Sputtering and heating of Titan's upper atmosphere

Abstract

Titan is an important endpoint for understanding atmospheric evolution. Prior to Cassini's arrival at Saturn, modelling based on Voyager data indicated that the hydrogen escape rate was large (1-3!10 28 amu s K1), but the escape rates for carbon and nitrogen species were relatively small (5!10 26 amu s K1) and dominated by atmospheric sputtering. Recent analysis of the structure of Titan's thermosphere and corona attained from the Ion and Neutral Mass Spectrometer and the Huygens Atmospheric Structure Instrument on Cassini have led to substantially larger estimates of the loss rate for heavy species (0.3-5!10 28 amu s K1). At the largest rate suggested, a mass that is a significant fraction of the present atmosphere would have been lost to space in 4 Gyr; hence, understanding the nature of the processes driving escape is critical. The recent estimates of neutral escape are reviewed here, with particular emphasis on plasma-induced sputtering and heating. Whereas the loss of hydrogen is clearly indicated by the altitude dependence of the H 2 density, three different one-dimensional models were used to estimate the heavy-molecule loss rate using the Cassini data for atmospheric density versus altitude. The solar heating rate and the nitrogen density profile versus altitude were used in a fluid dynamic model to extract an average net upward flux below the exobase; the diffusion of methane through nitrogen was described below the exobase using a model that allowed for outward flow; and the coronal structure above the exobase was simulated by presuming that the observed atmospheric structure was due to solar-and plasma-induced hot particle production. In the latter, it was hypothesized that hot recoils from photochemistry or plasma-ion-induced heating were required. In the other two models, the upward flow extracted is driven by heat conduction from below, which is assumed to continue to act above the nominal exobase, producing a process referred to as 'slow hydrodynamic' escape. These models and the resulting loss rates are reviewed and compared. It is pointed out that preliminary estimates of the composition of the magnetospheric plasma at Titan's orbit appear to be inconsistent with the largest loss rates suggested for the heavy species, and the mean upward flow extracted in the one-dimensional models could be consistent with atmospheric loss by other mechanisms or with transport to other regions of Titan's atmosphere.

Kinetic simulations of thermal escape from a single component atmosphere

“…The main differences arise from the presence of a gravity field. Although this problem has been of interest for years, and we are aware that non-thermal processes induced by UV photons and plasma bombardment occurring near or above the nominal exobase can dominate thermal escape, 6,33,34 the systematic study here will provide guidance in understanding atmospheric escape rates and leads to some surprising results. Gravity produces qualitative changes in the flow structure as compared with expectations based on analysis of free expansion at zero gravity.…”

Mars solar wind interaction: Formation of the Martian corona and atmospheric loss to space

“…The division between low‐ and high‐energy particles allows us to show their relative importance for the escape flux, although the efficiency of the slow particles is lower than the efficiency of high‐energy particles. It is interesting to highlight that the contribution of low‐energy particles has also been suggested as a significant potential source of heating and sputtering of Titan's upper atmosphere [ Johnson et al , 2006]. For both solar activities we have launched several thousand incident particles: 100,000 at low energy and 15,000 at high energy.…”

Preface

“…Titan's exosphere is discussed by Dandouras et al (2009), who describe the population of energetic neutral atoms there which result from bombardment by energetic ions populating Saturn's magnetosphere. The influence of Titan's space environment on its upper atmosphere and exosphere is also discussed by Johnson (2009) whose calculations suggest that magnetosphere coupling processes provide an important energy source for the atmosphere. Coates (2009) discusses the ion population discovered in the vicinity of Titan, of which the heavy negative ions are particularly noteworthy.…”