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Homes for life
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5.0 Introduction
Where should we look for life?
Finding 'Earth-like planets' in the 'Habitable (Goldilocks) Zone' is
frequently cited as the goal of planet hunters and are phrases often
used in the press.
Defining terminology is crucial in science – so what is an 'Earth-like
planet'? What is the 'Habitable Zone'?
We need a summary of various material from PHY106 to start.
5.1 Earth-like planets
What is an `Earth-like' planet?
Solid (rocky) body,
Large enough to retain a significant atmosphere for geological
timescales,
(Large) quantities of liquid water.
ie. suitable for 'life as we know it'
- Carbon-based using water as a solvent and living on the
surface of the planet.
5.1 Liquid water
In order to have liquid surface water the average surface
temperature of a planet must be around 270-370 K (although this
depends on atmospheric pressure, and doesn't take into account
possibly large variations in temperature across a planet).
The Earth has an average temperature of ~286 K (13 C) – but
geographical and temporal variations can be +/- 30 or 40 K.
(Note, the temperature on Mars can reach 35C near the equator,
but the atmospheric pressure is so low (~7 mbar) that liquid water
cannot exist on the surface – for long.)
5.1 The temperature of a planet
The surface temperature of a (rapidly rotating) planet with an albedo
alpha at a distance d from a star of luminosity L is:
Including, very importantly, the Greenhouse effect!
5.1 The greenhouse effect
The Earth's atmosphere is (almost) transparent to incoming Solar
radiation, but absorbs most of the IR radiation the ground emits after
absorbing Solar energy.
The atmosphere absorbs outgoing
IR radiation which heats the air, and
also the surface.
The most effective greenhouse
gasses are H2O, CO2, and O3
(also CH4, NO2 and CFCs).
5.1 Albedo
Albedo quantifies the reflectivity of a body
Venus ~0.7
Earth 0.37-0.39
Mars ~0.15
Albedos of various terrains:
Fresh snow 0.7 – 0.8
Old snow 0.5-0.6
Grass 0.20-0.25
Forest 0.05 – 0.10
Water 0.08 (plus high specific heat capacity)
Clouds 0.3-0.8
5.1 Retaining an atmosphere
In order to retain an atmosphere the escape velocity from the planet
must be large enough to stop gasses with typical velocities for the
planet's temperature from escaping.
5.1 The temperature of a planet
So, the surface temperature of a planet depends on
1. The luminosity of the star,
2. The (average) distance from the star,
3. The atmospheric composition and density,
4. The (average) albedo (clouds and surface).
5.2 Habitable zones
The `habitable zone' (HZ) is defined as the region around a star in
which an Earth-like planet has the right temperature for liquid water to
exist.
Simplistic calculations of the HZ assume that the planet is so Earth-like
that it has the same (~40K) greenhouse effect as the Earth and the
same albedo (~0.35).
5.2 Habitable zones
From our earlier discussion of planetary temperatures the current HZ in
the SS for present-day Earth is between ~0.95-1.6 au.
However, there are complications in even the simplistic model:
The HZ will move outwards as a star warms during its MS life, so a
zone that is continuously habitable will be far smaller than the current
HZ.
This is related to the Faint Young Sun problem we will return to later:
the Earth's surface temperature has been relatively constant over a
period in which the Sun's luminosity has increased by 25-30%.
5.2 Habitable zones around other stars
HZs obviously depend on the stellar luminosity, around low-mass
(ie. faint) stars the HZ may be so close to the star that the planet
becomes tidally locked (or close to it, cf. Mercury).
5.2 Gl581c
Gl581c has been argued to be a solid planet in the HZ. It is a 5
Earth-mass planet, 0.073au from a 0.013 Lsun (M3V) star. This
implies a surface temperature of:
If the surface is ice or cloud (alpha=0.8), the temperature could be
only 230K (and be sort-of Mars-like?)
If it has a Venus-like cloud cover (alpha=0.8), but a thick
atmosphere the temperature could be >500K?
Or it could be Earth-like (alpha=0.3, Tgreen=40K) and be at 360K
and 'habitable'?
5.3 Earth-Venus-Mars
Currently Venus is too hot (~740K), Mars is too cold (~210K), and the
Earth is 'just right' (~300K): hence 'Goldilocks Zone'.
But each of the three planets has changed significantly over time.
As we shall see in detail the Earth has evolved in many ways and has
been influenced by the presence of life.
Mars certainly had significant liquid surface water in the past, and
Venus probably did as well.
The current state of a planet is not the same as its past or future state
– planetary surfaces/atmospheres etc. evolve.
5.3 Venus – runaway greenhouse
The surface temperature of Venus is ~740K, most of which is due to an
extreme greenhouse effect (from a ~90 bar CO2 atmosphere). It is
thought that as the early Sun warmed surface temperatures rose, the
resulting feedback loop is:
Increasing T => oceans evaporate => more greenhouse effect from
H2O => increasing T => etc.
eventually all of the water was in the atmosphere, and the surface T
was high enough (few 100C) to cause the release of CO2 from rocks:
more CO2 => higher T => more CO2 release
5.3 Mars – a hot, warm past?
There is significant evidence that liquid water was present on Mars in
the distant (and possibly recent?) past.
5.3 Mars – runaway icehouse
But present-day Mars cannot support liquid water at the surface
(atmosphere of ~7 mbar, T~210 K). In order to have had water in the
past (especially when the Sun was 20-30% fainter), the atmosphere
must have been much thicker.
Impacts and the Solar wind erode planetary atmospheres, but volcanic
outgassing replaces the lost atmosphere. But Mars cooled
replenishment slowed and eventually stopped. Then as impacts/Solar
winds eroded Mars' atmosphere they reduced the greenhouse effect.
As water vapour froze out of the atmosphere the greenhouse effect
dropped further.
Eventually the temperature was low enough for CO2 to sublime out of
the atmosphere onto the polar caps, reducing the greenhouse effect
even more.
5.3 Earth – just right?
We think of Earth as being 'just right': with the perfect conditions for
life.
But the Earth has changed its atmospheric composition significantly
over time, but retained a roughly constant surface temperature despite
the warming Sun...
And we have to remember that we have evolved to suit the current
conditions on the Earth, so they are by definition ideal for us...
We will return to all these points in great detail later as they are
obviously of great importance to astrobiology.
5.4 HZ: orbital determinism?
The simple HZ is 'orbital determinism': that the orbit of a planet
determines its habitability.
This is true to some extent - it is difficult to imagine any circumstances
in which a planet in Mercury's orbit around the Sun would be anything
other than a parched, airless world.
But further away other factors become important:
1. Size: to what extent is Mars' fate due to size rather than distance?
2. Details: was Venus 'destined' to be a runaway greenhouse or could
it have avoided this (by being smaller? Larger? Wetter? Less
volcanically active? Did it fail to start a carbon cycle for some
reason?).
5.4 Limitations – Io, Europa and Titan
There are further limitations to the idea of a HZ:
It assumes that liquid water is a requirement for life – a liquid almost
certainly is, but it may be liquid methane or ethane (e.g. Titan).
It assumes liquid water has to be stable on the surface, and ignores
possible sub-surface oceans (e.g. Europa).
It assumes that the primary energy source is incident stellar radiation,
ignoring tidal forces as an energy force (e.g. Io and Europa).
5.4 Limitations – Enceladus & Triton?
As well as Europa, Saturn's moon Enceladus may have a subsurface
ocean as we see it is geologically active with eruptions of water.
Triton is also active with eruptions of what is thought to be nitrogen
(could life use liquid nitrogen???). (What about Ceres, Ganymede,
Titan's undersea water ocean etc. etc.?)
5.5 Galactic habitable zones
The concept of HZs can be extended to whole galaxies. A habitable
planet must be close enough to the centre to contain significant
amount of heavy elements to make planets and life, but far enough to
avoid regular encounters and supernovae/irradiation. We appear to
lie right in the centre of the MWs GHZ at ~8kpc (just coincidence?).
5.6 Summary
The habitable zone is the region around a star in which an Earth-like
planet could have liquid water.
The surface temperature of a planet is governed by the incident
stellar radiation, its albedo and the greenhouse effect of the
atmosphere.
The concept of the habitable zone is problematic because there are
places in the Solar System which may harbour life outside the
habitable zone. It is also possible to set the albedo and greenhouse
effect of a planet in such a way that liquid water can exist outside of
the 'standard' habitable zone.
Also, as a star warms during the MS the habitable zone will move.