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

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

Titan

History Titan was discovered in 1655 by Christiaan Huygens, a Dutchman Christiaan Huygens ( ) Huygens spacecraft ( )

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.

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

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

Spectra Spectra are plots of light intensity versus wavelength Wavelength

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

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.

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

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 

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

Atmospheric Composition Titan N2N2 CH 4 H2H2

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) Temperature profile

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

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 m 2 /s T i = 310 K T f = 273 K x ~ 0.2 m Q: How do you think a person would die on Titan?

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 –May give us clues as to how life began on Earth 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 This chemistry also produces a lot of “smog”, which makes it difficult to see the surface of Titan smog

CH 4 + N 2 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. hν, e - a Lots of ethane is predicted to be formed in Titan’s atm., enough to make an ocean!

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

Surface Pre-CassiniPost-Cassini Cassini gave us the first high-res views of the surface!

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!

Streams

Lakes

Sand dunes

Mountains Titan Mountains: 1.5 km Rincon Mountains: 1.9 km

Volcanoes?

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

Surface Titan looks like Earth, but remember… EarthTitan RockIce SandOrganic particles LavaWater Methane

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

Huygens found evidence for C 2 H 6, CO 2, C 2 N 2, C 6 H 6, and liquid methane at the surface Cassini found evidence for water ice, C 6 H 6, and “organics” at the surface Surface composition Spike indicates the presence of liquid methane at the surface.

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

Cloud of ethane over North Pole Source of the lakes?

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 CH 4 H2H2 C2H6C2H6 2CH 4 → C 2 H 6 + H 2

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

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

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

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