Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..469h&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, a part of NASA's Discovery Program, was designed to answer six questions [1]: (1) What planetary formational processes led to Mercury's high ratio of metal to silicate? (2) What is the geological history of Mercury? (3) What are the nature and origin of Mercury's magnetic field? (4) What are the structure and state of Mercury's core? (5) What are the radar-reflective materials at Mercury's poles? (6) What are the important volatile species and their sources and sinks near Mercury? MESSENGER is currently midway through a complex interplanetary cruise phase that involves three flybys of Mercury. The first of these, on 14 January 2008, provided important new information relating to several of the questions above [2-13]. Here we summarize observations made during the flyby that are most relevant to new insights about geological processes that have operated on Mercury and implications for the planet's history [3, 8-13]. The instruments that provided the most direct information on the geological history of Mercury during this first encounter were the Mercury Dual Imaging System (MDIS) [14], the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) [15], and the Mercury Laser Altimeter (MLA) [16]. Among the many specific questions remaining following the Mariner 10 mission to Mercury (1974- 1975) were (1) the level of mineralogical and compositional diversity of the crust, which appeared relatively bland in Mariner 10 data, (2) the nature of the rest of the huge Caloris impact basin seen only partially in Mariner 10 images, (3) the origin of the extensive plains observed on the surface (ponded impact ejecta or extrusive lava flows?), (4) the diversity and global distribution of tectonic features that have deformed the crust and their implications for strain as a function of time, and (5) the bombardment chronology and geological history of Mercury [1, 17-19]. The viewing geometry for the first MESSENGER encounter of Mercury [1] provided important information on these questions from image and remote sensing data on an additional 20% of the surface of Mercury not seen by Mariner 10, as well as comprehensive views of the Caloris basin and its surroundings. MESSENGER MDIS multi-spectral images [8-10] revealed a relatively low-reflectance surface with three broad units identified from reflectance and spectral slope in the wavelength range 0.4-1.0 μm. These new data helped to confirm the diversity of color units detected in re-processed Mariner 10 color-ratio images [20] and to extend the analysis to larger areas of Mercury. One of these new units is higher in reflectance and forms relatively red plains material that corresponds to a distinct class of smooth plains; these plains, on the basis of their sharp contacts with other units, are interpreted to have been emplaced volcanically. A second unit is represented by lowerreflectance material with a lesser spectral slope and is interpreted to form a distinct crustal component enriched in opaque minerals and possibly more common at depth. A spectrally intermediate terrain appears to form the majority of the upper crust in the newly observed area. Several other spectrally distinct units are found in local regions: (1) moderately high-reflectance, relatively reddish material associated with rimless depressions and located at several places along the interior margin of the Caloris basin rim; (2) highreflectance deposits observed in some impact crater floors; and (3) fresh crater ejecta that is less modified by space weathering than older surface materials. MASCS spectrometer data [9,15] show absorption and spectral slope properties of resolved spectra that are indicative of differences in composition and regolith maturation processes among color units defined by MDIS. Mid-ultraviolet to near-infrared reflectance observations of the surface revealed the presence of a previously unobserved ultraviolet absorption feature that suggests a low FeO content (<2-3 weight %) in silicates in average surface material. This result is supported by the lack of evidence for a key Fe2+ absorption band in spatially resolved spectra taken near the equator. A comprehensive view of the Caloris impact basin, the youngest known large impact basin on Mercury, was provided by the new MESSENGER data [8,10]. These observations support the interpretation that the surface of the Caloris interior is not composed of an extensive impact melt sheet, but rather has been resurfaced by volcanic plains. Evidence for a volcanic origin for the interior plains includes embayed craters on the basin floor, volcanic constructs, and diffuse deposits surrounding rimless depressions interpreted to be of pyroclastic origin [10,13]. Although the interior of Caloris basin appears similar in many respects to lunar mare basins flooded by mare basalts, the volcanic plains in Caloris are higher in reflectance than surrounding basin materials and lack spectral evidence for ferrous iron-bearing silicates. The new data permit the mapping throughout the basin interior of tectonic landforms observed in the eastern part of the basin by Mariner 10; contractional wrinkle ridges and extensional troughs occur in an annulus around the complete basin interior but have distributions and age relations different from their lunar basin counterparts, indicating a different stress history. A major surprise of the first flyby was the discovery of Pantheon Fossae, an extensive radial graben system located in the center of the Caloris interior, with individual graben up to hundreds of kilometres in length. This feature is unlike any structure seen in lunar basins. The MDIS color data and the geological interpretations from the image data helped to address a major question remaining from Mariner 10 [17]: Are the plains on Mercury formed by volcanic flooding, similar to the lunar maria, or did they form by impact ejecta ponding in a process similar to that thought to form the lunar light plains (Cayley Formation)? The new MDIS images show evidence for volcanic edifices and vents around the Caloris basin inner margin [10,13]. Impact crater morphologies and sizefrequency distributions derived from the new data [12] show that smooth plains exterior to Caloris display a crater density considerably less than that characterizing Caloris basin interior plains; this is interpreted to mean that the exterior plains are volcanic in origin and not Caloris impact ejecta. Moreover, morphologic evidence from regions exterior to Caloris shows that plains were emplaced sequentially inside and adjacent to numerous large impact craters and basins, often to thicknesses in excess of several kilometres [13]. A 3200-km-long Mercury Laser Altimeter profile [11] indicates that MLA data will be essential in further quantifying the thickness of plains. Features perhaps indicative of shallow magmatic intrusion include Pantheon Fossae, a possible radial dike swarm, and a floor-fractured crater, suggestive of shallow intrusion and floor uplift. Taken together, these observations from geomorphology, stratigraphy, color images, and impact crater size-frequency distributions support a volcanic origin for several regions of plains and appear to substantiate the important role of volcanism in the geological history of Mercury [20]. The global significance of contractional deformation was further underlined by new observations of numerous lobate scarps and wrinkle ridges. The average cumulative contractional strain is at least one third greater than that inferred from Mariner 10 images, and newly revealed stratigraphic relationships will permit an assessment of the time dependence of this strain [3]. These data provide new insight into the geological history of Mercury. In addition to improving our understanding of the diversity and character of the crust and the role of volcanism, the size distribution of impact craters on smooth plains matches that of lunar craters postdating the Late Heavy Bombardment, implying that the plains formed no earlier than 3.8 Ga [12]. The
Blewett David T.
Chapman Clark R.
Domingue Deborah L.
Evans Larry G.
Gillis-Davis Jeffery J.
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