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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 atmComposition: 96% CO2

Temperature: 470ºC; 880ºF (Not just 35ºC; 95ºF!)

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Parallel Histories

EarthSolar system nebula forms.

Proto-Earth develops.

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.

More bombardment, more water enrichment1.

Bombardment slows and stops.

VenusSolar system nebula forms.

Proto-Venus develops.

Heavy bombardment, including contributions from water-rich planetesimals from the outer solar system1.

Massive impact from an enormous object results in Venus having a very long rotation period (243 d).

More bombardment, more water enrichment1.

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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Parallel Histories

EarthVolcanic 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.

Oceans begin to accumulate; CO2 starts to be relocated from the atmosphere and into rocks.

VenusVolcanic outgassing begins; primarily CO2, H2O, small amounts of N2, and other compounds1.

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(?).

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 divergeAn 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 HistoriesEarth VenusH2O: accumulated in oceans; H2O: no oceans, Venus is dry;CO2: locked in rocks; CO2: remains in atmosphere;N2: remains in atmosphere. N2: insignificant, in atmosphere.

H2O continues to build in oceans; H2O is gone, crust is dry;Tectonics remain active; Dry crust not tectonic;CO2 (outgassed) stored in rocks. CO2 accumulates in atmosphere.

CO2 cycle stabilizes; CO2 accumulates in atmosphere;Greenhouse effect matures at stable level. Runaway greenhouse loop.

Life develops in oceans; Hellhole Venus;O2 crisis 545MYA. Venus repaves itself 750 MYA.

Life diversifies; Hellhole Venus;Occasional periodic extinctions. Hellhole Venus.

Today: the Earth is a lovely place. Hellhole Venus.

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Reality checks on Venusian waterClearly, 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, ANDThe 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 starToo 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 sizeToo 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. AtmospheresWithout 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 zoneInner 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 boundaryCertainly 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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UncertaintiesProblems 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!