TitanTitan

PTYS 206PTYS 206

(from original presentation by (from original presentation by Catherine Neish)Catherine Neish)

April 1, 2014April 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 (1629-1695)

Huygens spacecraft (1997-2005)

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/cm3)

Rock(3.0 g/cm3)

5150 km

Titan(1.88 g/cm3)

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 CH4 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 (2004 - today)

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

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

Atmospheric Composition

Titan

N2

CH4H2

• 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

Temperature profile

Titan Earth

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).

2

2

x

TD

t

T

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?

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

CH4 + N2

C2H6, C2H2, C3H8, HCN,

C2H4, HC3N, C2N2, 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 KhanA stately pleasure-dome decree:Where Alph, the sacred river, ranThrough caverns measureless to manDown to a sunless sea.

-Samuel Taylor Coleridge

Surface

Pre-Cassini Post-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…

Earth Titan

Rock Ice

Sand Organic particles

Lava Water

Water Methane

• 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

Surface composition

Mass spectrum at 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

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

CH4

H2

C2H62CH4 → C2H6 + H2

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