Page 14 Selenology Vol. 29 No.3 Fall 2010 Page 15 fluid basalts which leave few flow fronts. Io has an implications of the tidal heating found in the solar [16] Touma, J & Wisdom, J. (1998) Resonances [17] Kopal, Z. (1966). On the Possible Origin episodic heating cycle that results in massive heat system is still not well understood, applying what in the Early Evolution of the Earth-Moon System. of the Lunar Maria. Nature, 210 Apri loss in the eruptive phase as heat is dumped onto we do know to the Earth-Moon system could help Astronomical Journal, 115:1653-1663. the surface through basaltic outpourings. This heat explain a long standing mystery — the origin of the loss cools Io, making it less susceptible to Jupiter’s lunar mare flood basalts. tidal pull, thus reducing the effect of tidal heating by reducing the pliability of Io’s rigid body. The References Dionysius heating cycle then begins again; this process has [1] Gold, T (1955) The Lunar Surface. Nature, by Howard Eskildsen resulted in the resurfacing of Io with layer upon 115, 585-604. layer of flood basalts. A similar episodic nature is [2] Ronca, L.B., (1966) Meteoritic Impact and seen in the placement of the lunar mare basalts, Volcanism. Icarus 5, 515-520. Rayed crater Dionysius lies near the west- 50 km further than the radius of the outer circle on with layer upon layer of basalt flows building up [3] Ghods, A. & Arkani-Hamed, J. (2007) ern margin of Mare Tranquillitatis, not far from my photo (Author not listed 2009). A Clementine kilometer thick deposits [12]. It is noted that the Impact-induced convection as the main mechanism the crater pair of Ritter and Sabine, and south of image shows the dark rays in greater detail (Fig. C). time between flows was long enough to allow for for formation of lunar mare basalts. J. Geophys. Ariadaeus (Fig. A). Another photo of the area The USGS Geologic Map of the Julius Caesar the accumulation of regolith, evidence of the epi- Res. Vol. 112 E03005. reveals the rays and is marked with 20 km, 40 km, Quadrangle of the Moon depicts three medium sodic nature of the mare basalt flows [13]. [4] Taylor, R.S. & McLennan (2009) Planetary and 160 km diameter circles centered on the crater albedo surface units close to the crater and one dark Theory had predicted that Io’s heating would be Crusts: Their Composition, Origin and Evolution. for perspective concentrated at the poles. Surprisingly it was found Cambridge Univ. Press. (Fig. B). The 19 that the heating was actually being expressed in the [5] Grove, T.L. & Krawczynski, M.J. (2009) km diameter cra- equatorial regions of Io. Volcanism on Io is concen- Lunar Mare Volcanism: Where Did the Magmas ter has a slightly trated within 45 degrees of the great circle which Come From? Elements, vol. 5, no. 1, 29-34. asymmetrical is Io’s equator. The lunar maria are also distributed [6] Alfven, H. & Arrhenius, G. (1969). Two bright, continuous about a similar great circle. Generally speaking, the Alternatives for the History of the Moon. Science, ejecta ring nearly lunar maria are located within 29 degrees of a great 165. 40 km in diam- circle which is inclined 6 degrees to the moon’s [7] Kopal, Z. (1966). On the Possible Origin of eter. The bright present equator [14]. The mare distribution may the Lunar Maria. Nature, 210 April. continuous ejecta indeed have originally been produced around what [8] Kaula, W.M. (1971) Dynamical Aspects of appear to extend was the moon’s equator, however it is thought that Lunar Origins. Rev. Geophys. 9, 2, 217-238. slightly farther the uneven loading of the crust with basalt deposits [9] Kaula, W.M. (1973) Dynamically Plausible to the west than would have then caused a reorientation of the lunar Hypotheses of Lunar Origin. Nature Vol. 245 to the east. Both spin axis [15] [16]. The lunar mare distribution October 19. dark and bright is otherwise surprisingly similar to the volcanic [10] Peale, S.J., Cassen, P., and Reynolds, R.T., rays radiate near- distribution found on Io. Thus the lunar mare dis- (1979). Melting of Io by Tidal Dissipation. Science, ly symmetrically tribution could be thought to possibly represent a 203, pp. 892-894. from the crater historical lunar equator during a time of tidal heat- [11] Beatty, K.J., Petersen C.C. & Chaikin, A and most appear ing [17]. (1999) The New Solar System. Cambridge Univ. to end less than Since the end of the Apollo missions, science Press. 80 km from the has struck out to explore the other planets and [12] Woolfson, M.M. (2000) The origin & crater center, with moons of the solar system. The wonderful discov- evolution of the Solar System. Institute of Physics a few extending eries of this unmanned robotic exploration can be Publish., Philadelphia. slightly farther. applied to the moon to further our understanding of [13] http://www.nasa.gov/topics/moonmars/fea- Wikipedia, how- our closest neighbor. Can our modern understand- tures/apollo15_halo.html ever, describes ing of the manifestation of tidal heating on Io and [14] Ward, W.R. (1975) Past Orientation of the the rays as throughout the solar system possibly shed any new Lunar Spin Axis. Science vol. 189. extending more than 130 km from Dionysius and Region by Howard Eskildsen, Ocala Florida, USA, 2009/10/26, light on the pre-Apollo thinking about the origin of [15] Melosh, H.J. (1975) Mascons and the 00:09 UT, Seeing 6/10, Transparency 5/6, Meade 6” f/8 Refractor, 2X Barlow, DMK the lunar maria? Though the exact workings and Moon’s Orientation. Earth & Planet. Sci. Lett., 25 the center of the crater, which is 41AU02.AS, W-15 Yellow and IR Block Filters Page 16 Selenology Vol. 29 No.3 Fall 2010 Page 17

unless the impact had excavated completely through the darker layer and covered the halo with deeper bright-albedo material. Remote sensing stud- ies show that this is not the case. T.A. Giguere et al, used spectral analysis, FeO and TiO2 maps and optical matu- rity data based on Clementine UV-VIS imagery to study composition of the crater-related materials (Giguere et al 2005). The dark rays were indeed mare debris with minor amounts of highland material and dark mare materials were identified high in the inner walls of the eastern and southern portions of the crater Map-A-Planet Explorer: Moon—Clementine UVVIS Multispectral Mosaic— (see Fig. C). The bright Dionysius image rays were predominate- albedo unit (Fig. D). The quadrangle portrays the ly highland material medium albedo units as belonging to the smooth with variable amounts of mare components and the member of the Frau Mauro formation to the north, western and northern interior crater walls were of the Cayley formation to the west and undifferentiat- highland material. No solid evidence of dark-haloed ed terra material to the south and southwest (Morris craters was found on the adjacent Cayley deposits and Wilhelms 1967). The dark albedo mare basalts either. of the Procellarum group (in Mare Tranquillitatis) It is likely, then, that the impact creating lie to the east and embay the other formations. Dionysius occurred at the junction of the embay- It is apparent that the impact ejected dark-albe- ment of the highland formations by mare basalts do material as well brighter material to form the at the western margin of Mare Tranquillitatis as rays and, since they represent the material from the Dionysius by Howard Eskildsen, Ocala Florida, USA, 2010/01/28, 01:02 UT, Seeing 8/10, Transparency confirmed by the appearance of both formations least depth, both must have been present at or near 5/6, Meade 6” f/8 Refractor, 2X Barlow, DMK 41AU02.AS, No Filters high in the crater walls on Clementine images. This the pre-impact surface. If the darker material had would explain the presence of bright, and of dark, underlain the brighter, a dark-haloed crater (DHC) rays since both bright highland material and dark might have been expected. However, the material mare material would need to be present near the nearest the crater is bright so it could not be a DHC pre-impact surface to be scattered as light and dark rays over the surrounding terrain. If mare basalts References: were buried beneath the brighter formations, it Author not listed, 2009. Pitatus (crater), would be likely that some craters would excavate Wikipedia, URL: http://en.wikipedia.org/wiki/ Dionysius_(crater), (last date accessed, 29 June through the bright layers to reveal the dark basalt 2010). beneath and form dark-haloed craters. Therefore, Giguere, T. A., Hawke, B. R., Gaddis, L. R., the absence of distinct dark-haloed craters argues Blewett, D. T., Lucey, P. G., Peterson, C. A., Smith, against extensive cryptomare deposits (mare basalts G. A., Spudis, P. D. and Taylor, G. J., 2005. Remote buried under the brighter highland formations) Sensing Studies of the Dionysius Region of the Moon, LPS XXXVI, Abstract #1092. beneath the Cayley and other formations, but does Map-A-Planet Explorer: Moon- Clementine not preclude some minor interbedding of the bright UVVIS Multispectral Mosaic—Dionysius Image and dark layers. Morris, E. C. and Wilhelms, D. E., 1967. Though it is of small size, Dionysius reveals a Geologic Map of the Julius Caesar Quadrangle of great deal regarding the secrets of the Moon. the Moon, USGS. http://www.lpi.usra.edu/resourc- es/mapcatalog/usgs/I510/

Detail from Geologic Map of the Julius Caesar Quadrangle of the Moon by Elliot C. Morris and Don E. Wilhelms, 1967 http://www.lpi.usra.edu/resources/mapcatalog/usgs/1510/