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Lunar and Planetary Science XLVIII (2017)
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VOLCANIC INFILLINGS OF LARGE BASINS ON MERCURY AS INDICATORS OF MANTLE THERMAL STATE AND COMPOSITION. Sebastiano Padovan1,*, Nicola Tosi1,2, Ana-Catalina Plesa1, and Thomas
Ruedas1,3. 1Department of Planetary Physics, German Aerospace Center (DLR), Berlin, Germany, 2Department of
Astronomy and Astrophysics, Technische Universität, Berlin, Germany, 3Institute of Planetology, University of
Münster, Germany. *Contact: [email protected]
Introduction: The crust of Mercury is mostly the
cumulative result of partial melting in the mantle associated with solid-state convection [e.g., 1]. The details
of how the surface composition represents the result of
dynamical processes in the interior are difficult to elucidate. Explanations for the observed geochemically
varied surface include a heterogeneous mantle [2], the
effects of ancient giant impacts [3], an evolving mantle
composition [3, 4], or a combination of these processes. Here we explore the effects of large impacts on
mantle dynamics and associated melt production and
show that the properties of melt sheets within young
large basins on Mercury depend on the mantle thermal
state and composition.
Mantle convection: With the convection code
GAIA [5], we compute thermal evolution histories of
Mercury compatible with the expected amount of heat
producing elements (HPEs) in the mantle [1, 6] and
with the inferred thickness of the crust [6, 7]. We consider the effects of an insulating regolith layer, of different amounts of HPEs, and of variations in the reference viscosity. A large number of models produces a
crust which is compatible with the inferred range
Figure 1: Cumulative crustal thickness from convective melting as a function of time. Different curves refer to different combination of parameters, as indicated
in the legend. Most models are compatible with the
crustal thickness inferred from gravity and topography
data (grey band and horizontal black line) [6,7]. Colors indicate the depth of the source region of the partial melts forming the crust. Values for the HPEs are
found in [6].
(Figure 1). Independent of the choice of parameters,
crustal production is very rapid in the early phases of
evolution and is completed between about 0.5 and 2
Gy. It is important to note that as long as the mantle is
producing melt, the depth of the source region is
roughly constant during the evolution.
Impacts: We estimate the thermal anomalies in the
mantle generated by large impacts using scaling laws
[e.g., 8]. We simulate impactors with a velocity of 42
km/s and an impact angle of 45°, as appropriate for
Mercury [9]. The diameter of the impactors is varied in
order to produce basins with diameters in the range
from 715 km (Rembrandt) to 1550 km (Caloris).
The thermal anomaly associated with a basin forming event is located in the upper mantle and generates
convective currents and associated melting which postdate the impact event (Figure 2).
Figure 2: Temporal evolution of the temperature field
(background) and of the magnitude of the velocity
field (arrows) in the event of an impact forming a
Caloris-sized basin. The thermal anomaly warps the
isotherms and convective currents develop in the shallow mantle. The two snapshots are taken 3 (top) and 12
My (bottom) after the impact event.
Lunar and Planetary Science XLVIII (2017)
Post-impact volcanic eruptions: A large impact
facilitates the subsequent eruption of magma to the
surface. Any pre-existing crust is removed, thereby
eliminating a possible neutral buoyancy horizon created by a low density crust [10]. In addition, the fractured volume below the impact site [11] allows for an
easier upward migration of the melt.
Depending on the timing of the impact, the melt
erupting at the surface will be a combination of convective melt generated at depth (Figure 1) and shallow
melt resulting from the impact-induced convective currents (Figure 2). The volcanic infillings following an
impact happening early in the evolution of the planet,
when convection is still vigorous, are dominated by
convective melt. Later in the evolution, the erupted
melt shows the signature of the impact-induced shallow melt (Figure 3).
Figure 3: Melt production following an impact forming
a Caloris-sized basin. Results for the impact occurring
at 170, 500, 750, 1000, 1150 My. For each epoch the
cases for the impact occurring over an upwelling and
downwelling are plotted (the downwelling curve is
plotted over the upwelling curve). Thickness values refer to melt produced below the final basin. Colors indicate the source depth of the melt. The grey background
indicates qualitatively the mantle thermal state.
Young large basins on Mercury: The youngest
large basins on Mercury are Caloris (diameter d = 1550
km) and Rembrandt (d = 715 km). They both have volcanic infillings that postdate the basin itself by about
100 to 200 My [12, 13]. The thickness of the surficial
volcanic layer is between 500 and 800 m for Caloris
[14] and between 360 and 520 m for Rembrandt [13].
We can reproduce both the time delay between the
impacts and the emplacement of the volcanic plains in
the basins, and the thickness of the surficial volcanic
layer (see inset a in Figure 3), if an effusion rate of
about 30% is assumed and if the impacts that originated Caloris and Rembrandt happened when the melting
associated with mantle convection was feeble (i.e.,
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when most of the crust was already in place). This conclusion is corroborated by the dating of large volcanic
units on the surface of Mercury [15].
Conclusions and outlook: In this study we show
that the properties of the volcanic plains within the
young large basins of Mercury (Caloris and Rembrandt) depend on the mantle thermal state and composition. We predict the source depth of the volcanic
plains to be different from the source depth of older
surface units, a result that can help explaining the singular composition of the volcanic plains inside Caloris
[3, 16].
Acknowledgments: S.P. acknowledges support
from the German Academic Exchange Service
(DAAD). S.P. and N.T. acknowledge support from the
Helmholtz Gemeinschaft (project VH-NG-1017). A.P.
and T.R. acknowledge support from the German Research Foundation (DFG) through grants SFB-TRR
170 (A.P. and T.R.) and RU 1839/1 (T.R.). Computational time has been provided by the Nord-deutscher
Verbund für Hoch- und Höchstleistungsrechnen
(HLRN) through the project “Partial melting and the
thermo-chemical evolution of terrestrial planets”.
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