5 Habitable zones and Planetary atmospheres

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1 5 Habitable zones and Planetary atmospheres 5.1 Introduction Questions to answer: How does star affect the emergence and sustainability of life on a planet? What are the main properties of planetary atmospheres in the Solar system? How and what can we learn about atmospheres of extrasolar planets? 5.2 Habitable zones Unique Earth s properties enabling the life support: liquid water planetary environment (atmosphere, etc.) A circumstellar habitable zone is defined as encompassing the range of distances from a star for which liquid water can exist on a planetary surface. Pure water exists as a liquid between 273 K and 373 K, unless the pressure is too low. Therefore, the primary factor in determining a planet s habitability is temperature. The planet temperature is defined by the balance between absorption of stellar radiation (neglecting internal sources) re-radiation of it The stellar energy absorbed by the planet is 2 L* E (1 ) a Rp a, 2 4 d where L* is stellar luminosity, Rp is radius of the planet, d is distance between the planet and the star, and a is albedo. The amount of energy to be radiated by the planet can be expressed as L 4 R T, 2 4 p p e where Te is effective temperature of the planet. The balance between Lp and Ea defines the effective temperature of the planet. The effective temperature of the Earth is 255 K. It is lower than 273 K because the Earth is not a perfect black body and it traps some energy because of the greenhouse effect. If the Sun would be more luminous, to maintain the same effective temperature the Earth should be further away, and vice versa: Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-1

2 Figure 1: Left: the distance to the habitable zone versus stellar luminosity calculated from the balance between L p and E a. Right: the evolutionary track of a solar-type star starting on the Main Sequence and continued through the Red Giant and Horizontal Branch phases to the Asymptotic Giant Branch. The luminosity of the star increases by several orders of magnitude. 4 Gyr ago the solar luminosity was lower by 30%, as main-sequence stars move up along the sequence and further to the red giant phases habitable zone was closer to the Sun habitable zone is moving away from the Sun The continuous habitable zone (CHZ) is the region, in which a planet may reside and maintain liquid water throughout most of a star s life. Figure 2: Continuous habitable zone: yellow region is the habitable zone in the beginning of the main sequence, blue region is the habitable zone at late stages of the stellar evolution, and the green region is an intersection of the other two. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-2

3 Figure 3: Evolution of the solar luminosity (left) and the HZ (right). The most realistic model of the HZ is depicted by long dashes (Kasting et al. 1993). To estimate realistically the CHZ for the Sun, one needs to take into account albedo greenhouse effect The inner edge of the CHZ is determined by photodissociation of water and loss of hydrogen AU The outer edge of the CHZ is mainly defined by formation of CO2 clouds AU This contradicts however to the fact that surface of Mars was once carved by streams of some flowing liquid. The used climate model should be modified by adding other greenhouse gases (e.g. CH4) and perhaps by including a more dense cloud cover on Mars. Tidal heating of satellites around giant planets, such as Jupiter s satellite Europa, raises the possibility of liquid water existing below the surface of ice-covered satellites. Sun in time To understand the early phase of the planet formation and the evolution of the habitable zone in the Solar system, we need to study the effects of the young Sun on planets. This is possible with a sample of stars of about the same mass as the Sun but at different evolutionary stages. A study of such stars shows that the solar radiation has undergone dramatic variations since it arrived to the Main Sequence. For instance, the X-ray luminosity of the young Sun (~10 8 years) was 1000 times of the present level. Fig. 4 shows the X-ray luminosity of solar-type stars versus the age. In Fig. 5 this is compared with the radiation in other wavelengths. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-3

4 Figure 4: The X-ray luminosity of solar-type stars versus the age. Note the decrease by three orders of magnitude over 10 Gyr. Figure 5: The X-ray, UV and total luminosity of the Sun versus age as deduced from solar analogues. The young Sun had UV radiation factor of 10 to 100 higher than the present level, while its total luminosity was in fact only 70% of the present. Magnetic activity of the Sun (flares, coronal mass ejections, solar wind, etc.) was also much more prominent as compared to the Sun today. These factors have significantly influenced the planetary atmospheres, their composition and density. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-4

5 Effects of Solar magnetic activity on terrestrial planets Mercury 0.39 AU from the Sun Variation of the mean density with diameter of the terrestrial planets as well as the Moon (shown on the right) indicates that Mercury has a much higher mean density than expected given its size. This is due to the relative size of its iron core which is significantly larger than for any other terrestrial planet. One possible explanation is that Mercury s lighter mantle/crust was eroded away by the strong (~1,000 times present values) winds and the early Sun s higher extreme ultraviolet fluxes. The eroded and ionized material has been carried away with the solar wind. Figure 6: The mean density of terrestrial planets versus the diameter. Mercury significantly deviates from the common tendency. Venus 0.71 AU from the Sun No water or oxygen. The explanation is in photochemistry/photoionization effects: Venus has a slow rotation period (P= 243 days) and a very weak magnetic dynamo Venus is thus not protected from the Sun s plasma by planetary magnetic field Perhaps the young Sun s enhanced activity and UV flux played a major role: e.g. Photodissociation of water H2O H + OH, and subsequent loss of H. Earth 1 AU from the Sun A Young active Sun may have played a major role in the evolution of the Earth s atmosphere and possibly the origin and evolution of life. Destruction of methane (CH4) by the early Sun s strong UV radiation Formation of ozone (O3) via photoionization of O2 Photochemical reactions leading to the formation of organic molecules Erosion of the atmosphere due to enhanced solar wind: loss of H +, O ++, N + Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-5

6 Mars 1.52 AU from the Sun Today, Mars is a cold dry planet with a thin atmosphere rich in CO2. Mars also possesses a very weak magnetic field. There is also geological evidence of running water and possibly a permanent layer of permafrost. It is important to study the effects of the active young Sun on Mars: Loss of water and atmosphere Soil oxidation Possible early life Early Mars: liquid iron core produced a magnetic field strong enough to protect the young Martian atmosphere and surface water from the punishing effects of the young Sun s intense solar wind. (Lammer et al. 2003). Roughly 3.5 Billion years ago, Mars core solidified, shutting down the Martian magnetic dynamo. Consequences: Without a magnetic field, the outer Martian atmosphere was subjected to the ionizing effects and strong winds of the sun, and began to erode. At this time, water disassociates into 2H+O, where the lighter hydrogen is lost to the space while the heavier oxygen combines with iron on its surface Habitable zones around other stars In principle low-mass stars (dwarfs) are prime candidates for searches of planets in habitable zones: Long life times on the Main Sequence (>10Gyr, see figure below) Very abundant in the solar neighborhood (>70%) Better contrast star/planet for detection M.; F M.; G2 1.4 M F M G2 log L/L stable lifetime ~2.5 Gyr stable lifetime ~8 Gyr stable lifetime >20 Gyr stable lifetime >40 Gyr 0.7 M, K M.; K M, M M.; M e+9 1.0e e e+10 Age (years) Figure 7: Variations of the total luminocity for stars of different masses and spectral classes. Low-mass K-M dwarfs have long periods of constant luminosity and are therefore prime candidates for searches of planets in habitable zones. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-6

7 Stars later than F0 have main sequence lifetimes exceeding 2 Gyr. They are therefore potential candidates for harboring habitable planets. The HZ around an F star is larger and occurs farther out than for our Sun. The CHZ of F stars are narrower (log distance) than for the Sun because they evolve more rapidly. the HZ around K and M stars is smaller and occurs closer to the star. Nevertheless, the widths of all of these HZs are approximately the same in log distance. the CHZs around K and M stars are wider (in log distance) than for our Sun because these stars evolve more slowly. Planets orbiting late K stars and M stars may not be habitable, however, because they can become trapped in synchronous rotation as a consequence of tidal interaction. Lower UV flux of M dwarfs implies smaller planetary atmosphere erosion. However, young M dwarfs are extremely active and stay active for longer periods of time! Potential for very severe erosion of atmospheres due to X-rays, flares, etc. Mid-to-early K stars should be considered along with G stars as optimal candidates in the search for extraterrestrial life. Figure 8: Diagram showing the zero-age main-sequence habitable zone (solid curves) as a function of stellar mass. The long-dashed lines delineate the probable terrestrial planet accretion zone. The dotted line represents the distance at which an Earth-like planet in a circular orbit would be locked into synchronous rotation within 4.5 Gyr (Kasting et al. 1993). Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-7

8 5.3 Planetary atmospheres: Solar system General remarks The Solar system contains four major planets of the terrestrial type and four of the jovian type. The jovian planets are orbited by tens of satellites. Some of these have sizes that would easily classify them as planets were they to orbit the Sun directly. Pluto is now classified as a minor body. Substantial atmospheres are found on all major planets except Mercury, and on Titan, Saturn s largest satellite. Thin atmospheres are found on Mercury, Pluto and Triton, Neptun s largest satellite. The atmospheres are divided on three types: highly oxidized Earth-like atmospheres (CO2, O2) mildly reduced atmospheres of Titan, Pluto and Triton (CO/ CH4 ~ ) highly reduced atmospheres of the giant planets (CO/ CH4 ~ 10 6 ) The element is called oxidized, when it saturates all its chemical bounds with oxygen (CO2). The element is called reduced, when it saturates all its chemical bounds with hydrogen (CH4). Correspondingly, the atmosphere is called oxidized or reduced and the conversion process is oxidation or reduction. Three most abundant species of planetary atmospheres: Oxidized atmospheres: Venus Earth Mars Mildly reduced atmospheres: Titan Pluto Highly reduced atmospheres: Jupiter Saturn Uranus Neptune CO2 (0.96) N2 (0.78) CO2 (0.95) N2 (0.95) N2 (0.98) H2 (0.864) H2 (0.885) H2 (0.85) H2 (0.85) N2 (0.035) O2 (0.21) N2 (0.027) Ar (<0.07) CH4 ( ) He (0.136) He (0.115) He (0.15) He (0.15) * H2O is probably more abundant than CH4 below the visible clouds. SO2 ( ) Ar (0.0093) Ar (0.016) CH4 (0.04) CO (0.001) CH4 ( )* CH4 (0.005)* CH4 (0.02)* CH4 (0.02)* None of the known atmospheres resembles the ISM or comets Most massive (jovian) planets have preserved their initial composition, as it is impossible even for the lightest H to escape in significant quantities The atmospheres of the smallest planets are subject to the escape of part of the lightest gases Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-8

9 The escape process has important consequences for the time evolution of the composition the planetary atmospheres. The difference in atmospheres of terrestrial and jovian planets roots its origin in the planet formation history: o larger jovian planets were able to retain gases from a solar nebula o smaller terrestrial planets were unable to secure a significant amount of gases since the solar wind from the nearby sun blew the hydrogen and helium gases away o subsequent volcanism, outgassing and comet bombardment continuously replenished and added gases: H2O, CO2, N2, H2S, SO2. Atmospheric structure troposphere: T with altitude stratosphere: T with altitude mesosphere: T with altitude thermosphere: T with altitude Figure 9: Atmospheric structure: the variation of temperature with altitude for (a) Venus, (b) Earth, and (c) Mars. Note that only Earth has a stratosphere. Heating internal sources sunlight greenhouse effect Cooling molecular processes (ionization, dissociation, emission) convection Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-9

10 Venus The escape process influenced the formation and chemical evolution of the atmosphere on Venus (and other terrestrial planets). On early Venus liquid water was evaporated and transported to higher altitudes, where it was photodissociated. Most of the H atoms escaped, but the oxygen remained in the atmosphere and reacted with other species. The decrease of amount of liquid water reduced the capability to store carbondioxide (on Earth most CO2 is stored in the lithosphere as carbonates). An increasing CO2 abundance raised the surface temperature, as well as the rate of evaporation. This processes entered a runaway state, resulting in the observed dry CO2 rich atmosphere. thick atmosphere CO2 resides in the atmosphere chemical composition is determined by o equilibrium chemistry in the low atmosphere and at the surface (0 60 km) o photochemistry in the upper atmosphere ( km) thick and global cloud deck (concentrated sulfuric acid + others) slow atmospheric cycle of sulfur strong greenhouse effect surface pressure of 90 bars (1bar=10 5 Pa) surface temperature of 730 K no organic molecules (not even methane) Earth Venus and Earth are very similar in terms of size and density. In fact the total amount of CO2 is also similar for both planets, but CO2 resides in the lithosphere (carbonate rocks) many others are dissolved in water (NaCl in the oceans) the only planet with liquid water on the surface H2O cycle: interaction between the oceans and the atmosphere surface pressure of 1 bar surface temperature 288 K Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-10

11 Mars very thin atmosphere the surface pressure is about 6 mbar surface temperature varies from 170 K to 290 K transparent to all solar radiation no confirmed detection of any organic species chemical composition is similar to that of the Venus atmosphere Methane is not expected to be present in large amounts. If detected, it either originates from outgassing, biological sources, or both. Local enhancement of the methane abundance should be searched for. Outgassing events can trigger a whole series of chemical reactions in the presence of the UV radiation leading to synthesis of other organic molecules, such as formaldehyde and methanol. Titan Titan is one of the most interesting and complex objects in the Solar System when it comes to organic molecules in the atmosphere. Being a moon of Saturn, it is actually larger than both Mercury and Pluto. The atmosphere of Titan has some similarities to the Earth s. surface pressure of 1.5 bar N2 is the main atmospheric constituent CH4 is the second most important molecular species It is believed that the composition of the current Titan atmosphere is similar to that of the Earth s atmosphere before the appearance of life. Crucial differences to the Earth: low surface temperature on Titan, 94 K water vapor is kept out of the atmosphere. solid body is less massive by a factor of about 44 the atmosphere of Titan is less bound to the planet atoms and molecules escape easier Specific features: In the higher stratosphere, UV sunlight causes photolysis of methane and subsequent chemical reactions which lead to the formation of more complex molecules In the lower stratosphere and upper troposphere, low temperatures result in the condensation of organics onto the haze particles. A global haze layer exist in the lower stratosphere Organics could be transported down to the surface Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-11

12 The presence of areas (lakes, ponds) of liquid methane/ethane is possible Methane is important greenhouse gas. If its supply stopped, the atmosphere could collapse. The present abundance of methane and the rate of its destruction in the upper stratosphere requires a permanent source of methane, e.g. o evaporation from lakes o diffusion from a sub-surface ocean o cryovolcanism There could be a methane cycle in the atmosphere (clouds, rains) The Huygens space probe (NASA/ESA/ISA) landed on Titan on14 January 2005, collecting data as its parachutes slowed it down. The mission only lasted about 3 hours hours to descend to the surface, and then another half-hour on Titan s surface before the batteries ran out. During its descent, Huygens' camera captured many images, and its other five instruments sampled Titan's atmosphere to determine its composition. This is the first probe that has ever been landed in the outer solar system. Once Huygens landed, it measured the wind, weather, energy flux and surface features, relaying the information back to the Cassini spacecraft. Figure 10: Image of the Titan surface transmitted by the Huygens space probe. Pluto and Triton Pluto and Triton are very similar in terms of mass and composition, and are located in the outer, colder parts of the Solar System. Pluto s atmosphere was discovered in 1980s, from stellar occultation measurements. surface pressure of bar surface temperature 36 K large quantities of frozen nitrogen on the surface N2 in the atmosphere destruction of methane and ethane can produce more organics Triton s atmosphere was detected by Voyager 2 in 1989 N2 and methane are main components similar pressure as on Pluto photochemical hazes throughout the atmosphere Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-12

13 Giant planets Observable parts of the giant planet s atmospheres: methane is the most abundant organic molecule other organics if formed by UV photolysis of methane in the upper stratosphere molecular hydrogen is a background gas supplying hydrogen for chemical reactions the main stable products are ethane, acetelyne and polyacetelynes (C2nH2) vertical motions in the atmosphere transport the molecules to different layers at deeper layers (troposphere) the complex molecules transformed back to methane clouds and hazes composed of hydrocarbons clouds of ammonia ice and water The two smaller giants, Uranus and Neptune, have the highest methane abundances. These giants have accreted less of the solar composition gas after the formation of their ice-rich cores. The enrichment in the heavy elements is therefore much higher than for Jupiter and Saturn. Jovian system The icy satellites of Jupiter embedded in the energy-rich Jovian radiation belt complex are potential reservoirs of organic material. Detected molecules: Io Europa Ganymede Callisto SO2, SO3, H2S?, H2O? H2O, SO2, CO2, sulfate salts, H2O2, H2SO4 H2O, O2, CO2, CH, SO2, O3 H2O, SO2, SH, CO2, CH 5.4 Planetary atmospheres: Exoplanets Detection Atmospheres of extrasolar planets can be detected during transit events: Figure 11: The brightness of the star is reduced during the transit of the planet over the stellar disk. Some of the stellar light is blocked by the planet Some light goes through the planetary atmosphere Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-13

14 Figure 12: The light passing through the planetary atmosphere during the transit is partly absorbed and can be detected as an excess of line absorption in the stellar spectrum compared to the one outside the transit. Figure 13: Detection of the atmosphere on HD209458b by measuring the absorption excess in Na I lines (Charbonneau et al. 2000). Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-14

15 Figure 14: Detection of the absorption excess in various spectral lines due to planetary atmosphere. Left: Charbonneau et al. (2000), right : Schneider (2005). Day and night side of extrasolar planets NASA's Spitzer Space Telescope has made the first measurements of the day and night temperatures of a planet outside our solar system. The infrared observatory revealed that Andromedae b, a hot Jupiter circling very close to its host star with an orbital period of 4.6 days, is always as hot as fire on one side, and potentially as cold as ice on the other. Scientists believe the planet is tidally locked to its star. This means it is rotating slowly enough that the same side always faces the star, just as the same side of Earth's tidally locked moon always faces toward us, hiding its "dark side." However, since this planet is made of gas, its outer atmosphere could in principle be circulating much faster than its interior. The observed temperature difference between the two sides of Andromedae b is extreme about 1,400 degrees Celsius. This is unlike Jupiter, which is eventemperatured all the way around. Such a large temperature difference indicates the planet's atmosphere absorbs and reradiates sunlight so fast that gas circling around it cools off quickly, i.e. reemission is faster than the circulation than the time scale it takes to evenly distribute temperature within the atmosphere. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-15

16 Figure 16: The top graph consists of infrared data from NASA's Spitzer Space Telescope. It shows that Andromedae b, always has a giant hot spot on the side that faces the star, while the other side is cold and dark. All data points are actually the results of averaging observations over many orbits, always taken at the same phase (error bars indicated). The artist's concepts above the graph illustrate how the planet might look throughout its orbit if viewed up close with infrared eyes. The bottom graph and artist's concepts represent what astronomers might have seen if the planet had bands of different temperatures girdling it, like Jupiter, with no difference between the average temperatures of the sunlit and dark sides to detect. Evaporation of the planetary atmospheres Detection of an extended hydrogen envelope on HD209458b in Lyman alpha line with Hubble Space Telescope (HST): atmosphere extends over 200,000 km hydrogen is unbound to the planet and escapes with velocity ~ 100 km/s loss rate ~ 10 7 g/s explains very few detections of planets close to the parent star. Those planets should quickly evaporate, or become hydrogen-poor Neptune-mass planets. Figure 15: Hot Jupiters being extremely close to the host star loose their atmospheres at high rate and most probably look like comet-like objects (artist impression). Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-16

17 Predicted spectra of hot Jupiters Infrared spectra of hot Jupiters with solar metallicity are dominated by CO, H2O, CH4. The features in the visible are alkali absorption lines Na I and K I (see figure below). Higher metallicity of the planet increases the absorption of the incoming stellar flux leading to warmer atmospheres Ti-condensation is not possible gaseous TiO (and VO) is free in the atmosphere Strong absorption by TiO/VO leads to a stratosphere Temperature profiles in the planetary atmosphere for different metallicity ratios are shown on the right. Spectra of the planet with a stratosphere are dramatically different. They are still dominated by H2O and CO in the infrared. BUT: all molecular features are seen in emission, including strong CO emission at 4.5 μm (figure on the right): The planet is brighter if it has a stratosphere Planets with stratospheres are most probable to be detected directly Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-17

18 References: Roos-Serote, M. Organic molecules in planetary atmospheres, in Astrobiology: Future Perspectives, 2004, Kluwer Roush, T.L., & organics, ibid. Cruikshank, D.P. Observation and laboratory data of planetary Gilmour I., & Sephton M.A. An introduction to astrobiology, 2003, Cambridge Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-18