1 atmospheric radiation – lecture 11 phy2505 - lecture 20 comparative atmospheres: mars, earth...

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Atmospheric Radiation – Lecture 11

PHY2505 - Lecture 20

Comparative atmospheres: Mars, Earth & Venus

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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.

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Effective temperature

Venus Earth Mars

Distance from Sun (A.U.) 0.72 1 1.52

S=Flux, W/m2 2643 1370 593

r=Albedo 0.8 0.3 0.22

Effective Temperature, K 220 255 212

Actual observed Temperature, K 730 288 218

http://solarsystem.colorado.edu/cu-astr/home/lowRes.html

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Greenhouse hypothesesPrimary atmospheres: the region of the solar nebula where

terrestrial planets were formed was too hot for the condensation of volatiles such as CO2 or H2O.

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.

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Water on planets

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Greenhouse hypothesesVENUSTemperature of Venus initially higher than Earth Gases in atmosphere trap heat (greenhouse effect) Any water on surface evaporates and adds to greenhouse gasesSubsequently water is broken down and H escapesTemperature rises even more Runaway greenhouse effect

EARTHCO2 comparable to Venus but adsorbed in surface by way of Urey reactions

MARSGravity weaker than Earth, secondary atmosphere sustained large losses through atmospheric escapeReverse greenhouse effect: planet cold, water freezes reducing greenhouse gases, freezes more, cools more untillow pressure below the triple point of water

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Venus current atmosphere

Composition(near surface, by volume)

CO2 96.5%N2 3.5%

Minor species (ppm)SO2 - 150;

Argon (Ar) - 70;

Water (H2O) - 20;

Carbon Monoxide (CO) - 17; Helium (He) - 12;

Neon (Ne) - 7

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Earth current atmosphere

CompositionNitrogen 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%

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Mars current atmosphere

CompositionCarbon 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

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State of current measurements

Current data

MARS

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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 H2SO4 clouds?

Why are elevated winds so high?

Outgassing of SO2, CO2 reactions with surface, - is Venus cooling?

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Climate modelling: Venus

Two-stream radiative–convective model high-resolution spectral databases chemical/microphysical model of Venus’ clouds1. 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?

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Venus: results

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Venus: results

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Venus: results

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Climate problems: Mars

Water & faint young sun paradox: definite dramatic climate change ~ 2Ga

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Global dust storms – coupled feedbacks?

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Issues:

Past climate:

Producing enough CO2 to sustain liquid water

Currrent climate

Asymmetry of polar caps

Feedback due to cloud and dust

Orbital cycle

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1 atmospheric radiation – lecture 11 phy2505 - lecture 20 comparative atmospheres: mars, earth...

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