LA-UR- 0 Approved for public release; distribution is unlimited. Title: EVIDENCE OF WATER ICE NEAR THE LUNAR POLES Author(s): W. C. Feldman, LANUNIS-1 (SEE ATACHED SHEET FOR ADDITIONAL AUTHORS) Submitted to: Lunar and Planetary Sciences Conference 32nd Lunar and Planetary Institute Houston, TX March 12-16,2001 Los Alamos NATIONAL LABORATORY Los Alamos National Laboratory. an affirmative action/equal opportunity employer, is operated by the University of California forthe US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher’s right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (WOO) S. Maurice, Observatoire Midi-Pyrenees David J. Lawrence, LANLNS-1 R. C. Little, LANLR-5 Stefanie L. Lawson, LA"IS-1 Olivier Gasnault, LANLLNIS-1 Roger C. Wiens, LANLNS-1 Bruce L. Barraclough, LAWNS-1 Richard C. Elphic, LA"IS-1 Thomas H. Prettyman, LA"IS-1 John T. Steinberg, LANLNS-1 A. B. Binder, Lunar Research Institute Evidence for Water Ice Near the Lunar Poles. W.C. Feldman', S. Maurice', D.J. Lawrence', R.C. Little', S.L. Lawson', 0. Gasnault', R.C. Wiens', B.L. Barraclough', R.C. Elphic', T.H. Prettyman', J.T. Steinberg', and A.B. Binder3. 'Los Alamos National Laboratory, Los Alamos, New Mexico; 'Observatoire Midi-PyrCnCes, Toulouse, France; 3Lunar Research Institute, Tucson Arizona. Lunar Prospector epithermal neutron data small craters that harbor permanent shade [4]. were studied to evaluate the probable chemical The temperatures within many of these craters are state of enhanced hydrogen, [HI, reported low enough [5] that they can disable sublimation previously to be near both lunar poles [1,2]. as a viable loss process of [H'O]. It is therefore Improved versions of thermal and epithermal tempting to postulate that the enhanced hydrogen neutron data were developed for this purpose. within most regions of permanent shade is in the Most important is the improved spatial resolution form of water molecules. This postulate is obtained by using shortened integration times. A certainly viable within the bottoms of several new data set was created, Epi* = [Epithermal - large, permanently shaded craters near the south 0.057xThermal1, to reduce effects of composition pole. Predicted temperatures within them [5] fall variations other than those due to hydrogen. well below the 100 K temperature that is needed The Epi* counting rates are generally low to stabilize water ice for aeons. near both lunar poles and high over terrane near The picture is different near the north pole. recent impact events such as Tycho and Jackson. Here, there are relatively few permanently-shaded However, other lunar features are also associated craters that are large enough to harbor with high Epi* rates, which represent a wide temperatures that are sufficiently low to stabilize range of terrane types that seem to have little in water ice indefinitely against sublimation [5]. common. If we postulate that one property all Instead, the 'inter-crater' polar plains are a jumble bright Epi* features do have in common is low of many permanently-shaded craters that have [HI, then measured Epi* counting rates appear to diameters less than 10 km [4]. Although be quantitatively self consistent. If we assume simulations of temperatures within this class of that [H]=O above the top 9gthpercentile of Epi* craters show they are only marginally cold counting rates at 2" x 2" spatial resolution, then enough to indefinitely stabilize water ice [5], this [HI,,, = 55 ppm for latitudes equatorward of 175'1. terrane appears to have the highest [HI. This value is close to the average found in Nevertheless, predicted temperatures are close returned lunar soil samples, [HI,,, - 50 ppm [31. enough to that needed to permanently stabilize Using the foregoing physical interpretation of [H'O] to suggest that sublimation is indeed the Epi* counting rates, we find that the Epi* counts process that discriminates between polar terrane within most of the large craters poleward of +70° that contains enhanced [HI and those that do not are higher, and therefore [HI is lower, than that in (see, e.g., the temperature estimates for doubly- neighboring inter-crater plains, as shown in shaded craters [6]). If correct, then an important Figure 1. Fourteen of these craters that have areas fraction of the hydrogen near the north pole must larger than the LP epithermal spatial resolution be in the form of H20, which then resides within (55 km diameter at 30 km altitude), were singled these small craters. out for study. [HI is generally found to increase Estimates using our improved data set of [HI with decreasing distance from the poles (hence within craters near the south pole remain decreasing temperature). However, quantitative unchanged from those derived from our previous estimates of the diffusivity of hydrogen at low analysis [2], [HI = 1700&900 ppm. This temperature show that diffusion can not be an translates to [HzO]=1.5+0.8%. If all of the important factor in explaining the difference enhanced hydrogen in the north is in the form of between the relatively low [HI observed within HzO and is confined to the jumble of small the large sunlit polar craters and the relatively permanently-shaded craters identified by radar high [HI in neighboring inter-crater plains. [4], then we can estimate their water-ice fraction, A closer look at the 'inter-crater' plains near [H'O], using Figure la in [2]. We chose two the poles, shows that they are covered by many regions near the north pole for this purpose. They each have areas just larger than the surface foot-print of the LP epithermal neutron permanent shade), if correct, the resultant [HI spectrometer. The first was an inter-crater region content is significantly higher than what can be nestled between Rozhdestvenskiy and Plaskett, delivered from the solar wind to, and retained by, and the second covered the southeast comer of the lunar polar terrane. Such a large value for Peary. Using Figure 3 of [4], the first area by itself, rules in favor of the existence [HJof contains 232 km2 of measured permanent shade, significant water ice deposits at both lunar poles. and the second contains 129 km2. Adopting the References: [l] Feldman, W.C., et al. prescription used in Table 1 of [4] for estimating (1998) Science, 281, 1496-1500. [2] Feldman, actual from sampled shaded areas, multiplication W.C., et al. (2000) J. Geophys. Res., 105,4175- of sampled areas by 1.5 yields permanently 4195. [3] Haskin, L., and P. Warren (1991) in shaded areas that amount to 350 km2 in region 1, Lunar Sourcebook, a User's Guide to the Moon, and 200 km2 in the southeast comer of Peary. G. Heiken, D. Vaniman, B.M. French, eds., pp Comparison of the Epi* counts for both regions 357-474. [4] Margot, J.L., et al. (1999) Science, with the curves in Figure la of [2] yields an -284, 1658-1660. [5] Vasavada, A.R., et al. (1999) estimate for [H20] that is greater than 10%. Zcarus, 141, 179-193. [6] Carruba, V., A. Although this estimate is very uncertain Coradini (1999) Zcarus, 142,402-413. (reflecting our large uncertainty in area of E pi lherrrial* 127 128 130 132 134 13G 138 140 142 144 14G -180" North Pole ) 'io' South Pole ( -70' East Longitude East Longitude Figure 1. Overlays of stereographic projections of the Epi* neutron flux and Clementine 750 nm intensity poleward of +70° (left) and -70" (right) latitudes. .