Planetary Atmospheres IIPlanets and PlanetarySystemsRENEE CONDORI APAZA, JULIOVALDIVIA SILVA, Christopher P.McKayThe Advanced Studies Laboratories
What is an atmosphere?, What is its structure? Temperature of a planet, neglecting effects ofatmosphere (“no-greenhouse temperatures”) Generic atmospheric structure Global climate change Earth Venus Mars
• Planetary atmospheres as a balancing act:– Gravity vs. thermal motions of air molecules– Heating by Sun vs. heat radiated back into space– Weather as a way to equalize pressures at different places on aplanet’s surface• Atmospheres of terrestrial planets are very different nowfrom the way they were born– Formation: volcanoes, comets– Destruction: escape, incorporation into rocks, oceans– Huge changes over a billion years or less• Prospect of human-induced global warming on Earth needsto be taken seriously
• Earth diameter~ 12,000 km• Top of troposphere~ 12 km• Thickness ofatmosphere dividedby Earth diameter = 1 /1000
• About 10 km thick• Consists mostly ofmolecular nitrogen (N2)and oxygen (O2)• Fractions:– 78% Nitrogen– 21% Oxygen– 0.04% CO2
• Small amounts of gas are present even at > 300 km
• Create pressure that determines whether liquid watercan exist on surface• Absorb and scatter light• Create wind, weather, and climate• Interact with solar wind to create a magnetosphere• Can make planetary surfaces warmer throughgreenhouse effect
“No-greenhouse”temperaturealbedo = fraction of sunlightthat is reflected by a surface
Conclusion: for Venus and Earth, at least, something else isgoing on! (not just radiation into space.
• Ionization: Removal ofan electron• Dissociation: Destructionof a molecule• Scattering: Change inphoton’s direction• Absorption: Photon’senergy is absorbed
• carbon dioxide CO2• water vapor H20• methane CH4• others too (NO2, ....)• More greenhouse gases in atmosphere canlead to higher surface temperatures.
• What would happen to Earth’s temperature ifEarth’s surface were less reflective?a) It would go up.b) It would go down.c) It wouldn’t change• This is one of the factors exacerbating globalwarming.– As more arctic ice melts in summer, arcticocean absorbs more light, temperaturerises
• Ultraviolet lightcan break upO2 molecules,allowing ozone(O3) to form• Without plantsto release O2,there would beno ozone instratosphere toabsorb UVlight
• Ultra-violet light from Sun dissociates oxygenmolecules O2 to produce O in stratosphere• O combines with O2 to form O3 (ozone)• Ozone in stratosphere absorbs harmful ultravioletlight from Sun, providing land-based life with aprotective shield• Manmade aerosols (chlorofluorocarbons, CFCs)can inhibit ozone formation• Result: “Ozone hole”
Antarctic ozone hole is worst in late southern winter
?• Why is ozone hole much deeper and larger inthe Antarctic than in the arctic?– During winter, clouds form in the Antarctic ozonelayer.– Chemical reactions on ice particles in the cloudsactivate ozone destroying substances.– In spring, these substances take part in chemicalreactions that destroy ozone.• Arctic: warmer than Antarctic (fewer iceclouds), more irregular air circulation
• Huge changes took place over the 4.6 billionyears since planets formed!• Early atmospheres didn’t resemble currentones at all• Question: why are atmospheres of Venus,Earth, Mars so different?
Condensationonto surfaceChemicalreactionswith surfaceLarge impactsblast gas intospace
Moon Mercury• Very sensitive measurements show Moon andMercury have extremely thin atmospheres• Gas comes from impacts that eject surface atoms
• Why are they so different?• Were they always this different from each other?
First Atmosphere: Primordial elementsComposition - Probably H2, He Today these gases are relatively rare on Earth compared to otherplaces in the universe. Were probably lost to space early in Earths history becauseEarths gravity is not strong enough to hold lightest gasesEarth still did not have a differentiated core (solid inner/liquidouter core) which creates Earths magnetic field(magnetosphere = Van Allen Belt) which deflects solar wind.Magnetosphere protects any atmosphere from the solar wind. Once the core differentiated, gases could be retained.
Gases similar to those frommodern volcanoes (H2O, CO2,SO2, CO, S2, Cl2, N2, H2) and NH3(ammonia) and CH4 (methane)No free oxygen (O2 not found involcanic gases)Ocean Formation - As Earthcooled, H2O produced byoutgassing could exist as liquidCO2 could then dissolve inocean, be sequestered inmarine sediments
Today, atmosphere is ~21% free oxygen. How did oxygen reach this level? Oxygen Production Photochemical dissociation - breakup of water molecules by ultraviolet light Produced O2 levels 1-2% current levels At these levels O3 (Ozone) could form to shield Earth surface from UV Photosynthesis: CO2 + H2O + sunlight = organic compounds + O2 -produced by cyanobacteria, and eventually higher plants - supplied the restof O2 to atmosphere. Oxygen Consumers Chemical Weathering - through oxidation of surface materials (earlyconsumer) Respiration (much later) Burning of Fossil Fuels (much, much later) Once rocks at the surface were sufficiently oxidized, more oxygen could remainfree in the atmosphere
The Carbon Dioxide Cycle 1. Atmospheric CO2dissolves in rainwater2. Rain erodes mineralswhich flow into ocean3. Minerals combine withcarbon to make rockson ocean floor4. Subduction carriescarbonate rocks downinto mantle5. Rocks melt in mantleand outgas CO2 backinto atmospherethrough volcanoes
Cooling allows CO2 to build up in atmosphereHeating causes rain to reduce CO2 in atmosphere
• The first photosynthesis–Consumes CO2,release O2
Chemical building blocks of life could not have formed in thepresence of atmospheric oxygen. Chemical reactions that yieldamino acids are inhibited by presence of very small amounts ofoxygen. Oxygen prevents growth of the most primitive living bacteriasuch as photosynthetic bacteria, methane-producing bacteriaand bacteria that derive energy from fermentation. Conclusion - Since todays most primitive life forms areanaerobic (don’t need oxygen), the first forms of cellular lifeprobably had similar metabolisms. Today these anaerobic life forms are restricted to anoxic (lowoxygen) habitats such as swamps, ponds, and lagoons.
Potential source of the Earths ocean water iscomet-like balls of ice.Enter atmosphere at rate of about 20/second.Four billion years of such bombardment wouldgive enough water to fill the oceans to theirpresent volume.Possible problems with isotope ratios. Underactive research.
Solar Brightening• Sun very gradually grows brighter with time, increasingthe amount of sunlight warming planets
Changes in Axis Tilt• Greater tilt makes more extreme seasons, while smallertilt keeps polar regions colder.
Changes in Reflectivity• Higher reflectivity tends to cool a planet, while lowerreflectivity leads to warming
Changes in Greenhouse Gases• Increase in greenhouse gases leads to warming, while adecrease leads to cooling
• Global temperatureshave tracked CO2concentration for last500,000 years• Antarctic air bubblesindicate current CO2concentration ishighest in at least500,000 years
• Most of CO2 increase has happened in last 50 years!
Increased Glacier retreatsince the early 1990sArea of seasonally frozenground in NH has decreasedby 7% from 1901 to 2002IPCC Report 2007
CO2, CH4, N2O Concentrations- far exceed pre-industrialvalues- increased markedly since1750due to human activitiesRelatively little variationbetween last Ice Age andthe currentindustrial eraIPCC Report 2007
Impacts are worseAlready more flood and drought proneLarger share of the economy is in climate sensitivesectorsLower capacity to adaptLack of financial, institutional and technologicalcapacityClimate change is likely to impact disproportionatelyupon the poorest countries.. and the poorest people within countriesNet economic effects expected to be negative in mostdeveloping countries
• No evidence for platetectonics on Venus– No mid-ocean rifts– No subduction trenches• Volcanos spread evenlyacross surface instead ofat plate boundaries, as onEarth.• Lithosphere not brokeninto plates; probablybecause heat at surfaceslightly softens thelithosphere
• Venus has far fewer impact craters than Moon & Mercury, but more thanEarth.– The dense atmosphere protects it from smaller impacts• Geologic activity (volcanic resurfacing) has erased much of the evidence• Surface age is only about a billion years.• Rather uniform age implies that Venus was "resurfaced" by lava flowsduring a recent, relatively short period• This differs profoundly from Earths crustal history. What is it telling us?– Could Venus present crust only have formed that recently?– Could there have been a growing crust before 1 billion years ago that"turned over" as heat built up underneath, to lead to a new era of majorlava flows?– Why?
• Geomorphological evidence (*lots* of it)– River and flood channels, alluvial fans, slumps, canyons, ...• One more piece of evidence: shape of ocean basins
• Evidence of previousera when liquid waterwas plentiful• Today: Evidence for icemixed with soil in topmeter of ground
• Mars has not hadwidespread surfacewater for 3 billionyears• Greenhouse effectprobably keptsurface warmerbefore that• Somehow Marslost most of itsatmosphere
One possible scenario• Magnetic field may have preserved early Martianatmosphere .• Solar wind may have stripped atmosphere after fielddecreased because of interior cooling (no more moltencore)
Shortly after Mars formed, its surface temperature was ~ equal to its blackbodytemperature (around -55 C). As volcanoes dumped CO2 and H2O vapor into atmosphere, greenhouse effectincreased temperature above 0 C (freezing) so liquid water could exist. Liquid water was present, so rocks could efficiently remove CO2 from atmosphere. Two competing effects determined amount of CO2 in atmosphere: volcanoesadding CO2, and rocks absorbing CO2. Result: moderate level of CO2 . Greenhouse effect could keep surface T > 0 C, as long as volcanoes kept erupting. Eventually Mars core cooled and solidified (Mars is small). Volcanic activitysubsided. Magnetic field went away, solar wind particles eroded atmosphere. Once rate of eruptions tapered off, CO2 in the atmosphere started to fall. As the atmosphere thinned out, the greenhouse effect weakened. Eventually theaverage surface temperature dropped, and surface water froze.
Balancing act between injection and removal ofCO2 from atmosphereRole of liquid water in sequestering CO2 Venus too hot Mars too cold Earth just rightWhat is evidence for these scenarios? Howcould you test them?