Crater Central Peaks Research Articles

Understanding the Ries impact structure subsurface from high-resolution seismic data

Abstract The Ries impact structure (southern Germany) formed ca. 15 Ma and is 22–26 km in diameter, making it one of the youngest and best-preserved mid-size terrestrial impact craters, yet the subsurface has not been studied with modern geophysics. We present the first high-resolution seismic profiles of the Ries impact structure; the profiles show discontinuous intra-basement reflectors and a central crater floor without a significant central topographic high. The inner crystalline ring sits adjacent to, not on top of, the crater terrace zone. These morphologies indicate that during the crater modification stage, the rebounding central uplift at Ries rose and then collapsed without the continued outward motion required to form a fully developed peak ring. The Ries impact structure may be best considered a transitional complex crater form between a central-peak crater and a peak-ring crater as documented on the Moon and other rocky planets. A series of high-amplitude, discontinuous, topographically influenced reflectors overlying the basement implies that the suevite within the crater basin was emplaced via lateral transport.

The Paleogene Eagle Butte meteorite impact and dikes of seismic wave mobilized tuff in western Canada

AbstractMeteorite impact‐induced mobilization of a volcanic ash tuff resulting in 100s of m long lithified clastic dikes is a novel mechanism in the geologic record. The Paleogene Eagle Butte impact event in southeastern Alberta, western Canada, resulted in a 10–17 km diameter central peak crater, now partially eroded and buried without expression at the surface. Impact shock waves, degraded to elastic waves, mobilized sediment 10s of km beyond the crater and resulted in a cluster of 100s of m long, 10–20 cm thick, dikes consisting of volcanic ash. It is not well understood how effective meteorite impact‐induced waves would be as triggers for sediment mobilization at areas 10s of km distant from an impact crater. Emplacement of the Manyberries dike cluster provides insight into this issue and confirms that shock waves, degraded to elastic waves, can indeed liquefy and mobilize sediments at distances as much as 5 crater radii beyond an impact site. These impact‐induced waves triggered liquefaction and mobilization of a shallowly buried Campanian‐Maastrichtian deposit of volcanic ash and injected the tuff sediment along impact‐induced faults. The lithification of the dike tuff, consisting of the cristobalite–tridymite groundmass, resulted from partial dissolution of glass shards and precipitation of opaline cement. These weathering resistant dikes of indurated tuff sediment extend for as much as 800 m on recessive Campanian‐Maastrichtian shale ridges of the badlands topography.

U‐Pb dating of zircon and monazite from the uplifted Variscan crystalline basement of the Ries impact crater

AbstractImpact crater central peaks and peak ring complexes are important exploration targets for future missions to other planetary bodies, because they provide access to material uplifted from lower crustal levels. Material exposed there could also provide chronological constraints on crater formation events. Therefore, it is essential to understand if uplifted peak material preserves the chronological records of igneous and metamorphic protolith crustal rocks, or if such records are reset during impact events. To investigate this issue, we collected shocked gneiss and granite samples from uplifted crystalline basement megablocks in the 24 km diameter Ries impact crater in Germany, which is dated at ~14.8 Ma. Petrographic observations, electron beam imaging, and Raman spectroscopy suggest that these samples record the peak pressures of ~10–15 GPa. In situ U‐Pb dating shows that monazite U‐Pb systematics have not been affected by the Ries impact, as gneisses and granites yielded monazite U‐Pb dates of ~370 and 330 Ma, consistent with known Variscan metamorphic and magmatic events. The U‐Pb systematics of some zircon grains yielded U‐Pb dates of approximately 5–10 Ma, which is younger than the age of the Ries impact event. These young dates correspond to U‐rich metamict domains and may reflect recent Pb loss and/or U‐gain during postimpact hydrothermal alteration or weathering. These observations indicate that dating uplifted crystalline material in impact craters on other bodies might provide useful petrological and chronological constraints on the underlying target rocks rather than directly dating impact events, for which sampling impact melt and impact melt‐bearing lithologies should remain the primary target.

Formation of Tridymite and Evidence for a Hydrothermal History at Gale Crater, Mars

AbstractIn August 2015, the Curiosity Mars rover discovered tridymite, a high‐temperature silica polymorph, in Gale crater. The existing model for its occurrence suggests erosion and detrital sedimentation from silicic volcanic rocks in the crater rim or central peak. The chemistry and mineralogy of the tridymite‐bearing rocks, however, are not consistent with silicic volcanic material. Using data from Curiosity, including chemical composition from the Alpha Particle X‐ray Spectrometer, mineralogy from the CheMin instrument, and evolved gas and isotopic analyses from the Sample Analysis at Mars instrument, we show that the tridymite‐bearing rocks exhibit similar chemical patterns with silica‐rich alteration halos which crosscut the stratigraphy. We infer that the tridymite formed in‐place through hydrothermal processes and show additional chemical and mineralogical results from Gale crater consistent with hydrothermal activity occurring after sediment deposition and lithification.

The anatomy of fresh complex craters on the mid‐sized icy moons of Saturn and self‐secondary cratering at the rayed crater Inktomi (Rhea)

AbstractCassini mapping of Saturn’s mid‐sized icy moons of well‐preserved complex craters in the 45–95 km size range provides insight into cratering processes at lower surface gravity and on icy targets. These craters are characterized by steep rim scarps, rugged hummocky floor deposits of curvilinear ridges and scarps, and rugged conical central peaks. Ponded impact melt or related deposits are not observed on any floor or ejecta units of any of these complex craters, indicating that melt production may be much lower than predicted. Mantling ejecta units drape over pre‐existing topography, grading into concentric zones of secondary craters at ~1 crater diameter from the rim, demonstrating that secondaries occur on mid‐sized icy planetary bodies. The largest and youngest bright‐ray system is the 49 km central peak crater Inktomi, target of a mapping campaign down to 34 m pixel scales. In addition to classical secondary craters up to 3 km, several hundred small craters <1 km in size form an unusual densely spaced cluster across the eastern floor and ejecta deposit. The terrain‐indiscriminate distribution of these eastern floor craters indicates they are not explosive volatile release pits, such as mapped on Vesta or Mars, but self‐secondary craters formed by the fallback of ejected blocks back into the crater itself during crater formation. Self‐secondaries likely formed by an irregular and stochastic but poorly understood impact process, such as spallation influenced by irregular surface topography. Self‐secondary craters of this type could influence the interpretation of crater counts on large fresh impact craters and basins elsewhere.

Spectral investigation of Ceres analogue mixtures: In-depth analysis of crater central peak material (ccp) on Ceres

The dwarf planet Ceres is an airless body composed of Mg-phyllosilicates, NH4-phyllosilicates, Mg/Ca-carbonates and a dark component. The subsurface of Ceres, investigated by the material composing the peak of complex craters (ccp, crater central peak material; Galiano et al., 2019), reveals a composition similar to the surface, with an increasing abundance of phyllosilicates in the interior. A moderate trend between age of craters' formation and spectral slope of ccps suggests that younger ccps show a negative/blue slope and older ccps are characterized by positive/red slope. To investigate the causes of different spectral slope in ccps, different grain-sized Ceres analogue mixtures were produced and spectrally analysed. First, the end-members of the Ceres surface (using the antigorite as Mg-phyllosilicate, the NH4-montmorillonite as NH4-phyllosilicate, the dolomite as carbonate and the graphite as dark component), were mixed, obtaining mixtures with different relative abundance, and identifying the mixture with the reflectance spectrum most similar to the average Ceres spectrum. The selected mixture was reproduced with grain size of 0–25 μm, 25–50 μm and 50–100 μm. The three mixtures were heated and spectrally analysed, both with an acquisition temperature of 300 K (room temperature) and 200 K (typical for surface Ceres temperature during VIR observations).The best analogue Ceres spectrum is coincident with a mixture composed of 18 M% (mass percentage) of Dolomite, 18 M% of Graphite, 36 M% of Antigorite and 28 M% of NH4-montmorillonite, after experiencing a heating process.The heating process produces: 1) a darkening and reddening of spectrum, as consequence of the devolatilization of OH group in phyllosilicates and a more dominant effect of opaque phase; 2) a deepening in the intensity of the 3.4 and 4.0 μm band, as well as the 2.7 and the 3.1 μm band, likely due to the loss of absorbed atmospheric water; 3) narrowing of 3.1 μm band and the shift of band center toward longer wavelength (i.e. at 3.06 μm) coincident with mean Ceres spectrum, related to the loss of absorbed atmospheric water.The analysis of the best Ceres analogue mixture, reproduced at different grain size and after heating process, reveals a weakening of 2.7, 3.1, 3.4 and 4.0 absorption bands in coarser samples, likely related to large size of dark grains which reduce the spectral contrast. Furthermore, spectra of coarser mixtures are more red-sloped, suggesting that this trend is more affected by the dark component.The best analogue Ceres mixture produced in this work is almost coincident with the mean spectrum of Haulani ccp, the youngest ccps on Ceres and therefore representative of less altered material on Ceres.The redder spectral slope observed in the older ccps is probably the consequence of the space weathering effects on the original material composing the peak.

Aqueous Processes From Diverse Hydrous Minerals in the Vicinity of Amazonian‐Aged Lyot Crater

AbstractAmazonian‐aged Lyot crater is the best‐preserved and deepest peak‐ring impact crater (diameter, D = 220 km) in the northern lowlands of Mars. Morphological features including scouring channels emanating from its ejecta and small channels within the crater have been examined previously to understand hydrological activity associated with the crater. In this study, we analyze images acquired by the Compact Reconnaissance Imaging Spectrometer for Mars on board the Mars Reconnaissance Orbiter to investigate the mineralogical record in Lyot and its surroundings, which are presently enriched in ground ice, to understand the associated aqueous processes, their relative timing, and a possible role for ground ice in hydrous mineral formation. We find diverse hydrous minerals, including Fe/Mg phyllosilicates, chlorite, illite/muscovite, and prehnite in Lyot crater walls, central peak, and ejecta, as well as in two craters to the west of Lyot. The exposure and distribution of the hydrous minerals suggest that they are related to the impact process, either exposed by the excavation of hydrothermally altered rocks or formed through syn‐depositional hydrothermal alteration immediately after impacts. The Lyot impact induced channel formation to the north, but no mineralogical evidence of aqueous alteration associated with the channels is observed. The sinuous channels within Lyot, diverted by bedrock units with hydrous mineral detections, did not cause mineralization but likely represent the last stage of water activity in Lyot crater. The separate episodes of water activity indicate flow of liquid water on Mars' surface during the Amazonian but limited to no aqueous alteration to generate hydrous minerals.

Spectral analysis of the Cerean geological unit crater central peak material as an indicator of subsurface mineral composition

The dwarf planet Ceres is a heavily cratered rocky body, and complex craters with a central peak are widely observed on its surface. These types of craters form when a large body impacts the surface, generating extreme temperatures and pressures. During the impact event a large volume of rock is raised from the subsurface and a central uplift is formed. The material composing the central uplift is called crater central peak material (ccp) and the spectral analysis of such geologic areas can provide information about the composition of Ceres’ subsurface. Reflectance spectra of 32 ccps, acquired by the VIR spectrometer on board the NASA/Dawn spacecraft, were analyzed and shows absorption bands located at about 2.7, 3.1, 3.4 and 4.0 µm which are also common on the Cerean surface. These absorptions are related, respectively, to Mg-phyllosilicates, NH4-phyllosilicates and Mg/Ca-carbonates.The spectral parameters considered in this work are: spectral slopes estimated between 1.2 µm and 1.9 µm, band depths at 2.7-, 3.1-, 3.4- and 4.0-µm, and band centers near 4.0-µm. The ccps spectral parameters were analyzed in conjunction with other Cerean parameters, such as the estimated depth of excavation of the material composing the central peak, in order to search for correlations and information about Ceres’ subsurface.Central peak material located polewards show stronger 2.7- and 3.1-µm band depths with respect to those at the equatorial region, suggesting that subsurface deposits closer to poles are probably richer in Mg- and NH4-phyllosilicates. The 3.4-µm spectral feature is also deeper in ccps located at poleward latitudes, similar to the phyllosilicates. Conversely, the 4.0-µm band does not show this trend with latitude.An increase in both 3.1- and 3.4-µm band depths with the estimated depth of excavation indicates that the spectral feature at 3.4-µm is the result of different contributions from carbonates and NH4-phyllosilicates, as expected. However, depending on their relative influence, the shape of the 3.4-µm spectral feature can vary.Phyllosilicates and carbonates are the resulting products of aqueous alteration of chondritic material and, given the increasing abundance of such minerals (in particular ammoniated phyllosilicates) with depth of excavation, it is likely that our investigation involved subsurface layers nearby the boundary between the volatile-rich crust and the silicate-rich mantle.Na-carbonate is found in the crater central peak material of Ernutet, Haulani and Ikapati, characterized by an estimated depth of excavation of about 6–9 km, where deposits of sodium carbonates could be locally present.

Mineralogical Diversity and Geology of Humboldt Crater Derived Using Moon Mineralogy Mapper Data.

Moon Mineralogy Mapper (M3) spectroscopic data and high‐resolution imagery data sets were used to study the mineralogy and geology of the 207 km diameter Humboldt crater. Analyses of M3 data, using a custom‐made method for M3 spectra continuum removal and spectral parameters calculation, reveal multiple pure crystalline plagioclase detections within the Humboldt crater central peak complex, hinting at its crustal origin. However, olivine, spinel, and glass are observed in the crater walls and rims, suggesting these minerals derive from shallower levels than the plagioclase of the central peak complex. High‐calcium pyroxenes are detected in association with volcanic deposits emplaced on the crater's floor. Geologic mapping was performed, and the age of Humboldt crater's units was estimated from crater counts. Results suggest that volcanic activity within this floor‐fractured crater spanned over a billion years. The felsic mineralogy of the central peak complex region, which presumably excavated deeper material, and the shallow mafic minerals (olivine and spinel) detected in Humboldt crater walls and rim are not in accordance with the general view of the structure of the lunar crust. Our observations can be explained by the presence of a mafic pluton emplaced in the anorthositic crust prior to the Humboldt‐forming impact event. Alternatively, the excavation of Australe basin ejecta could explain the observed mineralogical detections. This highlights the importance of detailed combined mineralogical and geological remote sensing studies to assess the heterogeneity of the lunar crust.

Detection of Mg-spinel bearing central peaks using M3 images: Implications for the petrogenesis of Mg-spinel

Mg-spinel bearing lithologies, lacking abundant mafic materials, have been discovered with images acquired by the Moon Mineralogy Mapper (M3) aboard Chandrayaan-1. We conducted a systematic screening of lunar crater central peaks for the presence of Mg-spinel to address its distribution and petrogenesis. 38 Mg-spinel bearing crater central peaks were identified in this study out of 166 craters investigated. The results suggest that Mg-spinel is common in the lunar crust and appears to be extensive in the middle part of the lunar crust underneath Procellarum KREEP Terrane (PKT). Mg-spinel neither exclusively originated from deep layers (>10 km) nor necessarily coexist with the appearance of olivine or pyroxene. 15 Mg-spinel bearing central peaks originated from depth less than 10 km. Nine investigated central peaks only contain Mg-spinel and plagioclase without any detectable mafic materials. All those observations imply that the origin of Mg-spinel is possibly related to Mg-suite plutonism and assimilation between high Mg′ magma with anorthositic crust. The extensive distribution and Mg-suite related petrogenesis indicates that Mg-spinel bearing lithologies might represent a new member of Mg-suite rocks.

遊星百景 その6 ～月面クレーター中央丘の地質～

遊星百景 その6 ～月面クレーター中央丘の地質～

Analysis of MESSENGER high‐resolution images of Mercury's hollows and implications for hollow formation

AbstractHigh‐resolution images from MESSENGER provide morphological information on the nature and origin of Mercury's hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low‐reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales <20 m indicate that the average depth of hollows is 24 ± 16 m. We propose that hollows cease to increase in depth when a volatile‐depleted lag deposit becomes sufficiently thick to protect the underlying surface. The difficulty of developing a lag on steep topography may account for the common occurrence of hollows on crater central peaks and walls. Disruption of the lag, e.g., by secondary cratering, could restart growth of hollows in a location that had been dormant. Images at extremely high resolution (~3 m/pixel) show that the edges of hollows are straight, as expected if the margins formed by scarp retreat. These highest‐resolution images reveal no superposed impact craters, implying that hollows are very young. The width of hollows within rayed crater Balanchine suggests that the maximum time for lateral growth by 1 cm is ~10,000 yr. A process other than entrainment of dust by gases evolved in a steady‐state sublimation‐like process is likely required to explain the high‐reflectance haloes that surround many hollows.

Ancient and recent clay formation on Mars as revealed from a global survey of hydrous minerals in crater central peaks

AbstractClay minerals on Mars have commonly been interpreted as the remnants of pervasive water‐rock interaction during the Noachian period (>3.7 Ga). This history has been partly inferred by observations of clays in central peaks of impact craters, which often are presumed uplifted from depth. However, combined mineralogical and morphological analyses of individual craters have shown that some central peak clays may represent post‐impact, possibly authigenic processes. Here we present a global survey of 633 central peaks to assess their hydrous minerals and the prevalence of uplifted, detrital, and authigenic clays. Central peak regions are examined using high‐resolution Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution Imaging Science Experiment data to identify hydrous minerals and place their detections in a stratigraphic and geologic context. We find that many occurrences of Fe/Mg clays and hydrated silica are associated with potential impact melt deposits. Over 35% of central peak clays are not associated with uplifted rocks; thus, caution must be used when inferring deeper crustal compositions from surface mineralogy of central peaks. Uplifted clay‐bearing rocks suggest the Martian crust hosts clays to depths of at least 7 km. We also observe evidence for increasing chloritization with depth, implying the presence of fluids in the upper portions of the crust. Our observations are consistent with widespread Noachian/Early Hesperian clay formation, but a number of central peak clays are also suggestive of clay formation during the Amazonian. These results broadly support current paradigms of Mars' aqueous history while adding insight to global crustal and diagenetic processes associated with clay mineral formation and stability.

Lunar central peak mineralogy and iron content using the Kaguya Multiband Imager: Reassessment of the compositional structure of the lunar crust

Ryder and Wood (1977) suggested that the lunar crust becomes more mafic with depth because the impact melts associated with the large Imbrium and Serenitatis basins are more mafic than the surface composition of the Moon. In this study, we reexamine the hypothesis that the crust becomes more mafic with depth; we analyze the composition of crater central peaks by using recent remote sensing data and combining the best practices of previous studies. We compute the mineralogy for 34 central peaks using (1) nine-band visible and near-infrared data from the Kaguya Multiband Imager, (2) an improved version of Hapke's radiative transfer model validated with spectra of lunar soils with well-known modal mineralogy, and (3) new crustal thickness models from the Gravity Recovery and Interior Laboratory data to examine the variation in composition with depth. We find that there is no increase in mafic mineral abundances with proximity to the crust/mantle boundary or with depth from the current lunar surface and, therefore, that the crust does not become more mafic with depth. We find that anorthosite with very low mafic abundance (“purest anorthosite” or PAN) is a minority constituent in these peaks, and there is no clear evidence of a distinct PAN-rich layer in the middle crust as previously proposed. The composition of most of the central peaks we analyze is more mafic than classically defined anorthosites with an average noritic anorthosite composition similar to that of the lunar surface.

Characterization of melt and ejecta deposits of Kepler crater from remote sensing data

AbstractWe used Moon Mineralogy Mapper (M3), Arecibo and Mini‐RF radar, and Diviner radiometer data with Lunar Reconnaissance Orbiter (LRO) Camera and Kaguya Terrain Camera images to characterize the target, ejecta, and impact melt‐rich lithologies in and around lunar central peak crater Kepler. M3 data indicate the impact melt rocks of crater floor to be high‐Ca pyroxene dominated, distinct from the low‐Ca pyroxene‐dominated crater wall. The central uplift is high‐Ca pyroxene dominated, and has higher albedo. These observations are consistent with thin mare basalts underlain by noritic Imbrium ejecta, underlain by gabbroic crustal material. M3 data reveal an enigmatic, splash‐like feature of melt‐rich material on the southeastern (uprange) crater wall and flank. M3 data also highlight halos around Kepler. In detail the halos are slightly variable, but in broad terms they define a consistent feature, offset to the inferred downrange direction, and interpreted to reflect the distribution of glass‐bearing impact breccia. The radar data sets show most of the proximal ejecta to be radar‐bright. However, Diviner rock abundance data do not indicate the presence of blocks on the surface nor can they be seen using LRO Narrow Angle Camera images. Thus, the blocks giving rise to the enhanced radar signal are buried. Beyond the radar‐bright zone, a subtle radar‐dark halo emerges, coincident with a region of very low rock abundance in Diviner data. This multidisciplinary approach provides a robust analysis of the main characteristics of a lunar complex crater and reveals previously unidentified features related to the distribution of impact melt.

Quantifying the attenuation of structural uplift beneath large lunar craters

Terrestrial crater observations and laboratory experiments demonstrate that target material beneath complex impact craters is uplifted relative to its preimpact position. Current estimates suggest maximum uplift is one tenth of the final crater diameter for terrestrial complex craters and one tenth to one fifth for lunar central peak craters. These latter values are derived from an analytical model constrained by observations from small craters and may not be applicable to larger complex craters and basins. Here, using numerical modeling, we produce a set of relatively simple analytical equations that estimate the maximum amount of structural uplift and quantify the attenuation of uplift with depth beneath large lunar craters.

A global inventory of central pit craters on the Moon: Distribution, morphology, and geometry

The origin of central pit craters on the Moon has long been an enigma, and a primary reason is that their geographic distribution and morphometric characteristics were unknown. We investigated a global inventory of lunar central pit craters using high-resolution image and topography data obtained from the Lunar Reconnaissance Orbiter. 56 certain and 35 probable central pit craters are found on both the lunar maria and highlands. The certain pit craters are ∼9–57km in diameter. The average diameter ratio between the central pits and their parent craters is ∼0.12 and the average depth/diameter ratio for the central pits is ∼0.072. With irregular-shaped rims, the central pits have conical profiles and some have flat floors. The central pits occur on both crater floors and central peaks. The floor pits are generally larger, deeper, and with more irregular shape compared with summit pits. Both the summit and floor pit craters have formed in every lunar stratigraphic epoch from Nectarian to Copernican. Target properties of background terrains affect the morphology and size of central pits, but they do not determine whether or not a central pit forms during a cratering event. The lunar central pits may have formed by deformation of central peaks caused by some mechanical processes during or soon after the cratering process of their parent craters.

Bulk mineralogy of lunar crater central peaks via thermal infrared spectra from the Diviner Lunar Radiometer: A study of the Moon's crustal composition at depth

The central peaks of lunar impact craters are thought to be composed of uplifted material originating from varying depths of the crustal column. The interpreted crystallization sequence of the early lunar magma ocean resulted in an anorthositic upper crust that may become progressively more mafic as it approaches the olivine‐rich mantle. Emissivity spectra from the Lunar Reconnaissance Orbiter (LRO) Diviner Radiometer are used to derive the wavelength location of the Christiansen Feature (CF), which is sensitive to bulk silicate mineralogy. Here a survey of CF values has been performed for the central peaks of 135 complex craters, providing global and regional observations of the heterogeneity of crustal compositions. Crustal thickness models give context to the preimpact depth of the central peak material and its proximity to the crust‐mantle boundary. This study has identified six craters with potentially ultramafic compositions within their central peaks. More common occurrences of mafic material, found in a wide variety of crater central peaks, show a silicate composition roughly similar to mare basalt or an olivine‐bearing gabbro. The range of central peak CF values is similar to that of the rest of the lunar surface. Bulk mineralogy of the central peak material does not appear to be correlated with its crustal depth of origin, suggesting both lateral and vertical heterogeneity in crustal composition rather than a gradual transition from felsic to mafic composition. It is likely that the Moon's extensive cratering history has continually overturned the original crust, erasing any original systematic dependence of composition on depth or proximity to the mantle.

INTERESTING MOON’S SURFACE FEATURES. OUR STUDY AND SOME INFERENCES

In this article the results of the long-term CCD observations of the Moon are presented. We describe some unusual features existing on the Moon's surface and discuss their possible origin. Observations of the Moon's surface even with the small-size telescopes appear to be quite promising. It could be that the author of the present study succeeded in discovering peculiar circular structures in some places on the Moon's surface, extraordinarily round shape of some dark patches of the lunar maria and the dark stripe of unclear origin on the shore of Mare Humorum. It is likely that peculiar craters of unknown origin, which are localized on the fissures, are the centre of activity in the crater Alphonsus, rather than the crater's central peak as it

A spectroscopic analysis of Martian crater central peaks: Formation of the ancient crust

The earliest formed crust on a single plate planet such as Mars should be preserved, deeply buried under subsequent surface materials. Mars' extensive cratering history would have fractured and disrupted the upper layers of this ancient crust. Large impacts occurring late in Martian geologic history would have excavated and exposed this deeply buried material. We report the compositional analysis of unaltered mafic Martian crater central peaks with high‐resolution spectral data that was used to characterize the presence, distribution and composition of mafic mineralogy. Reflectance spectra of mafic outcrops are modeled with the Modified Gaussian Model (MGM) to determine cation composition of olivine and pyroxene mineral deposits. Observations show that central peaks with unaltered mafic units are only observed in four general regions of Mars. Each mafic unit exhibits spectrally unmixed outcrops of olivine or pyroxene, indicating dunite and pyroxenite dominated compositions instead of basaltic composition common throughout much of the planet. Compositional analysis shows a wide range of olivine Fo# ranging from Fo60 to Fo5. This variation is best explained by a high degree of fractionation in a slowly cooling, differentiating magma body. Pyroxene analysis shows that all the sites in the Southern Highlands are consistent with moderately Fe‐rich, low‐Ca pyroxene. Mineral segregation in the ancient crust could be caused by cumulate crystallization and settling in a large, potentially global, lava lake or near surface plutons driven by a hypothesized early Martian mantle overturn.