Laser altimeter for Enceladus: Scientific objectives and design approaches

Kurzfassung

Enceladus, a moon of Saturn, presents one of the most promising environments in the Solar System to discover habitable conditions beyond Earth. Following the observation of cryovolcanic activity by the Cassini mission, Enceladus has become a focal point for planetary exploration. Both NASA and ESA have identified Enceladus as a primary target for future exploration endeavors. A significant proportion of scientific questions regarding Enceladus can be addressed through the measurement of its geodetic parameters (e.g., rotation state, tidal deformation) and surface characterization in terms of topography and reflectance. A laser altimeter is an ideal instrument for accomplishing this task. Laser altimeters have been widely employed on space missions, particularly for targets that have not been explored by orbiting spacecraft (e.g., Ganymede, Mercury, and numerous asteroids). Furthermore, when it comes to detecting relatively small tidal deformations, a laser altimeter is capable of providing the required level of accuracy. However, the capabilities of a laser altimeter extend beyond this application. Specifically, for Enceladus, a laser altimeter can address the following scientific objectives: Measure the global shape/topography. Characterize potential landing sites in terms of landing safety. Measure the surface roughness at baselines as small as a few meters. Measure the libration amplitude. Determine the tidal Love number h2 by measuring the vertical tidal deformation. Constrain the thickness and the rheological properties of the ice shell. Quantify the tidal heat dissipation by measuring the tidal phase lag. Determine the extent of the sub-surface ocean and the size of the silicate core. Determine the ice particle size on the surface by measuring the surface albedo. For the purpose of particle size measurement, we exploit a water ice absorption band situated at 1040 nm, which is in close proximity to the laser wavelength of 1064 nm, where albedo measurements are conducted. The depth of this absorption band exhibits a correlation with the diameter of ice particles, although the decrease in reflectance (for larger particle diameters) is moderate, resulting in a surface albedo exceeding 80% within the absorption band (Stephan et al., 2021). The accomplishment of the aforementioned objectives would enable addressing key scientific questions pertaining to the formation and evolution of Enceladus, as well as the elucidation of the mechanism driving its cryogenic activity. Instrument concepts with multiple laser beams and detector arrays, both with classical avalanche diodes as well as single-photon counting diodes, are under consideration and will be traded-off against the science objectives and the required resources. One potential design approach for a laser altimeter tailored to Enceladus can be obtained by utilizing the concept of the Ganymede Laser Altimeter (GALA, Hussmann et al., 2019), currently en route to Jupiter. By adapting the design of GALA to Enceladus it is anticipated that the power, mass, and volume of the instrument can be significantly reduced due to the twofold increase in surface albedo compared to Ganymede. Furthermore, improvements in volume and mass budgets are expected to be achieved through the implementation of a combined optical design for the transmitter and receiver. In our study, we use recently discovered orbits around Enceladus, which provide comprehensive global coverage and minimal distances to the surface (Parihar et al., 2024). The average altitude for these orbits is approximately 100 km, with a mean ground velocity of 100 m/s. Employing the same pulse divergence of 100 µrad (full cone) as utilized in GALA, we would obtain footprints of 10-meter diameter on the surface. The relatively slow ground velocity permits moderate sampling frequencies on the order of 20 Hz, thereby enabling overlapping footprints along the track (with a 5 m distance between footprint centers). Given the high surface albedo, the aperture of the receiver telescope and the pulse energy of the emitted pulse can be significantly decreased. Considering an aperture of 8 cm, the pulse energy can be reduced by approximately a factor of 10 compared to GALA, while maintaining equivalent detection performance. This would allow for the utilization of a low optical-power laser with an output power of only a few tens of mW, thereby significantly reducing the costs associated with the instrument.