At r h > 25 au , the coma is too weak to be well measured in the data . It is possible that more volatile species ( N 2 , CH 4 , or CO ) could have dominated sublimation at these times . These latter species , however , have orders of magnitude higher vapor pressure ( and specific sublimation rates ) than CO 2 and N H 3 do at T = 78 K , the η = 1 subsolar temperature for r h = 25 au . Any surface abundance in their pure ice forms on the surface of BB must be very low relative to the CO 2 /N H 3 that appear to dominate sublimation at r h ≤ 25 au . Perhaps these more - volatile species were heavily sublimated from BB during its previous perihelion passage to ≈ 18 au , , leaving behind a crust that is . Left : Following Eq . 18 and assuming radiative equilibrium temperatures , we plot the log of Afρ v th √ T vs 1 / T , which should yield a straight line if the coma 's scattering strength is proportional to the sublimation rate of a single species following the Clausius - Clapeyron ( CC ) relation . For either the fast - or slow - rotator bounds on radiative equilibrium ( η = 0.75 , 1 ) , the data are well fit by such a form . Right : Relative probability of the enthalpy of sublimation ∆H in a fit of Eq . 18 to the measured values of Afρ , marginalizing over the scaling constant . The central red curve gives the nominal case , with η = 1 and a dust velocity scaling with the thermal velocity . The left blue curve assumes a fast - rotating limit ( η = 0.75 ) and the right dashed curve assumed dust velocities scaling with radiation pressure ( v d ∝ T 4 ) . The enthalpies of sublimation of potential cometary volatiles are marked : the data strongly favor CO2 or N H3 as the driver of BB 's mass loss to date .