Saturday, 19 March 2011


Mercury is the most difficult planet to observe due to its proximity to the Sun. Though its phases were discovered by Zupus in 1639, the first map of its surface was not drawn until 1882. Ground-based observations led to a mistaken perception that Mercury rotates synchronously with its orbital motion, which would leave one half of the planet permanently dark.[3]

The viability of an atmosphere on Mercury had been debated before Mariner 10's 1974 flyby.[4] On the one hand proximity to the Sun would induce a high temperature in any putative atmosphere, leading to its rapid escape.[5] On the other hand the dark hemisphere would be very cold, causing many gases to freeze. To prevent the freeze, the atmosphere would either need to be relatively thick (about 5 mbar for carbon dioxide) in order to transport heat from the illuminated hemisphere, or would need to be composed of gases such as argon with low condensation temperatures.[6]

By the 1960s some evidence had accumulated indicating that Mercury might have a thin atmosphere. Polarimetric measurements by Bernard Lyot and Audouin Dollfus implied a surface pressure of 0.1–10 mbar,[7] and some spectroscopic results suggested a carbon dioxide atmosphere with a few mbars of surface pressure.[7] Additionally microwave observations showed that the temperature of Mercury's night side was higher than what had been considered possible.[6]

However all these pieces of evidence were later called into doubt. The discovery that Mercury's rotation is not synchronous explained elevated night temperatures,[6] while improved spectroscopic measurements sharply lowered the upper limit on the carbon dioxide presence to less than 0.0001 mbar.[8] The accuracy of the polarimetry was questioned as well.[7] By 1974 a consensus had formed that Mercury, like the Moon, had virtually no atmosphere. The Mariner 10 flybys confirmed that conclusion, discovering only a tenuous surface-bound exosphere (i.e. a thin shell of gases whose base coincides with the planetary surface).

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