1. Habitability

2. What does life need?
Elements – Life on Earth uses about 25 of the 92 naturally occurring elements.
Energy – An energy source for metabolism (from sun, chemical reaction, etc.)
Liquid Water – dissolves organic molecules, transports chemicals, involved in metabolic reactions.
Could you use something other than water? Maybe. Water is good because:
It stays liquid over a large range of temperatures.
It expands (floats) as it freezes.
Electrical charge distribution makes it polar. (affects what dissolves in water, cell membranes don’t)

3. Solar System Habitability
Moon and Mercury – least likely
Venus – highly unlikely
Jovian Planets – Unlikely
Small moons, asteroids, comets, Pluto – Unlikely
Mars and large moons – most likely
These are places where what we think life needs may be in place.

4. A Habitable Zone
Venus and Mercury are too hot, Mars is a little too cold.
Inner boundary at 0.84 AU (if moderate greenhouse) or 0.95 AU if water is lost from the upper atmosphere.
Outer boundary 1.4 AU AU (little/no greenhouse effect to strong greenhouse effect)
Earth lies within both the conservative and non-conservative estimates.
Mars lies within the non-conservative estimates (optimistic)
Of course, the sun has changed its luminosity over time. The Sun is brightening, therefore we would expect the habitable zone to MOVE OUTWARD over time. (See Fig 9.5 on p.237) (Of course this is true only if it depends only on distance to the Sun.)
But, Mars has become less habitable and Earth has been habitable for some 4 billion years. Hmmmm. (Venus may have had water once upon a time, but we have no evidence of that now.) (So it must not by ONLY DISTANCE TO THE SUN that is responsible.)
Let’s explore the EVOLUTION OF THE HABITABLE ZONE over time.
Understanding the evolution requires understanding how and why Venus, Earth, and Mars evolved along such DIFFERENT PATHS.
A region of our Solar System where temperatures (in principle) would allow for liquid water to exist on the SURFACE of a planet (of the correct type).

5. Comparative Planetology: Venus, Earth, and Mars
WHY IS THE GREENHOUSE EFFECT SO DIFFERENT ON THE 3 PLANETS?
Venus is roughly the same size as Earth with roughly the same amount of carbon dioxide as Earth. Most of Earth’s carbon dioxide is locked in the rocks or dissolved in the oceans. On Venus it is nearly all in the atmosphere. This is the primary source of the intense greenhouse effect on Venus. (There are other contributing factors, but CO2 is the major one.)
Earth put the CO2 back in the rocks after outgassing by means of the CO2 cycle (WATER)! Venus lacks a CO2 cycle (no oceans).
If Venus started out so much like Earth, WHY DOESN’T VENUS HAVE OCEANS?
The Greenhouse Effect makes a large difference in the Venusian atmosphere, but very little difference in the Martian atmosphere.

6. Venusian Oceans? No water today! (no ice or water, almost no vapor)
Either:
Venus didn’t have much water (unlikely), or
Venus lost it. (how?)
Venus likely had lots of water (or vapor) in its youth.
No ozone layer, UV would break water
Check on amount of Deuterium!
Deuterium: 1/50,000 hydrogen atoms on Earth
135 x more abundant (relative to ordinary Hydrogen) on Venus than on Earth
This suggests that a great deal of water was broken apart and lost to space. (We don’t know EXACTLY how much. Lower limit is a global layer several meters deep.)

7. Runaway Greenhouse Effect
If we moved Earth to the position of Venus, we would end up with the same situation as Venus thanks to the runaway greenhouse effect.
On Earth, the water rained down and dissolved the CO2 which then got locked in the rocks. This put a brake on the runaway greenhouse.
On Venus, either the water never rained down (too warm) or the water evaporated (again too warm). No brake for the runaway greenhouse and it gets hotter and hotter, spiraling out of control.
Whatever water Venus had in the past is long gone… along with any habitability.

8. Surface Habitability Factors
Distance to the Sun (Venus got too warm and had a runaway greenhouse)
Planetary Size (Mars cooled too quickly, geologically inactive)
Atmospheric Loss (Mars has no magnetic field protection and low level of volcanism for replacement, Venus is losing too)
All 3 planets (Mars, Earth, Venus), started roughly the same.
Mars froze Venus baked Earth still has liquid water
What factors caused the difference?
With Venus – distance to the Sun

9. Outside the Zone The concept of a habitable zone
precedes discussions of life on Europa or Titan
Only took into account liquid water AT THE SURFACE
Only talked about heating by the Sun
Only used the liquid water
Liquid water subsurface is possible for Europa and Mars.
The heat source for Europa would be tidal.
What about another liquid? (ethane?)
These things will make us reconsider where to look for habitability. Moons of Jovian type planets, planets with other heat sources, planets with other liquids either on or under the surface.
So a lot of “habitable” worlds may not be within the “habitable zones” of their stars. So, is the concept obsolete?
The concept is still useful in looking for intelligent life in the universe! (subsurface would be nearly impossible for us to detect, and may never rise to advanced civilizations. We look to the surface habitable planets for hopes of more advanced species.)

10. Future Habitability Habitability WILL end on Earth.
As the Sun brightens, we’ll lose water.
In 3-4 billion years, Earth will be hot enough for a runaway greenhouse.
When the Sun dies (about 5 billion years from now), it will become a red giant. (>100x its present radius) We will bake!
Then it will eject it’s outer layers in a minor (!!) explosion which will likely destroy the planet.
We’ll start the water losing process in about 1 billion years. (Not a given, too many variable in climate change)
We don’t know any way to stop the progress toward runaway greenhouse, but by then Mars will be in the habitable zone. LET’S MOVE!

11. Could we survive? Only if we move first.
And if our new planet dies? Keep moving!
A new star! (beyond our reach at present)
When it dies, keep moving.
Maybe we’ll make it that long.