Patrice Smith
Studying lunar mineralogy provides information about the Moon’s origin and evolution. Of particular interest is the presence of absorbed lunar water or water-like molecules(H2O/OH) and its implications for the volatile inventory available to future astronauts to the Moon looking for resources in future lunar exploration missions. Three spacecraft have detected H2O/OH absorption features near 3-um on the Moon. Despite the agreement on the existence of the 3-um absorption features, differences in the corrections of the Moon’s thermal contribution has led to two different conclusions on the diurnal strength of these features near the lunar sub-solar point. Thus, we have collected and analyzed ground-based observations of the sub-solar point at multiple observing timescales to provide an independent study on the behavior of the Moon’s 3-um absorption features.
The ground-based observations of the Moon were made using the NASA InfraRed Telescope Facility (IRTF), and the location focused on was the sub-solar point where the Sun’s light rays hit the Moon’s surface perpendicularly. Data were collected using the long wavelength cross dispersed mode (LXD_short) with wavelength range of 1.76um to 4.2um and resolving power of 2,500.
To get a representative spectrum of the 3-um absorption feature, the contribution of the thermal radiation imparted on the spectrum needs to be corrected. Thermal models identify the thermal excess of a spectrum and can be used to approximate the flux contribution that needs to be removed. In this study, the Near-Earth Asteroid Thermal Model was used to remove the Moon’s thermal excess in the sub-solar point spectra.
Our preliminary thermally corrected spectrum shows an absorption feature around 3um. A confirmed detected presence of OH/H2O for the subsolar point would establish hydration of minerals near the lunar noon time of day.
Using the NASA IRTF different locations on the Moon have been observed at different lunar times of day. A lunar day is the equivalent measure of New Moon to New Moon being sunset to sunrise and Full Moon being noon. The locations observed were the poles, mid latitiudes, mare, highlands, and sub-solar point. The data taken were spectra in the near infrared from 1.67um to 4.2um. Around 3um is where a water/hydroxyl absorption line can be found. The SpeX instrument on the IRTF produced low to medium resolution spectra that was spatially resolved and could be reduced in the software SpeXTool. After data reduction, which included flatfield and wavelength calibrations, extracting and combining spectra, standard star corrections, telluric corrections, and merging multi-order spectra, thermal emission from the blackbody radiation of the Moon still dominated the spectra.
To isolate spectral features due to the absorption of surface material, the infrared reflected sunlight needed to be isolated from the thermal infrared radiation due the Moon’s blackbody emission. While previous models had been employed with differing results, the model used here is the Near Earth Asteroid Thermal Model. The assumption of NEATM is that the application be to a spherical body with negligible thermal emission out it’s backside and that it be non-rotating. An important adjustment made during the application of the model is the beaming parameter which is a tweaking of the model to account for the light scatter change due to the bolderiness of the surface. Here, we present initial evidence of a water/hydroxl absorption feature in the spectrum of a subsolar observation.