Transcript Lecture 09a: Habitable zones - Sierra College Astronomy Home Page

The Solar System’s Habitable Zone
Goals
• Learn about the solar system’s habitable zone
Venus and Mars as a bookends of the Sun’s habitable zone
• Venus’ runaway greenhouse effect
• The importance of atmospheres for habitable zones
•
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The habitable zone
The range of distances around a star, at which a planet could
potentially have conditions that would allow for abundant amounts
of liquid water on the planet’s surface.
Important reminders:
1. Being in a habitable zone may be a necessary, but not sufficient, condition for
life (i.e., the Moon);
2. We shall see that whether a planet is habitable may change over time, as the
planet’s characteristics, and the central star’s power output, both change over
time.
3. This assumes a solar power source, and does not consider cases such as
Europa (subsurface oceans warmed by tidal power sources), or Martian
volcanic zones (warmed by geothermal energy).
4. We are also discriminating against life forms that are based upon other
elements (boron, silicon), or that use other fluids (methane, ethane,
ammonia).
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Life beyond the habitable zone
But what about life on rogue interstellar worlds?
– Earth-sized;
– Ejected from solar system with atmosphere intact;
– Thick hydrogen atmosphere which acts as a blanket;
– Slow to cool, especially if geologically active;
– Could have surface oceans for billions of years.
The low energy budget of such extreme conditions would likely lead to only simple life
forms.
In a small survey of part of the sky, 10 have been found.
This means there could be more than 100 billion in our whole galaxy.
Numbers suggest only a small fraction formed alone in space.
Most were ejected from their systems.
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Venus’ Story
Recall from our previous studies that Venus is Earth’s twin…
M=0.815×MEarth
R=0.949× REarth
D=0.723 a.u.
Based upon the similarities of cratering histories (inferred from our
studies of Mercury, the Moon, asteroids, and Mars), we believe that
Venus experienced similar impacts through time as the Earth did.
Then why is Venus the solar system’s hell hole?
Atmosphere: 90 atm
Composition: 96% CO2
Temperature: 470ºC; 880ºF (Not just 35ºC; 95ºF!)
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Parallel Histories
Earth
Venus
Solar system nebula forms.
Solar system nebula forms.
Proto-Earth develops.
Proto-Venus develops.
Heavy bombardment, including
contributions from water-rich
planetesimals from the outer solar
system1.
Heavy bombardment, including
contributions from water-rich
planetesimals from the outer solar
system1.
Massive glancing impact from the
proto-Moon results in the capture of
our Moon.
Massive impact from an enormous
object results in Venus having a very
long rotation period (243 d).
More bombardment, more water
enrichment1.
More bombardment, more water
enrichment1.
Bombardment slows and stops.
Bombardment slows and stops.
1It
is a reasonable assumption that the nature of the planetesimals striking
Venus and the Earth were similar in composition.
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Earth
Parallel Histories
Venus
Volcanic outgassing begins; primarily
CO2, H2O, small amounts of N2, and
other compounds1.
Volcanic outgassing begins; primarily
CO2, H2O, small amounts of N2, and
other compounds1.
Because of its high rotation rate and
resulting high magnetic field, solar
stripping removes negligible amounts
of the atmospheric gases.
Because of its low rotation rate and
resulting low magnetic field, solar
stripping removes more amounts of
atmospheric gases than in the Earth’s
case, but this is still negligible.
Oceans begin to accumulate; CO2
starts to be relocated from the
atmosphere and into rocks.
Oceans begin to accumulate(?); CO2
starts to be relocated from the
atmosphere and into rocks(?).
1It
is a reasonable assumption that the nature of the outgassing from Venus
and the Earth were similar in composition.
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Paths diverge
An important process affects Venus, but not the Earth
Slightly closer to the Sun, the ultraviolet radiation striking Venus is
slightly more intense. This radiation ionizes water vapor in the upper
atmosphere:
H2O + photon  H2 + ½ O2
The H2 escapes because of its low mass (more on that in a bit). The O2
is removed by solar stripping.
The O2 that is not stripped becomes chemically bound into the rocks.
This process robs Venus of its water—Venus loses its oceans and dries
up!
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Diverging Histories
Earth
Venus
H2O: accumulated in oceans;
CO2: locked in rocks;
N2: remains in atmosphere.
H2O: no oceans, Venus is dry;
CO2: remains in atmosphere;
N2: insignificant, in atmosphere.
H2O continues to build in oceans;
Tectonics remain active;
CO2 (outgassed) stored in rocks.
H2O is gone, crust is dry;
Dry crust not tectonic;
CO2 accumulates in atmosphere.
CO2 cycle stabilizes;
Greenhouse effect matures at stable level.
CO2 accumulates in atmosphere;
Runaway greenhouse loop.
Life develops in oceans;
O2 crisis 545MYA.
Hellhole Venus;
Venus repaves itself 750 MYA.
Life diversifies;
Occasional periodic extinctions.
Hellhole Venus;
Hellhole Venus.
Today: the Earth is a lovely place.
Hellhole Venus.
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Reality checks on Venusian water
Clearly, the histories of Venus and the Earth diverge because of the
differences in their crust and atmospheric H2O content. The Earth
has 10,000× the water that Venus has!
Q1: Are we sure Venus’ water is not hiding in the crust?
Q2: Are we sure Venus’ water has left the planet?
A1:
We know Venus has active volcanoes, because of the sulfuric acid
(H2SO4) in Venus’ atmosphere. (Sulfuric acid is corrosive, and
would leave the atmosphere in 100 million years.) Active volcanoes
must be replenishing it by outgassing sulfur dioxide (SO2).
If water was in the crust of Venus, the volcanoes would be pumping
it back into the atmosphere.
However, they aren’t! So Venus’ water is not hiding in the crust.
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Evidence of disassociation
Q2: Are we sure Venus’ water has left the planet?
A2:
Consider a molecule of H2O that is disassociated into H2 and O2.
In thermal equilibrium, all the molecules have the same energy:
Kinetic Energy = ½ mv2
Low mass molecules have a higher velocity. H2 is very low mass,
so it can escape the gravitational field of Venus.
Heavy hydrogen (deuterium) is rare (~1: 50,000). But it would
have a hard time escaping Venus’ gravity.
Deuterium is enhanced by 100× on Venus, suggesting vast
amounts of H2O loss.
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Runaway Greenhouse Effect
What would happen to the Earth, at Venus’ position in the solar
system?
The temperature would rise ~30ºC, to 45ºC (113ºF);
Evaporation rates would increase, AND
The hotter atmosphere could hold more water;
The H2O driven into the atmosphere would (as greenhouse gas) heat
the Earth still more;
 the Earth gets hotter;
 more evaporation, and more H2O in the atmosphere;
 the Earth gets hotter;
 more evaporation, and more H2O in the atmosphere;
 the Earth gets hotter;
 more evaporation, and more H2O in the atmosphere;
 the Earth gets hotter;
 more evaporation, and more H2O in the atmosphere;
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Runaway Greenhouse Effect
Ultimately, as a result of this positive feedback, the oceans
would vaporize.
UV photons would begin the process of disassociating the water
vapor, and in time the hydrogen would escape and the oxygen
would be locked in our surface rocks.
Earth: what a hell hole!
Was Venus once habitable?
Over the last 5 billion years, the Sun has slowly brightened by
30%. Long, long ago, water may have been stable on the
Venusian surface.
Is it possible that Venusian life migrated to the upper
atmosphere, where it survives to this day?
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Three factors that determine surface habitability
1. Distance from the central star
Too close, and the temperature of the planetary surface rises. Even a relatively
small temperature increase can result in runaway greenhouse effects.
Too far, and the temperature of the planetary surface drops. Greenhouse effects can
help keep a planet warm.
The range of habitable distances from the star depend upon the luminosity of the
central star. More on this important topic, to follow!
2. Planetary size
Too small, the planet will cool too fast. When it solidifies, it will lose its magnetic
field. With no magnetic field, its atmosphere will be stripped.
Small planets will lose tectonics more rapidly, which would end the CO2 cycle.
Rotation is also important in generating that magnetic field?
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Three factors that determine surface habitability
3. Atmospheres
Without an atmosphere, liquid water will not be stable. Low-mass planets cannot
hang onto their atmospheres.
Are Jovian planets necessary to disturb the orbits of ice-rich planetesimals towards
inner terrestrial proto-planets?
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Our solar system’s habitable zone
Inner boundary
Certainly smaller than 1 a.u. (Earth)
Larger than 0.7 a.u. (Venus)
Models suggest runaway greenhouse at 0.84 a.u.
But…in hotter settings, water vapor circulating above the ozone layer
might become disassociated, to be lost. In time, the water could be robbed
from a planet: this is called a “moist greenhouse effect.”
The moist greenhouse effect may occur at ranges of 0.95 a.u!
Outer boundary
Certainly larger than 1 a.u. (Earth)
Smaller than 1.5 a.u. (Mars)
But…if Mars were larger, with more atmosphere and more greenhouse
effect, it might be within the habitable zone (1.7 a.u.).
But…even if a planet has a thick atmosphere, if it is far from the star its
CO2 atmosphere could deposit out as snow, so the outer boundary to the
habitable zone may be 1.4 a.u.
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Uncertainties
Problems and uncertainties with our models are frustrating, but they are being
improved.
Next class, we venture into the dangerous, slippery realm where science and
politics overlap!
There be dragons ahead!
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