Radar Altimetry of Mercury: A Preliminary Analysis - Brown ...

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400 HARMON ET AL.' RADAR ALTIMETRY OF MERCURYscarps, ridges and moderate-sized craters, to the 1000-2000km scales of large impact basins and major upland regions.When used in tandem with Mariner 10-derived images andmaps, radar altimetry has allowed us to better constrain andmore closely examine previous interpretations of the geologyof the planet.Crater depth measurements appear to confirm earlier findingsfrom shadow measurements [Gault et al., 1975; Malin andDzurisin, 1977], although the slightly shallower radar depthswarrant further study. Both radar and shadow measurementssuggest that Mercurian craters are shallower, on the average,than lunar craters. Only one large (800 km) basin has beenfound to show a radar altimetric signature, the overall shallownessof which suggests significant modification by isostaticrelaxation or volcanic filling.Altimetry over the southern environs of Caloris Basin indicatesthat the circum-Caloris smooth plains extend well intothe unimaged hemisphere. Both imaged and unimaged sectionsof these plains are strongly down-bowed, suggesting thatthere has been subsidence under the load of the smooth plainsfill. This and other similarities to lunar maria offer evidence,albeit circumstantial, in favor of a volcanic origin for thesmooth plains around Caloris. Although the Arecibo data areconsistent with the hypothesis that there is an irregular annulusof smooth plains surrounding Caloris Basin, new photographicimages of the region to the west of the Mariner 10terminator will be required to establish this definitively.Some of the most important results of this study bear uponthe tectonic evolution of Mercury. We have obtained altimetricprofiles across three features mapped as arcuate scarps.One of these features appears to be a ridge rather than a scarpwhile the other two features, though showing some crosssectionalasymmetry consistent with thrust-faulting, have profileswhich are more ridge-like than is suggested by images.These results should be factored into models that account forsuch structures as compressional tectonic features. Altimetryhas allowed us to document the existence of at least one largefault zone associated with 3 km of reliefi This fault zone alsoappears to be intimately connected with one of the two broadtopographic bulges in the equatorial zone of Mercury. Thesebulges rise roughly 3 km above the mean equatorial radiusand are aligned along an axis within about 10 ø of the so-called"hot poles" of Mercury. These bulges present an interestingalternative to the hypothesis that the smooth plains in oraround Caloris control the dynamical figure of Mercury. Thequestion of whether their origin is due to tidal stresses or tosome endogenic process is crucial to understanding the tectonichistory of the planet.Radar observations of the equatorial zone of the unimagedhemisphere reveal uplands and lowlands, a number of largecraters, and some topographically smooth areas. The topographyof this hemisphere shows no marked differences with respectto the imaged hemisphere and no evidence has beenfound for the existence of another Caloris-type impact struc-ture.The discussion we have included in this paper is intended tobe descriptive and preliminary. There are several areas whichwarrant more intensive study. Further geophysical modellingof the Caloris region should include the radar topography as aconstraint. It is clear, however, that a reasonably completeunderstanding of this complex and interesting region of Mercurywill require an orbiting spacecraft with altimetric, gravimetric,and imaging capabilities. The Arecibo altimetry hasprovided some new information on the topography of ridges,scarps and faults; more detailed interpretation of these find-ings is in order. Finally, we note that the possibilities for studyof the topography of Mercury with earth-based radar havenot been exhausted, and continued observations should bemade of the unimaged hemisphere and of areas in the imagedhemisphere deemed interesting on the basis of existing data.APPENDIX' THE MERCURIAN COORDINATE GRIDSince the spatial resolution of the Arecibo radar altimetry iscomparable with uncertainties or inconsistencies in the cartographicgrid systems, an assessment of the grid systems is inorder.Longitude CoordinateThe USGS shaded-relief map of the H-6 quadrangle adoptsthe longitude reference used in the Mariner 10 cartography[Davies and Batson, 1975], in which the 20 ø longitude meridianis defined as passing through the small reference craterHun Kal. Davies and Batson [1975] state that the IAU 0 ølongitude meridian (defined by referencing to the subsolarmeridian on January 1, 1950) is at 359.5 ø ___ 0.5 • on the Mariner10 grid. Comparisons of Arecibo altimetry features withthe USGS H-6 quadrangle showed the Arecibo longitudes tobe systematically larger by approximately 0.4 ø . This differenceimplies that the 0 ø longitude reference used in the Areciboaltimetry reduction programs is at approximately 359.6øW inthe Mariner 10 (Hun Kal) system, which is consistent with theDavies and Batson [1975] estimate of the position of the IAUmeridian. Comparisons of Arecibo longitudes with longitudestabulated in the Astronomical Almanac [U.S. Naval Observatory,1984], which uses the 1979 IAU system, shows preciseagreement to within 0.01 øA position check of the longitudes of altimetry features withtheir locations on the USGS H-7 and H-8 shaded-relief mapsshowed no systematic discrepancies such as that found incomparisons with the USGS H-6 map. This contrast impliesthat there is an inherent misregistration of the USGS H-7 andH-8 maps relative to H-6. A check of the common boundaryof the H-6 and H-7 relief maps does, in fact, show such amismatch. These findings were confirmed by spot checks betweenthe USGS relief maps and the Mercury Control Net[Davies and Katayama, 1976]. This check showed preciseagreement between USGS and Control Net longitudes for theH-6 quadrangle, but that the USGS longitudes were approximately0.5 ø larger than Control Net longitudes for the H-7and H-8 quadrangles.Since the USGS shaded-relief maps provide the best meansof interpreting the radar altimetry, it was decided to adjust theArecibo profiles to conform to the USGS longitudes. Accordingly,the Arecibo profiles on the H-6 quadrangle (Figure 2a)were shifted east by 0.4 ø, whereas no shift was made to profileson the other quandrangles.LatitudeCoordinateBoth the Arecibo and USGS coordinate systems follow theIAU convention placing the Mercurian spin axis normal tothe orbital plane. The best estimate of the pole position fromMariner 10 data is offset by 2 ø from the IAU pole, with anerror oval encompassing the IAU pole [Klaasen, 1976]. Invokingsome dynamical studies by Peale [1969], Klaasen concludedthat the spin axis was most likely aligned within approximately1 ø of the orbit normal. This, then, admits thepossibility of errors in subradar latitude of the order of adegree.
HARMON ET AL.: RADAR ALTIMETRY OF MERCURY 401Acknowledgments. We are grateful to the staff of Arecibo Observatoryfor their assistance in various aspects of this project. In particular,we would like to thank R. Velez, T. Crespo, A Hine, and G. Giles.We would also like to thank T. Forni and J. Chandler for theirassistance in the ephemeris generation. One of us (I.I.S.) gratefullyacknowledges support from National Science Foundation grantPHY-8243330. D.L.B. wishes to thank the staff and administration ofArecibo Observatory for their help and for support during his stay. Aportion of this work was supported by NASA grant NGR-40-002-116from the Planetary Geology Program to J.W.H. The NationalAstronomy and Ionosphere Center (Arecibo Observatory) is operatedby Cornell University under contract with the National ScienceFoundation and with support from the National Aeronautics andSpace Administration.REFERENCESAsh, M. E., I. I. Shapiro, and W. B. 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