Chapter 10 notes

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Department
Astronomy
Course
ASTRON C12
Professor
Geoffrey Marcy
Semester
Fall

Description
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 level  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 precipitation).  Similarly, equatorial regions experience high rainfall because they receive more sunlight.  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
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