Abstract. Earth appears to have been warm during its early history despite the fainmess of the young Sun. Greenhouse warming by gaseous CO2 and H20 by itself is in conflict with constraints on atmospheric CO2 levels derived from paleosols for early Earth. Here we explore whether greenhouse warming by methane could have been important. We find that a CH 4 mixing ratio of 10 -4 (100 ppmv) or more in Earth' s early atmosphere would provide agreement with the paleosol data from 2.8 Ga. Such a CH 4 concentration could have been readily maintained by methanogenic bacteria, which are thought to have been an important component of the biota at that time. Elimination of the methane component of the greenhouse by oxidation of the atmosphere at about 2.3 -2.4 Ga could have triggered the Earth's first widespread glaciation.

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70 degrees to -27 degrees ), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20 degrees . Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 10(2) new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earth's moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Miranda's surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

From Space Telescope Imaging Spectrograph (STIS) observations of Uranus in 2012, when good views of its north polar regions were available, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60• N and beyond. This depletion in the north polar region of Uranus in 2012 is similar in magnitude and depth to that found in the south polar regions in 2002. This similarity is remarkable because of the strikingly different appearance of clouds in the two polar regions: we have never seen any obvious signs of convective activity in the south polar region, while the north has been peppered with numerous small clouds thought to be of convective origin. Keck and Hubble Space Telescope imaging observations close to equinox at wavelengths of 1080 nm and 1290 nm, with different sensitivities to methane and hydrogen absorption but similar vertical contribution functions, imply that the depletions were simultaneously present in 2007, and at least their gross character is probably a persistent feature of the Uranus atmosphere. The depletion appears to be mainly restricted to the upper troposphere, with the depth increasing poleward from about 30• N, reaching ∼4 bars at 45• N and perhaps much deeper at 70• N, where it is not well constrained by our observations. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modeling, it suggests that sparse localized convective events may be important in cloud formation. A more complex meridional circulation pattern may be necessary to reproduce the observed cloud distribution, but microwave observations appear to be most compatible with a single deep circulation cell. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55• N and 82• N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH 4 /H 2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity. While there has been no significant secular change in the brightness of Uranus at continuum wavelengths between 2002 and 2012, there have been significant changes at wavelengths sensing methane and/or hydrogen absorption, with the southern hemisphere darkening considerably between 2002 and 2012, by ∼25% at mid latitudes near 827 ...

Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometers show that the morphology and motions of large-scale features change with depth in the cloud deck. Poleward meridional velocities, seen in both spectral regions, are much reduced in the NIR In the south polar region the markings in the two wavelength bands are strongly anticorrelated. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large-scale planetary waves. No optical lightning was detected.