  Observations of exoplanetary spectra are leading to unprecedented constraints
on their atmospheric elemental abundances, particularly O/H, C/H, and C/O
ratios. Recent studies suggest that elemental ratios could provide important
constraints on formation and migration mechanisms of giant exoplanets. A
fundamental assumption in such studies is that the chemical composition of the
planetary envelope represents the sum-total of compositions of the accreted gas
and solids during the formation history of the planet. We investigate the
efficiency with which accreted planetesimals ablate in a giant planetary
envelope thereby contributing to its composition rather than sinking to the
core. From considerations of aerodynamic drag causing `frictional ablation' and
the envelope temperature structure causing `thermal ablation', we compute mass
ablations for impacting planetesimals of radii 30 m to 1 km for different
compositions (ice to iron) and a wide range of velocities and impact angles,
assuming spherical symmetry. Icy impactors are fully ablated in the outer
envelope for a wide range of parameters. Even for Fe impactors substantial
ablation occurs in the envelope for a wide range of sizes and velocities. For
example, iron impactors of sizes below ~0.5 km and velocities above ~30 km/s
are found to ablate by ~60-80% within the outer envelope at pressures below
10^3 bar due to frictional ablation alone. For deeper pressures (~10^7 bar),
substantial ablation happens over a wider range of parameters. Therefore, our
exploratory study suggests that atmospheric abundances of volatile elements in
giant planets reflect their accretion history during formation.
