16:00 〜 16:15
[SIT18-08] Evidence and consequences of deep Martian mantle layering
★Invited Papers
キーワード：Mars core size, Mars evolution, Mars mantle layering
The identification of deep reflected phases in the seismic recordings of the InSight mission [1] as core-reflected phases have led to the first seismic detection of the Martian core [2]. This has let to core size estimates spanning the higher end of InSight pre-mission estimates, implying a large fraction of Sulfur in the core together with smaller fractions of O, C, and H. However, these fractions lie beyond the experimental petrological range [3]. In addition, the recent detection of P-diffracted phase [4] requires a significant reduction of seismic velocities in the deep mantle, which is difficult to explain with compositionally homogeneous mantle models [5].
The presence of a well-separated metallic core indicates that Mars experienced an early global magma ocean stage whose crystallisation likely led to the formation of a compositionally distinct layer at the bottom of the mantle [6]. Such a basal mantle layer (BML) is expected to be heavily enriched in heat-producing elements and in iron, leading to long-term stability with little mixing between the layer and the overlying mantle [7].
We tested the compatibility of deep Martian mantle layering with InSight seismic [8] and geodetic [9] data, along with other observational constraints. We conducted Monte Carlo Markov chain inversions in which the long-term thermo-chemical history of Mars’ main envelopes is embedded into the forward problem [10]. This approach allows for more consistent and better-constrained profiles than in classical inversions, and allows reconstructing the long-term history of the planet. Our inversion approach also considers an enriched silicate layer above the core-mantle boundary and we invert for the layer thickness and for its thermal conductivity [11]. We used the most recent travel time dataset that contains considerably more shallow and deep phases (including ScS and Pdiff) compared to previous studies.
The BML leads to the presence of a fully molten silicate layer above the core, overlain by a partially molten layer. The fully molten silicate layer acts as a seismic extension of the iron core and triggers S-reflections above the core-mantle boundary, in agreement with theoretical predictions [7]. This results in a core 100-200 km smaller than previous estimates that assume a compositionally homogeneous mantle [2].
The smaller core inferred in models that account for a BML is considerably denser than previous estimates. This revised core density can be explained by fewer amounts of S and other light elements within the experimental petrological range.
Our results show that the presence of a BML is compatible with seismic, geodetic and petrological
experimental data [12]. The development of a fully molten silicate layer that triggers deep S-wave reflections above the core also reduces the travel time of P-diffracted waves along the CMB, yielding a good data fit for the differential travel time between PP and Pdiff phases. The fully molten layer is overlain by a partially molten silicate layer that accommodates tidal dissipation. The resulting structure is compatible with geodetic data [13].
[1] Banerdt, W. et al., Nature geoscience, 13, 183-189 (2020).
[2] Stähler, S., et al., Science 373, 443–448 (2021).
[3] Pommier, A. et al., (2022) Front. Earth Sci., doi: 10.3389/feart.2022.956971 (2022).
[4] Horleston, A. et al., The Seismic Record, 2, 88-99 (2022).
[5] Posiolova, L. et al., Science, 378, 412-417, (2022).
[6] Elkins-Tanton, L. et al., JGR, doi:10.1029/2005JE002480 (2003).
[7] Samuel, H. et al., JGR, doi:10.1029/2020JE006613 (2021).
[8] Lognonné, P. et al., Nature geoscience, 13, 213-220 (2020)
[9] Folkner, W. et al., Space Sci. Rev., 214, 100 (2018).
[10] Drilleau, M. et al., G. J. Int., 226, 1615-1644 (2021).
[11] Samuel et al., AGU Fall meeting #830599, https://agu.confex.com/agu/fm21/prelim.cgi/Paper/830599 (2021)
[12] Samuel et al., EPSC meeting, Vol. 16, EPSC2022-297, https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-297.html (2022).
[13] Le Maistre, S., et al., LPSC abstract #1611 (2023).
The presence of a well-separated metallic core indicates that Mars experienced an early global magma ocean stage whose crystallisation likely led to the formation of a compositionally distinct layer at the bottom of the mantle [6]. Such a basal mantle layer (BML) is expected to be heavily enriched in heat-producing elements and in iron, leading to long-term stability with little mixing between the layer and the overlying mantle [7].
We tested the compatibility of deep Martian mantle layering with InSight seismic [8] and geodetic [9] data, along with other observational constraints. We conducted Monte Carlo Markov chain inversions in which the long-term thermo-chemical history of Mars’ main envelopes is embedded into the forward problem [10]. This approach allows for more consistent and better-constrained profiles than in classical inversions, and allows reconstructing the long-term history of the planet. Our inversion approach also considers an enriched silicate layer above the core-mantle boundary and we invert for the layer thickness and for its thermal conductivity [11]. We used the most recent travel time dataset that contains considerably more shallow and deep phases (including ScS and Pdiff) compared to previous studies.
The BML leads to the presence of a fully molten silicate layer above the core, overlain by a partially molten layer. The fully molten silicate layer acts as a seismic extension of the iron core and triggers S-reflections above the core-mantle boundary, in agreement with theoretical predictions [7]. This results in a core 100-200 km smaller than previous estimates that assume a compositionally homogeneous mantle [2].
The smaller core inferred in models that account for a BML is considerably denser than previous estimates. This revised core density can be explained by fewer amounts of S and other light elements within the experimental petrological range.
Our results show that the presence of a BML is compatible with seismic, geodetic and petrological
experimental data [12]. The development of a fully molten silicate layer that triggers deep S-wave reflections above the core also reduces the travel time of P-diffracted waves along the CMB, yielding a good data fit for the differential travel time between PP and Pdiff phases. The fully molten layer is overlain by a partially molten silicate layer that accommodates tidal dissipation. The resulting structure is compatible with geodetic data [13].
[1] Banerdt, W. et al., Nature geoscience, 13, 183-189 (2020).
[2] Stähler, S., et al., Science 373, 443–448 (2021).
[3] Pommier, A. et al., (2022) Front. Earth Sci., doi: 10.3389/feart.2022.956971 (2022).
[4] Horleston, A. et al., The Seismic Record, 2, 88-99 (2022).
[5] Posiolova, L. et al., Science, 378, 412-417, (2022).
[6] Elkins-Tanton, L. et al., JGR, doi:10.1029/2005JE002480 (2003).
[7] Samuel, H. et al., JGR, doi:10.1029/2020JE006613 (2021).
[8] Lognonné, P. et al., Nature geoscience, 13, 213-220 (2020)
[9] Folkner, W. et al., Space Sci. Rev., 214, 100 (2018).
[10] Drilleau, M. et al., G. J. Int., 226, 1615-1644 (2021).
[11] Samuel et al., AGU Fall meeting #830599, https://agu.confex.com/agu/fm21/prelim.cgi/Paper/830599 (2021)
[12] Samuel et al., EPSC meeting, Vol. 16, EPSC2022-297, https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-297.html (2022).
[13] Le Maistre, S., et al., LPSC abstract #1611 (2023).