10.1 the concept of a habitable zone
The orbit is an ellipse, not a circle, and Earth’s distance from the Sun varies from a minimum of about 147.1 million kilometers each Jan to a maximum of about 152.1 million km
It has little effect on weather. But there must be some distance from the sun that would deep it to be too hot.
How does a planet's location affect its prospects for life?
There is some range of distances that is “just right” for a world like Earth, it is called the habitable zone.
the habitable zone is the range of distances from the Sun within which we could in principle move our planet without fundamentally changing the characteristics that make it home
to abundant life.
Note that this definition captures the essence of what we mean by a habitable zone, but there are several important caveats to keep in mind:
1. The concept of a habitable zone is based on the range of distances at which worlds similar to Earth could exist. In other words, a habitable zone is a zone in which it is
possible for a world to have abundant liquid water on its surface.
2. Simply being in a star’s habitable zone is not sufficient to make a world habitable. The Moon presents an obvious case in point: As a companion to our planet, it is located at
essentially the same distance from the Sun as Earth, but it is not habitable.
3. Habitable zones evolve with time. In particular, because stars like the Sun tend to brighten as they age, we expect a star's habitable zone to move outward over time.
In summary, at any particular time, a star’s habitable zone is the range of distances around it at which a planet could potentially have surface
temperatures that would allow for abundant liquid water.
No place we looked hat have surface water. There is a distinction between surface life in a habitable zone and subsurface life.
We can try to find outside our Sun, from telescopes.
Surface life may well create spectral signatures in a planet’s atmosphere that would allow us to detect it, but subsurface life is hard to find because of atmospheric changes that
affects what we see in a telescopic image, it is called spectral signature.
Could life exist outside the habitable zone?
There are several ways that habitability might present itself outside the habitable zone. One is in small pockets of subsurface groundwater.
If this was true it makes habitability even more common, because so many moons have subsurface water.
Another intriguing possibility concerns surface habitability on Earth size planets outside habitable zones.
Usually Earth size planets have an atmosphere for some time until the hydrogen escape int space. Sometimes these planets form and get thrown into space before the hydrogen escape, and if that happens, the atmosphere will remain and making it sustainable
condition for life!
These free floating Earths, should be quite common.
Also, a place with liquid medium can also sustain life, like liquid methane, for example. They will all evaporate when atmosphere pressure is low, so if there is
surface liquid, it already means there is an atmosphere.
In summary, it’s quite possible that the majority of habitable worlds in the universe are not located within stellar “habitable zones.”
But these places seem unlikely to rise complex life or advanced civilizations.
10.2 Venus: An example in potential habitability
Why is venus too hot?
The surface of Venus is far too hot for liquid water
It makes its orbits at 72% of Earth's distance, and because of the inverse square law, the intensity of the sunlight is 1/(72^2) = 1.9 times of Earth.
It's just a little warmer. But Venus is very hot because high temperature produced extreme greenhouse effect, produced by atmospheric carbon dioxide.
200000 times more carbon dioxide than Earth.
Same amount of outgassing as Earth from volcanoes.
We had the same amount of carbon dioxide. On Venus, all of it went to the atmosphere. On Earth, it was locked up in carbonate rocks or dissolved in the oceans, and went
through chemical reactions, turing into minerals like limestone. This happened on Earth because Earth had liquid to allow the carbon dioxide cycle.
Water outgassed on Earth went to the oceans. Water outgassed in Venus doesn't exist, because it was baked out long ago.
Venus never had much water to begin with, it had such high temperature that water vapour could not condense into solid particles.
UV light form the sun broke apart the water molecules, and hydrogen atoms escaped to space.
Venus lacks a magnetic field because of slow rotation, so it makes it easier for hydrogen to fly away.
Also, Water on Earth came from planetesimals from outer solar system, but no such planetesimals crashed Venus the same way.
Few impact craters = always have geological activities.
2 Evidence for ongoing geological activities:
1. Sulfuric acid in the clouds, which only comes from volcanic outgassing.
2. Lava flow evidence
Deuterium is this isotope of hydrogen that has a neutron in addition to a proton. They are called heavy water and as water but heavier, so they don't fly away as easy as hydrogen
This is the case: some of the deuterium stayed on Venus, suggesting that water must have existed in the past on Venus but broken apart and got lost in space.
Comets come to Venus and they bring deuterium. That's why there is more deuterium on Venus, because the new water comes and hydrogen gets stripped away again! Most of Earth's water is turned into rain and back into the oceans.
UV light does break apart water but it doesn't penetrate far into the atmosphere and touch water.
If we were as close to the sun as Venus, the Runaway Greenhouse Effect will occur:
higher temperature increases evaporation, causing more water vapour, which strengths the greenhouse effect.
Causing: Oceans will evaporate, carbonate rocks will decompose because of the heat, releasing the trapped carbon dioxide and then the greenhouse effect will increase.
We'll be even hotter or just as hot as venus.
On Earth, it was cool enough for water to rain down to make oceans in which car bon dioxide could dissolve and undergo chemical reactions that locked it away in carbonate
rocks. As a result, our atmosphere was left with only enough greenhouse gases to make our planet pleasantly warm.
Earth does lie in the habitable zone, why is why we didn't suffer from a runaway greenhouse effect.
Conclusion: The boundary for Sun's habitable zone must lie beyond the orbit of Venus but within the orbit of Earth.
Could venus have once been habitable, and could life still exist there?
Some 4 billion years ago, the intensity of sunlight at Venus was probably only about 40% greater than it is at Earth today. Rain might well have fallen on Venus, and oceans could
have formed. It’s even conceivable that life could have arisen on the young Venus or been transported there on meteorites from Earth.
And then it got hotter as the Sun gradually brightened with age.
enus’s entire surface appears to have been “repaved” by tectonics and volcanism; this repaving would have covered up any shorelines or other geological evidence of past oceans.
So we don't know anything about the place.
10.3 surface habitability factors and the habitable zone
We know that distance from the Sun is not the only habit ability factor.
what factors influence surface habitability?
few stars are much more luminous than the Sun, about 10% of all stars have luminosities similar to that of the Sun, and the vast majority of stars are considerably dimmer than the
Sun. Brighter = wider habitable zone.
The wider habitable zones of brighter stars might seem to increase the odds of finding planets within these zones
Problem:The brightest stars turn out to have extremely short lifetimes—millions of years rather than billions of years—and hence may not offer enough time to nurture biology.
Plate tectonics is necessary for long term habitability on any world, so the size requirement for surface habitability is a size that allows plate tectonics to exist.
This is because the climate that's good has the carbon dioxide cycle, which depends on the cycle of plate tectonics.
We do not fully understand how planetary size is related to plate tectonics. Beyond that, we don’t know how large a planet must be to allow for the presence of liquid water over extended periods of time.
A third crucial factor in surface habitability, after size and distance, is the presence of an atmosphere.
It has to have sufficient pressure