10.1 Atmospheric Basics
An atmosphere is a layer of gas that surrounds a world
Collisions of individual atoms or molecules in an atmosphere create pressure
that pushes in all directions
Planetary atmospheres exist in a perpetual balance between the downward
weight of their gases and the upward push of their gas pressure
One bar (measurement of atmospheric pressure)= 1.03 kg/cm = pressure at sea
Greenhouses gases absorb infrared light, trapping this infrared light and slowing
its escape to space (greenhouse effect). They do this by absorbing and reemitting
infrared radiation, heating the lower atmosphere)
Troposphere stratosphere thermosphere exosphere
Interactions between atmospheric gases and energy from the sun: most light
comes from the sun in visible light, sun also emits significant amounts of UV light
and X rays, and planetary surface emits infrared light
The sky appears blue because the atmosphere scatters blue light much more
than red light, and the blue light reaches you from all directions when the Sun is
overhead. The sky is red at sunset or sunrise because sunlight passes through a
greater amount of atmosphere to reach you, which means most blue light is
scattered away, leaving red light.
Aurora: charged particles trapped in the magnetosphere that gains energy from
variations in the solar wind, which then follows the magnetic field all the way
down to Earth’s atmosphere where it collides with the atoms and molecules to
radiate and produce moving lights. Most common near the magnetic poles and
best viewed at high latitudes.
10.2 Weather and Climate
Low-pressure regions draw air inward and storms generally occur here.
Larger size and faster rotation contribute to a stronger Coriolis effect.
Thunderstorms are common on summer afternoons when the sunlight-warmed
surface drives strong convection (strong convection means more clouds and
Similarly, equatorial regions experience high rainfall because they receive more
Solar brightening: warm climates with time
Factors that can cause long-term climate change
o Changes in axis tilt: make seasons more or less extreme/ extra summer
warmth caused by greater tilt prevents ice from building up, which then
reduces the planet’s reflectivity and thereby makes the whole planet
warmer (Earth’s past period of smaller axis tilt correlate well with the
times of past ice ages)
o Changes in reflectivity: microscopic dust particles released by volcanic
eruptions (aerosols) can reflect sunlight (major volcanic eruptions
followed by planetwide cooling) o Changes in greenhouse gas abundance: if the planet warms from the
increase in greenhouse gas, increased evaporation and sublimation may
add substantial amounts of gas to the planet’s atmosphere, leading to an
increase in atmospheric pressure
A planet gains atmospheric gas through outgassing, evaporation/sublimation,
and surface ejection
A planet loses atmospheric gas through (condensation, chemical reactions,) solar
wind stripping, and thermal escape.
10.3 Atmospheres of the moon and Mercury
No atmosphere because their gas densities are far too low for sunlight to be
scattered or absorbed.
The only ongoing source of gas is the surface ejection- however this gas never
accumulates because some of them are blasted upward fast enough to achieve
escape velocity. The rest bounce around (a few dozen times), arcing into the
before crashing back down and being absorbed back into the surface.
10.4 The atmospheric history of Mars
Mars’ elliptical orbit affects its seasons
The atmospheric pressure increases at the summer pole and decreases at the
winter pole (carbon dioxide condenses into dry ice at the winter pole and
sublimates into carbon dioxide gas at the summer pole)
Dust devils, swirling winds of air that rise up from the ground, are common
during summer in either hemisphere.
Light scattered by the suspended dust (from dust storms) tends to give the
Martian sky a yellow-brown color
Mars’s axis tilt varies extremely because of the effect of Jupiter’s gravity (Mars is
closer to Jupiter’s orbit than Earth is) and because Mars’s two tin moon