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V Homes for life http://sgoodwin.staff.shef.ac.uk/phy229.html 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.