1. 1 Atmospheric Radiation – Lecture 11 PHY2505 - Lecture 20 Comparative atmospheres: Mars, Earth & Venus

2. 2 Atmospheric Radiation – Lecture 20 Comparative planetology Comparative planetology is the name given to an approach to studying the planets. This approach is based on the idea that the individual planets can be better understood by comparing the physical processes of all the planets. The basic physical ideas in our physical models for one planet must hold true in general for the other planets. Comparing the atmospheres of planets, particularly their thermal structures, gives us insight into the processes that drive climate. The terrestiral planets: Venus, Earth and Mars, formed at a similar time under similar conditions and yet their climates vary dramatically. A question is whether relatively small changes to the thermal structure in the Earth’s atmosphere could push it into the climate regime of either of its nearest neighbours.

3. 3 Atmospheric Radiation – Lecture 20 Effective temperature VenusEarthMars Distance from Sun (A.U.)0.7211.52 S=Flux, W/m226431370593 r=Albedo0.80.30.22 Effective Temperature, K220255212 Actual observed Temperature, K730288218 http://solarsystem.colorado.edu/cu-astr/home/lowRes.html

4. 4 Atmospheric Radiation – Lecture 20 Greenhouse hypotheses Primary atmospheres : the region of the solar nebula where terrestrial planets were formed was too hot for the condensation of volatiles such as CO 2 or H 2 O. These molecules either arrived as trace species, adsorbed on or captured in the interiors of the solids that gradually accreted to form the planets, or they were brought in by comets, from the region of the solar system beyond the snow line.

5. 5 Atmospheric Radiation – Lecture 20 Water on planets

6. 6 Atmospheric Radiation – Lecture 20 Greenhouse hypotheses VENUS Temperature of Venus initially higher than Earth Gases in atmosphere trap heat (greenhouse effect) Any water on surface evaporates and adds to greenhouse gases Subsequently water is broken down and H escapes Temperature rises even more Runaway greenhouse effect EARTH CO 2 comparable to Venus but adsorbed in surface by way of Urey reactions MARS Gravity weaker than Earth, secondary atmosphere sustained large losses through atmospheric escape Reverse greenhouse effect: planet cold, water freezes reducing greenhouse gases, freezes more, cools more until low pressure below the triple point of water

7. 7 Atmospheric Radiation – Lecture 20 Venus current atmosphere Composition (near surface, by volume) CO 2 96.5% N 2 3.5% Minor species (ppm) SO 2 - 150; Argon (Ar) - 70; Water (H 2 O) - 20; Carbon Monoxide (CO) - 17; Helium (He) - 12; Neon (Ne) - 7

8. 8 Atmospheric Radiation – Lecture 20 Earth current atmosphere Composition Nitrogen 78.08% Oxygen 20.95% *Water 0 to 4% Argon 0.93% *Carbon Dioxide 0.0360% Neon 0.0018% Helium 0.0005% *Methane 0.00017% Hydrogen 0.00005% *Nitrous Oxide 0.00003% *Ozone 0.000004%

9. 9 Atmospheric Radiation – Lecture 20 Mars current atmosphere Composition Carbon Dioxide (CO2) - 95.32% Nitrogen (N2) - 2.7% Argon (Ar) - 1.6% Oxygen (O2) - 0.13% Carbon Monoxide (CO) - 0.08% Minor (ppm): Water (H2O) - 210 Nitrogen Oxide (NO) - 100 Neon (Ne) - 2.5 Krypton (Kr) - 0.3 Xenon (Xe) - 0.08

10. 10 Atmospheric Radiation – Lecture 20 State of current measurements Current data MARS

11. 11 Atmospheric Radiation – Lecture 20 Climate problems: Venus Past climate: Magellan mapping of surface suggests recent geological activity: whole surface resurfaced 700M years ago – has this produced climate change? Current climate: Is the current climate stable? What governs formation of H 2 SO 4 clouds? Why are elevated winds so high? Outgassing of SO 2, CO 2 reactions with surface, - is Venus cooling?

12. 12 Atmospheric Radiation – Lecture 20 Climate modelling: Venus Two-stream radiative–convective model high-resolution spectral databases chemical/microphysical model of Venus’ clouds 1. How do variations in atmospheric water and sulfur dioxide affect cloud structure and planetary albedo? How do these, in turn, affect the temperature at the surface? 2. How does the equilibration of atmospheric sulfur dioxide with surface minerals affect cloud structure and surface temper-ature, and over what timescales? 3. How have changes in atmospheric water abundance due to exospheric escape of hydrogen and volcanic outgassing af-fected cloud structure and surface temperature, and over what timescales? 4. What was the effect on Venus’ cloud structure and sur-facetemperature of an epoch of rapid plains emplacement by widespread, global volcanism?

13. 13 Atmospheric Radiation – Lecture 20 Venus: results

14. 14 Atmospheric Radiation – Lecture 20 Venus: results

15. 15 Atmospheric Radiation – Lecture 20 Venus: results

16. 16 Atmospheric Radiation – Lecture 20 Climate problems: Mars Water & faint young sun paradox: definite dramatic climate change ~ 2Ga

17. 17 Atmospheric Radiation – Lecture 20 Climate problems: Mars Global dust storms – coupled feedbacks?

18. 18 Atmospheric Radiation – Lecture 20 Climate problems: Mars Issues: Past climate: Producing enough CO 2 to sustain liquid water Currrent climate Asymmetry of polar caps Feedback due to cloud and dust Orbital cycle

19. 19 Atmospheric Radiation – Lecture 20 Climate problems: Mars