Abstract
The upper atmosphere of Uranus has been observed to be slowly cooling between 1993 and 2011. New analysis of near-infrared observations of emission from H3+ obtained between 2012 and 2018 reveals that this cooling trend has continued, showing that the upper atmosphere has cooled for 27 years, longer than the length of a nominal season of 21 years. The new observations have offered greater spatial resolution and higher sensitivity than previous ones, enabling the characterization of the H3+ intensity as a function of local time. These profiles peak between 13 and 15 h local time, later than models suggest. The NASA Infrared Telescope Facility iSHELL instrument also provides the detection of a bright H3+ signal on 16 October 2016, rotating into view from the dawn sector. This feature is consistent with an auroral signal, but is the only of its kind present in this comprehensive dataset.This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H3+, H5+ and beyond’.
Highlights
William Herschel’s discovery of Uranus in 1781, announced in this journal [1], marked the first addition to the roster of planets in our Solar System since antiquity
This study extends the baseline of observations with ones obtained in 2012– 2018, bringing the total period of H+3 observations of Uranus to 27 years, longer than an individual solar season at Uranus (21 years), adding data from the NASA Infrared Telescope Facility (IRTF), Keck, the
This study has revealed that the upper atmosphere of Uranus has cooled consistently between 1992 and 2018, at a rate of 8 ± 1 K yr−1
Summary
William Herschel’s discovery of Uranus in 1781, announced in this journal [1], marked the first addition to the roster of planets in our Solar System since antiquity. They discovered that the upper atmosphere of Uranus had dramatically cooled from 715 ± 47 K in 1992 to 534 ± 39 K in 2008 This slow yet consistent cooling was initially interpreted to be related to seasonal solar irradiance, modulating the effectiveness of the Joule heating by changing ionospheric conductivity, where the solstice (1986) would appear hotter than the equinox (2007) since the illuminated area on the planet over the course of a day is greater by about a factor 2 at equinox. These facilities enable the characterization of the H+3 emission across the disc of Uranus
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