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Observations of exoplanetary spectra are leading to unprecedented constraintson their atmospheric elemental abundances, particularly O/H, C/H, and C/Oratios. Recent studies suggest that elemental ratios could provide importantconstraints on formation and migration mechanisms of giant exoplanets. Afundamental assumption in such studies is that the chemical composition of theplanetary envelope represents the sum-total of compositions of the accreted gasand solids during the formation history of the planet. We investigate theefficiency with which accreted planetesimals ablate in a giant planetaryenvelope thereby contributing to its composition rather than sinking to thecore. From considerations of aerodynamic drag causing `frictional ablation' andthe envelope temperature structure causing `thermal ablation', we compute massablations for impacting planetesimals of radii 30 m to 1 km for differentcompositions (ice to iron) and a wide range of velocities and impact angles,assuming spherical symmetry. Icy impactors are fully ablated in the outerenvelope for a wide range of parameters. Even for Fe impactors substantialablation occurs in the envelope for a wide range of sizes and velocities. Forexample, iron impactors of sizes below ~0.5 km and velocities above ~30 km/sare found to ablate by ~60-80% within the outer envelope at pressures below10^3 bar due to frictional ablation alone. For deeper pressures (~10^7 bar),substantial ablation happens over a wider range of parameters. Therefore, ourexploratory study suggests that atmospheric abundances of volatile elements ingiant planets reflect their accretion history during formation.

Efficiency of Planetesimal Ablation in Giant Planetary Envelopes