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Unformatted text preview: escape speed: gas molecules can escape from planet if travelling fast enough Air molecules are always in motion speed depends on temperature and mass of molecules hotter => faster => easier to escape lighter => faster => easier to escape If a planet's atmosphere is hot enough, light elements will escape Earth is hot enough for hydrogen and helium to escape Atmosphere and temperature atmosphere absorbs energy from sunlight X-rays and ultraviolet radiation are absorbed at different altitudes average surface temperature is about 300 K if no atmosphere, would be about 270 K (below freezing) This temperature difference comes from the "greenhouse effect" atmosphere traps heat near Earth's surface...
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