ANALYSIS OF MERCURIAN CRATERS BY MEANS OF CARTOGRAFIC METHOD. Kozlova E.A. 1, Sitnikov B.D 1., Rodionova J.F. 1, Shevchenko V.V. 1 1. Sternberg State Astronomical Institute, Universitetskiy prospect 13, Moscow 119992, Russia. E- mail: katk@sai.msu.ru The maps of density distribution of craters with different degree of rim degradation were compiled with use of Morphological catalogue of Mercurian craters created in Sternberg State Astronomical Institute on the base of Mariner 10 data. Coordinates of craters in the catalogue are changed in accordance with new data of Mercury North Pole. A treatment of data bank is fulfilled with the use of software in regime of the choice of ensemble of the objects, compiling maps of density distribution of craters. The maps of density distribution of craters with different degree of rim degradation permit to determine the regions of concentration a young and old craters. We turned the Molveide projection on -20º along meridians. The scale of the map show the density of craters - the quantity of craters referenced to 1 million km 2. Also we have calculated the area and position the permanently shaded areas in these craters. We used model truncated cone for calculations for craters with diameter > 15 km, and spherical form for craters with diameter <15 km. The total area of a constant shadow in area of North Pole of Mercury has made from above 27000 km 2, in area of the South Pole - more than 13000 km 2. The bank of data of Morphological catalogue of Mercurian craters in diameters 10 km and more including 6500 craters have been compiled for 45% of the surface of Mercury in Sternberg State Astronomical Institute on the base of Mariner 10 data [1]. Each crater exposed on photomap of Atlas of Mercury was measured and his morphologic features were determined with the use of large scale photos. The morphological features include: the degree of rim degradation of crater; terraces and faults on the inner slopes; peaks, hills and ridges on the crater floor; fissures and chains; character of floor; lava; ray system of crater; local terrain (plain, highland) and special features. Sometime it was difficult to determine the morphological features of craters because of the poor quality of the pictures obtained in specific illumination. Coordinates of craters were measured with the accuracy + 0.1º, and diameters with the accuracy + 2 km. Then the coordinates of craters in the catalogue were recalculated in accordance with new data of Mercury North Pole [2]. We used the next system of symbols for the designation of the morphologic features of craters (Fig. 1).
Feature name Meaning Catalogue notation Feature name Meaning Catalogue notation Rim degradation highly preserved 1 one ridge and E rim many peaks preserved rim 2 circular ridge Ac smoothed rim 3 circular ridge Bc and one hill degraded rim 4 circular ridge Cc and many hills wholly degraded rim 5 circular ridge and peak Dc circular ridge Ec and many peaks Terraces and no terraces, no 0 Fissures and unclear 0 faults fault chains unclear 1 no fissures and 1 no chains one terrace 2 one chain 2 one fault 3 many chains 3 one terrace and 4 one fissure 4 fault many terraces 5 one chain and 5 one fissure many terraces and fault 6 many fissures 6 one chain and 7 many fissures Peaks, hills and no peaks, no 0 Character of unclear 1 ridges hills, no ridges floor unclear 1 flat 2 rough 3 one hill 2 Lava no lava 0 many hills 3 unclear 1 one peak 4 lava on the floor 2 one peak and one hill 5 one peak and 6 floor is fully 3 many hills covered lava many peaks 7 Ray system no rays 0 many peaks and one hill many peaks and hills 8 unclear 1 9 rays 2 Local terrain unclear 1 one ridge A plain 2 one ridge and one hill B highland 3 transitional zone 4 one ridge and many hills one ridge and one peak C Special features dark material 1 D scarp 2 Figure 1. System of classification for craters of Mercury.
Figure 2. Crater Brahms (58,9º N, 177º W, D=114 êì). For example crater Brahms (fig. 2) has such note in our morphologic catalogue: 58917711425903103. The first six figures are the latitude and longitude, the next three is diameter in km, then the figure 2 is the degree of rim degradation, 5 designs many terraces, 9 - many peaks and hills, 0 no fissures and chains, 3 rough floor, 1 unclear lava, 0 no rays, 3 crater is on highland. A treatment of data bank is fulfilled with the use of software in regime of client-server on the choice of ensemble of the objects, compiling maps of density distribution of craters. The maps of density distribution of craters with different degree of rim degradation permit to determine the regions of concentration a young and old craters (fig. 3 and 4). Figure 5 show the quantity of craters of different diameters in % on Mercury and on the Moon. Diameter, km 10-20 20-40 40-80 80-160 >160 Mercury 48 30 16 5 1 Moon 59 27 10 3 1 Figure 5. Comparison of quantity craters (%) of different diameters on Mercury and on the Moon..
Morphological features Mercury Moon Rim degradation 1 3 19 2 12 27 3 46 27 4 29 21 5 10 6 Terrace 38 6 Faults 13 1,5 Hills 27,5 12,9 Central peaks 7,3 5,5 Ridges 0,9 3 Chains 10,5 12 Fissures 0,3 2 Flat floor 26,2 7 Rough floor 35,3 71,5 Dark material on a part of floor 8,5 11 Dark material on total floor 3,5 0,1 Local terrain: highland 82 94 Local terrain: plains 11 3 Local terrain: transitional zone 6,5 3 Figure 6. Comparison of quantity of craters (in %) with different morphological features at Mercury and the Moon. The maps of craters density distribution was compiled by the follow method. It was necessary to do a choice the objects with different morphological features. The special language of inquiry was composed to determine not only the morphology of craters, but also the region of their disposition on the surface of the planet. It was possible to fulfile the treatment of some selections for analysis of cross-correlation dependences. It is important for examination of hypothesis about the nature of the different regions of the surface of Mercury. The language of inquiry is represented is a system of panels called by clients supplements. An ensemble of a objects compiled on inquiry is a number of craters introduced a sum of casual composition and a trend. The task is to separate of these two components and to compile the map of the trend. At the first stage the function of the crater density distribution was calculated at the knobs of regular network. Each point of the network was examined as a centre of the circle with the angular radius of 4 degree. The object from choice was considered belonging to the circle if one of its element has a cross with it. The function significance was equal the ratio of the number of objects to the area of the circle. At the second stage a spherical regression of the crater density distribution function on the full system of orthogonal spherical function was fulfilled step by step. Trigonometric function sin (kx) and cos (ky) and the system of multi-member by Legendre from spherical coordinates was used as a system of spherical functions. Picked out trend was mapped in the different projections: polar and stereographic. Programming allows to change the parameters of the maps in wide limits. It is possible to compile the maps and investigate some interesting regions in details.
Figure 3. The density of distribution of craters with 1, 2 and 3 degrees of rim degradation
Figure 4. The density of distribution of craters with 4 and 5 degrees of rim degradation
The surface of Mercury was studied more than once with ground-based radars. The studies performed in 1991- -1992 using radio telescopes of the Gold Stone and Arecibo observatories showed that the radio echo arriving from some regions in the polar areas of the planet has properties similar to those of the echo coming from icy satellites of Jupiter and from the southern polar cap of Mars. It was suggested that areas with unusual properties are clusters of volatile elements in the "cold traps" of the polar regions of the planet. Such areas were designated by roman letters from A to Z [5]. We adopted morphometric data for Mercurian craters from [6]. According to these data, fresh craters (which we refer to preservation class 1 according to the terminology of the "Morphological Catalog of Mercurian Craters") with diameters ranging from 225 m to 15 km are simple bowl-shaped craters with the d / D (depth-to-diameter) ratio almost exactly equal to 1/5. The older, modified simple craters we referred to preservation class 2. Pike [6] identifies two classes of complex craters (i.e., craters with terraces, peaks, and rings) on Mercury: mature and immature. Immature craters occupy the diameter interval from 10 to 29 km and are, on the whole, smaller and have more complex morphology compared to simple craters. Their characteristic feature is a flat floor. According to the terminology of the "Morphological Catalog of Mercurial Craters," these craters belong to preservation class 3. Mature Mercurian craters differ from immature craters mostly by greater sizes. According to Pike [6], mature Mercurial craters are craters with diameters ranging from 30 to 175 km. According to the terminology of the "Morphological Catalog of Mercurian Craters," these craters belong to preservation classes 2 and 3. We analyzed a total of 831 craters with diameters >10 km in the polar regions of Mercury, including 337 and 494 craters in the regions of the north and south poles of Mercury, respectively. We found permanently shadowed areas in 109 craters in the northern hemisphere and in 144 craters of the southern hemisphere of Mercury. We estimate the total permanently shadowed area in the northern polar region of Mercury at 21 190.6 km 2. According to our data, the total permanently shadowed area in the region of the south pole of Mercury is equal to 24 488.8 km 2. Te total permanently shadowed area in the polar regions of Mercury is equal to 45 679.4 km 2 [7]. To compute the temperature in the crater we analyzed the direct solar flux coming to an illuminated element of the crater surface, the thermal flux from the interiors of the planet, the flux reflected from the illuminated surface of the crater, the secondary reflected light flux and the infrared flux incident onto crater surface element. We do not take into account the thermal flux from the adjacent elements of the crater; however, this flux can be ignored because of the low thermal conductivity of regolith on the Mercury. In addition, we also ignore the effect of the solar wind, because it barely penetrates into permanently shadowed areas [7]. Among the Mercurial craters with anomalous reflective properties considered in this paper, the craters coinciding with regions D and E may also contain, in addition to sulfur deposits, water-ice deposits not covered by a regolith layer and deposits of other volatiles, such as CO 2 and NH 3. The maximum temperatures in shadowed areas of craters with regions X, C, J, and F do not exceed 110 E, thereby preventing the existence of the deposits of such volatile compounds as CO 2 and NH 3 but allowing water-ice deposits to be preserved for a long period. In craters with regions G, N, M, M2, K, K2, B, L, L2, P, J2, Q2, V, V2, and U, the maximum temperature in nonilluminated areas does not exceed 110 E---the threshold above which water ice cannot stably exist if not covered by a regolith layer. The anomalous properties of the reflected signal in craters R, T, P2, S, and Q are most likely due to sulfuric deposits, because the maximum temperatures exceed 170 K even in shadowed areas. References: [1] (1977) Atlas of Mercury 1:5 000 000 Topographic series. USGS. [2] Harmon J.K. and Perillat P.J. (2001) Icarus. V.149, p. 1-15. [3] J.F Rodionova. et al (1987) Morphologicheskiy Katalog Kraterov Luny. Moskva, Nauka. 187 p. [4] J.F. Rodionova et al (1993) Morfometriya lunnyh kraterov. Astronomicheskie Aspekty Osvoenia Luny I Poisk Vnezemnyh Resursov, p.93-102. [5] Harmon, J.K. and Slade, M.A., (1992) Science, V. 258, p. 640--643. [6] Pike J.R. (1971) Icarus,V. 15, p. 384--395. [7] Kozlova E. A. (2004) Solar System Research, V. 38, p. 335 346.