Lecture #21 April 17, 2008: Science and Red Mars
A space elevator is a structure that goes from a planet’s surface to geosynchronous
orbit (36,000 km above surface of Earth).
Appears in many SF novels, notably Arthur C. Clarke’s The
Fountains of
Planetary climates and terraforming
Venus is too hot..
Earth (“Terra”) is just right!
Mars is too cold…
Why? Water … vs. CO2
The “Greenhouse effect”: transparent to visible light but opaque to infrared.
CO2 is a “greenhouse gas.” It tends to trap heat.(Water vapor and methane are also important greenhouse gases).
The Greenhouse effect is good. Without it Earth would be about 30 ºC colder.
Global warming is too much Greenhouse effect (about 3%).
Too much CO2 will definitely cause global warming. But how much is too much?
Many complicated feedbacks in the system. Hence, some uncertainty and “controversy.”
From Red Mars (part 2)
“X claims a, Y claims b. They make their arguments to support their claims, with any number of points. But when their listeners remember the discussion, what matters is simply that X believes a and Y believes b. People then form their judgement on what they think of X and Y.”
Venusian atmosphere: 95% CO2 and 90 times thicker
than Earth’s atmosphere. RUNAWAY Greenhouse effect
Terrestrial atmosphere: 79% N2, 20% O2, plus a touch of CO2, water vapor, etc.
Just-right Greenhouse effect
Martian atmosphere: very thin, 95% CO2 Minimal
Greenhouse effect
So why does Venus have so much CO2, Mars so little, and Earth “just right”?
A: The “carbon cycle”
Water (rain) washes CO2 out of the atmosphere into sediments
Volcanoes release CO2 from sediments into atmosphere
Venus lost its water billions of years ago (broken apart by solar UV)
Earth has both active volcanoes and plenty of water!
Mars is smaller than Earth and Venus; volcanoes died billions of years ago
Terraforming Mars: add a thicker atmosphere to trap in heat
Next topic: where could we find life?
At this time, more than 200 hundred planets have been discovered around other stars!
http://exoplanets.org/
Most exoplanets ( = planet outside our solar system) discovered by Doppler shift of primary star
This method is most sensitive to heavy (Jupiter-sized) planets close to the primary
Limits on stars that can host life-as-we-know-it:
Must live long enough for life to evolve
Must have a reasonable temperature range for planets
Not too much ultraviolet!
all these point to G-class stars as ideal
Brief history of life on earth
13.6 billion years ago: Universe begins.
4.5 billion years ago: solar system & earth forms
3.5-3.9 billion years ago: first life on earth.
AGE OF BACTERIA AND ALGAE (prokaryotes, simples cells)
1.9 billion years ago: first eukaryotes (complex cells)
650 million years ago: first multi-celled organisms
NEARLY 4 BILLION YEARS JUST TO GET A JELLYFISH
M,K,G + maybe F class stars (up to 1.6 solar masses)
Our Sun’s habitability zone is about 0.7 to 1.5 A.U.
A K-class star has a habilitability zone half this size
and an M-class start has a habilitability zone only 1/10th this size
(A,B, and O class stars have much larger habitability zones, but have too short life times and also emit high amounts of UV radiation)
What I want you to know
1. Over 200 hundred “exoplanets” (planets outside our solar system) have been discovered.
2. These are mostly Jupiter-sized gas giants and unlikely to support life.
3. Evolution of life appears to take billions of years – so massive, hot, short-lived stars cannot support life.
4. Very small, cool stars live a long time, but are so cool that their “habilitability zones” are very small, and unlikely to find a planet there!