High-Resolution Morphological and Spectrophotometric Analysis of the Moon Using JANUS Observations from the LEGA Flyby

Introduction The JANUS (Jovis, Amorum ac Natorum Undique Scrutator) imaging system [1], onboard ESA’s JUICE mission, had the unique opportunity to observe both Moon and Earth during the Lunar-Earth Gravity Assist (LEGA) maneuver on August 19–20, 2024. This flyby represented the first opportunity to operate JANUS under conditions like those expected in the Jovian system. The JANUS telescope, a modified Ritchey-Chrétien design with a 103.6 mm aperture and 467 mm focal length, uses a Teledyne-e2v CMOS detector and 13 filters spanning 340–1080 nm. During LEGA, the instrument observed the Moon’s dayside, capturing imagery across a wide latitudinal and longitudinal track. Observational planning considered operational constraints, such as filter switching times, data volume limits, and rapid spacecraft motion. The collected data supported performance verification, calibration refinement, and validation of data processing tools, effectively preparing JANUS for its forthcoming scientific operations in the Jovian system. In addition, such a flyby enables us to conduct unique scientific investigations of the Moon thanks to high-resolution and multifilter images acquired.

 

JANUS Observation of the Moon During the Moon flyby, due to operational constraints, JANUS was switched on 1 hour before the beginning of image acquisition. To ensure thermal stability at the time of observation, the S/C survival heaters were used to bring the telescope to the optimal temperature. Imaging began shortly before crossing the lunar terminator and continued beyond limb crossing to capture stray light. Observations covered latitudes 17°S–16°N and longitudes 107°E–7°W, encompassing prominent lunar features such as the LaPérouse and Langrenus craters, Mare Fecunditatis, Sinus Asperitatis, and the southern portion of Mare Tranquillitatis. Initial imaging used the panchromatic filter with lossless compression, followed by multi-filter sequences (4 to 13 filters) under varying solar incidence angles (90° to 29°), enabling detailed coverage and performance assessment. Among the regions observed by JANUS, our investigation focuses on two specific areas, detailed below.

 

Langrenus crater: Langrenus is a prominent lunar impact crater located at approximately 8°S, 61°E, on the eastern boundary of Mare Fecunditatis. Measuring ~132 km in diameter and ~2.7 km in depth, it exhibits a well-preserved, terraced rim and steep inner walls, characteristic of complex craters. A central peak structure rises ~1 km above the crater floor, indicative for gravity-driven crater modification. During the LEGA flyby, the JANUS camera acquired high-resolution imagery of Langrenus (~20 m/pixel) using four distinct filters: Blue (450/60 nm), Red (646/60 nm), NIR1 (910/80 nm), and NIR2 (1015/130 nm), as shown in Fig.1. These data support both morphological and spectrophotometric analyses of the crater. A detailed geological map is in preparation to distinguish the geomorphological and structural units across the crater. Langrenus also serves as a stratigraphic window into the adjacent titanium-rich mare basalts. A Digital Terrain Model (DTM) derived from the combination of JANUS imagery and other lunar dataset will enable refined topographic analysis. Complementary spectral analysis is underway to identify the presence of mafic minerals such as olivine and pyroxenes. As illustrated in Fig. 1, the derived RGB composite reveals bluish regions consistent with pyroxene-bearing material, offering valuable insights into the compositional diversity of the site.

Mare Fecunditatis Area: We are conducting a detailed analysis of the boundary between Mare Fecunditatis and Mare Tranquillitatis, focusing on the transition zone between the lunar maria and adjacent highlands. This region was imaged by the JANUS instrument at spatial resolutions ranging from 30 to 60 m/pixel, using various filter sequences. The observations reveal a variety of geological features, including wrinkle ridges, small impact craters, and rounded volcanic domes.

As illustrated in Fig. 2, a representative segment of the Mare Fecunditatis–highland boundary was imaged using five JANUS filters (Violet 380/80 nm, Blue 450/60 nm, Red 646/60 nm, NIR1 910/80 nm, and NIR2 1015/130 nm). From these data, both standard RGB and Clementine-like RGB composites were generated (e.g., R=750/430, G=750/1015, B=430/750). The resulting imagery demonstrates strong consistency with previous mission datasets while offering enhanced spatial resolution for improved geological interpretation.

Ongoing spectral analysis will further investigate compositional variations across the mare–highland interface, with the goal of identifying localized mineralogical concentrations and enhancing our understanding of lunar crustal evolution.

Discussion and future works

The JANUS imaging system, as demonstrated during its observation of the Moon during the LEGA maneuver, has proven its capability to capture high-resolution, multi-filter imagery under conditions similar to those expected in the Jovian system. The ability to observe lunar features across a broad latitudinal and longitudinal range provides a comprehensive dataset for both performance verification and scientific investigations. Detailed analyses of the Langrenus crater and the Mare Fecunditatis region highlight the potential for understanding lunar geological processes, especially in terms of compositional and structural variations. Fresh highlands materials are blue, fresh mare materials are yellowish, and mature mare soils are purplish or reddish. Fresh crater rims appear cyan and due to its color fresher and mature basalt units can be distinguished to their yellowish and reddish color (Fig.2) partly showing sharp geological boundaries. In the next future, we will advance the investigation of JANUS observations and integrate with other lunar datasets, such as topographic and spectral data, will allow for an enhanced interpretation of the lunar surface, enabling insights into the history and evolution of both impact and volcanic processes.

Acknowledgement: JANUS has been funded by the respective Space Agencies: ASI (lead funding agency), DLR, Spanish Research Ministry and the UK Space Agency. Main hardware-provider Companies and Institutes are Leonardo SpA (Prime Industry), DLR-Berlin, CSIC-IAA and Sener. PI and Italian team members acknowledge ASI support in the frame of ASI-INAF agreement n. 2023-6-HH.0. We gratefully acknowledge funding from National Institute of Astrophysics through the INAF - Mini Grant RSN3 RIFTS project (d.d. 5/2022). Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA

References [1] Palumbo, P., et al., 2025, SSR. [3] Grasset, O., et al., (2013). PSS, 78, 1-21.