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
Greenhouse and icehouse Earth producing more CO and heating up the Earth's atmosphere. Earth is more commonly placed in a greenhouse state throughout the epochs, and the Earth has been in this state for approximately 80% of the past 500 million years, which makes understanding the direct causes somewhat difficult. An "icehouse Earth" is the earth as it experiences an ice age. Unlike a greenhouse Earth, an icehouse Earth has ice sheets present, and these sheets wax and wane throughout times known as glacial periods and interglacial periods. During an icehouse Earth, greenhouse gases tend to be less abundant, and temperatures tend to be