The Solar System

1
The Solar Systems Habitable Zone

• Goals
• Learn about the solar systems habitable zone
• Venus and Mars as a bookends of the Suns
  habitable zone
• Venus runaway greenhouse effect
• The importance of atmospheres for habitable
  zones

2
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 planets surface.
• Important reminders
• Being in a habitable zone may be a necessary, but
  not sufficient, condition for life (i.e., the
  Moon)
• We shall see that whether a planet is habitable
  may change over time, as the planets
  characteristics, and the central stars power
  output, both change over time.
• 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).
• We are also discriminating against life forms
  that are based upon other elements (boron,
  silicon), or that use other fluids (methane,
  ethane, ammonia).

3
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.

4
Venus Story

• Recall from our previous studies that Venus is
  Earths twin
• M0.815MEarth
• R0.949 REarth
• D0.723 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 systems hell hole?
• Atmosphere 90 atm
• Composition 96 CO2
• Temperature 470ºC 880ºF (Not just 35ºC 95ºF!)

5
Parallel Histories

• Earth
• Solar 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.

Venus Solar 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.
6
Parallel Histories

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

Venus Volcanic 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 Earths 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.
7
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 waterVenus loses
  its oceans and dries up!

8
Diverging Histories

• Earth Venus
• H2O 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.

9
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 arent! So Venus water is not
  hiding in the crust.

10
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.

11
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

12
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?

13
Three factors that determine surface habitability

• 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!
• 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?

14
Three factors that determine surface habitability

• 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?

15
Our solar systems 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.
• Butin 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)
• Butif Mars were larger, with more atmosphere and
  more greenhouse effect, it might be within the
  habitable zone (1.7 a.u.).
• Buteven 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.

16
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!