The chemical composition of impact craters on Titan

Abstract

<p>We investigate the spectral behavior of nine Titan impact craters in order to constrain their surface composition using Visual and Infrared Mapping Spectrometer (VIMS) data and a radiative transfer code (RT) [e.g. 1] in addition to emissivity data. Past studies have looked at the chemical composition of impact craters either by using qualitative comparisons between craters [e.g. 2;3] or by combining all craters into a single unit [4], rather than separating them by geographic location or degradation state. Here, we use a radiative transfer model to first estimate the atmospheric contribution to the data, then extract the surface albedos of the impact crater subunits, and finally constrain their surface composition by using a library of candidate Titan materials. Following the general characterization of the impact craters, we study two impact crater subunits, the ‘crater floor’ and the ‘ejecta blanket’. The results show that Titan’s mid-latitude plain craters: Afekan, Soi, and Forseti, in addition to Sinlap and Menrva are enriched in an OH-bearing constituent (likely water-ice) in an organic based mixture, while the equatorial dune craters: Selk, Ksa, Guabonito, and Santorini, appear to be purely composed of organic material (mainly unknown dune dark material). This follows the pattern seen in [4], where midlatitude alluvial fans, undifferentiated plains, and labyrinths have surface spectra consistent with a mixture of tholin-like spectral features and water ice-like spectral features, while the equatorial undifferentiated plains, hummocky terrains, dunes, and variable plains appear to have spectra similar to a dark material and tholin-like mixture in their very top layers. These observations also agree with the evolution scenario proposed by [3] wherein the impact cratering process produces a mixture of organic material and water-ice, which is later “cleaned” through fluvial erosion in the midlatitude plains. This cleaning process does not appear to operate in the equatorial dunes, which seem to be quickly covered by a thin layer of sand sediment (with the exception of the freshest crater on Titan, Sinlap). Thus, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.</p> <p>[1] Hirtzig, M., et al. (2013). Icarus, 226, 470–486; [2] Neish, C.D., et al. (2015), Geophys. Res. Lett. 42, 3746–3754; [3] Werynski, A., et al. (2019), Icarus, 321, 508-521; [4] Solomonidou, A., et al. (2018), J. Geophys. Res, 123, 2, 489-507</p>

Geomorphological Map of the Soi Crater Region on Titan

“…The craters located close to the equator (including Santorini) appear organic in composition, as they are rich in the dark organic material and tholin‐like material, without the presence of any water ice (Solomonidou, Neish, et al., 2020). The Soi crater rim and ejecta appears primarily composed of tholin‐like material, followed by water‐ice, a mixture that is characteristic of the plains craters located around the 30°N latitude zone (Solomonidou, Neish, et al., 2020). The Soi crater region impact craters offer good examples of dune material “invading” the interior of the craters.…”

“…Craters appear to get filled with aeolian material and eventually are buried, which could explain the relatively few number of craters on Titan (e.g., Hedgepeth et al., 2020; Neish et al., 2015, 2016; Wood et al., 2010). The craters located close to the equator (including Santorini) appear organic in composition, as they are rich in the dark organic material and tholin‐like material, without the presence of any water ice (Solomonidou, Neish, et al., 2020). The Soi crater rim and ejecta appears primarily composed of tholin‐like material, followed by water‐ice, a mixture that is characteristic of the plains craters located around the 30°N latitude zone (Solomonidou, Neish, et al., 2020).…”

“…Relatively few impact craters are found (Hedgepeth et al., 2020; Neish & Lorenz, 2012; Neish et al., 2013, 2015; Werynski et al., 2019; Wood et al., 2010), confirming a geologically young surface. Organic materials, produced by long‐term photochemical processing of methane and nitrogen in the upper atmosphere, cover much of Titan (Barnes et al., 2008, 2011; Brossier et al., 2018; Clark et al., 2010; Krasnopolsky, 2009; Lavvas et al., 2008; Mackenzie et al., 2014; Soderblom et al., 2007; Solomonidou, Neish, et al., 2020; Solomonidou et al., 2018; Wilson & Atreya, 2004). Titan's equatorial zones are dominated by massive organic sand seas (Lopes et al., 2010; Lorenz, Wall, et al., 2006; Radebaugh et al., 2008; Rodriguez et al., 2014), whereas the mid‐latitudes are dominated by “undifferentiated plains”: patterns of wind deposition on Titan show that, for both northern and southern hemispheres, winds transport material from both the equatorial regions and high latitudes toward the mid‐latitudes (a belt at ∼35°), where materials are eventually concentrated and deposited as undifferentiated plains (Lopes et al., 2016; Malaska, Lopes, Hayes, et al., 2016; Solomonidou, Neish, et al., 2020).…”

“…Organic materials, produced by long‐term photochemical processing of methane and nitrogen in the upper atmosphere, cover much of Titan (Barnes et al., 2008, 2011; Brossier et al., 2018; Clark et al., 2010; Krasnopolsky, 2009; Lavvas et al., 2008; Mackenzie et al., 2014; Soderblom et al., 2007; Solomonidou, Neish, et al., 2020; Solomonidou et al., 2018; Wilson & Atreya, 2004). Titan's equatorial zones are dominated by massive organic sand seas (Lopes et al., 2010; Lorenz, Wall, et al., 2006; Radebaugh et al., 2008; Rodriguez et al., 2014), whereas the mid‐latitudes are dominated by “undifferentiated plains”: patterns of wind deposition on Titan show that, for both northern and southern hemispheres, winds transport material from both the equatorial regions and high latitudes toward the mid‐latitudes (a belt at ∼35°), where materials are eventually concentrated and deposited as undifferentiated plains (Lopes et al., 2016; Malaska, Lopes, Hayes, et al., 2016; Solomonidou, Neish, et al., 2020). A variety of channel networks and fluvial valleys have been seen by both the Cassini orbiter and by the Huygens probe during its descent, suggesting that liquid hydrocarbons have flowed energetically across Titan's surface (Birch et al., 2016; Burr, Drummond, et al., 2013; Burr, Perron, et al., 2013; Burr et al., 2009; Langhans et al., 2012; Radebaugh et al., 2018).…”

Planetary Caves: A Solar System View of Processes and Products

“…Importantly, with an increasing resolution, more detailed and perhaps smaller isolated patches of these terrains could be revealed (Sotin et al., 2017). Visible or near‐infrared imaging over multiple wavelengths could also provide spectral data that could distinguish compositional classes over these terrains (e.g., Solomonidou et al., 2020). While great achievements have been made to extract altimetry data from SAR swaths (Corlies et al., 2017; Stiles et al., 2009), acquisition of high‐resolution elevation data inside many labyrinth terrains was not possible; this was largely due to limited availability of swaths that enabled the development of stereogrammetric imagery (Kirk et al., 2009).…”