Titan is Saturn's moon, and is almost as large as Mercury--Titan's diameter is 5150 km.  In many ways Titan is like a planet.
 
 
 

The nature of Titan
 

Titan has a very thick atmosphere (second only to Venus) which obscures the surface.  Such thick cloud cover and atmosphere might signal significant greenhouse warming.  The presence of a greenhouse effect would mean thatTitan may be warmer than would be expected for a satellite at its distance from the Sun.

For some of the best ground based images of Titan taken by Professor Close click here

Saturn is located 9.5 A.U. from the Sun--->Saturn's moon should have a surface temp of below 100 K--->water would only exist as ice and there would be no biological activity--->of no interest.

BUT Titan's possible greenhouse means warmer surface temperatures and possible liquid water.

Voyager (1982) spacecraft flyby has gave scientists the first good data on Titan (see below for more info from the recent Cassini mission).

Voyager discoveries:

1.  Titan's mass and radius determined and then used to calculate its density:

    1.9 g.cm³

whereas the Earth's density=5.2 g/cm³ and the Moon's density=3.3 g/cm³

Such a density means that Titan is very light.
 

2.  Considering the density value of Titan, it is most likely Titan is composed of mainly ICE.  More than half of its volume must be ice.

Remember: the density of water=1 g/cm³.
 

3.  Titan's atmosphere consists of mainly N₂.
 

4.  Temperature on the surface of Titan=94 K.

Temperature 40 km above the surface=70 K.

Temperature rises again at higher altitudes.  This atmospheric temperature profile is similar to the Earth's.

*Such a relatively low surface temperature (94 K) rules out the presence of a strong greenhouse.

BUT even at fairly low temps, organic molecules are involved in chemical processes and for this reason Titan makes for an interesting natural laboratory.

Why does Titan have such thick clouds?

Titan's atmosphere is predominantly N₂, but also contains small amounts of methane (CH₄).  It has been theorized that Titan has global scale oceans of methane which continually resupply the Titan atmosphere with the small amounts of methane observed.  Further, it has been thought that the evaporation of methane in the atmosphere has condensed into clouds, creating Titan's thick cloud cover.  However, observations tell us Titan's atmosphere is not saturated with methane so the clouds cannot consist of methane.

Another hypothesis given to explain Titan's thick clouds is that the global oceans that may be present consist of ethane and nitrogen as well as methane.  This would fit well with the chemical compounds observed in the atmosphere BUT observations contradict the presence of any such global oceans.

Instead, it is more likely that the ethane-methane mixture does not reside at the surface in large open bodies of liquid.  Rather, it percolates up through the cracks and pores in the surface ice.  Then, it evaporates into the atmosphere helping to create the thick cloud cover.
 

Water on Titan

• Low surface temperature (94 K) means water on Titan is in the form of ice.  BUT in the past, could there have been liquid water?

• A large asteroid impact could have provided enough heat to melt the ice near the surface.

• It has been estimated that the impacts that have occurred throughout time would have provided enough heat energy so that every part of Titan's surface would have been melted at least once.

• Given refreezing time, a given piece of surface would have stayed liquid 100-1000 years.

• Presence of liquid water--even relatively briefly--has implications for prebiotic chemistry.

Titan exobiology

• There is probably no liquid water today on Titan.  An exception would be the possible existence of short-lived transient reservoirs located near impact sites.

• Titan is too small to have interior heating--->no hydrothermal systems.

• Liquid methane-ethane large scale lakes/oceans may exist (as groundwater versus global scale) but are probably not conducive to biological activity as methane and ethane are relatively non-reactive chemically.  Unlike water they do not have the ability to dissolve a wide variety of compounds.

• However, despite this, a substantial amount of atmospheric organic chemistry that is related to terrestrial prebiotic chemistry is occuring on Titan.

• An atmospheric haze of organic molecules is formed by chemical reactions involving methane and ethane and driven by the Sun--->the organic molecules that saturate the atmosphere condense out and settle into the methane-ethane oceans/lakes--->sediment of these organic molecules is formed--->heat (from occasional impact events) will form ponds consisting of a slurry of water, methane, ammonia, and organic compounds--->this slurry is very similar to what might have been present on prebiotic Earth.

For all the most up to date news about Titan from the Cassini-Huygens spacecraft click here

The Galilean satellites

The Jupiter system is composed of:

Jupiter-----------Io--------------Europa--------------Ganymede----------------Callisto
 

Jupiter has a source of energy like our Sun.  However, while the Sun's energy is nuclear in nature, Jupiter's  energy is given off during gravitational contraction of the planet as it cools.

Both Io and Europa are similar in size to the Moon.

Io--->r=1815 km
Europa--->r=1569 km
Moon--->r=1738 km

Ganymede and Callisto are both similar in size to Titan.

Ganymede--->r=2631 km
Callisto--->r=2400 km
Titan--->r=2575 km

The Galilean satellites--water ice, liquid water and tidal heating

This density value tells us that Io probably has an iron core.

There appears to be no ice on Io.

Europa density=3.01 g/cm³

Combine this value with an observed highly reflective surface and it can be concluded that the surface of Europa is composed of pure water ice.

The water abundance of Europa is a little more than 15%.

Ganymede density=1.93 g/cm³
Callisto density=1.83 g/cm³

Probably 50-57% and 52-58% of Ganymede and Callisto respectively are made up of ice.

Scientists find both Io and Europa interesting because they experience tidal heating.

• Tidal heating on Io is analagous to the raising of tides on Earth by Moon.
• On Io, tides are created by the movement of Io in its orbit around Jupiter.
• Io's orbit is not exactly circular and Jupiter's gravitational pull results in the distortion of Io's shape.
• This distortion causes Io to heat up.
• The interior of Io heats up to the melting point.
• Active volcanism is produced at the surface.

Focus on Io

• Io's volcanism is interesting because where there's volcanism there are hydrothermal systems.  As we've seen, hydrothermal systems on Earth are associated with the formation and evolution of life.

• Most of the volcanism on Io is sulfuric, but there is evidence for some silicate (rock-based) volcanism.

• There is no evidence of water, nor any evidence that Io ever had water.  It may have been too warm on Io as it formed for water to condense.

Question: Could there be hidden water?

Possibly---and therefore exobiological activity could exist.
 

**However, it seems despite the active volcanism, there is neither water nor life on Io.
 

Focus on Europa

• Europa is the next moon out from Io.

• Probably contains about 15% or more water based on its density.

• It is thought the water:

OR

2.  Could be on the surface in the form of an ocean of liquid water covered with ice.

This is idea of a sub-surface ocean of liquid water is now quite popular. For more information on Europa click here for a start

It has not been possible to prove (2).  But we do know there is abundant water ice on the surface and it may be present beneath the surface too.
 
 

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