Thermal Segregation and Reddening in Europa's Double Ridges

Abstract

Europa's double ridges often display lower albedo and redder color than their surroundings. Their unique topography may cause sublimation-driven darkening due to illumination and self-heating, a process known as thermal segregation. We apply an advanced 3D thermophysical model, including shadowing and self-heating through mutual exchange of radiation, to digital elevation models of double ridges at a range of latitudes and orientations. Results show that self-heating in ridge troughs can markedly increase temperatures and sublimation rates, with a difference in maximum trough temperatures of up to 20 K, which may have implications for detection of endogenic heat. Incorporating a simple exosphere model and assuming an initial 10% concentration of 1 $\mu$m non-ice particles, we find thermal segregation can produce reddening in the form of dark lag layers from the equator to the middle latitudes, but is generally negligible at 60 degrees or higher. Lag formation timescales in ridge troughs are 10 - 100 yr to produce an optically thick layer. Modeling suggests that low-albedo lag layer formation provides positive feedback, further increasing surface heating. These effects may also darken Europa's surface in areas surrounding the ridges. However, the net mass balance controlling sublimation and lag formation is highly sensitive to the global water exosphere density: values $\sim 10^{16}$ molec/m$^{2}$ produce reddening in the trough and ablation of $\sim1~\mu\mathrm{m~yr^{-1}}$ of material, while values $\sim10^{18}$ molec/m$^{2}$ result in net deposition of $\sim 10~\mu\mathrm{m~yr^{-1}}$. Model predictions of resulting low albedo material in double ridge troughs are provided, which can be tested with eventual data from Europa Clipper.