The Penetration of Solar Radiation Into Carbon Dioxide Ice

Abstract

Icy surfaces behave differently to rocky or regolith‐covered surfaces in response to irradiation. A key factor is the ability of visible light to penetrate partially into the subsurface. This results in the solid‐state greenhouse effect, as ices can be transparent or translucent to visible and shorter wavelengths, while opaque in the infrared. This can lead to significant differences in shallow subsurface temperature profiles when compared to rocky surfaces. Of particular significance for modeling the solid‐state greenhouse effect is the e‐folding scale, otherwise known as the absorption scale length, or penetration depth, of the ice. While there have been measurements for water ice and snow, pure and with mixtures, to date, there have been no such measurements published for carbon dioxide ice. After an extensive series of measurements we are able to constrain the e‐folding scale of CO2 ice for the cumulative wavelength range 300 to 1,100 nm, which is a vital parameter in heat transfer models for the Martian surface, enabling us to better understand surface‐atmosphere interactions at Mars' polar caps.

The Penetration of Solar Radiation Into Granular Carbon Dioxide and Water Ices of Varying Grain Sizes on Mars

“…Results for the e ‐folding scale for granular ice deposits fit well with the previously published snow and slab ice results of ζ snow = 11.2 ± 3 mm (Chinnery et al, 2019), and on average ζ CO2 slab = 47.6 ± 2 mm, ranging up to 65.1 ± 6.3 mm for the best quality ice samples (Chinnery et al, 2018). Further to this, Portyankina et al (2018) made light intensity measurements over the wavelength range 450–900 nm in CO 2 ice formed under Martian pressure and temperature conditions, which we have used to calculate the penetration depth of ζ CO2 slab = 66.7 mm .…”

“…In accordance with the methodology used by Kaufmann and Hagermann (2015), for small ice grains, the measurements taken at 5‐mm depth were used as the zero point and subsequent values normalized accordingly (so that calculations are made over the change in ice thickness, rather than absolute thickness). However, in our previous study (Chinnery et al, 2018), it was found that a minimum ice depth of 8 mm was needed to remove the effects of albedo from slab ice measurements, and a greater range of ice thickness measurements was required due to the increased transparency of compact ice. The granular ice samples were measured in increments of 5 mm due to the copper ring sizes used to adjust sample height, and so at larger grain sizes, the reference depth was increased to 10 mm, and depth range was also increased (see Table 1).…”

“…Top: The average (mean) e ‐folding scale is plotted for each grain size and ice composition. The e ‐folding scale of H 2 O and CO 2 snow is plotted (data from Chinnery et al, 2019), in addition to the results from slab ice experiments, of which the CO 2 slab ice e ‐folding scale was published in Chinnery et al (2018). Vertical error bars show the spread of data ±2 mm for experimental errors, horizontal errors denote the grain size ranges.…”

“…So that direct comparisons can be made between ice compositions, light intensity measurements using slabs of water ice have also been undertaken, following the same methodology used for the CO 2 ice measurements (as detailed in Chinnery et al, 2018). The model gives a good prediction of the e ‐folding scale up to the point at which slab ice is plotted.…”

“…CO 2 ice samples were prepared by condensing CO 2 directly from the gas phase within a pressure vessel cooled by liquid nitrogen, following the methodology detailed in Chinnery et al (2018) and based on that used by Kaufmann and Hagermann (2017). This formed large cylinders of CO 2 ice, which were then broken up and sieved.…”