Atmosphere of Triton asymmetry was actually observed during star occultations by Triton in 1990s. Before Voyager 2 arrived, a nitrogen and methane atmosphere with a density as much as 30% that of Earth had been suggested. This proved to be a great overestimate, similar to the predictions of the atmospheric density of Mars, but like on Mars, a denser early atmosphere is postulated. Voyager 2 flew past Triton five hours after closest approach to Neptune in mid-late August 1989. During the flyby, Voyager 2 took measurements of the atmosphere, finding methane and nitrogen in the atmosphere. In the 1990s, observations from Earth were
Flash-gas (petroleum) to its combustion. As of 2009, energy-related CO emissions made up more than 80% of total U.S. greenhouse gas emissions. Flare and flash gas both contain GHGs and therefore contribute to their release into the atmosphere, estimates of their emission levels during petroleum production vary due to largely unmeasured release of flash gas and other forms of fugitive emissions. One study estimated a 40% increase in the atmospheric concentration of the GHG carbon dioxide (CO) is due directly to human activities since 1750. GHGs other than CO which are produced during petroleum operations are also a concern to the environment.