Selection and Characteristics of the Dragonfly Landing Site Near Selk
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The Planetary Science Journal, 2:24 (13pp), 2021 February https://doi.org/10.3847/PSJ/abd08f © 2021. The Author(s). Published by the American Astronomical Society. Selection and Characteristics of the Dragonfly Landing Site near Selk Crater, Titan Ralph D. Lorenz1 , Shannon M. MacKenzie1 , Catherine D. Neish2,3 , Alice Le Gall4 , Elizabeth P. Turtle1 , Jason W. Barnes5 , Melissa G. Trainer6 , Alyssa Werynski2, Joshua Hedgepeth2, and Erich Karkoschka7 1 Johns Hopkins University Applied Physics Laboratory 1001 Johns Hopkins Road, Laurel, MD 20723, USA 2 Department of Earth Sciences, The University of Western Ontario, London, Canada 3 The Planetary Science Institute, Tucson, AZ, USA 4 LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Paris, France 5 Department of Physics, University of Idaho, Moscow, ID 83844-0903, USA 6 NASA Goddard Space Flight Center, Greenbelt, MD, USA 7 Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ, USA Received 2020 September 21; revised 2020 November 30; accepted 2020 December 2; published 2021 February 8 Abstract The factors contributing to the initial selection of a dune site near the Selk impact structure on Titan as the first landing site for the Dragonfly mission are described. These include arrival geometry and aerodynamic/ aerothermodynamic considerations, illumination, and Earth visibility, as well as the likely presence of exposed deposits of water-rich material, potentially including materials where molten ice has interacted with organics. Cassini observations of Selk are summarized and interpreted: near-infrared reflectance and microwave emission data indicate water-rich materials in and around the crater. Radar topography data shows the rim of Selk to have slopes on multi-km scales reaching only ∼2° degrees, an order of magnitude shallower than early photoclinometric estimates. Unified Astronomy Thesaurus concepts: Saturnian satellites (1427) 1. Introduction intended to offer a comprehensive or final interpretation of Cassini data of the Selk region, but rather to motivate more Dragonfly is a relocatable lander destined for Titan (Lorenz detailed studies. et al. 2018; Turtle et al. 2018) selected in 2019 as the fourth mission in NASA’s New Frontiers Program. Exploiting Titan’s dense atmosphere and low gravity, Dragonfly uses a multi-rotor 1.1. Requirements mobility system to explore diverse sites over tens to hundreds fi fl of kilometers. The scienti c goals above for Dragon y indicate desired The scientific motivation for Dragonfly is to determine the aspects of the destination region. As discussed in Lorenz ( ) composition of surface materials where prebiotic chemistry is 2019 , the provision of mobility permits partial decoupling of fi expected to be most advanced. Of particular interest are the scienti c desirability of a target site from the topographic cryolava flows or in impact melt deposits. Laboratory studies features of a site for safe landing. An obvious implicit have shown that Titan haze analogs quickly hydrolyze to yield requirement for a successful mission is that safe landing terrain ( known building blocks of life, such as amino acids and exists. Precedent from lunar and Mars exploration e.g., ) fi nucleobases (e.g., Neish et al. 2008, 2009, 2010). Neish et al. Golombek et al. 2003 argues that characteristics de ning °– ° (2018) note that impact melts will likely have encountered safety are slopes less than 10 15 , and a low probability of (“ ”) (∼ ) higher temperatures (273 K and above) than cryolava deposits encountering obstacles rocks on a lander scale 1m. fi (perhaps little higher than 176 K). Chemical kinetics would Geological analogy suggests that dune elds are highly lead to orders of magnitude higher reaction rates, and thus probable to satisfy these criteria. While steeper slopes can much richer prebiotic synthesis, in the impact melt case. obviously exist on dune slip faces, these are readily detected fl The present paper lays out the geometrical aspects of and avoided, and interdune ats are ideal for landing as sand Dragonfly’s arrival which defined the accessible area on Titan may partly mantle any gullies or rocks. It may be recalled that and why the Selk impact structure and nearby dune fields were mantling was considered a reassuring consideration in Viking fi selected as the most promising scientific targets. The Cassini landing site de nition, as being likely to diminish crater or ( ) data available on Selk and its environs are summarized, and other slopes, e.g., Masursky & Crabill 1976 . Another implicit, how this information was used during Dragonfly’s initial but notable, constraint is that adequate data exist at a candidate — formulation and Phase A study (2016–2019) to define the landing site to assert it as being suitable it should be initial landing site. It should be recognized that the Dragonfly remembered that Titan radar coverage at 1 km resolution or mission is still undergoing detailed definition (Phase B) and better only amounts to about 40% of the surface, and other some minor details, such as the exact dimensions and position Titan data sets are even more limited. of the landing ellipse, are subject to ongoing and future The limited available resolution of remote sensing data at fi refinement. It should be understood also that this paper is not Titan puts landing site identi cation at Titan today on a somewhat analogous footing as the identification of the Viking lander sites in 1976, e.g., Stooke (2012): large scale landforms Original content from this work may be used under the terms fi of the Creative Commons Attribution 4.0 licence. Any further can be identi ed and the relevant geological processes distribution of this work must maintain attribution to the author(s) and the title deduced, but the lander-scale terrain characteristics must be of the work, journal citation and DOI. estimated by geological analogy and with some limited 1 The Planetary Science Journal, 2:24 (13pp), 2021 February Lorenz et al. Figure 1. (a) Phase A interplanetary trajectory with launch in 2025 and Titan arrival in 2034. The launch date has since been prescribed to be 2027, although the arrival date and conditions remain the same. (b) Titan arrival in a Saturn-fixed frame (the arrival direction thus is now rotated relative to (a) as the vector difference of the interplanetary heliocentric arrival velocity and Saturn’s orbital velocity). The vehicle is aimed at the trailing hemisphere of Titan, to minimize the entry velocity. The resultant arrival direction defines the hemisphere accessible to Dragonfly. Figure 2. Schematic illustration of the combined delivery constraints. Given the incoming direction on a given epoch, the aerodynamic/aerothermodynamic constraints define a toroidal region where the flight path angle is in an allowable range. To perform descent while in Earth communication and with illumination requires being on the dayside at entry. Requiring a period of some days on the ground before sunset eliminates the late afternoon sliver of the daylight region. The logical AND combination of the entry and illumination constraints defines the accessible region for that entry epoch. The accessible regions for a range of arrival epochs (defined by the allowable Titan-relative entry velocity) can be combined in a logical OR sense to yield the ultimate set of possible target locations. quantitative constraints from radar data and scattering models. There are geometric constraints associated with any specific Although some large rocks proved to exist at both sites, e.g., mission opportunity, specifically landing date. Landing during “Big Joe” near Viking 1, and Viking 2 had one landing foot polar winter places severe restriction on science operations that somewhat perched, both Vikings landed successfully. The might require illumination (notably, mobility)(Barnes et al. contemporary Mars site selection process (e.g., Golombek et al. 2020), and prevents direct-to-Earth communication and so 2003) exploits global high-resolution high-precision data sets would require a much more expensive architecture with a relay on slope, topographic roughness, rock abundance etc. which spacecraft. In the context of the New Frontiers 4 competition, are simply not available for Titan until a future orbiter mission which specified launch in 2025 (and thus, in practical terms, performs comparable mapping. arrival in the mid-2030s during northern winter) this precluded 2 The Planetary Science Journal, 2:24 (13pp), 2021 February Lorenz et al. Figure 3. SAR imaging of Selk. The T95 SAR swath is the highest resolution data available (300 × 390 m). The incidence-corrected backscatter product ID on the PDS is BIFQI09N199_D253_T095S01_V02. Heavy dissection of the rim is evident, and a few dunes appear to be present on the crater floor. To the south, linear dunes taper out and re-emerge. The general E-W alignment of linear dunes seen all over Titan is prominent at right, although a second orthogonal set is present at left: this may be a result of “flow-straightening” by the crater causing a transverse dune pattern. The dune pattern to the north is predominantly E-W linears but with some irregularities. The T36 image is of considerably lower quality. The T98 image (lower right) is of superficially low quality but highlights roughness around the rim. exploration of Titan’s northern seas as in the proposed Titan load that the vehicle must tolerate. Programmatic choice of Mare Explorer (TiME) mission (Stofan et al. 2013). aeroshell diameter, in turn limited by launch vehicle fairing Additional geometric constraints emerge from implementa- size, and of thermal protection system (TPS) material on the tion choices in a mission concept. First, a single-element in situ heat shield, will in turn map to constraints on entry speed. mission at Titan should logically take advantage of Titan’s Because the arrival velocity of the spacecraft in the Saturnian atmosphere for entry directly from the interplanetary arrival system adds vectorially with Titan’s orbital motion, it is trajectory, avoiding the need for propulsive orbit insertion.