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Atmospheres: Composition AST111, Lecture 6a This natural color image shows Titan's upper atmosphere -- where methane molecules are being broken apart by solar ultraviolet light and the byproducts combine to form compounds like ethane and acetylene. The haze preferentially scatters blue and ultraviolet wavelengths of light, making its complex layered structure more easily visible at the shorter visible wavelengths. A movie sequence of images, shows movement of the haze layers over the course of a few hours (see PIA06223). Lower down in the atmosphere, the haze turns into a globe-enshrouding smog of complex organic molecules. This thick, orange-colored haze absorbs visible sunlight, allowing only perhaps 10 percent of the light to reach the surface. The thick haze is also inefficient at holding in and then re-radiating infrared (thermal) energy back down to the surface. Reverse greenhouse! Titan has a thicker atmosphere than Earth, but the thick global haze causes the greenhouse effect there to be somewhat weaker than it is on Earth. Images taken with the Cassini spacecraft wide-angle camera using red, green and blue. The images were obtained at a distance of approximately 9,500 kilometers (5,900 miles) from Titan on March 31, 2005. This lecture ❑Observed molecules in different atmospheres ❑Phases of molecules ❑Condensation and the Clausius-Clapeyron equation ❑Oxidizing vs Reducing atmospheres ❑ Using heavy noble gases as tracers of initial composition ❑More on greenhouse effect Earth, Venus and Mars • Earth’s atmosphere is primarily Nitrogen (78% by volume) and Oxygen (21%) but also contains CO2, H20, Ar. • Mars and Venus are dominated by CO2(98%) • Ozone has been identified both on Mars and Earth • Spectra from the planets varies with latitude. • All three planets have deep CO2 absorption bands, much of the atmospheric opacity is from CO2 • On Earth much of the atmospheric opacity is from water. Titan • • • • Titan is often discussed with terrestrial planets because its atmosphere is thick and contains many of the same molecules Titan is dominated by N2 which shows up in the UV spectrum as strong line emissions. There is also some CH4, CO2, CO and many hydrocarbons Unidentified molecular haze, Keck adaptive optics image of Titan, Saturn's largest moon, possibly produced by obtained in infra-red light. The bright crescent at the southern rim (bottom of image) is due to scattering of photoelectric processes sunlight by hazes in Titan's atmosphere. Titan's surface consists of highlands or continents made of ice. Some of the dark regions seen in the northern hemisphere are lakes or seas made of liquid hydrocarbons. Titan’s Haze Cassini July 29, 2004 Titan is encircled in purple stratospheric haze. This image shows two thin haze layers. The outer haze layer is detached and appears to float high in the atmosphere. The image was taken using a spectral filter sensitive to wavelengths of ultraviolet light centered at 338 nanometers. The image has been falsely colored: The globe of Titan retains the pale orange hue our eyes usually see, and both the main atmospheric haze and the thin detached layer have been brightened and given a purple color. The best observations of the layer are made in ultraviolet light because the small haze particles scatter short wavelengths more efficiently than longer visible or infrared wavelengths. The haze is at altitudes above 400 kilometers, where ultraviolet light breaks down methane and nitrogen molecules. The products are believed to react to form more complex organic molecules containing carbon, hydrogen and nitrogen that combine to form the very small particles seen as haze. Titan's Mottled Surface PIA20016 From NASA's Cassini spacecraft, acquired during the mission's "T-114" flyby on Nov. 13, 2015. The visual and infrared mapping spectrometer (VIMS) made these observations, blue represents wavelengths at 1.3 microns, green 2.0 microns, and red 5.0 microns. A view at visible wavelengths would show only Titan's hazy atmosphere. Near-infrared wavelengths penetrate the haze and reveal the moon's surface. Planets and Satellites with Tenuous Atmospheres • All objects contain atmospheres of some kind, however it could be so tenuous that is very difficult to detect • Continuous bombardment of energetic particles from the solar wind, micro-meteorites and magnetosphere kicks up atoms and molecules from the surface. • Sputtering forms a corona which can sometimes be seen in resonant emission lines • For small bodies, the acceleration from gravity is small so the tenuous atmospheres can have large scale heights Mercury and the Moon • Mercury’s atmosphere was first detected from Space with Mariner 10’s airglow spectrometer. Later on ground based observations in Sodium have also detected it. • It consists of atomic species Na, K, Mg, O • A few thousand per cm3. • H, He are captured from the solar wind. • Apollo spacecraft detected a similar atmosphere on the Moon with mass and UV spectrometers • The Moon’s atmosphere is denser in the day time. Lunar Exosphere Images of the lunar sodium exosphere during total lunar eclipses. (Mendillo et al., Observational Test for the Solar Wind Sputtering Origin of the Moon's Extended Sodium Atmosphere, Icarus, 137, 12-23, 1999.) Mercury’s tenuous atmosphere Spectroscopic measurements of sodium in Mercury's atmosphere are shown above. High abundance of Na is red, low abundance blue. The dotted line around the planet represents the "seeing disk". Spectrograph slit measurements have been interpolated to portray an "image". From Anne Sprague’s website. Pluto and Triton • Both extremely cold 40-60K • However, surface temperatures are high enough to partly sublime some ices N2, CH4(methane), CO2 • The amount of vapor on these objects can be estimated from the equations for vapor pressure equilibrium. • N2 is believed to be the dominant molecule in Pluto’s atmosphere • We knew Pluto has an atmosphere because of stellar occultations. Io • Io’s atmosphere consists primarily of sulfur dioxide! There is also SO2 ice on the surface. • Other gases SO, Na, K, O. • Cl inferred from its presence in the plasma torus. Io’s plasma torus Four ultraviolet observations of the Io torus spanning 1 October to 14 November, 2000, during the Cassini Jupiter flyby. The torus is a donut of glowing gas orbiting Jupiter, composed mostly of sulfur and oxygen gases from Io's volcanic eruptions. Because the atoms giving off the light are trapped by Jupiter's tilted magnetic field, the torus wobbles back and forth during the course of a Jupiter day. Europa and other icy satellites • Europa is covered by water ice. • Has an oxygen rich atmosphere. • Sputtering knocks H2O apart. Hydrogen is more likely to escape. This leaves oxygen behind. • A tenuous Oxygen atmosphere has also been detected on Ganymede Atmospheres of Giant planets Mostly composed of Molecular hydrogen H2 At UV wavelengths opacity provided by Rayleigh scattering, you can only see the upper atmosphere • Optical and IR light is reflected from clouds • Opacity at longer wavelengths provided by molecular hydrogen excitation and by scattering from cloud particles • Ammonia absorbs in the radio Thermal infrared spectra reveal presence of H2O, CH4 (methane), NH3 (ammonia) and H2S (hydrogen Sulphide) on Jupiter. However mixing ratios measured by Galileo probe are difficult to account for and the abundances are less than expected Noble gasses such as Ar, Kr, Xe are 2.5 times solar! • • Saturn, Neptune and Uranus • • • • • • • CH4 and NH3 are detected on Saturn but only CH4 on Uranus and Neptune Heavy elements in general are more abundant on the more distant planets than Jupiter. There is evidence that the amount of gas incorporated into each planet depends on radial distance from the Sun. The balance of CO vs CH4 and N2 vs NH3 depends on temperature and pressure Even in Jupiter the upper atmosphere contains CO rather than CH4 Convection causes methane to rise and be heated, where it is converted to CO. Convection also accounts for the production of clouds at different heights. As the pressure drops, water clouds, followed by ammonia sulfide NH4SH followed by ammonia clouds. URANUS https://en.wikipedia.org/wiki/Atmosphere_of_Uranus#/media/ File:Tropospheric_profile_Uranus_new.svg Primitive Atmospheres • Young stars are observed to be surrounded by gas clouds • Young stars probably collapse from gas clouds. • The primitive solar nebula probably had abundances (composition) similar to the Sun. • However, how/when gas depletion occurred is complex. Grain growth during driving of photoevaporative winds? Inert Gases as Tracers of Early Conditions • Inert gases such as Ne are not produced by radioactive decay and are too heavy to escape thermally. • The amount of Ne probably hasn’t changed much. • However Ne/H, Ne/O, Ne/N abundance ratios on Earth, Mars and Venus are much smaller than Solar abundances. • Either primitive atmospheres were removed, swept away, or primitive atmospheres were initially very small. Secondary Atmospheres • Volcanos outgas, particularly in CO2 and H2O. • Over a long period of time, much of our atmosphere could have been made from volcanic outgasing. • Venus, Mars and the Earth all have evidence for volcanoes. • Iron rusts, removing oxygen. This could have caused a hydrogen rich atmosphere at early times. • There is little evidence for an oxygen rich atmosphere for 1/3 of the Earth’s lifetime. • Oxygen is produced by disassociation of water by UV light, and by photosynthesis. Phases of molecules depend upon pressure and temperature Liquid water Evolution of surface temperatures of Venus, Earth and Mars Cloud formation High: Temperature low Saturation partial pressure of water is low, Atmosphere cannot hold much water gas At this height the partial pressure of water is equal to the saturation pressure. Water condenses out of the air. Low: Temperature high Saturation Partial pressure of water is high. Atmosphere can hold lots of water gas Saturation Vapor Pressure The saturation partial vapor pressure at temperature T is given by the Clausius-Clapeyron equation. P = CL e Ls /(Rgas T ) Rgas the gas constant 8.314 𝗑107 erg K-1 mole-1 CL a constant — units pressure (bar) Ls the latent heat units erg/mole When the partial pressure of a molecule exceeds the saturation vapor pressure, the molecule condenses out of the atmosphere. A cloud forms. At the base of the cloud the partial pressure is equal to the saturation vapor pressure given by the Clausius Clapeyron equation. Cloud heights • Consider gas with a constant fraction of a particular molecule (e.g., water). Pressure and temperature decreases with height. Partial pressure is the contribution to the total pressure from water molecules. P = (Σnspecies)kT • The saturation vapor pressure is the maximum partial vapor pressure that this molecule can have. At some height the pressure is low enough that the partial pressure of the molecular is equal to the saturation pressure and molecules condense / clouds form. • The height where clouds form can be also be used to estimate the partial pressure or abundance of particular molecules. Cloud base example P = CL e Ls /(Rgas T ) The base of a methane (CH4 ) cloud in Uranus’s atmosphere is at a pressure level of 1.25 bars and a temperature of T=80K. The saturation vapor pressure of methane is given by the Clausius-Clapeyron equation with CL = 4.658 𝗑 104 bar and Ls =9.71 𝗑 1010 erg mole-1 Rgas the gas constant 8.314 𝗑107 erg K-1 mole-1 A. Estimate the partial pressure of CH4 at the cloud base. B. Estimate the volume mixing ratio of methane at the cloud base. Cloud base example(continued) The base of a methane (CH4 ) cloud in Uranus’s atmosphere is at a pressure level of 1.25 bars and a temperature of T=80K. The saturation vapor pressure is given by the Clausius-Clapeyron equation with CL = 4.658 𝗑 104 bar and Ls =9.71 𝗑 1010 erg mole-1 Rgas the gas constant 8.314 𝗑107 erg K-1 mole-1 A. Estimate the partial pressure of CH4 at the cloud base. The partial pressure is equal to the saturation vapor pressure at the cloud base because there methane has started to condense. The saturation vapor pressure is that given by the Clausius-Clapeyron equation. = 0.021 bar The ratio of the partial pressure to the total pressure is 0.021bar/ 1.25 bar = 0.017 Cloud base example (continued) B. Estimate the volume mixing ratio of methane at the cloud base. The volume mixing ratio is the fraction of molecules compared to total in a given volume The ratio of the partial pressure to the total pressure is 0.021bar/1.25 bar = 0.017 The idea gas law is P=nkT So the partial pressure divided by the total pressure gives the fraction of the number of molecules that are methane. The volume mixing ratio of methane is 0.017 Formation of Clouds • • • • Earth primarily has water clouds Giant planets have clouds of NH3, H2S, CH4 Mars has CO2 clouds Venus has H2SO4 clouds How do clouds modify the radiative energy balance? • Clouds can be highly reflective. Why Venus has a very high albedo. • Clouds block outgoing IR radiation, adding to greenhouse effect • Latent heat of condensation changes thermal structure of updrafts • As clouds condense, energy is released and this affects the lapse rate (dT/dz). Oxidizing vs Reducing Atmospheres CH 4 + H 2O 2NH 3 + 2H 2O H 2S + 2H 2O CO + H 2O CH 4 ↔ ↔ ↔ ↔ ↔ CO + 3H 2 N 2 + 3H 2 SO 2 + 3H 2 CO 2 + H 2 C + 2H 2 Giant planets Terrestrial planets High Pressure Low Pressure Atmospheric escape Carbon Dioxide on Earth • Volcanic outgasing brings CO2 into the atmosphere. • Carbon dioxide is removed from the Earth’s atmosphere by chemical reaction between CO2 dissolved in water with silicates. Urey weathering reaction. • The weathering rate is higher when the temperature is higher. The more CO2 removed, the cooler the planet stability. Carbon dioxide on Mars • Lack of plate tectonics means Mars is no longer producing CO2 and what was in the atmosphere could be condensed onto the poles, or absorbed as Carbon into the rock. Without much water, there is not much weathering, CO2 is not added to the atmosphere. • There is evidence of previous vulcanism so Mars must have had copious CO2 and water. . • Mars could have had a denser atmosphere and more water in the past. • Loss of water could be caused by impacts or large obliquity or large eccentricity of orbit. Carbon dioxide on Venus • Venus is extremely dry. • Oceans slowly evaporated and water disassociated slowly (moist greenhouse) (however current evaporation rate is estimated to be insignificant). • Or runaway greenhouse: All water turns to steam, high convection and water is disassociated at high altitude. • Once all the water on the surface of Venus is gone, then CO2 cannot be removed by weathering. The greenhouse effect ✓ L 16⇡ d2 ◆ 14 278K Teq ⇠ =p d(in AU) The terrestrial planets emit most of their light at infrared wavelengths. ❑ They would all be brightest near a wavelength of 10 µm. ❑ Solar heating arrives mostly at visible wavelengths, where the atmosphere is transparent. Created for Global Warming Art by Robert A. Rohde The greenhouse effect (continued) ❑ Infrared light is absorbed very strongly by molecules in the atmosphere, notably by water and CO2. ❑ Light can only escape directly to outer space through “windows”, of which the most important lie at wavelengths 8-13, 4.4-5, 3-4.2, 2-2.5, 1.5-1.8, and 1-1.4 µm. The greenhouse effect (continued) ❑ Hotter blackbodies shine more at shorter wavelengths, so if not enough light escapes at 3-5 and 8-13 µm, the surface heats up until enough of the emission leaks out in the shorterwavelength windows. ❑ This effect warms all three of the atmosphere-bearing planetary surfaces. The greenhouse effect (continued) If there’s liquid water on the surface, the greenhouse effect can be self-stabilizing, as water droplets form clouds that reflect sunlight. (CO2 forms neither droplets nor clouds.) If temperature rises, → more water evaporates into atmosphere → more clouds form → albedo increases → less sunlight reaches surface → temperature drops. And vice versa. But on Venus, sunlight and the greenhouse effect was sufficient to evaporate all of the water, leaving no liquid bodies on the surface. The greenhouse effect (continued) ❑ Liquid water dissolves carbon dioxide, both from the atmosphere and from rocks, creating carbonic acid: — H2CO3 (in solution H3O+ + HCO3 ) ❑ From there the carbon can be incorporated in carbonate minerals that can form readily in liquid water. • These days, this is done most readily on Earth by oceandwelling organisms, creating CaCO3 ❑ Thus if there is a lot of liquid water, carbon from CO2 will eventually be locked up in carbonate minerals, rather than allowed to be present in the atmosphere. • This is the case, for example, on Earth. • On Venus, though, the lack of liquid water let the CO2 remain in the atmosphere. The greenhouse effect (continued) ❑ Under solar ultraviolet illumination, water molecules high in the atmosphere dissociate readily, producing atomic hydrogen and oxygen. • Oxygen goes on to react with other molecules; hydrogen does not. ❑ Hydrogen is too light to be retained by Venus’s gravity, so it escapes, relatively quickly. • Soon there’s no more water, or possibilities for making any more water! A dead world. • And all the carbon and oxygen winds up in CO2, the atmosphere pressurizes, and the greenhouse effect cranks up to 735 K. A sterilized world. Titan’s Haze Inverse Greenhouse effect! Review • Primary vs secondary atmospheres, Oxidizing vs Reducing atmospheres • The composition of the atmospheres of different planets • Tenuous atmospheres • The formation of clouds, cloud base height • Phase diagrams • The Clausius-Clapeyron equation