Ultraviolet Spectrometer Observations of Uranus

Abstract

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 10(29) per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 10(20) per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x10(11) watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10(-7) at a vertical column abundance of molecular hydrogen of 10(23) per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

The aurorae of Uranus past equinox

“…A supplementary 16% average contribution of H Lyα [Broadfoot et al, 1986] may be added to obtain an exhaustive estimate of the total flux radiated by H and H2. Column 8 of Table 1 lists the H2 auroral peak brightnesses, for the 2012 and 2014 campaigns but also for the 1998 and 2011 ones.…”

“…The Hubble Space Telescope (HST) recently succeeded in re-detecting the Far UltraViolet (FUV) aurorae of Uranus in 2011 and then in 1998 [Lamy et al, 2012] (hereafter L12), long after their discovery by the UV Spectrometer (UVS) of Voyager 2 in 1986 [Broadfoot et al, 1986]. These detections included the first images of Uranus' aurorae and provided a new means to remotely investigate the poorly known magnetosphere of Uranus from Earth, awaiting for any future in situ exploration [Arridge et al, 2011].…”

The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant planets

“…The innermost dusty ζ ring appears to have moved several thousand kilometres outward between the Voyager 2 flyby and recent Earth-based observations , but this changing ring has not been studied closely. Ring particles could be lost to the planet by the drag force from the extended exosphere of Uranus (Broadfoot et al, 1986) and may lead to similar effects as the 'ring rain' at Saturn . Finally, Voyager's single high-phase image of the rings revealed a plethora of otherwise unknown dust structures (Murray and Thompson, 1990).…”

The physics of planetary rings

“…Ýòà ìåëêàÿ ïûëü ðàñïðåäåëåíà íåîäíîðîäíî è îáðàçóåò ðÿä êîëüöåâûõ ñòðóêòóð, êîòîðûå îòíîñÿòñÿ ïî íàøåé êëàññèôèêàöèè ê âòîðè÷íûì êîëüöàì [28]. Âàaeíåéøèì ôàêòîðîì, óäàëÿþùèì ïûëü èç ïëîòíûõ êîëåö, ÿâëÿåòñÿ çíà÷èòåëüíîå àýðîäèíàìè÷åñêîå òðåíèå îò âåðõíèõ ñëîåâ àòìîñôåðû Óðàíà [33].…”

“…Íî â ñèñòåìå Óðàíà íà êîëüöà äåéñòâóåò åùå îäèí ìîùíûé ðàçðóøèòåëüíûé ôàêòîð, îòêðûòûé «Âîÿäaeåðîì-2»: àýðîäèíàìè÷åñêîå òðåíèå ÷àñòèö î âåðõíèå ñëîè î÷åíü ïðîòÿaeåííîé àòìîñôåðû Óðàíà. Ìèêðîííûå ÷àñòèöû ïàäàþò íà ïëàíåòó çà íåñêîëüêî ñîòåí ëåò [33], à âðåìÿ aeèçíè ñàìèõ êîëåöíåñêîëüêî ìèëëèîíîâ ëåò [33]. Êàê ïîêàçàíî â [95], óñòîé÷èâûì ÿâëÿåòñÿ òîëüêî ìàññèâíîå êîëü-öî ε, ðàñïîëîaeåííîå äàëüøå âñåõ îò ïëàíåòû è îêðóaeåííîå äâóìÿ ïà-ñòóõàìè: Êîðäåëèåé è Îôåëèåé.…”