Lunar and Planetary Science XXX 1892.pdf

BRIGHTNESS TEMPERATURES OF THE LUNAR SURFACE: THE CLEMENTINE LONG-WAVE INFRARED GLOBAL DATA SET. S. L. Lawson and B. M. Jakosky, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-0392 ([email protected]).

Introduction: Observers have been measuring the with emissivity e, Stefan-Boltzmann constant s, MoonÕs thermal emission for many years [1, 2, 3, 4]. albedo A, solar constant So, and Sun-Moon distance R In 1930, Pettit and Nicholson first measured the full in AU. Assuming unit emissivity, the Lambertian Moon surface emission to be non-Lambertian [1]. They temperature T is the temperature of a black body found the temperature along the lunar equator radiating the same power that is absorbed from the empirically varied as cos1/6(i), as opposed to the Sun. cos1/4(i) variation expected from a Lambertian surface; Figure 1 shows a comparison of LWIR data with the temperature away from the subsolar point decreased calculated Lambertian temperatures as a function of more slowly than would be expected from a latitude. The variation of brightness temperature as Lambertian surface. In the 1960s, Sinton [3] measured cos1/6(i) is also plotted. The data points in Figure 1 are the subsolar point brightness over a lunation and also LWIR image-averaged temperatures for ten Clementine found that the angular brightness dependence was not orbits (152-156 and 168-172) where the Sun was that of a spherical Lambertian surface. Both these within one degree of local noon (i » latitude). No observations have been interpreted as an effect of assumptions about lunar surface thermal properties average lunar surface roughness. The limb of the full were made when reducing the plotted LWIR data. The Moon is brighter than that expected from a smooth red (upper) line represents the cos1/6(i) temperature surface because the randomly oriented surfaces of rough variation of Pettit and Nicholson [1], whereas the green particles radiate more efficiently in the direction of the (lower) line shows the Lambertian temperature observer than a similar smooth surface would. We find variation of cos1/4(i). The 2s errors for the LWIR data that nadir-looking observations of the lunar surface do are approximately 8 K near the equator and increase to not exhibit this anomalous behavior, but instead vary over 40 K at high latitudes. as cos1/4(i). The scientific payload on the Clementine spacecraft included a Long-Wave Infrared (LWIR) camera with a single passband of width 1.5 mm centered at a wavelength of 8.75 mm. The Clementine orbit deviated ±30û from sun synchronous, and for two lunar months dayside nadir-looking images were obtained near local noon [5]. Contiguous pole-to-pole imaging strips were obtained with approximately 10% overlap between adjacent frames. However, significant longitude gaps exist between successive orbital passes. During the systematic mapping phase of the Clementine mission, approximately 220,000 thermal-infrared images of the lunar surface were obtained. Observed LWIR radiances can be converted to brightness temperatures which provide information on various physical properties of Figure 1. LWIR image-averaged temperatures and the lunar surface. equilibrium models as a function of latitude. For the Analysis: A simple thermal equilibrium model models, i = latitude, e = 1, A = 0.15, and R = 1 AU. was used to calculate expected brightness temperatures of the lunar surface. Because the lunar surface layer is Discussion: The albedo used for the model highly insulating and the LWIR observations were temperatures in Figure 1 is 0.15, which is a highlands very near local noon, conduction into the regolith was average. The longitude ranges for the Clementine neglected. The Moon was assumed to be a smooth orbits plotted in Figure 1 are 350û-5û and 30û-45û. At spherical object in instantaneous equilibrium with the these longitudes, there are maria at mid-latitudes. solar insolation, and the local temperature T varied Since maria have lower albedos than highlands (values with solar incidence angle i as: typically ranging from 0.07 to 0.10), it is expected that 14/ TT= cos ( i ). the equatorial data in Figure 1 are warmer than the SS The subsolar temperature T was calculated from models. It is clear from Figure 1 that the LWIR SS temperatures do not follow a cos1/6(i) temperature 4 S o variation, but instead are well fit by the Lambertian esTA=-()1 SS 2 temperature model. This is expected due to the fact R that the LWIR observations are nadir-looking, where Lunar and Planetary Science XXX 1892.pdf

BRIGHTNESS TEMPERATURES OF THE LUNAR SURFACE: S. L. Lawson and B. M. Jakosky

the effects of surface relief average out over a single due to the large errors there. Figure 2 clearly shows frame. A cratered surface may appear warmer than a that the Moon is hotter at the equator than it is at the smooth surface when viewed from overhead during the poles. Many lunar features stand out in Figure 2, such lunar day. In addition, a surface with rocks may appear as Mare Orientale, Mare Crisium, and the Apennine cooler than a smooth surface. These two effects on Mountains. At equatorial latitudes, the nearside maria observed temperature can be thought to cancel when are warmer than the farside highlands. Figure 2 positive and negative relief are viewed together from demonstrates that the dayside lunar thermal emission overhead. is largely governed by albedo and by the intensity of Figure 1 demonstrates that a Lambertian surface is solar radiation. adequate for modeling nadir-looking temperature Conclusions: The LWIR data from noontime observations of the lunar surface. The LWIR data were orbits demonstrate that the Lambertian temperature observed at incidence angles of ±30û from local noon, model of cos1/4(i) is a fair approximation for nadir- and at different Sun-Moon distances. Thus, in order to looking temperatures. Deviations from the Lambertian compare LWIR data from all of the Clementine orbits model are likely due to surface roughness effects and we used the following formulation: variations in infrared emissivity. In addition, the 2 LWIR global data set reveal the dayside lunar thermal 44 cos(inoon ) Rmeasured emission to be largely governed by albedo, by the TT= , noon measured 2 intensity of solar radiation, and by the emission angle cos(i ) R measured noon of observations. where inoon is the image latitude minus the latitude of References: [1] Pettit, E. and S.ÊB. Nicholson the sub-solar point and Rnoon is 1 AU. Clementine (1930). Astrophys. J. 71, 102-135. [2] Sinton, W. month 2 data was multiplied by a scale factor, but the (1962). Chapter 11 in Physics and Astronomy of the resulting temperatures remained within the 2s errors. Moon, ed. Kopal. [3] Saari, J.ÊM. and R.ÊW. Shorthill Figure 2 shows the LWIR image-averaged (1972). Moon 5, 161-178. [4] Mendell, W.ÊW. and temperatures adjusted to local noon and interpolated F.ÊJ. Low (1975). Proc. Lunar Sci. Conf. 6th, 2711- onto a new grid of one square degree intervals. Data 2883. [5] Nozette, S., et al. (1994). Science 266, gaps (longitudinal stripes) were filled via a simple 1835-1839. smoothing routine and high latitudes were neglected

Figure 2. LWIR global mosaic of image-averaged temperatures adjusted to local noon, interpolated onto a new grid of one square degree intervals, and smoothed over longitudinal gaps.