METO 637

Lesson 21

Mars• Much of the surface is very old and cratered but

there are also younger rift valleys, ridges, hills and plains. No plate tectonics.

• There is no evidence of recent volcanic activity.• There is clear evidence of erosion including

large flood plains and small river systems. Liquid water is the obvious fluid.

• Evidence points to wet episodes, which occurred only briefly and about 4 billion years ago.

• Mars has a dense core, probably of iron with a high fraction of sulfur (iron sulfides)

Mars

• Smaller than the Earth, at a distance of about 1.5 AU from the Sun.

• Surface gravity is 3.71 m/sec compared with the Earth, 9.82 m /sec.

• Escape velocity is 5.0 km/sec (cf 11.2)• Weak magnetic field, but not global.• Has a highly eccentric orbit about the Sun.

Energy at the surface varies by 1.45. This brings about a temperature difference between aphelion and perihelion of 30 degrees.

Mass spectrometer data for Mars

Mars

• Mean insolation at the surface is about one half that at the Earth.

• As a result it is a colder planet – mean temperature of 220 K.

• Too cold for water to flow. Lack of an ocean results in an arid and dusty climate.

• Atmospheric pressure at the surface is an average of 7 mb. Atmospheric mass is less than one per cent of the Earth’s. However this is enough mass to support strong winds and vast dust storms.

• During the winter months only, ice crystal clouds appear

Surface pressure recorded by the Viking Landers

Mars

• Bulk atmosphere is composed of carbon dioxide (95%), with minor amounts of nitrogen (2.7%) and argon(1.6%).

• Trace amounts of molecular oxygen and water. • Mars was much like Earth in its early history.

Almost all of its carbon dioxide was used up to form carbonate rocks. But lacking plate tectonics it cannot recycle the rocks back into the atmosphere.

• The greenhouse effect is much smaller than that of the earth – low density of carbon dioxide.

Water vapor abundance as a function of latitude and season

Viking orbiter

Carbon dioxide stability

• Mars, like Venus, has an atmosphere whose bulk chemistry is dominated by carbon photolysis:

CO2 + hν(λ<204 nm) → CO + O• As on Venus we must again invoke a catalytic

route for the recombination of CO and O, since the direct recombination is spin forbidden.

• Water vapor is about ten times more abundant than on Venus. HOX can thus provide a recombination mechanism that is fast enough tocompete with CO2 photolysis.

Carbon dioxide stability

H + O2 + M → HO2 + M

O + HO2 → O2 + OH

CO + OH → CO2 + H

CO + O → CO2

And

O + O2 + M → O3 + M

H + O3 → O2 + OH

CO + OH → CO2 + H

CO + O → CO2

Carbon dioxide stability

• The odd-hydrogen compounds are supplied by photochemical decomposition of water vapor:

H2O + hν → OH + H

O(1D) + H2O → OH + H• The O(1D) is derived from the photodissociation

of CO2, O3, and O2

• There is a small net sink for H2O at low altitudes:

H + HO2 → H2 + O2

• This supply of H2 limits the rate at which H escapes from the exosphere.

Schematic of the HOX cycle

Model (lines) and Viking measurements (symbols) of major constituents

1D model predictions

1D model predictions