The Cometary Tail

The Moon lost its volatiles into a cometary tail. The Earth kept its because its atmosphere was too compact to escape.

After the giant impact that formed the Moon, both the proto-Earth and the proto-lunar disk were incandescent — magma oceans covered by rock vapor atmospheres. Both were hot enough to vaporize volatile elements. Yet Earth retained its volatiles and the Moon did not. The standard explanation invokes the Moon’s lower gravity, but the mechanism by which volatiles actually left has remained unclear.

Nie and Bhatt (arXiv:2603.05322) show that the asymmetry is hydrodynamic, not just gravitational. Earth’s post-impact atmosphere was dominated by heavy carbon species (CO) and was spatially compact — it behaved as a closed system, unable to launch a sustained outflow. The proto-lunar disk atmosphere was dominated by light hydrogen (H, H₂) and was spatially extended — it developed into a hydrodynamic outflow analogous to the solar wind.

The key physics: in the partially dissociated disk atmosphere, recombination of atomic hydrogen (2H → H₂) releases heat that keeps the gas nearly isothermal as it expands. An isothermal atmosphere that extends past the escape radius doesn’t slowly leak — it activates a bulk outflow, a wind. The disk atmosphere didn’t evaporate molecule by molecule. It blew itself away.

The outflow was strong enough to propel volatiles from inside the Roche limit out of Earth’s gravity entirely, creating a cometary tail of proto-lunar volatiles streaming into interplanetary space. The Moon’s volatile depletion isn’t subtraction — it’s expulsion.

The structural insight: two magma ocean atmospheres, both hot, both vapor-rich, but one compact and one extended. The compact one retains everything. The extended one loses everything. Composition (heavy CO vs. light H₂) determines spatial extent, spatial extent determines whether a wind activates, and the wind is all-or-nothing. A continuous variable (composition) triggers a discontinuous outcome (retention vs. loss).

Nie and Bhatt, “Hydrodynamic outflows of proto-lunar disk volatiles,” arXiv:2603.05322 (2026).