UV degradation of accreted organics on Mars: IDP longevity, surface reservoir of organics, and relevance to the detection of methane in the atmosphere

Abstract

[1] Reanalysis of the Viking Lander results on Mars has suggested a surface reservoir of organic carbon at the ppm level. The size of this putative reservoir could be explained if the source of carbon on Mars is meteoritic in origin and is destroyed primarily by UV irradiation, yielding methane. By combining a numerical UV model for the surface of Mars with published laboratory measurements of organic UV photolysis, the times required to completely convert the carbon within individual particles to methane may be calculated. For interplanetary dust particles (IDPs) initially containing 10 wt% carbon, lifetimes of organics range from 3.9 years for a 0.2 mm diameter particle at equatorial latitudes to 4900 years for a 200 mm diameter particle at polar latitudes, and implies a median time for IDP organics by UV photolysis of 320 years at equatorial latitudes and 1500 years at polar latitudes. Assuming no redistribution of organics over the surface, the IDP organic reservoir at the surface would range from 1.1 Â 10 À6 kg m À2 at equatorial latitudes to 6.6 Â 10 À6 kg m À2 at polar latitudes. If accreted carbon is evenly mixed with the soil, up to 3.4 ppm of organic carbon at the VL1 landing site can be explained from a meteoritic origin and up to 4.9 ppm at the VL2 landing site. Derived from the IDP organic reservoir, small fluctuations in methane would exist due to variations in UV irradiation with latitude and L S . Production of methane is expected to range up to 0.35 pptv sol À1. Citation: Moores, J. E., and A. C. Schuerger (2012), UV degradation of accreted organics on Mars: IDP longevity, surface reservoir of organics, and relevance to the detection of methane in the atmosphere,

UV production of methane from surface and sedimenting IDPs on Mars in light of REMS data and with insights for TGO

“…In order to determine the amount of methane emitted under these conditions, the quantum efficiency (QE) of the conversion of IDP organic carbon to methane is needed. Moores and Schuerger (2012) developed an equation for QE as their equation 1 which captured the variation observed in methane flux under the experimental conditions described by Schuerger et al (2012). This equation is reproduced below:…”

“…These values are compared to the pre-REMS radiative transfer modeling work of Moores and Schuerger (2012) that derived values from Moores et al (2007), which used a fixed optical depth of 0.5, in the lower panel of Figure 3. On average, the D&A code described here produces energies that are lower than the previous model by 35%.…”

“…These refined results will be presented in section 3, while this section will review the UV-CH 4 model that was used to produce values of methane produced from irradiation of IDPs on the surface Moores and Schuerger, 2012) when coupled to the UV radiative transfer model (Moores et al, 2007), which provided energy fluxes reaching the Martian Methane in the light of REMS surface. Note that throughout this section and the remainder of the paper, methane production rates will be stated in terms of ppbv sol …”

“…Furthermore, as noted by Moores and Schuerger (2012), measurements of methane emissions from the surface can be used to constrain the quantity and distribution of organic carbon at the surface of Mars. As such, this paper provides the needed update to the model, the specifics of which will be discussed in section 2.…”

PALLAS: Planetary Analogues Laboratory for Light, Atmosphere, and Surface Simulations

“…Oro & Holzer, 1978;ten Kate et al, 2005ten Kate et al, , 2006Stalport et al, 2009;Moores & Schuerger, 2012). This UV photodegradation is predominantly a surface process, since UV does not penetrate a rocky surface deeper than a few 100 nanometres, depending on composition (180 nm has been used as a reference value (Jeong et al, 2003;Schuerger et al, 2011).…”