Evidence of sub-surface energy storage in comet 67P from the outburst of 2016 July 03

Abstract

On 2016 July 03, several instruments onboard ESA's Rosetta spacecraft detected signs of an outburst event on comet 67P, at a heliocentric distance of 3.32 au from the Sun, outbound from perihelion. We here report on the inferred properties of the ejected dust and the surface change at the site of the outburst. The activity coincided with the local sunrise and continued over a time interval of 14-68 min. It left a 10-m-sized icy patch on the surface. The ejected material comprised refractory grains of several hundred microns in size, and sub-micron-sized water ice grains. The high dust mass production rate is incompatible with the free sublimation of crystalline water ice under solar illumination as the only acceleration process. Additional energy stored near the surface must have increased the gas density. We suggest a pressurized sub-surface gas reservoir, or the crystallization of amorphous water ice as possible causes.

Summer outbursts in the coma of comet 67P/Churyumov–Gerasimenko as observed by Rosetta–VIRTIS

“…On 2016 July 3, the Star Tracker cameras on Rosetta detected fast particles travelling at a speed of (25 ± 10) m s −1 at the beginning of the event and slower ones at velocity of (0.41 ± 0.05) m s −1 1 h after (Agarwal et al 2017). Similar velocity values were measured for the outburst of 2016 February 19 (Grün et al 2016).…”

“…where I obs (W m −2 sr −1 μm −1 ) is the observed scattered light intensity by the instrument, a is the dust particle size, and F i (λ) is the incident solar flux. An approximate method to compute the filling factors is to assume an albedo for the quiescent dust coma and a higher albedo for the outburst dust particles (Agarwal et al 2017;Bockelée-Morvan et al 2017). For the quiescent dust coma albedo, we use the quantity wp(g) calculated by Ciarniello et al (2015) where w = Q sca /Q ext is the single scattering albedo at 0.55 μm.…”

“…Other estimations of the dust mass ejected from 67P/CG outbursts have been derived from OSIRIS images obtained less than few minutes after outburst onset (Grün et al 2016;Knollenberg et al 2016;Vincent et al 2016;Agarwal et al 2017;Gicquel et al 2017). Even if they use different methods and assumptions, all estimations derived outburst mass release of several tons.…”

“…From 2014 to 2016, several outbursts were observed by Rosetta (Tubiana et al 2015;Grün et al 2016;Knollenberg et al 2016;Vincent et al 2016;Agarwal et al 2017;Della Corte et al 2017). These observations provided a detailed characterization of the nature and evolution of 67P/CG outbursts and determined, in a few cases, their periodicities.…”

“…On 2016 July 3, several instruments onboard Rosetta detected an outburst when 67P/CG was at 3.32 au from the Sun after perihelion. Agarwal et al (2017) reported the inferred properties of the ejected dust, and that the surface change at the site of the outburst occurred at local sunrise, lasting 14-68min. The outburst left a 10 m-sized icy patch on the nucleus surface.…”