We present Hubble Space Telescope observations of active asteroid 313P/Gibbs ( formerly P/2014 S4 ) taken over the five month interval from 2014 October to 2015 March .
This object has been recurrently active near perihelion ( at 2.4 AU ) in two different orbits , a property that is naturally explained by the sublimation of near surface ice but which is difficult to reconcile with other activity mechanisms .
We find that the mass loss peaks near 1 kg s ^ { -1 } in October and then declines over the subsequent months by about a factor of five , at nearly constant heliocentric distance .
This decrease is too large to be caused by the change in heliocentric distance during the period of observation .
However , it is consistent with sublimation from an ice patch shadowed by local topography , for example in a pit like those observed on the nuclei of short-period comet 67P/Churyumov-Gerasimenko .
While no unique interpretation is possible , a simple self shadowing model shows that sublimation from a pit with depth to diameter ratio near 1/2 matches the observed rate of decline of the activity , while deeper and shallower pits do not .
We estimate the nucleus radius to be 700 \pm 100 m ( geometric albedo 0.05 assumed ) .
Measurements of the spatial distribution of the dust were obtained from different viewing geometries .
They show that dust was ejected continuously not impulsively , that the effective particle size is large , \sim 50 \mu m , and that the ejection speed is \sim 2.5 m s ^ { -1 } .
The total dust mass ejected is \sim 10 ^ { 7 } kg , corresponding to \sim 10 ^ { -5 } of the nucleus mass .
The observations are consistent with partially shadowed sublimation from \sim 10 ^ { 4 } m ^ { 2 } of ice , corresponding to \sim 0.2 % of the nucleus surface .
For ice to survive in 313P for billion-year timescales requires that the duty cycle for sublimation be \lesssim 10 ^ { -3 } .