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History - [8 • Gradual erosion of the atmosphere by solar...

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History Mars' atmosphere is believed to have changed over the course of the planet's lifetime, with evidence suggesting the possibility that Mars had large oceans a few billion years ago. [7] As stated in the Mars Ocean Hypothesis , atmospheric pressure on the present day Martian surface only exceeds that of the triple point of water (6.11 hectopascals (0.0886 psi)) in the lowest elevations; at higher elevations water can exist only in solid or vapor form. Annual mean temperatures at the surface are currently less than 210 K (−63 °C; −82 °F), significantly lower than what is needed to sustain liquid water. However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface. Possible causes for the depletion of a previously thicker Martian atmosphere include: Catastrophic collision by a body large enough to blow away a significant percentage of the atmosphere;
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Unformatted text preview: [8] • Gradual erosion of the atmosphere by solar wind; [9] and • On-going removal of atmosphere due to electromagnetic field and solar wind interaction. [8] Structure Mars' atmosphere is composed of the following layers: • Lower atmosphere: This is a warm region affected by heat from airborne dust and from the ground. • Middle atmosphere: Mars has a jetstream , which flows in this region. • Upper atmosphere, or thermosphere: This region has very high temperatures, caused by heating from the Sun. Atmospheric gases start to separate from each other at these altitudes, rather than forming the even mix found in the lower atmospheric layers. • Exosphere: Typically stated to start at 200 kilometres (120 mi) and higher, this region is where the last wisps of atmosphere merge into the vacuum of space. There is no distinct boundary where the atmosphere ends; it just tapers away....
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