Abstract
We report the discovery of a large ( ∼ 8500 km diameter) infrared-bright storm at Neptune’s equator in June 2017. We tracked the storm over a period of 7 months with high-cadence infrared snapshot imaging, carried out on 14 nights at the 10 m Keck II telescope and 17 nights at the Shane 120 inch reflector at Lick Observatory. The cloud feature was larger and more persistent than any equatorial clouds seen before on Neptune, remaining intermittently active from at least 10 June to 31 December 2017. Our Keck and Lick observations were augmented by very high-cadence images from the amateur community, which permitted the determination of accurate drift rates for the cloud feature. Its zonal drift speed was variable from 10 June to at least 25 July, but remained a constant 237.4 ± 0.2 m s−1 from 30 September until at least 15 November. The pressure of the cloud top was determined from radiative transfer calculations to be 0.3-0.6 bar; this value remained constant over the course of the observations. Multiple cloud break-up events, in which a bright cloud band wrapped around Neptune’s equator, were observed over the course of our observations. No “dark spot” vortices were seen near the equator in HST imaging on 6 and 7 October. The size and pressure of the storm are consistent with moist convection or a planetary-scale wave as the energy source of convective upwelling, but more modeling is required to determine the driver of this equatorial disturbance as well as the triggers for and dynamics of the observed cloud break-up events.
Highlights
17 The Voyager 2 spacecraft flyby of Neptune in 1989 revealed an extremely dynamic, turbulent atmo18 sphere (Smith et al 1989; Tyler et al 1989)
1 We report the discovery of a large (∼8500 km diameter) infrared-bright storm at Neptune’s equator 2 in June 2017
The geometric albedos we found were somewhat higher than the K-band value of 0.058 ± 0.016 reported by Roddier et al (1997) but in good agreement with Dumas et al (2003), who obtained 0.084 ± 0.002 in the Hubble Space Telescope (HST) F160W filter at 1.6 μm and 0.075 ± 0.010 in the HST F204M filter at 2.04 μm. Data were obtained on 17 nights from the Shane 120-inch reflecting telescope at the UCO Lick Observatory on Mount Hamilton, California
Summary
17 The Voyager 2 spacecraft flyby of Neptune in 1989 revealed an extremely dynamic, turbulent atmo sphere (Smith et al 1989; Tyler et al 1989). Clouds form when humid (rich in condensible species) upwelling air reaches a low enough temperature that condensation occurs, meaning that convective upwelling results in localized cloud systems and down welling regions tend to remain relatively cloud-free Combining these physical principles with the observed cloud patterns and deep atmosphere brightness temperature maps led to a hypothesis for Neptune’s convection in which air rises from as deep as 40 bars into the stratosphere at midlatitudes, and subsides over the poles and at the equator (de Pater et al 2014), explaining the cloud bands at Neptune’s midlatitudes and the relative paucity of clouds at the equator.
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