Microsymposium 36, MS087, 2002

PERMANENTLY SHADED AREAS AT THE LUNAR POLES DURING THE PERIOD OF REGRESSIONOF THE LINE OF NODES V. V. Shevchenko1, E. A. Kozlova1. 1.Sternberg State Astronomical Institute, 119899,Moscow, Russia.

The orbit of the Moon is very nearly circular(eccentricity ~ 0.05) with a mean separation from theEarth of about 384,000 km, which is about 60 Earthradii. The plane of the orbit is tilted about 5,13° degreeswith respect to the ecliptic plane. The Moon's rotationaxis is not fixed in space. The regression of the line ofnodes of the Moon’s orbit has amplitude of 9” inobliquity and varies over a period of 18,6 years [1]. Themaximum altitude of the Sun above the celestial equatorat midday is 1,53 °. So, the high of the limb of the Sunabove the horizon is given by

β = 90°- ϕ + 1,53° + δ

There ϕ - is latitude of the crater, δ =0,25°- is the half ofvisual diameter of the Sun.

Taking account of these movements, only centralpart of fresh deepest craters near poles might lie inconstant shadow during the all period of regression ofthe line of nodes of the Moon’s orbit. Water ice depositscan preserve in such craters.

We used the frustum of cone model for large craterswith diameter larger than 10 km (see the fig. 1).Geometrical parameters characterizing our model arethe diameter of crater D, the depth of crater h, the floordiameter d and the wall slope α. Parameters have beenstudied in [2] according the size and degree ofdegradation of crater. Positive direction of X- axis is thedirection of lunar equator, S- axis is the direction of thecenter of the Sun, and Z- axis is the normal direction oflunar surface.

Figure 1. The permanently shaded areas for lunarsummer marked in blue and for the all period ofregression of the line of nodes of the Moon’s orbitmarked in black.

We researched the regions with latitude higher than60° for both hemispheres of the Moon. 950 craters and1127 craters with diameters larger than 10 km wereconsidered for North and South Polar regions,respectively [3]. The areas, permanently shaded duringthe all period of regression of the line of nodes of theMoon’s orbit Sp, exist in 88 craters in the North Pole

region, and in 103 craters in the South Pole region. Theareas of these terrains are 0,0089% and 0,0131% of totallunar surface, respectively. We have investigated thecraters according to area of permanently shaded regions.Our estimates of the percentage of Sp listed in Table 1:

Table 1.The areas of permanently shaded regions forboth hemispheres of the Moon.

Sp N, km2 S, km2% totallunarsurface

Sp>100 km2 1955,57 3315,27 0,0139100km2>Sp>50km2

596,31 707,19 0,0035

50km2>Sp>20km2

597,72 643,58 0,0033

20 km2>Sp>10km2

93,04 141,78 0,0006

10 km2>Sp>0 139,01 180,23 0,0009Sum 3381,65 4988,05 0,022

Some papers [4,5] had shown that effect ofsignificant portion of the enhanced hydrogen near bothpoles is corresponding to water ice deposits. Thedistributions of the Sp >10 km2 are shown at the fig 2.and fig.3. The crater Peary in North Polar Region isdemonstrating the enhanced hydrogen (H), especially insoutheastern corner. According to our calculations, thePeary is not contains the Sp areas. But two small craters(d = 10 km) are lie in southeastern part of Peary bottom.These craters are contains the permanently shaded areasSp > 20 km2 and can contains water ice deposits. Themuch concentrations of hydrogen are overlain a fewsmall craters (D< 30 km) near the North Pole. Ourestimates of permanently shaded areas in these craters isSp >50 km2 (fig. 2). The maximum enhanced hydrogennear North Pole is overlain the heavily cratered regionnear the crater Rozhdestvenskiy. There is no craterswith diameter D > 10 km, but there is many small,isolated craters, which can contains water ice.

H is enhanced in several craters (50 km>D>10 km)near the South Pole. These craters are contains thepermanently shaded areas Sp > 50 km2 and can containswater ice deposits. The largest concentrations ofhydrogen near the South Pole are overlay the cratersFaustini, Cabeus and unnamed crater at (87,20 S, 358,90

E), which contains the Sp > 100 km2 (fig. 3).So, the largest concentrations of hydrogen are

coinciding with areas, permanently shaded during theperiod of 18,6 years.

References: [1] Kulikov K.A. et al. (1972) Osnovylunnoy astrometrii.”Nauka”. [2] Rodionova J.F. etal.,(1988) An essential morphometric characteristics of

PERMANENTLY SHADED AREAS AT THE LUNAR POLES. V. V. Shevchenko

lunar cratres.,Trudy GosudarstvennogoAstronomicheskogo Instituta im. P. K. Shternberga,Tom LX, p.179-183. [3] Rodionova J.F. et al.,(1987)Morfologicheskiy katalog kraterov Luny, ed Moscow

University, 177p. [4] Feldman W.C. et al., (1998)Science 281, p.1496-1500. [5] Feldman W.C. et al.,(2001) JGR.

Figure 2. The distribution ofthe permanently shaded regionsSp > 10 km2 at the North Poleof the Moon accordingly theareas. Such regions are lies inthe small fresh craters or in thecraters, lies on the bottom oflarge deep craters. The zonewhere water ice can preserve isaround 15° the North Pole.

Figure 3. The distribution ofthe permanently shaded regionsSp > 10 km2 at the South Poleof the Moon accordingly theareas. The water ice depositscan exist in some large (D>20km) craters near the South Pole.The maximum permanentlyshaded area is in the latitudinalzone 85°-90°.