LRO Diviner Nonlinear Response and Opposition Effect Corrections

Abstract

Aboard the Lunar Reconnaissance Orbiter, the Diviner Lunar Radiometer Experiment measures thermal radiation to determine the brightness temperature of the lunar surface. As with the Mars Climate Sounder (upon which Diviner is based), we use pre-flight calibration data to correct for the nonlinear response in Diviner’s detectors, which in-turn accounts for much of the detector non-uniformity within channels. Furthermore, channels 8 and 9 exhibit unexpectedly high brightness temperatures close to the equator around midday, with even higher brightness temperatures when observing lunar highlands as opposed to maria. Unexpectedly high brightness temperatures around midday at the equator is reminiscent of the opposition effect known to exist on the Moon at low phase angles in Visual to Near Infra-Red (VNIR) wavelengths. Diviner channel 2 data (which detects solar radiation reflected by the Moon) shows this opposition effect, which is more pronounced in the highlands than the maria. We interpret a correlation we observe between channel 2 detected radiance and channel 8 and 9 brightness temperature as due to incomplete blocking of reflected solar radiation. This leads us to an opposition effect correction for Diviner channels 8 and 9 dependent on Diviner’s solar channel data. Whether this is a direct leak of VNIR light upon the detectors, or solar heating of blocking filters, which then radiate infrared radiation upon the detectors, is yet to be determined. We can use the nonlinearity and opposition effect corrections to recharacterize the spectral emissivity of the lunar regolith, which we can then compare to laboratory spectra.

Introduction

The Lunar Reconnaissance Orbiter (LRO)’s Diviner Radiometer has observed the Moon for the past eight years, continuously measuring lunar radiance across 9 channels (of 21 thermophile detectors each) from the visible to far infrared wavelengths (0.35 to 400 \(\mu\text{m}\)) (Paige 2009). This data is used to characterize the Moon’s thermal environment and composition. Within the same channel, we expect thermophile detectors to return measurements with only slight differences due to the spread of the detectors’ views of the lunar surface. However, the spread of observed brightness temperatures is greater at higher temperatures of the longer wavelength channels. Furthermore, we see channels 8 (50-100 \(\mu\text{m}\)) and 9 (100-400 \(\mu\text{m}\)) measurements exceed those of channels 3-5 (7.8, 8.25, and 8.55 \(\mu\text{m}\) respectively) at low solar phase angle (the angle between the observer, the observed, and the sun), where channels 3-5 were designed to detect the emissivity maximum of lunar materials. Herein, we exhibit probable causes of these issues, and demonstrate how they can be corrected.

Instrument Overview

Diviner is composed of 9 channels, as shown in Table 1. Each of these nine channels has 21 detectors that would ideally be observing the same thing at the same time. This paper focuses on the nonlinear correction of the non-Solar channels.

Diviner spectral channel passbands and measurement functions (Paige 2009)
Channel Number Channel Type Channel Name Passband \(\mu\text{m}\) Measurement function
1 (A1) Solar High Sensitivity Solar 0.35-2.8 Reflected solar radiation, high sensitivity
2 (A2) Solar Reduced Sensitivity Solar 0.35-2.8 Refelcted solar radiation, reduced sensitivity
3 (A3) 8 \(\mu\text{m}\) 7.8 \(\mu\text{m}\) 7.55-8.05 Christiansen feature
4 (A4) 8 \(\mu\text{m}\) 8.25 \(\mu\text{m}\) 8.10-8.40 Christiansen feature
5 (A5) 8 \(\mu\text{m}\) 8.55 \(\mu\text{m}\) 8.38-8.68 Christiansen feature
6 (A6) Thermal 13-23 \(\mu\text{m}\) 13-23 Surface temperature (most sensitive channel for >178 K)
7 (B1) Thermal 25-41 \(\mu\text{m}\) 25-41 Surface temperature (most sensitive channel for 43-69 K)
8 (B2) Thermal 50-100 \(\mu\text{m}\) 50-100 Surface Temperature (most sensitive channel for 43-69 K)
9 (B3) Thermal 100-400 \(\mu\text{m}\) 100-400 Surface temperature (most sensitive channel for <43 K)