Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                            
September 6, 1991
(Phone:  202/453-1547)

Diane Stanley
Ames Research Center, Mountain View, Calif.
(Phone:  415/604-9000)


RELEASE:  91-143

SCIENTISTS DISCOVER ANTI-GREENHOUSE EFFECT ON TITAN


     An anti-greenhouse effect on a solar system body has been 
described for the first time by scientists from NASA's Ames 
Research Center, Mountain View, Calif., and the Paris Observatory.

     Drs. Christopher McKay and James Pollack of Ames, with 
France's Dr. Regis Courtin, also describe the temperature 
structure and energy balance on Titan, Saturn's largest moon.  
Using data from the Voyager 1 spacecraft and computational models, 
the scientists determined the source, or controls, of the 
temperature profile.

     Their results are in the current issue of Science magazine.  
They say the temperature model of the organically-rich, thick 
atmosphere on Titan may help in basic studies of the greenhouse 
effect on Earth.

     Their study provides the first description of a greenhouse 
and anti-greenhouse effect existing simultaneously, in tension 
with each other.  Greenhouse effects on Venus and Earth have been 
discussed extensively.

     The study results define an anti-greenhouse effect that 
reduces Titan's surface temperature by 16 degrees Fahrenheit.  The 
effect is produced by a thick, organic haze in Titan's upper 
atmosphere that absorbs solar light, but transmits reflected 
infrared radiation.

     Titan's greenhouse effect, they report, increases the surface 
temperature by 38 degrees Fahrenheit.  The effect is caused by 
atmospheric gases reflecting infrared energy back to Titan's 
surface.  The surface temperature of Titan, minus 290 degrees 
Fahrenheit, is therefore 22 degrees warmer than it would be 
without atmospheric effects.

                             - more -
                                      
                                      
                                      
                              - 2 -


     Between the size of Earth's moon and Mars, Titan is the only 
satellite in the solar system with a thick atmosphere.  It's 
atmospheric pressure is similar to Earth's - only one and one half 
times greater - and it has the same primary atmospheric ingredient 
- nitrogen.  Also, the major greenhouse gas on each body (water on 
Earth and methane on Titan) condenses on its surface.

     Titan's greenhouse effect depends on a link between hydrogen 
and methane, like the Earth's greenhouse effect depends on carbon 
dioxide and water vapor, says McKay.

     As humans increase carbon dioxide on Earth and it gets 
warmer, more water goes into the atmosphere.  Water vapor on Earth 
is a more potent greenhouse gas than carbon dioxide, McKay said.

     Similarly, on Titan, hydrogen acts like carbon dioxide, he 
said.  It does not condense.  If you increase it, it will pull the 
methane out of suspected methane-rich oceans or lakes.  On Titan, 
the condensed surface liquid, methane (like water on Earth) then 
becomes the most potent greenhouse gas.

     McKay, Pollack and Courtin were modelling the greenhouse 
effect on Titan when they realized that they needed a 
counterpoint.  They then isolated the temperature dynamic of the 
upper atmospheric haze.

     Earth has no comparable mechanism.  However, stratospheric 
ozone acts, in a small way, as a highly selective anti-greenhouse 
shield.  It absorbs only a specific region of solar light, 
ultraviolet, and lets infrared radiation pass through on its way 
out.

     Debris from volcanic explosions or extraterrestrial impacts 
also temporarily scatters solar light and acts as an 
anti-greenhouse agent.  But the gases (mostly sulfur dioxide) 
eventually produced by these events are greenhouse agents, letting 
in solar light and reflecting infrared radiation back to space.

     As carbon dioxide and water vapor increase in the Earth's 
atmosphere, more clouds will form.  Scientists want to know how 
they will warm or cool the planet.

     Just as Titan's greenhouse/anti-greenhouse effect is a good 
comparison model for Earth, studying the formation of clouds on 
Titan and their effect on the greenhouse balance may help answer 
these questions, McKay suggested.

                             - end -