In un paese come l`Italia, caratterizzato da una forte densità abitativa

Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2012-2013
THE SURFACE OF MERCURY: INTERPRETING REMOTE SENSING IMAGES AND
SPECTRAL SIGNATURES.
Ph.D. candidate: SABRINA FERRARI, III course
Tutor: Doc. MATTEO MASSIRONI Prof. FABRIZIO NESTOLA
Cycle: XXV
Abstract
Knowing the evolution and composition of the surface of Mercury enables us to discern several processes that operated
during the formation of the inner Solar System (e.g. crustal accretion, impact gardening and global cooling of Terrestrial
Planets). The elaboration and classification of remote sensing multi-band images allowed the interpretation of the
stratigraphy, composition, age and structural evolution of Rembrandt basin and scarp system, i.e. the largest impact basin
cross-cutting by contractional structure of the southern hemisphere of Mercury. The interaction of different scale processes
has been demonstrated. Since the knowledge of the surface composition of Mercury is based on the interpretation of spectra,
Thermal Infra-Red spectra of representative mineral phases have been measured in laboratory at the temperatures of the
planetary surface. Significant changes in the high temperature spectra have been detected. Failure to allow for these effects
leads to errors in the estimation of chemical compositions by Infra-Red spectra.
Introduction
The special yield of remote sensing data from the on-going Mercury Surface Space ENvironment
GEochemistry and Ranging (MESSENGER) NASA mission (Solomon et al., 2001) currently allows a
detailed analysis of terrains and structural forms of the surface of Mercury. During its second and third
flybys, the Mercury Dual Imaging System (MDIS) (Hawkins et al., 2007) of MESSENGER imaged a
previously unseen, well-preserved basin named Rembrandt in Mercury’s southern hemisphere. With a
diameter of 715 km, Rembrandt is among the largest and youngest impact basins recognized on Mercury
(Watters et al., 2009; Fassett et al.,2012). Its interior is partially resurfaced by smooth plains interpreted to
be of volcanic origin (Denevi et al., 2009) and is crossed by a 1000-km-long reverse fault system (Watters
et al., 2009; Byrne et al., 2012). Notably, the Rembrandt basin area recorded most of the activities that
modified the surface of Mercury (e.g. basins formation and impact gardening, global and basin-related
tectonics and resurfacing) and represents a good case study for understanding length and sequence of such
processes. The further recognition of unusual kinematic indicators of strike-slip motion along the cited
fault system (Massironi et al., 2012) motivated a more detailed work focused on the complex
relationships between the Rembrandt basin and the scarp. To this end, MESSENGER MDIS mosaics and
derived Digital Terrain Models (Preusker et al., 2011) have been used to map in fine detail the basin units
and the structures of Rembrandt Basin area, in order to obtain better insight in the kinematic development
of the Rembrandt scarp system and whether its development was influenced by the interaction between
global- and basin-scale processes. Then, crater counts have been performed on each unit, in order to
estimate the ages by applying the Model Production Function (MPF) of Marchi et al. (2009). The dataset
has been constantly integrated with fresh images thus the morphological interpretations have been everimproved.
The knowledge of the composition of the surface of Mercury is currently based on the interpretation
of spectra provided by MESSENGER (e.g. Visible and Near Infra-Red spectra, X-ray spectra and
Gamma-ray spectra). The next European Space Agency and Japan Aerospace Agency mission to
Mercury, named BepiColombo, will carry on board the Mercury Radiometer and Thermal Infrared
Spectrometer (MERTIS) (Hiesinger and Helbert, 2010) that will be able to provide spectra from 7 to 14
µm. This range of wavelength is very effectively to identify the fine-scale structural properties of silicates
(e.g. stretching and bending motions in the silicon-oxygen anions, metal–oxygen and lattice vibrations,
Hamilton, 2010 and reference therein). In addition, for mineral families as olivines and pyroxenes, the
emissivity peak positions (bands) are a good indicator of the composition. Previous interpretations of the
spectra of Mercury have not considered the possible effects induced by the extreme daily surface
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2012-2013
temperature range. Indeed, the temperature of the surface of Mercury can range in 44 earth-days between
70 and 725 K, depending on latitudes. Assuming that such variations can cause significant crystal
structure changes, X-ray diffraction and Thermal Infra-Red spectroscopy have been conducted in
laboratory in order to verify the influence of the thermal expansion on the spectral signatures. These insitu measures have been collected up to 725 K on 8 minerals assumed to be present on the Mercury
surface (Warell et al., 2010) showing considerable variations.
The structural evolution of Rembrandt basin and scarp system of Mercury
The sets of Experimental Data Records (EDS) returned by MESSENGER need a preliminary
processing to be usable by common cartography software. Thus, the EDS of the interested area have been
converted and calibrated using the Integrated System for Imagers and Spectrometers (ISIS). The 28
obtained images have been used to build up mosaics with different condition of illumination, which
represent a very useful set to detect structures. Single images and mosaics have been analysed in the ESRI
ArcMap Geographic Information System (GIS) in order to provide a geological and structural map of the
Rembrandt area (Fig. 1).
The basin has been subdivided into three main units, based on different primary morphological
characteristics: the Hummocky Material, a mixture of impact melts and breccias, the Proximal Ejecta
fallen beyond the rim, and the volcanic Interior Plains, which flooded the crater floor after the impact.
Fig. 1: Geological and structural map of the Rembrandt basin and scarp system.
Marchi et al. (2009) chronological model has been applied in order to obtain the age assessment of
major domains of the Rembrandt basin: the impact-forming Hummocky Material and the smooth Interior
Plains. Since the difference between the inferred ages is negligible, the adopted method cannot
discriminate whether the Interior Plains are impact melt formed during basin formation or volcanic
material emplaced soon after the basin formation. However, both previous works (Watters et al. 2009)
and the morphological observations reported in this study support a volcanic origin for the Interior Plains.
Concerning structural mapping, the basin displays a large variety of tectonic patterns (Watters et al.,
2009). Over 250 radial contractional and extensional features of variable lengths (5 to 60 km) have been
mapped; these structures describe a partial fan-like pattern and are mainly bounded by concentric
contractional ridges. The overall net of structures manifests within the Interior Plains, postdating the
smooth plains emplacement, and represents multiple episodes of deformation of the basin interior.
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2012-2013
The wide scarp crossing Rembrandt basin has been regarded as the surface expression of a large-scale
thrust fault system, which features a back-thrust and displays several kinematic indicators of lateral
movements along its length. On the basis of these lateral shear indicators (Massironi et al., 2012), the
structure has been divided into two main branches, obtaining a branch confined within the basin and a
branch developed out of the basin. The overall kinematics suggests a SE-vergent thrust, which was
followed by minor compressional structures detected within younger craters and possibly associated to a
prolonged slowing down phase of the global contraction. Given the nature of fault growth, the shape of
the scarp, and its transpressional character, one might expect fault vertical displacements (throws) to be
greatest near the fault center, close to the meeting point of the branches. However, the vertical
displacements obtained elaborating the Digital Terrain Model of Preusker et al. (2011) show an overall
decrease toward the NE tip. In particular, the scarp developed inside the basin displays notably lower
vertical displacements than the scarp developed outside. On the whole, the two scarps have
morphological characteristics that possibly reflect independent evolution.
The proposed sequence of events places the Rembrandt basin-forming impact during the thrust
activity. This scenario requires a pre-existing contractional ridge, which was promoted by the early global
cooling of the planet. The subsequent formation of the Rembrandt basin may have again promoted lower
crustal flow towards the basin centre. The related perturbation of the regional stress field promoted a
change in the scarp strike inside the basin and the development of later strike-slip structures. Sustained
growth of the Rembrandt scarp led to the formation of the second transpressive branch, whose
development was passively controlled by the basin structure; this secondary scarp accumulated less strain
than the older ridge. This scenario satisfies each of the observations described in this study, and may be
the most likely process through which the complex Rembrandt basin-and-scarp system formed. In a
broader sense, this sequence suggests that the action of global contractional tectonism might be strongly
influenced on local scales by the stress fields induced by impact craters and basins.
Thermal emissivity under extreme temperature ranges: implication for the surface of Mercury
As a consequence of the thermal expansion induced by the daily variation of the surface temperature,
the spectral signature of minerals assumed to be present on the Mercury surface could be significantly
affected. In order to determine the relationship between thermal expansions and spectral signatures of
such minerals within a wide temperature range, a multi-methodological approach that comprehends both
X-ray diffraction and Thermal Infra-Red (TIR) spectroscopy has been scheduled. Eight samples of the
possibly constituents of the surface of Mercury are currently under investigation. Single phases of
plagioclases (including a labradorite personally collected in July 2010 from the Flakstadøy Basic
Complex at Lofoten Island, Norway), clinopyroxenes and forsterite have been deeply characterized at
room conditions by powder and single-crystal X-ray diffraction and analyzed by electron microprobe
(Wavelength Dispersive Spectroscopy method). After that, the phases have been reduced toward two
different grain-sizes that are presumed representative of the shallow terrains of Mercury: 30 m and 100
m (Emery et al., 1998).
All the reduced samples have been investigated in the TIR spectroscopy range (7-14 µm) at the
Planetary Emissivity Laboratory (PEL) in Berlin (Maturilli et al., 2008), where they can be heated into a
special vacuum chamber by induction, avoiding to heat the entire environment and simulating the
increasing temperature of the surface of Mercury (in-situ analyses). The emissivity measurements have
been performed at Earth atmospheric pressure and temperature and then under vacuum at three different
steps of induced temperature. As the samples to be investigated must dried before definitive
measurements, a pre-measure sample treatment was conducted at PEL, testing a heating-time in oven and
acting on the vacuum condition timing necessary to obtain suitable samples.
Notably, the thermal expansion induced by the increasing temperature affected all the collected
spectra, shifting their characteristic bands toward lower wavenumbers (or higher wavelength). The shifts
on the emissivity spectra of heated plagioclases have been previously noted by Helbert and Maturilli
(2009) but not ascribed to the increase of the unit-cell parameters. Since the positions of the bands are
distinguishing of each composition, a spectrum collected at high temperatures records the thermal
expansion simulating a phase enriched in larger cations.
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2012-2013
Figure 2. Emissivity TIR spectra for Fo92 olivine between
1000 and 800 cm-1 collected in vacuum and at 352 K (dashed
curve) and at 773 K (solid curve); straight lines capture the
positions of the emissivity minima and display their shift to
lower wavenumbers with increasing T.
The emissivity spectra of the heated Forsterite
sample are shown in Fig. 2, as example.
Seemingly, an emissivity spectrum collected
from the surface of Mercury at high daily
temperatures will record the thermal expansion of
the surface minerals. Far to allow for this effect
leads to errors in the estimation of chemical
composition of the cited phases. In parallel, X-ray
diffraction analyses have been conducted under
non-ambient conditions up to 775 K on Forsterite
(in collaboration with Dr. Günther J. Redhammer)
and on the samples of clinopyroxenes (in
collaboration with Dr. Matteo Alvaro) in order to
know the relationship between thermal expansion
and shift of the spectra signatures.
The described laboratory experimental protocol
allows collecting and combining in-situ spectra
and X-ray diffraction data on further minerals, in
order to explain any anomaly or complexity
showed by spectra collected from the surface of
Mercury.
References
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HAMILTON, V.E. 2010. Thermal infrared (vibrational) spectroscopy of Mg–Fe olivines: A review and
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FASSETT, C. I., et al., 2012. Large impact basins on Mercury: Global distribution, characteristics, and
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MARCHI, S., MOTTOLA, S. CREMONESE, G. MASSIRONI, M. and MARTELLATO, E. 2009. A
new Chronology for the Moon and Mercury. Astronomical Journal, 137, 4936-4948.
MASSIRONI, M., et al., 2012. Strike-slip kinematics on Mercury: evidences and implications. In: 43rd
Lunar Planet. Sci. Conference. Abstract. The Woodlands, Texas.
MATURILLI, A., et al., 2008. The Berlin emissivity database (BED). Plan. Space Sci., 56, 420-425.
PREUSKER, F., et al., 2011. Stereo topographic models of Mercury after three MESSENGER flybys.
Plan. Space Sci., 59, 1910-1917.
SOLOMON, S.C., Mc NUTT, R.L. and GOLD, R.R. 2001. The MESSENGER mission to Mercury:
scientific objectives and implementation. Plan. Space Sci., 49, 1445-1465.
WARELL, J., et al., 2010. Constraints on Mercury’s surface composition from MESSENGER and
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2012-2013
SUMMARY OF ACTIVITY
Courses:
F. NESTOLA: “Metodologie analitiche”, Dipartimento di Geoscienze, Università degli Studi di Padova.
M. FLORIS: “Introduzione alle tecniche GIS”, Dipartimento di Geoscienze - Università degli Studi di Padova.
F. PESARIN, L. SALMASO: “Statistica applicata alla sperimentazione scientifica”, Dipartimento di tecnica e gestione dei
Sistemi Industriali, Università degli Studi di Padova.
F. CAMARA: “Risoluzione strutturale di sostanze inorganiche a struttura cristallina ignota”, Dipartimento di Geoscienze,
Università degli Studi di Padova.
CENTRO DI SERVIZI INTERDIPARTIMENTALE CUGAS: “Corso teorico-applicativo sulle tecniche SEM ed ESEM, II
edizione”, Università degli Studi di Padova.
FIRST EGU SUMMER SCHOOL: “Structural Analysis of Crystalline Rocks, Nevessee Area/South Tyrol, Italy.
R. ANGEL: “Elasticity and Structure of Minerals”, Dipartimento di Geoscienze, Università degli Studi di Padova.
P. TAROLLI, M. CAVALLI: “Geomophometry - Analisi quantitativa della superficie terrestre", Centro Interdipartimentale di
Ricerca in Cartografia, Fotogrammetria, Telerilevamento e S.I.T., Università degli Studi di Padova.
R. ANGEL: “Scientific Communication”, Dipartimento di Geoscienze, Università degli Studi di Padova.
10th EMAS 2012 Regional Workshop on Electron Probe Microanalysis Today - Practical Aspects, Dipartimento di
Geoscienze, Università degli Studi di Padova.
Communications (current year):
FERRARI, S. The Geology and structure in and around Rembrandt Basin, Mercury. 2011-2012 DTM Lecture Series,
Carnegie Institution of Washington, Washington D.C.
CARLI, C., CAPACCIONI, F., MATURILLI, A., AMMANNITO, E., FERRARI, S., NESTOLA, F., HELBERT, J.,
MASSIRONI, M., SGAVETTI, M. and SERVENTI G. Studying basalts spectra in the VNIR and MidIR: what we could learn
integrating data from VIHI and MERTIS the spectrometers onboard BepiColombo. EGU 2012 General Assembly, Wien
(Austria).
FERRARI, S., POZZOBON, R., CASTELLUCCIO, A., MASSIRONI, M. and DI ACHILLE, G. DTM analysis and
displacement estimates of a major mercurian lobate scarp. EGU General Assembly 2012, Wien (Austria).
Posters (current year):
FERRARI, S., M. MASSIRONI, C. KLIMCZAK, P. K. BYRNE, G. CREMONESE and S. C. SOLOMON, Complex history
of the Rembrandt basin and scarp system, Mercury. European Planetary Science Congress 2012, Madrid (Spain).
DI ACHILLE, G., POPA, C., MASSIRONI, M., FERRARI, S., GIACOMINI, L., MAZZOTTA EPIFANI, E., POZZOBON,
R., ZUSI, M., CREMONESE, G. and PALUMBO, P. Mapping Mercury’s tectonic features at the terminator: implications for
radius change estimates and thermal history models. 43rd Lunar Planet. Sci. Conference. Abstract. The Woodlands (Texas).
MASSIRONI, M., DI ACHILLE, G., FERRARI, S., GIACOMINI, L., POPA, C., POZZOBON, R., ZUSI, M.,
CREMONESE, G. and PALUMBO, P. Strike-slip kinematics on Mercury: evidences and implications. 43rd Lunar Planet. Sci.
Conference. Abstract. The Woodlands (Texas).
Publications:
MASSIRONI M., GIACOMINI L., FERRARI S., MARTELLATO E., CREMONESE G., MARCHI S. and CORADINI A.
Benefits of the Proposed Magia Mission for Lunar Geology. Earth, Moon, and Planets 107 (2-4) 267-297 (2010).
HELBERT, J., NESTOLA, F., FERRARI, S., MASSIRONI, M., MATURILLI, A., REDHAMMER, G. J., CAPRIA, M. T.,
CARLI, C., CAPACCIONI, F. and BRUNO, M., submitted. The chameleon-like surface of Mercury, Earth Planet. Sci.Lett.
MASSIRONI, M., DI ACHILLE, G., FERRARI, S., GIACOMINI, L., POPA, C., POZZOBON, R., ZUSI, M.,
CREMONESE, G., PALUMBO. P., submitted. Strike-slip kinematics on Mercury: evidences and implications, Geology.
Grants (current year):
HIGH TEMPERATURE EXPERIMENTS TO PLANETARY EMISSIVITY LABORATORY (PEL) AT DLR BERLIN,
GERMANY, July 2012, granted by 7th Framework Programme / EuroPlanet Research Infrastructure (EC Grant agreement
number 228319).
Lab activity:
HIGH TEMPERATURE EXPERIMENTS to Planetary Emissivity Laboratory (PEL) at Deutsches Zentrum fuer Luft- und
Raumfahrt (DLR) Berlin, Germany, 8 weeks:
December 2010, granted by 7th Framework Programme / EuroPlanet Research Infrastructure
April 2011 instrumental set-up .
June and July 2011, emissivity measurements
July 2012, granted by 7th Framework Programme / EuroPlanet Research Infrastructure
MESSENGER PREDOCTORAL FELLOW at the Department of Terrestrial Magnetism (DTM), Carnegie Institution of
Washington, Washington D.C., 9 weeks.
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