Evolution of an Early Titan Atmosphere: Comment

TL;DR

This paper argues that solar heating of Titan's upper atmosphere, not surface temperature, primarily drives atmospheric escape, revising previous models and supporting nitrogen retention over early solar system timescales.

Contribution

It demonstrates that upper atmospheric solar heating dominates escape processes, challenging prior models based on surface temperature and providing an estimate consistent with nitrogen retention.

Findings

Escape driven mainly by solar heating of the upper atmosphere.

Previous models overestimated escape rates by considering surface temperature.

Estimated escape rate (~10^4 kg/s) supports nitrogen retention over Titan's early history.

Abstract

Escape of an early atmosphere from Titan, during which time NH3 could be converted by photolysis into the present N2 dominated atmosphere, is an important problem in planetary science. Recently Gilliam and Lerman (2014) estimated escape driven by the surface temperature and pressure, which we show gave loss rates that are orders of magnitude too large. Their model, related to Jeans escape from an isothermal atmosphere, was used to show that escape driven only by surface heating would deplete the atmospheric inventory of N for a suggested Titan accretion temperature of ~355 K. Therefore, they concluded that the accretion temperature must be lower in order to retain the present Titan atmosphere. Here we show that the near surface atmospheric temperature is essentially irrelevant for determining the atmospheric loss rate from Titan and that escape is predominantly driven by solar heating of the upper atmosphere. We also give a rough estimate of the escape rate in the early solar system (~10^4 kg/s) consistent with an inventory of nitrogen being available over the time period suggested by Atreya et al. (1978) for conversion of NH3 into N2.