Astronomy190 - Topics in Astronomy Astronomy and Astrobiology Lecture 6 : Atmospheric Science Ty Robinson

Questions of the Day How do atmospheres help make a world habitable? Why/how do the compositions of the atmospheres of Mars, Venus and Earth differ? How does the greenhouse effect work to warm a world’s surface? What factors affect the ability of a world to maintain its atmosphere?

Height of Atmosphere = 100km Scale Radius of Earth = 6,300km Height of Atmosphere = 100km

Height 1 Pressure [bars] At depth in the ocean? >1,000 bars Atmospheric Pressure Height 1 Pressure [bars] At depth in the ocean? >1,000 bars

Questions? What would happen to Earth’s oceans if we did not have an atmosphere? our oceans would sublimate the oceans will remain liquid, but expand our oceans would freeze through nothing Answer = c

An atmosphere is required for a world to have liquids on its surface! Water Phase Diagram An atmosphere is required for a world to have liquids on its surface! [bars] water on the Moon

Earth’s Atmospheric Composition Gas Amount [%] Nitrogen 78 Oxygen 21 Water 1 - 4 Carbon Dioxide 0.04 Methane 2 x 10-4 Ozone 7 x 10-6 Nitrogen fixing, diazotroph [di : two ; azot : Nitrogen ; troph : eat] Dimethyl sulphide

Surface Pressure [bars] Venus, Earth and Mars Surface Pressure [bars] 0.01 1 100 Composition [%] Nitrogen 3 72 Oxygen 0.1 21 Water 0.03 1 - 4 0.01 Carbon Dioxide 95 0.04 97 Methane ? 2 x 10-4 Ozone 7 x 10-6

Volcanic Gasses Nitrogen Water (> 60%) CO2 (>10%) SO2, H2S, H2 …

Earth’s Atmospheric Structure : Troposphere Questions Why is the surface the warmest part of the troposphere? Absorption of solar radiation at the surface Lapse Rate 8º C per km What happens to water vapor as you lift it higher into the atmosphere? The gas cools and condenses to form clouds.

Earth’s Atmospheric Structure : Stratosphere Question Why does the temperature increase with height in the stratosphere? UV light O3 (absorption) O2 O

Earth’s Atmospheric Structure : Mesosphere How do the structure and composition of the atmosphere determine surface conditions?

Transmission/Absorption of UV and Vis Light In the UV and visible… (Sun) (heating) Ozone Layer Cloud (scattering) Cloud (scattering) (heating) Surface

Albedo (Bolometric) Albedo = Reflected Light / Incident Light Albedo 0.15 0.37 0.65 4πR2σTe4 = S(1-A) πR2 Effective Temperature [ºC] -55 -17 -43 Now for the infrared (IR)…

Transmittance : how well a gas transmits radiation Greenhouse Gasses Transmittance : how well a gas transmits radiation Low transmittance implies the gas is a good absorber Important greenhouse gasses (on Earth): H2O CO2 O3 CH4

In the infrared… Ozone Layer (absorption) Cloud (absorption) Greenhouse Effect I In the infrared… Ozone Layer (absorption) Cloud (absorption) (absorption) Cloud (absorption) (emission) Surface

In the infrared… Ozone Layer (emission) Cloud (emission) (emission) Greenhouse Effect II In the infrared… Ozone Layer (emission) Cloud (emission) (emission) Cloud (emission) Surface (absorption)

Effective Temperature [ºC] -55 -17 -43 Surface -50 15 470 Greenhouse Surface Temperatures Effective Temperature [ºC] -55 -17 -43 Surface -50 15 470 Greenhouse Warming [ºC] 5 32 513

Surface Pressure [bars] Atmospheric Escape Why do worlds even have atmospheres? Surface Pressure [bars] 0.01 1 100

Questions? What factor(s) determine a world’s ability to retain an atmosphere? temperature, gravity, magnetic field and atmospheric composition planetary composition gravity and atmospheric composition magnetic field Answer = a

velocity gravity Earth’s magnetic field prevents the Gas Escape I Earth’s magnetic field prevents the Solar wind from stripping away our atmosphere. velocity Each molecule in Earth’s upper atmosphere has a velocity (which depends on its mass and surrounding temperature). If this velocity is large enough, a molecule can escape Earth’s gravity. gravity

Mercury: weak magnetic field, low gravity, high Gas Escape II Mercury: weak magnetic field, low gravity, high temperature : no atmosphere Venus: no magnetic field, high gravity, relatively high temperature : thick atmosphere even after a great deal of gas has escaped Moon: no magnetic field, low gravity, same distance from the Sun as Earth : no atmosphere

Gas Escape III H atoms Earth Features on the Earth’s disk (dayglow from the sunlit atmosphere, a northern auroral oval, and equatorial airglow) are due to the emission of atomic O at 130.4 and 135.6 nm and emission of molecular N2 (140-170 nm). 93,000 metric tonnes of hydrogen escapes each year. 9.3x10^10 g/(3 x 10^7 s/yr) ~ 3x10^3 g/s = 3 kilograms per second

Why does Mars have such a thin atmosphere? Gas Escape IV Question Why does Mars have such a thin atmosphere? Massive impacts can also drive off atmospheric gasses, especially on worlds with low gravity.

Questions of the Day How do atmospheres help make a world habitable? Why/how do the compositions of the atmospheres of Mars, Venus and Earth differ? How does the greenhouse effect work to warm a world’s surface? What factors affect the ability of a world to maintain its atmosphere?

Quiz 1 - Consider two stacked sedimentary layers. The lower layer has been determined to be 1.5 Gyrs old. Most likely, how was this age determined? What can we deduce about the upper layer? 2 - Describe the different processes that can affect visible radiation as it passes through Earth’s atmosphere. 3 - What is one thing you did not understand from today’s lecture?