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RISS 인기검색어
- 검색키워드
- 저자 : Garrick-Bethell, Ian (검색결과 16 건)
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The tidal–rotational shape of the Moon and evidence for polar wander
Garrick-Bethell, Ian,Perera, Viranga,Nimmo, Francis,Zuber, Maria T. Nature Publishing Group, a division of Macmillan P 2014 Nature Vol.512 No.7513
The origin of the Moon’s large-scale topography is important for understanding lunar geology, lunar orbital evolution and the Moon’s orientation in the sky. Previous hypotheses for its origin have included late accretion events, large impacts, tidal effects and convection processes. However, testing these hypotheses and quantifying the Moon’s topography is complicated by the large basins that have formed since the crust crystallized. Here we estimate the large-scale lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the spherical harmonic degree-2 topography is consistent with a crust-building process controlled by early tidal heating throughout the Moon. The remainder of the degree-2 topography is consistent with a frozen tidal–rotational bulge that formed later, at a semi-major axis of about 32 Earth radii. The probability of the degree-2 shape having both tidal-heating and frozen shape characteristics by chance is less than 1%. We also infer that internal density contrasts eventually reoriented the Moon’s polar axis by 36 ± 4°, to the configuration we observe today. Together, these results link the geology of the near and far sides, and resolve long-standing questions about the Moon’s large-scale shape, gravity and history of polar wander.
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Troctolite 76535: A sample of the Moon's South Pole-Aitken basin?
Garrick-Bethell, Ian,Miljković,, Katarina,Hiesinger, Harald,van der Bogert, Carolyn H.,Laneuville, Matthieu,Shuster, David L.,Korycansky, Donald G. Academic Press 2020 Icarus Vol.338 No.-
<P><B>Abstract</B></P> <P>Lunar samples returned by the Apollo program have provided insights into numerous solar system processes. However, no samples were returned from the lunar farside, where one of the Moon's most geologically important features resides: the 2500-km-diameter South Pole-Aitken basin (SPA). Here, we explore the hypothesis that lunar troctolite 76535 was excavated by SPA. This hypothesis is motivated by the sample's low peak shock pressure (<6 GPa), its substantial depth of origin (45–65 km), and its ancient <SUP>40</SUP>Ar/<SUP>39</SUP>Ar age of 4.25 Ga. We use hydrodynamic simulations of crater formation to show that for vertically incident impactors, SPA is the only known basin that can excavate material from the depth and shock pressure range relevant to 76535. The thermal history of 76535 also rules out excavation where a magma ocean was locally present. However, for the vertical impacts modeled, delivery of 76535 to the Apollo 17 site, where it was collected, requires a second impact event that preserved the sample's low shock state. An alternative interpretation of the SPA origin is that 76535 originates from the Serenitatis, Fecunditatis, or Australe basins, if the inferred origin depth of 76535 is in error by ~20 km, or its inferred peak shock pressure is in error by a factor of ~2. These basins could also be candidates for excavating 76535, if oblique impacts yield lower shock pressures of material excavated from the relevant depth. If troctolite 76535 is in fact a sample of SPA, we find that its 4.25 Ga excavation age and the density of large (100–300 km diameter) impact basins within and on the rim of SPA are consistent with the monotonically decaying chronology.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Simulations of vertically incident impactors show that only the South Pole-Aitken basin could have excavated the 4.25-billion-year old Apollo sample 76535. </LI> <LI> Oblique impact modeling and additional petrological studies are required to rule out the Serenitatis, Australe, and Nectaris basins. </LI> <LI> Local basal crustal temperatures when 76535 was excavated were 500-740°C, consistent with the absence of a magma ocean. </LI> <LI> When coupled with a 4.25 Ga formation age, the density of 100-300 km impact craters on the South Pole-Aitken basin is consistent with a monotonically decreasing impactor population since solar system formation. </LI> </UL> </P>
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Iron content determines how space weathering flux variations affect lunar soils
McFadden, James,Garrick-Bethell, Ian,Sim, Chae K.,Kim, Sungsoo S.,Hemingway, Doug Elsevier 2019 Icarus Vol.333 No.-
<P><B>Abstract</B></P> <P>Previous work has established that the solar wind and micrometeoroids produce spectral changes on airless silicate bodies. However, the relative importance of these two weathering agents, the timescales over which they operate, and how their effects depend on composition have not yet been well determined. To help address these questions we make use of the fact that solar wind and micrometeoroid fluxes vary with latitude on the Moon. Previous work has shown that this latitudinally varying flux leads to systematic latitudinal variations in the spectral properties of lunar soils. Here we find that the way in which a lunar soil's spectral properties vary with latitude is a function of its iron content, when we consider soils with 14–22 wt% FeO. In particular, a 50% reduction in flux corresponds to a significant increase in reflectance for 14 wt% FeO soils, while the same flux reduction on 21 wt% FeO soils is smaller by a factor of ~5, suggesting that this brightening effect saturates for high FeO soils. We propose that lower iron soils may not approach saturation because grains are destroyed or refreshed before sufficient nano- and micro-phase iron can accumulate on their rims. We compare our results to the spectral variations observed across the Reiner Gamma swirl, which lies on a high‑iron surface, and find it has anomalous brightness compared to our predictions. Swirls in Mare Marginis, which lie on a low iron surface, exhibit brightness differences that suggest reductions in solar wind flux between 20 and 40%. Our inferences suffer from the limited latitudinal extent of the maria and the convolution of micrometeoroid flux and solar wind flux changes with latitude. Superior constraints on how space weathering operates throughout the inner solar system would come from in situ measurements of the solar wind flux at lunar swirls.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We find that the lunar maria brighten at higher latitudes, and the total brightening is a function of soil iron content. </LI> <LI> Above a soil iron content of ~21 wt% FeO, there is a negligible change in brightness with latitude. </LI> <LI> We use this finding to predict the brightness of the Reiner Gamma swirl, and find that it is anomalously bright. </LI> </UL> </P>
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Nayak, Michael,Hemingway, Doug,Garrick-Bethell, Ian Elsevier 2017 Icarus Vol.286 No.-
<P>A number of magnetic anomalies are present along the northern edge of the lunar South Pole-Aitken (SPA) basin. A variety of hypotheses for their formation have been proposed, but an in-depth study of their properties has not been performed. Here we use two different methods to invert for their source body characteristics: one that completely searches a small parameter space of less than ten uniform strength dipoles per anomaly, and another that uses grids of hundreds of dipoles with variable magnetization strengths. Both methods assume uniform magnetization directions at each anomaly and with one exception, produce nearly the same results. We introduce new Monte Carlo methods to quantify errors in our inversions arising from Gaussian time-dependent changes in the external field and the uncertain geometry of the source bodies. We find the errors from uncertainty in source body geometry are almost always higher. We also find a diverse set of magnetization directions around SPA, which we combine with other physical arguments to conclude that the source bodies were likely magnetized in a dynamo field. Igneous intrusions are a reasonable explanation (Purucker et al., 2012) for the directional variability, since they could be intruded over different magnetic epochs. However, the directional variability also implies either surprisingly large amounts of true polar wander or a dynamo not aligned with the lunar spin axis. We also explore the possibility that true polar wander caused by the SPA impact could allow iron-rich SPA ejecta to record a diverse set of magnetic field directions. Some of this material may have also become 'sesquinary' ejecta and re-impacted across the Moon on 10(4)-10(6) year timescales to capture such changes. No completely satisfactory answer emerges, except that the dipole-axis of the lunar dynamo may have been variable in direction. Published by Elsevier Inc.</P>
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Poppe, A.R.,Fatemi, S.,Garrick-Bethell, I.,Hemingway, D.,Holmstrom, M. Academic Press 2016 Icarus Vol.266 No.-
<P>Remanent magnetization has long been known to exist in the lunar crust, yet both the detailed topology and ultimate origin(s) of these fields remains uncertain. Some crustal magnetic fields coincide with surface albedo anomalies, known as lunar swirls, which are thought to be formed by differential surface weathering of the regolith underlying crustal fields due to deflection of incident solar wind protons. Here, we present results from a three-dimensional, self-consistent, plasma hybrid model of the solar wind interaction with two different possible source magnetizations for the Reiner Gamma anomaly. We characterize the plasma interaction with these fields and the resulting spatial distribution of charged-particle weathering of the surface and compare these results to optical albedo measurements of Reiner Gamma. The model results constrain the proposed source magnetizations for Reiner Gamma and suggest that vertical crustal magnetic fields are required to produce the observed 'dark lanes.' (c) 2015 Elsevier Inc. All rights reserved.</P>
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