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



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

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







George Hill determined Titan’s mass in the 1880s by examining the motions of Hyperion, another of Saturn’s moons

  • 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


Titan is made of ~50% ice and ~50% rock

  • Titan is made of ~50% ice and ~50% rock



Gerard Kuiper discovered that Titan had an atmosphere in 1944 by taking spectra of Titan

  • Gerard Kuiper discovered that Titan had an atmosphere in 1944 by taking spectra of Titan



Spectra are plots of light intensity versus wavelength

  • Spectra are plots of light intensity versus wavelength



There are three types of 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


Kuiper saw CH4 absorption bands on Titan:

  • Kuiper saw CH4 absorption bands on Titan:



Two spacecraft have visited Titan:

  • Two spacecraft have visited Titan:



What did we learn?

  • What did we learn?

    • Atmosphere
      • Composition (N2)
      • Temperature profile
      • Chemistry
    • Size of moon
  • What were the limitations?

    • Voyager couldn’t see Titan’s surface 




Prior to Voyager, no one knew what Titan’s atmospheric composition was

  • 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




Titan’s temperature profile was measured from a radio occultation by Voyager (atm. acts like a lens)

  • 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)




Titan’s surface temperature is 95 K!

  • 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).


Titan boasts a rich atmospheric 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




Voyager could not see through Titan’s smog to the 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)




What have we learned?

  • 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!














Titan looks like Earth, but remember…

  • Titan looks like Earth, but remember…



Huygens found evidence for C2H6, CO2, C2N2, C6H6, and liquid methane at the surface

  • 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



Huygens found evidence for C2H6, CO2, C2N2, C6H6, and liquid methane at the surface

  • 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



Titan has clouds of methane and ethane

  • Titan has clouds of methane and ethane





Unlike Earth, Titan continually loses its methane through reactions in the atmosphere

  • 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


Cassini got more detailed information about Titan’s atmospheric chemistry

  • Cassini got more detailed information about Titan’s atmospheric chemistry

    • Compounds detected up to mass 100! Very complex organics…


The Huygens probe descended through Titan’s atmosphere. On its way down, it took pictures and measured…

  • The Huygens probe descended through Titan’s atmosphere. On its way down, it took pictures and measured…

  • Movie: Huygens’ descent through the atmosphere





There is still much more to learn about Titan!

  • 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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