Titan

1. Titan PTYS 206 (from original presentation by Catherine Neish) April 1, 2014

2. Introduction • Titan is Saturn’s largest moon, and the second largest moon in the solar system

3. Titan

4. History • Titan was discovered in 1655 by Christiaan Huygens, a Dutchman Christiaan Huygens (1629-1695) Huygens spacecraft (1997-2005)

5. History • George Hill determined Titan’s mass in the 1880s by examining the motions of Hyperion, another of Saturn’s moons • Titan is 0.41 times the mass of Mercury, but 1.18 times the volume of Mercury Q: How can Titan be bigger than Mercury, but so much less massive? A: Because Titan is made mostly of ice, and therefore is less dense than Mercury.

6. Ice (1.0 g/cm3) Rock (3.0 g/cm3) Titan (1.88 g/cm3) History • Titan is made of ~50% ice and ~50% rock liquid water? 5150 km

7. History • Gerard Kuiper discovered that Titan had an atmosphere in 1944 by taking spectra of Titan What are spectra???

8. Wavelength Spectra • Spectra are plots of light intensity versus wavelength

9. Spectra • There are three types of spectra: • Continuous spectrum • Light of all wavelengths that is emitted from a hot, dense gas • Emission spectrum • Light of specific wavelengths that is emitted from excited molecules • Absorption spectrum • Light of specific wavelengths that is absorbed by molecules in a cool gas in front of a hot source

10. Wavelength Spectra Kuiper saw CH4 absorption bands on Titan: Using spectra, we can discover molecules on planets and stars remotely, without needing to visit them. Methane absorption bands

11. Spacecraft Missions Two spacecraft have visited Titan: Voyager 1 (1980) Cassini-Huygens (2004 - today)

12. Voyager • What did we learn? • Atmosphere • Composition (N2) • Temperature profile • Chemistry • Size of moon • What were the limitations? • Voyager couldn’t see Titan’s surface 

14. Atmospheric Composition • Prior to Voyager, no one knew what Titan’s atmospheric composition was • Kuiper thought that methane might comprise most of Titan's atmosphere • Voyager discovered that Titan’s atmosphere was mostly nitrogen (N2), like the Earth • Methane therefore plays a role in Titan's atmosphere somewhat like that of water in Earth's atmosphere • It is a species that changes phase from liquid to vapor, depending on local weather

15. Atmospheric Composition Titan H2 CH4 N2

16. Temperature profile • Titan’s temperature profile was measured from a radio occultation by Voyager (atm. acts like a lens) • The occultation also allowed us to measure Titan’s size • The radio waves cut out when Voyager went behind Titan’s solid surface (D = 5150 km)

17. Temperature profile Titan Earth ozone A: Yes! Methane is a greenhouse gas. Q: Does Titan have a greenhouse effect?

18. Temperature profile • Titan’s surface temperature is 95 K! • How long would it take a human to freeze at these temperatures? • According to the diffusion equation, it would take a human ~1/2 hour to freeze solid (though your exterior would freeze instantly). D ~ 1 x 10-6 m2/s Ti = 310 K Tf = 273 K x ~ 0.2 m Q: How do you think a person would die on Titan?

19. Chemistry • Titan boasts a rich atmospheric chemistry! • Light from the Sun (hν) and high-energy electrons (e-) break apart the N2 and CH4 in Titan’s atmosphere to make many complex organic molecules • May give us clues as to how life began on Earth ex. C2H6, C2H2, C3H8, HCN, C2H4, HC3N, C2N2 • This chemistry also produces a lot of “smog”, which makes it difficult to see the surface of Titan smog

20. hν, e- Lots of ethane is predicted to be formed in Titan’s atm., enough to make an ocean! a CH4 + N2 C2H6, C2H2, C3H8, HCN, C2H4, HC3N, C2N2, etc.

21. Surface • Voyager could not see through Titan’s smog to the surface • Our best pre-Cassini view came from the Hubble Space Telescope • HST looked through methane “windows” in Titan’s atmosphere (wavelengths where methane does not absorb all the light) Xanadu In Xanadu did Kubla Khan A stately pleasure-dome decree: Where Alph, the sacred river, ran Through caverns measureless to man Down to a sunless sea. -Samuel Taylor Coleridge

22. Surface Cassini gave us the first high-res views of the surface! Pre-Cassini Post-Cassini

23. Cassini-Huygens • What have we learned? • Surface • First views of the surface! • Strangely Earth-like… but no ethane ocean…. • Composition • Atmosphere • Clouds and weather • Chemistry • What are the limitations? • Very limited surface science - need to return with a balloon!

24. Streams

25. Lakes

26. Sand dunes

27. Mountains Titan Mountains: 1.5 km Rincon Mountains: 1.9 km

28. Volcanoes?

29. Craters There are very few craters on Titan. This implies Titan has a young surface.

30. Surface Titan looks like Earth, but remember…

31. Mass spectrum at surface Surface composition • Huygens found evidence for C2H6, CO2, C2N2, C6H6, and liquid methane at the surface • Cassini found evidence for water ice, C6H6, and “organics” at the surface

32. Surface composition • Huygens found evidence for C2H6, CO2, C2N2, C6H6, and liquid methane at the surface • Cassini found evidence for water ice, C6H6, and “organics” at the surface Spike indicates the presence of liquid methane at the surface.

33. Weather • Titan has clouds of methane and ethane • Titan has a “methanological” cycle like the hydrological cycle on Earth • Expect ~1 cm of rainfall per year

34. Cloud of ethane over North Pole Source of the lakes?

35. H2 CH4 C2H6 Weather • Unlike Earth, Titan continually loses its methane through reactions in the atmosphere • All the methane would be gone in 10 million years! • Need a source of methane 2CH4→ C2H6 + H2

36. Atmospheric Chemistry • Cassini got more detailed information about Titan’s atmospheric chemistry • Compounds detected up to mass 100! Very complex organics…

37. Huygens • The Huygens probe descended through Titan’s atmosphere. On its way down, it took pictures and measured… • Temperature and pressure profiles • Wind speeds • Atmospheric and surface composition • Movie: Huygens’ descent through the atmosphere

38. Huygens saw a lot of evidence for fluid flow on Titan: Streams Rounded “rocks” Stream-cut hills

39. Future missions • There is still much more to learn about Titan! • Where is the ethane? • What is the source of Titan’s methane? • Is there really volcanism? If so, what is the lava like? • How complex are the organics on Titan’s surface? • Are there biological molecules, like amino acids, or simple lifeforms on the surface? • In the future, we’d like to return to Titan with a balloon to make more detailed measurements of the surface