Lecture Notes: 

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Lecture #23 April 10

• LIFE ZONE: 


Inward and outward from the earth's orbit extends the life zone of the 
solar system. Whether it extends to Mars, we can not be sure. Perhaps it 
once included Venus, but now life seems very unlikely on that beastly hot 
planet .

• EQUILIBRIUM TEMPERATURE



We can estimate the equilibrium surface temperature of a bare planet 
without an atmosphere by balancing the energy received from the sun with 
that emitted by the planet into space. 

The difference between estimated and observed temperatures is due to the 
atmospheres of these planets, Venus having the largest atmosphere and the 
largest difference. That huge difference is due to the run-away greenhouse 
effect.

• POSITIVE AND NEGATIVE FEEDBACK

Both Venus and Mars have atmospheres which differ from that of the earth 
because of positive feedback. Because of the runaway green house on Venus, 
the outgassing of carbon dioxide from its surface rocks and evaporation of 
oceans, the atmosphere has grown to its present size of 94 times greater 
than the earth's. 

In the case of Mars, positive feedback has amplified processes that caused 
the atmosphere to freeze out. The atmosphere is not perhaps contained 
beneath the surface as permafrost. In the atmospheres of Venus and Mars, 
about 96% of the gas is carbon dioxide. 

Earth's atmosphere has various negative feedback processes which stabilize 
it. Its nitrogen and oxygen are largely the result of the massive 
transformation of our atmosphere by plant life. 

• LIFE ON MARS

Mars seems a slightly colder cousin of Earth; the tilt of its rotational 
axis and the duration of its day are similar to those of the earth. 
Because its year is 687 days, seasonal changes are slower than on the 
earth. 

Life has always seems a tantalizing possibility on Mars because of its 
changing polar caps, the wave of darkening that proceeds to the equator 
every spring, and temperature near the equator that are close to or 
slightly above freezing. The great volcanoes of the planet as well as 
comet droppings in the past probably left enough water on the planet to 
cover its surface 10 to 1000 meters deep. Dried stream beds confirm that 
water once flowed across its surface. 

Percival Lowell's famous search for life on Mars was triggered by 
Schiaparelli's use of the term "canali" to describe lines which he thought 
we saw on its surface. Lowell was a victim of wishful thinking and optical 
illusions. His book "Mars, the Abode of Life" inspired H. G. Wells to 
write "The War of the Worlds", which was adapted for radio by Orson Wells. 

The Viking landers searched for life and found nothing, primarily because 
the laboratory was run by a robot. If a good experimental biologist had 
been along, he/she may well have discovered life. Recent studies of Mars 
meteorites which were collected in the Antarctic, provide hints of fossil 
life on that planet.