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• 1 2016-05-18T10:01:59+00:00 Atmosphere 15 image_header 2019-12-03T16:17:39+00:00 Atmosphere refers to a layer of gasses surrounding a planet or other body that is held in place by the gravity of that body. More massive planets have stronger gravitational attraction and thus can hold onto thicker, more gas-rich atmospheres.  Conversely, smaller planets like Mars, or even moons like Saturn's Titan have thinner atmospheres.
  
  Planetary atmospheres initially form by outgassing of the planet’s interior.  Rocks and molten rocks (magmas) have gasses (like carbon dioxide or water) dissolved within them.  Release of the gasses may be gradual or catastrophic, such as during a volcanic eruption or impact event.

  Earth

  The make up of the Earth’s atmosphere is a byproduct of biological activity. Photosynthetic organisms have harvested light and carbon dioxide and created an oxygen-rich atmosphere.  Earth’s atmosphere consists mainly of nitrogen (78.08%), oxygen (20.95%), argon (0.93%), and carbon dioxide (0.039%).  There is also variable amounts of water vapor (~1% at sea level and ~0.4% averaged over the whole surface). In addition to providing us with breathable air, the atmosphere protects life by absorbing ultraviolet solar radiation and reducing temperature extremes from day and night.  Earth also has an active hydrosphere, which refers to the combined mass of water found on, under, and over the surface of a planet in the form of oceans, ice caps, and groundwater.
   

  Venus and Mars

  For comparison, atmospheres of Venus and Mars are mainly composed of carbon dioxide and small quantities of nitrogen, argon, oxygen, and sulfur dioxide.
  
  Rovers snapping sunset images from the surface of Mars see blue sunsets, which are a product of the scattering of light by dust in the Mars atmosphere and its dry, carbon dioxide-rich composition  
  
  The atmosphere of Venus is notably thick with an atmospheric pressure at the surface equivalent to pressures of nearly 1 kilometer under the Earth’s oceans.  The thick, carbon dioxide-rich atmosphere of Venus is credited with its runaway greenhouse effect and creating surface temperatures of at least 462˚C.

  Mercury

  The thickness of a body’s atmosphere is also influenced by temperature.  Tiny Mercury has the thinnest atmosphere of the eight planets circling the Sun.  Mercury is constantly bombarded by solar weather. Much of the hydrogen and helium in Mercury’s thin atmosphere originated from the Sun.  In addition, Mercury’s negligible atmosphere contains oxygen and the volatile metals sodium and potassium.