Developing Oxford’s Enceladus Thermal Mapper (ETM)

Introduction: Enceladus Thermal Mapper (ETM) is an Oxford-built high-heritage instrument that is being developed for outer solar system operations. ETM is based upon the design of Lunar Thermal Mapper (LTM, launched on Lunar Trailblazer, Fig. 1). It has a strong heritage story, including MIRMIS (on Comet Interceptor), Compact Modular Sounder (on TechDemoSat-1) and filters shared with Lunar Diviner (on Lunar Reconnaissance Orbiter).

 

ETM is a miniaturized thermal infrared multispectral imager, with space for 15 spectral channels (bandpasses) that can be tailored to the mission requirements. It consists of a five-mirror telescope and optical system and an uncooled microbolometer detector array. Real-time calibration is achieved using a motorized mirror to point to an onboard blackbody target and empty space. ETM has an IFOV of 35 mm, so assuming a 100 30 km orbit it will have a spatial resolution of 40 to 70 m/pixel and a swath width of 14 - 27 km.

 

ETM Updates: Through UK Space Agency funding we have developed three areas of ETM: its filter profile, radiation tolerance and sensitivity to Enceladus-like surfaces.

 

Filters: ETM is a push broom thermal mapper, which works by the detector being swept over a surface. Each of the detector’s 15 channels is made up 16 rows, which are coadded to increase the signal to noise. A recently completed preliminary study has updated ETM’s bandpasses to include filters between 6.25 mm and 200 mm to enable it to detect Enceladus’ polar winter (<50 K), nighttime (50-60 K), daytime (70-80 K) and active region temperatures (>170 K). Depending on the mission goals not all channels need to be utilised to achieve this, making some available for additional studies (e.g. searching for salt).

 

Radiation: The radiation environments of Enceladus are vastly different to those of the Moon. Recent radiation testing and analysis showed that the majority of ETM’s existing design is already highly radiation tolerant. With some additional shielding and one component change all parts can reach the radiation hardness required to operate in the Saturn-system. The additional shielding may be provided by the spacecraft structure, depending on the adopted design.

 

Sensitivity: ETM’s sensitivity to cryogenic surfaces is currently predicted through a well-characterised model. However, as part of the LTM calibration campaign we plan to directly measure its sensitivity to <50 K surfaces, comparable with those observed on Enceladus. The experiment and the required bespoke components have been designed, and the additional equipment required has been procured. We anticipate the testing will continue through 2025 and 2026 as part of the LTM calibration campaign.

 

Conclusion: Enceladus Thermal Mapper has strong heritage for remote sensing of airless bodies in the solar system. This work has strengthened the instrument design making it even more suitable for long-term operations in outer solar system environments and observing surfaces at cryogenic temperatures. This makes ETM the ideal choice for future missions to study surfaces of outer solar system moons, asteroids, comets and other such targets.

 

Acknowledgements: We thank the UK Space Agency’s Bilateral Program for its support, making this work possible.