Lunar Heat Flow Radiometer

Motivation: The Lunar Heat Flux Radiometer (HFR) will peer into the lunar subsurface using microwave wavelengths (~10-200cm, 300MHz to 6GHz and a 90Ghz surface radiometer), providing a measurement of geothermal heat flux without need for drilling. Though this is primarily aimed to be a landed instrument for DALI, this instrument can equally be used as an orbital measurement. We place its current state of development at TRL 4, but we can quickly bring it to TRL 6+ by leveraging our team’s experience with Earth-orbiting radiometer instruments. The upwelling surface emission at microwave wavelengths is a function of the sub-surface temperature profile and dielectric material properties. For solid planetary bodies, surface transparency in microwave wavelengths varies mainly due to changes in minerology (dominated on the Moon by the dielectric properties of the mineral ilmenite) and density (i.e., subsurface rocks). Physical temperatures vary mainly with regolith thermal inertia (influenced by the presence of subsurface rocks or ice) and, at depth, geothermal heat. By observing radiation from multiple frequencies between 300MHz and 6GHz, our instrument will measure: 1) dielectric properties, 2) depth of bedrock, buried rocks and ground ice, and 3) subsurface temperatures that constrain geothermal heat flux. China’s Chang’E 1 and 2 orbiter missions carried two 4-channel (3.0, 7.8, 19.35, and 37 GHz) microwave instruments into lunar orbit. These measurements have been used to reconstruct regolith layer thickness, dielectric properties, subsurface temperatures, and geothermal heat flow. Although it has given tantalizing information and provide an excellent proof of concept as to the possibilities of a microwave instrument, it used off-the-shelf frequencies not specifically optimized for subsurface observations. Our goal is to design a purpose-built instrument to measure near surface density, dielectric properties, and geothermal heat flux.

We will develop and test a prototype instrument for remote lunar heat flow. This includes developing the ultra-wideband antenna, radiometer front-end with embedded calibration sources and the spectrometer backend. The instrument will capable of measuring calibrated upwelling brightness in the range of 300 MHz to 6 GHz. The instrument will leverage technology developed for Earth and planetary instruments, including ultra-low power CMOS spectrometer chips. We will first perform a study to refine the instrument performance parameters and create a set of key and driving requirements. Design studies for each sub-system will lead to sub-system prototyping efforts and finally a fully functional engineering model that will undergo testing to raise the TRL to 6.

Relevance: The proposed effort is relevant to the DALI call in the following ways: 1. By enabling new measurements of regolith stratigraphy and heat flow we answer central objectives of “The Scientific Context for the Exploration of the Moon”, specifically Characterizing the thermal state of the interior heat engine, and determining the physical properties and the vertical extent and structure of the regolith. 2. In addition to surface lunar investigations, the HFR could be used as an orbital instrument. 3. The TRL of the proposed instrument is raised from 4 to 6 by retiring technology risks and allowing insertion into future lunar mission instrument calls. 4. The effort continues to drastically lower the weight and power of the key instrument technology. More »