11/9/2011
Atmospheres of the Moon, Mercury and Mars
Mercury and the Moon
• Do Mercury and the Moon even have
atmospheres?
– Sort of
• Mercury and the Moon are not entirely
devoid of gas.
• Densities are so low that scattering and
absorption of photons is rare.
– Dark skies, no troposphere, stratosphere or
thermosphere
Mercury and the Moon
• The total amount of gas in the atmospheres
of Mercury and the Moon is very small.
• The low densities mean that collisions
between particles are rare
– Atmospheres extend several thousand kilometers
from their surfaces.
Mercury and the Moon
Source and Loss Processes
• Initial supply: volcanic outgassing
– Bodies are now geologically dead
• Most gas lost by solar wind stripping or
thermal escape
– High day-side temperatures and low mass
• The only ongoing source of atmospheric gas
is surface ejection
– Most gas escapes into space immediately
– Other atoms bounce off the surface a few times
before escaping
Ice
• Liquid water cannot exist on Mercury or the
Moon
– What about ice?
• Daytime temperatures are too high for ice to
survive
• Bottoms of polar craters are essentially in
perpetual shadow
– Evidence for ice
Mars
• Outgassing initially supplied a substantial
early atmosphere to the larger terrestrial
planets
– Makes atmospheric history much more complex
– Differences must arise from source and loss
processes
• Size is the most important factor
– Controls the level of outgassing and determines
the roles of other loss processes
• Mars is only 40% larger than Mercury
– Surface shows a much more complex history
Mars’ Atmosphere
Mars Today
• Mars’ surface resembles deserts and volcanic
plains on Earth
– Atmosphere makes it much different
• Pressure less than 1% than that on Earth
means water is not stable on the surface
• Mostly carbon dioxide, but small amount of
atmospheric gas produces only a weak
greenhouse effect
• Average temperature of only -50°C
• Little oxygen means no ozone layer
Martian Seasons and Winds
• Mars has seasons very similar to Earth’s due
to its similar axis tilt
– Seasons last almost twice as long because Mars’
year is almost twice as long as Earth’s
• Also affected by the eccentricity of its orbit.
– More eccentric than that of Earth
– Southern hemisphere experiences more extreme
seasons.
Martian Seasons
Martian Seasons and Winds
• Seasonal changes create the most significant
feature of Mars’ weather:
– Winds blow from the summer pole to the winter
pole
• Temperatures are cold enough at the winter
pole that CO2 condenses into dry ice at the
polar cap
• Frozen CO2 at the summer pole sublimates
– Differences in pressure drive strong winds
between the polls
Martian Seasons and Winds
• The direction of the pole-to-poles winds
changes with the seasons.
• These winds can sometimes trigger massive
dust storms.
– Especially during the extreme southern summer
• Storms can cover the entire surface of the
planet.
– Settling dust can change the reflectivity of large
portions of the surface
Martian Dust Storms
Martian Dust Devils
Water Ice on Mars
• Mars’ polar ice caps are largely composed of
frozen water
– A thin layer of dry ice, a few meters thick at most
• Much of the surface might also contain a
layer of water ice covered by dust.
Martian Polar Ice Cap
Surface Ice
The Martian Sky
• The color of the Martian sky is affected both
by scattering (like Earth’s atmosphere) as
well as absorption by dust.
• Scattering would tend to make the sky blue,
but low densities means it would be almost
black
• Absorption has the opposite effect
– Dust absorbs blue light and reflects red light
• The effect is that Mars’ sky is often pinkishbrown, but can be yellow, blue, or even
green.
Martian Sky
Climate and Axis Tilt
• Mars’ rotation axis varies much more wildly
than Earth’s
– From as little as 0° to as much as 60°
• Causes much more severe climate change
– Perpetually frozen when axis tilt is small
– Relatively warm when axis tilt is large
• Changes in global average temperature can
change atmospheric pressure
– Probably not enough for liquid water to be stable
on the surface
Why did Mars Change?
• We know that Mars once had a significant
amount of water on its surface, so what
happened?
– Axis tilt not enough
• Mars must have once had a much thicker
atmosphere with a stronger greenhouse
effect.
– Calculations and theoretical models predict that
outgassing could have created an atmosphere
400 times as dense as it is today
Why did Mars Change?
• Current models are unable to account for
enough carbon dioxide to provide the
needed greenhouse effect to support liquid
water.
– Additional warming may have been provided by
methane or carbon dioxide ice clouds
• The big question, then, is where all this gas
went.
Why did Mars Change?
• Mars must have lost most of its carbon
dioxide over time, which would weaken the
greenhouse effect until the planet froze
over.
– Some froze in the poles
– Some may have bonded to surface rock
– Most was probably lost to space
• Mars probably once supported a strong
magnetosphere similar to Earth’s.
– Substantially weakened as the interior cooled and
solidified
Why did Mars Change?
• Without a magnetosphere, the atmosphere
is subject to solar wind stripping.
• Much of Mars’ water is also probably gone
for good.
– No stratosphere means UV light easily reaches
the surface
– Breaks apart water molecules
• Leftover oxygen that doesn’t escape into
space is largely bound to the iron-rich
surface material
– “red planet”
Why did Mars Change?
• So, mot of Mars’ atmospheric changes are
due to its small size
– Interior cooling
– Low gravity
• Distance from the Sun is the last nail in
Mars’ coffin.
– If it were significantly closer to the Sun, it might
still be able to support liquid water.
Next Time…
Atmosphere of Venus
But first…
Quiz 9
1) Which of the following is not a source of
atmospheric gas?
a)
b)
c)
d)
Outgassing
Evaporation
Sublimation
Condensation
2) Why are Mars’ seasons more extreme than Earth’s?
a)
b)
c)
d)
It is farther from the Sun.
Its atmosphere is less dense.
Its axis tilt is significantly greater than Earth’s.
Its orbit is more eccentric than Earth’s.