Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b absorbs over 50 % of the stellar Ly \alpha emission during transit .
Given the planet ’ s atmospheric composition and energy-limited escape rate , this hydrogen outflow is expected to entrain heavier atoms such as C and O .
We searched for C and Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130M observations made during the planet ’ s extended H i transit .
These observations show no transit absorption in the C ii 1334,1335 Å and Si iii 1206 Å lines integrated over [ -100 , 100 ] km s ^ { -1 } , imposing 95 % ( 2 \sigma ) upper limits of 14 % ( C ii ) and 60 % ( Si iii ) depth on the transit of an opaque disk and 22 % ( C ii ) and 49 % ( Si iii ) depth on an extended , highly asymmetric transit similar to that of H i Ly \alpha .
C ^ { + } is likely present in the outflow according to a simulation we carried out using a spherically-symmetric , photochemical-hydrodynamical model .
This simulation predicts a \sim 2 % transit over the integrated bandpass , consistent with the data .
At line center , we predict the C ii transit depth to be as high as 19 % .
Our model predicts a neutral hydrogen escape rate of 1.6 \times 10 ^ { 9 } g s ^ { -1 } ( 3.1 \times 10 ^ { 9 } g s ^ { -1 } for all species ) for an upper atmosphere composed of hydrogen and helium .