Transcript 637Lesson21 - Atmospheric and Oceanic Science

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 to
compete 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