Elizabeth P. Turtle

Saturn's moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan's northern winter and spring were investigated in detail by the Cassini-Huygens spacecraft (2004-2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan's late northern summer. Using JWST's mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan's climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.

The Europa Imaging System (EIS) consists of a Narrow-Angle Camera (NAC) and a Wide-Angle Camera (WAC) that are designed to work together to address high-priority science objectives regarding Europa’s geology, composition, and the nature of its ice shell. EIS accommodates variable geometry and illumination during rapid, low-altitude flybys with both framing and pushbroom imaging capability using rapid-readout, 8-megapixel (4k × 2k) detectors. Color observations are acquired using pushbroom imaging with up to six broadband filters. The data processing units (DPUs) perform digital time delay integration (TDI) to enhance signal-to-noise ratios and use readout strategies to measure and correct spacecraft jitter. The NAC has a 2.3° × 1.2° field of view (FOV) with a 10-μrad instantaneous FOV (IFOV), thus achieving 0.5-m pixel scale over a swath that is 2 km wide and several km long from a range of 50 km. The NAC is mounted on a 2-axis gimbal, ±30° cross- and along-track, that enables independent targeting and near-global (≥90%) mapping of Europa at ≤100-m pixel scale (to date, only ∼15% of Europa has been imaged at ≤900 m/pixel), as well as stereo imaging from as close as 50-km altitude to generate digital terrain models (DTMs) with ≤4-m ground sample distance (GSD) and ≤0.5-m vertical precision. The NAC will also perform observations at long range to search for potential erupting plumes, achieving 10-km pixel scale at a distance of one million kilometers. The WAC has a 48° × 24° FOV with a 218-μrad IFOV, achieving 11-m pixel scale at the center of a 44-km-wide swath from a range of 50 km, and generating DTMs with 32-m GSD and ≤4-m vertical precision. The WAC is designed to acquire three-line pushbroom stereo and color swaths along flyby ground-tracks.

Double ridges are sprawling features observed globally across the icy surface of Europa. They consist of two topographic highs flanking a trough. The topographic relief of the ridges is approximately 100 m, and the ridges extend up to hundreds of kilometers in length. The interior structure and dynamics of Europa's ice shell are currently poorly constrained. Therefore, accurate models for the formation of these prominent surface features can be useful for determining how the ice shell operates. We hypothesize that double ridges form as a result of incremental ice wedging. We use both analytical and numerical finite element models to quantify the deformation that occurs as an ice wedge grows incrementally within the ice shell. We show that incremental growth of the ice wedge results in surface deformation that matches the size and shape of typical Europan double ridges, including their topographic relief and surrounding troughs. We find that as the depth of the ice wedge increases, double ridges become broader and shorter. We explore the possibility of local and non‐local sources for the liquid water that freezes to produce the wedge and ultimately argue in favor of local sources of liquid water within the ice shell.

The Europa Imaging System (EIS; Fig. 1) for NASA's Europa Clipper Mission combines a narrow-angle camera (NAC) and a wide-angle camera (WAC) to explore Europa and address high-priority geology, composition, ice shell and ocean science objectives. Both cameras have framing and pushbroom imaging capability, including broadband color filters. EIS data will be used to generate: cartographic and geologic maps; regional and high-resolution topography; GIS, color, and photometric data products; a database of plume-search observations; and control points tied to radar altimetry. Our science goals are to: constrain the formation processes of landforms by characterizing geologic structures, units, and global cross-cutting relationships; identify relationships between surface and sub-surface structures and potential near-surface water detected by ice-penetrating radar; investigate compositional variability between and among landforms and correlate composition be-tween individual features and regional units; search for evidence of recent or current activity, including potential erupting plumes; constrain ice-shell thickness from global shape measurements via limb fits; characterize surface clutter (off-nadir surface reflections that may appear to be subsurface interfaces) to aid interpretation of deep and shallow radar sounding; characterize scientifically compelling landing sites and hazards by determining the nature of the surface at meter scales. Figure 1: EIS NAC (left) and WAC (right) on the spacecraft nadir deck. The NAC is mounted on a 2-axis gimbal. The DPUs are in the spacecraft vault. EIS Narrow-angle Camera (NAC) The NAC has a 2.3° x 1.2° field of view (FOV) with a 10-μrad instantaneous FOV (IFOV), and from an altitude of 50-km achieves 0.5-m pixel scale over a 2-km-wide swath. Its 2-axis gimbal, ±30° cross- and along-track, enables targeting independent of spacecraft orientation, allowing near-global (>90%) mapping of Europa at ≤100-m pixel scale (to date, only ~14% of Europa has been imaged at ≤500 m/pixel), as well as regional stereo imaging. The gimbal slew rate is designed to be able to perform very high-resolution stereo imaging from as close as 50-km altitude during high-speed (~4.5 m/s) flybys to generate digital topographic models (DTMs) with ≤4-m ground sample distance (GSD) and ≤0.5-m vertical precision. The NAC will also perform high-phase-angle observations to search for potential erupting plumes; a pixel scale of 10 km from 106 km range means that the NAC can take advantage of good illumination geometry for forward scattering by potential plumes even when the spacecraft is distant from Europa. EIS Wide-angle Camera (WAC) The WAC has a 48° x 24° FOV with a 218-µrad IFOV, and is designed to acquire 3-line pushbroom stereo and color swaths along flyby ground-tracks. From an altitude of 50 km, the WAC achieves 11-m pixel scale over a 44-km-wide swath, generating DTMs with 32-m GSD and 4-m vertical precision. These data also support characterization of surface clutter for interpretation of radar deep and shallow sounding. Detectors and Electronics The cameras have identical rapid-readout, radiation-hardened 4k x 2k CMOS detectors and can perform both pushbroom and framing imaging. Color observations are acquired by pushbroom imaging using six broadband filters (Table 1; Fig 2), allowing mapping of surface units and correlation with geologic structures, topography, and compositional units from other instruments. Radiation-hard data processing units (DPUs) take advantage of the CMOS rapid, random-access readout and use real-time processing for pushbroom imaging, including: WAC 3-line stereo, digital time delay integration (TDI) to enhance signal-to-noise ratios (SNR), and readout strategies to measure and correct jitter. Table 1: NAC and WAC broad-band, stripe filters Filter Wavelength (nm) Key Uses Clear NAC: 350–1050 WAC: 370–1050 Mapping, stereo, context imaging, best SNR for faint targets, e.g., plume searches NUV NAC: 355–400 WAC: 375–400 (See Figure 2) BLU 380–475 (See Figure 2) GRN 520–590 (See Figure 2) RED 640–700 (See Figure 2) IR1 780–920 (See Figure 2) 1µm 950–1050 (See Figure 2)   Figure 2: EIS broadband filter wavelengths (top) and WAC filter layout (bottom). Summary EIS data sets and collaborative science with other investigations will provide insight into Europa’s global geology, ice shell, and the potential for recent or current activity, to fulfill the goal of exploring Europa to investigate its habitability. Updates will be provided on flight instrument performance from ground testing and expected datasets from the planned tour at Jupiter.

The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus’s five largest “classical” moons—Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon, Puck—but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved data sets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons’ surface compositions, and the processes that modify them, is limited to disk-integrated data sets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and to investigate Uranus’s ring moons and irregular satellites, close-up observations and measurements made by instruments on board a Uranus orbiter are needed.

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.

C-LIFE is a landed camera suite, suitable for Ocean World surfaces, consisting of a color Context Reconnaissance Stereo Imager and an LED flashlight that can also identify biogenic material through fluorescence. The C-LIFE design leverages ongoing work (with industry partners Space Dynamics Lab, SRI International and Ball Aerospace) from our ICEE-2 and COLDTech awards in low-temperature detector qualification, radiation modeling and mechanical design. It takes advantage of our development work on the Descent Imager for Europa Hazard Avoidance and Radiation Durability (DIEHARD) and camera development on other missions. The C-LIFE camera head is mounted on the Europa Lander’s high gain antenna, which provides tilt and pan capability. Minimal electronics in the camera head control and read out the detector and LEDs. Electronics in the lander vault perform most camera functions and image processing. C-LIFE contains no moving parts. In lieu of a focus mechanism, C-LIFE utilizes high F/number optics and a field of view elongated in the vertical direction with progressive focus from top (infinity e.g. imaging horizon) to bottom (close e.g. imaging sample delivery port). Strip filters (that match Europa Clipper’s Europa Imaging System) on the detectors and partly-overlapping images provide color coverage. Heaters warm C-LIFE to operating temperatures, allowing self-heating to take over, but otherwise the camera is unheated in order to conserve energy. We combine two independent eyes into one mechanical housing (Figure 1) with a dual periscope design, which reduces the mechanical envelope, shielding mass, heating energy and total cabling distance. We use LEDs in three bands to illuminate and to excite fluorescence. Fluorescence excited by these three bands can identify the presence of key metabolic biomarkers and discriminate the quantities of live cells, dead cells and spores in a terrestrial setting. Figure 1. A dual-periscope design, with eyes 20 cm apart, folds optical trains (incoming rays in gray) to mutually shield focal planes. Fold mirrors locate LEDs inside the camera for shielding.

Titan has an active methane-based hydrologic cycle¹ that has shaped a complex geologic landscape², making its surface one of most geologically diverse in the Solar System. Despite the differences in materials, temperatures and gravity fields between Earth and Titan, many of their surface features are similar and can be interpreted as products of the same geologic processes³. However, Titan’s thick and hazy atmosphere has hindered the identification of its geologic features at visible wavelengths and the study of its surface composition⁴. Here we identify and map the major geological units on Titan’s surface using radar and infrared data from the Cassini orbiter spacecraft. Correlations between datasets enabled us to produce a global map even where datasets were incomplete. The spatial and superposition relations between major geological units reveals the likely temporal evolution of the landscape and provides insight into the interacting processes driving its evolution. We extract the relative dating of the various geological units by observing their spatial superposition in order to get information on the temporal evolution of the landscape. The dunes and lakes are relatively young, whereas the hummocky or mountainous terrains are the oldest on Titan. Our results also show that Titan’s surface is dominated by sedimentary or depositional processes with a clear latitudinal variation, with dunes at the equator, plains at mid-latitudes and labyrinth terrains and lakes at the poles.