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

Transcription

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 ( ) Huygens spacecraft ( )

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 History Titan is made of ~50% ice and ~50% rock liquid water? Ice (1.0 g/cm 3 ) Rock (3.0 g/cm 3 ) Titan (1.88 g/cm 3 ) 5150 km

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

8 Spectra Spectra are plots of light intensity versus wavelength 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 Spectra Kuiper saw CH 4 absorption bands on Titan: Wavelength Methane absorption bands Using spectra, we can discover molecules on planets and stars remotely, without needing to visit them.

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

12 Voyager What did we learn? Atmosphere Composition (N 2 ) Temperature profile Chemistry Size of moon What were the limitations? Voyager couldn t see Titan s surface L

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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 (N 2 ), 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 CH 4 H 2 N 2

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 Q: Does Titan have a greenhouse effect? A: Yes! Methane is a greenhouse gas.

18 Temperature profile Titan s surface temperature is 95 K! How long would it take a human to freeze at these temperatures? T t = D 2 x T 2 D ~ 1 x 10-6 m 2 /s T i = 310 K T f = 273 K x ~ 0.2 m According to the diffusion equation, it would take a human ~1/2 hour to freeze solid (though your exterior would freeze instantly). 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 N 2 and CH 4 in Titan s atmosphere to make many complex organic molecules ex. C 2 H 6, C 2 H 2, C 3 H 8, HCN, C 2 H 4, HC 3 N, C 2 N 2 May give us clues as to how life began on Earth This chemistry also produces a lot of smog, which makes it difficult to see the surface of Titan smog

20 a CH 4 + N 2 hν, e - C 2 H 6, C 2 H 2, C 3 H 8, HCN, C 2 H 4, HC 3 N, C 2 N 2, etc. Lots of ethane is predicted to be formed in Titan s atm., enough to make an ocean!

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 Earth Rock Sand Lava Water Titan Ice Organic particles Water Methane

31 Surface composition Huygens found evidence for C 2 H 6, CO 2, C 2 N 2, C 6 H 6, and liquid methane at the surface Mass spectrum at surface Cassini found evidence for water ice, C 6 H 6, and organics at the surface

32 Surface composition Huygens found evidence for C 2 H 6, CO 2, C 2 N 2, C 6 H 6, and liquid methane at the surface Spike indicates the presence of liquid methane at the surface. Cassini found evidence for water ice, C 6 H 6, and organics 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 Weather Unlike Earth, Titan continually loses its methane through reactions in the atmosphere H 2 CH 4 2CH 4 C 2 H 6 + H 2 C 2 H 6 All the methane would be gone in 10 million years! Need a source of methane

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 Streams Huygens saw a lot of evidence for fluid flow on Titan: 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

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