Efficiency of planetesimal ablation in giant planetary envelopes
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Abstract
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.
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1365-2966
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Science and Technology Facilities Council (ST/N000927/1)