mantle. • Mercury formed nearer the protosun, where it was hotter and more volatile substances didn't condense as readily. • New observations by Messenger cast doubt on this theory

22. Mercury’s Magnetic Field • Mercury has a weak magnetic field (1% of Earths) • This magnetic field causes a magnetosphere similar to Earth’s but weaker • Some solar wind can penetrate Mercury’s magnetosphere and reach the surface

23. Venus Mercury and Venus 3/08/12

24. Venus – Our “Sister” Planet Basic Facts • 2nd planet from the Sun • Brightest of all planets as seen from Earth • Only the Sun and moon are brighter • Semi-major axis = 0.72 AU • Radius = 6052 km (0.95Rearth) • Mass = 4.9x1024kg (0.82Mearth) • Density = 5240 kg/m3 • Rotational period = 243 days • Retrograde (why?)

25. Venus – Our “Sister” Planet Basic Facts (cont.) • Atmosphere • Mostly C02 • Runaway greenhouse effect • Very dense and massive • 90 times the pressure at Earth’s ruface • Russian Venera probes were either crushed during descent or survived only an hour due to high pressures • Surface temperature = 750K • Earth = 300K, Mercury=700K • No significant intrinsic magnetic field

26. 2 Russian spacecraft landed on Venus and took the only pictures from its surface(they then failed shortly after landing due to extreme pressure and temperature) Mercury and Venus 3/08/12

27. Venus surface temperature – 750K • Hotter than Mercury! • So hot, that rocks almost glow! • A plastic construction helmet subjected to brief exposure of the surface temperature of Venus 

28. Venus Rotation • Venus rotates very slowly • Its rotation rate was not known until fairly recently • Cannot see the surface • determined from radar imaging • Most planets and moons have prograde rotation, Venus is opposite • The reason is not known for certain, but may be due to a large impact, or tidal forces Mercury and Venus 3/08/12

29. Venus clouds rotate faster than the planet • The clouds of Venus rotate in the same direction as the surface • But MUCH faster • It takes 4 days for them to go around planet • 220 mph Mercury and Venus 3/08/12

30. Atmospheric Circulation • The circulation of the Venusian atmosphere is dominated by two huge convection currents in the cloud layers, one in the northern hemisphere and one in the southern hemisphere. • Nearly equal day and night temperatures • And equal pole and equator temperatures Mercury and Venus 3/08/12

31. Venus Atmospheric Components: A Comparison with Earth Mercury and Venus 3/08/12

32. Venus’s Runaway Greenhouse effect • Venus very likely had oceans in the distant past. • As the Sun became brighter (it was dimmer in the past), Venus got hotter and the water began to evaporate • The increased water vapor (a greenhouse gas) led to an enhanced greenhouse effect and a larger temperature • Also, since there was less water to dissolve CO2 and SO2, these gases were free in the atmosphere (rather than in the ground) • These are also greenhouse gases • It got even hotter ! • And the cycle continued, until it reached the temperature it is today. Mercury and Venus 3/08/12

33. On Venus, greenhouse gases are NOT recycled On Earth, greenhouse gases are recycled Mercury and Venus 3/08/12