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Iron oxide-hydroxide minerals in Martian dust provide crucial insights into Mars’ past climate and habitability. Previous studies attributed Mars’ red color to anhydrous hematite formed through recent weathering. Here, we show that poorly crystalline ferrihydrite (Fe 5 O 8 H · nH 2 O) is the dominant iron oxide-bearing phase in Martian dust, based on combined analyses of orbital, in-situ, and laboratory visible near-infrared spectra. Spectroscopic analyses indicate that a hyperfine mixture of ferrihydrite, basalt and sulfate best matches Martian dust observations. Through laboratory experiments and kinetic calculations, we demonstrate that ferrihydrite remains stable under present-day Martian conditions, preserving its poorly crystalline structure. The persistence of ferrihydrite suggests it formed during a cold, wet period on early Mars under oxidative conditions, followed by a transition to the current hyper-arid environment. This finding challenges previous models of continuous dry oxidation and indicates that ancient Mars experienced aqueous alteration before transitioning to its current desert state. Mars’ distinctive red colour attributed to ferrihydrite, a type of rust mineral. This finding suggests Mars experienced a cold and wet environment before transitioning to its current desert state.

To date, the parent body of mesosiderites remains uncertain. Some candidate bodies have been proposed, including 4 Vesta and 16 Psyche. In this study, we performed a comprehensive spectroscopic survey of mesosiderites and pyroxene+metal mixtures to determine the factors that control their reflectance spectra in the 0.3–2.5 μm region and to help better identify possible parent bodies. We also applied these results to several well-known parent-body targets. We found that the mesosiderite powder and pyroxene powder on a metal slab both show significant differences in their spectra compared to Psyche. Although the spectra of the 10:90 orthopyroxene+metal powder was similar to that of Psyche, this does not necessarily mean that there is only a small amount (∼10%) of pyroxene on the surface of Psyche. Taking into account the density of Psyche, we infer that there might be a large amount of low-iron pyroxene on its surface. Looking in more detail at the spectral metrics, we note that some forms of some mesosiderites match some aspects of Psyche. Since some spectral metrics and ferrosilite content of howardites, diogenites, and mesosiderites are similar, the possibility of Vesta being a mesosiderite parent body cannot be ruled out either. However, the higher metal content of mesosiderites favors them, over howardites and diogenites, being linked to Psyche.

China’s Tianwen-2 spacecraft was launched on 2025 May 29 and will arrive at the Earth quasi-satellite (469219) Kamo‘oalewa in 2026 July. We previously reported that Kamo‘oalewa develops an LL-chondrite-compositional, highly space-weathered surface. Here, using the light-curve data and the Cellinoid model, we modeled Kamo‘oalewa’s shape, rotation period, and pole orientation. We then estimated the global distribution of regolith critical size using the balance method of gravity, cohesive force, and centrifugal force. Furthermore, in the temperature range of 253.15−473.15 K, we measured the thermal parameters of laser-irradiated LL chondrite powder that best matches Kamo‘oalewa’s spectrum, estimating Kamo‘oalewa’s thermal inertia and skin depth (lower limit of regolith thickness). Using the radiative transfer mixing model, we also estimated the sub-micrometer-sized iron (SMFe0) content in Kamo‘oalewa’s regolith. Finally, using the MIT online spectral classification tool for the laser-irradiated LL chondrite powder, we obtained a virtual spectral type of Kamo‘oalewa. Our model gives a size of 68 × 46 × 39 m, a rotation period of 27.66 minutes, and a pole orientation of 134 .° 7 longitude and −11 .° 4 latitude for Kamo‘oalewa. Regolith grains with a size <2 cm can remain stable over 93.8% of the global surface area of Kamo‘oalewa. Laser-irradiated LL chondrite powder shows a low thermal inertia (163.14−232.31 J m−2 K−1 s−1/2), corresponding to a thermal skin depth of 3.1−3.5 mm on Kamo‘oalewa. An SMFe0 content of 0.29 ± 0.05 wt.% is required to match Kamo‘oalewa’s spectrum. The virtual spectral type of Kamo‘oalewa is given as “Sqw.”

Iron‐bearing minerals are a major component of materials on the lunar surface, and many of them can be distinguished based on the diagnostic absorption features in visible and near‐infrared reflectance spectra. The relationship between the 1 µm absorption (Band I) center and the band area ratio (BAR), defined as the ratio of 2–1 µm absorption areas, provides a sensitive way to estimate the relative abundance of olivine (Ol) and pyroxene. In the plot of the BAR value versus the Band I center, the Ol‐orthopyroxene (Opx) mixing line (derived from terrestrial materials) is strictly applicable only to Ol‐Opx mixtures. Based on published database of laboratory spectra and compositional data for lunar rocks and mineral separates, this study investigated the spectral characteristics of a variety of common Fe2+‐bearing lunar minerals and rocks, such as clinopyroxene (Cpx), ilmenite‐rich basalt, pyroxene‐bearing anorthosite, and glass‐rich impact melt. The lunar Ol‐Cpx‐Opx mixing line for rocks and minerals is determined, which contains less curvature than the Ol‐Opx mixing line, consistent with both the higher Fe2+ content of lunar mafic silicates and the presence of appreciable Cpx. This study suggests that some of the pyroxene‐bearing lunar materials that are rich in ilmenite, glass, or plagioclase can also be distinguished by this method. These results indicate that the plot of the BAR value versus the Band I center is a useful tool for spectral analysis of lunar composition and mineralogy, especially for those having pyroxene‐dominated spectra. Plain Language Summary Iron‐bearing minerals are a major component of materials on the lunar surface, many of which can be distinguished by their different diagnostic absorption features in reflectance spectra. The relationship between two spectral parameters, which are the wavelength position of the 1 µm absorption (Band I) center and the band area ratio (BAR, defined as the ratio of 2–1 µm absorption areas), provides a sensitive method for estimating the relative abundance of two common iron‐bearing minerals (olivine and orthopyroxene). However, there is a systematic offset in the identification of lunar materials. Based on published laboratory spectral database, this study investigated the spectral characteristics of various common iron‐bearing lunar minerals and rocks and determined the distribution trend of lunar olivine and pyroxene (orthopyroxene and clinopyroxene) mixtures in the plot of the BAR value versus the Band I center. This study also suggests that some of the pyroxene‐bearing lunar materials rich in ilmenite, glass, or plagioclase can also be distinguished by this method. This result indicates that this method is useful for spectral analysis of composition and mineralogy on the lunar surface. Key Points Variation of two spectral parameters of lunar ferrous mineral mixtures with composition was analyzed Some ilmenite‐rich basalts at specific particle sizes can be distinguished from basalts poor in ilmenite Mare basalts can be distinguished from pyroxene‐bearing mixtures rich in glass or plagioclase

The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument onboard the Mars 2020 Perseverance rover detected so far some of the most intense fluorescence signals in association with sulfates analyzing abraded patches of rocks at Jezero crater, Mars. To assess the plausibility of an organic origin of these signals, it is key to understand if organics can survive exposure to ambient Martian UV after exposure by the Perseverance abrasion tool and prior to analysis by SHERLOC. In this work, we investigated the stability of organo-sulfate assemblages under Martian-like UV irradiation and we observed that the spectroscopic features of phthalic and mellitic acid embedded into hydrated magnesium sulfate do not change for UV exposures corresponding to at least 48 Martian sols and, thus, should still be detectable in fluorescence when the SHERLOC analysis takes place, thanks to the photoprotective properties of magnesium sulfate. In addition, different photoproduct bands diagnostic of the parent carboxylic acid molecules could be observed. The photoprotective behavior of hydrated magnesium sulfate corroborates the hypothesis that sulfates might have played a key role in the preservation of organics on Mars, and that the fluorescence signals detected by SHERLOC in association with sulfates could potentially arise from organic compounds.

Spectral characterization of asteroids relies on meteorite spectral calibration to constrain their surface mineralogy. We have conducted a comprehensive spectroscopic study of LL chondrites to shed light on the factors that control their reflectance spectra in the 0.3–2.5 μm region. Our results show that the ratio of the area of an absorption feature in the 2 μm region to that of Band I (termed the band area ratio (BAR)) decreases with increasing petrologic grade. Grain-size variations can cause systematic changes in the spectral slope, albedo, absorption depths, and BAR of LL chondrites. The chip spectra of LL chondrites have a blue spectral slope and shallow absorption depths when compared to their corresponding powder spectra. Phase-angle variation has a systematic effect on the spectral slope and albedo of LL chondrite spectra. The shock effect can cause systematic changes in the albedo and absorption depths of LL chondrites. Whether or not the metal/magnetic particles are removed has little effect on the spectra of LL chondrites. Terrestrial weathering significantly reduces the 0.5/0.6 μm reflectance ratio of LL chondrites, but this ratio has no clear trend with weathering grade. We also applied these results to four well-known asteroid mission targets. Our results suggest that the spectral difference between smooth terrain and rough terrain of 25143 Itokawa may be due to the presence of fine-grained regolith in the smooth terrain. The contents of olivine and pyroxene in 4179 Toutatis, 99942 Apophis, and 469219 Kamo’oalewa are comparable to those in L chondrites, LL chondrites, and LL chondrites, respectively.

The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument, which is mounted on the Mars 2020 Perseverance rover, detected Raman signals in spectral regions relevant to organics, plausibly polycyclic aromatic hydrocarbons (PAHs), co-located with sulfate minerals, in the Quartier abrasion of the Issole outcrop in the Jezero crater floor on Mars. In order to ascertain the plausibility of the organic origin of these signals, it is essential to determine whether organics can withstand the effects of ambient Martian ultraviolet (UV) radiation after they are exposed by the Perseverance’s abrasion tool and prior to the analysis by SHERLOC. In this work, the stability under Martian-like UV irradiation of PAHs like 2,6-dihydroxynaphthalene and benzo[a]pyrene in hydrated magnesium sulfate, one of the main sulfate phases present in Quartier, is investigated. Our findings indicate that the spectroscopic features of 2,6-dihydroxynaphthalene and benzo[a]pyrene in hydrated magnesium sulfate remain unaltered when exposed to UV radiation comparable to that experienced at Jezero crater over a period of dozens of Martian days (or sols). Consequently, due to the photoprotective properties of this mineral, after the abrasion exposes them to the radiation, these compounds can still be detectable by the SHERLOC measurement and also by SuperCam because some organic bands fall in its infrared spectral range. In addition, photoproducts due to the UV exposure for both PAHs were detected. These results corroborate the hypothesis that the Raman signals detected by SHERLOC co-located with sulfates may arise from organic compounds.

Most near-Earth objects are thought to originate from the collisional fragments of the main asteroid belt. One question that remains to be resolved is the proportion of near-Earth objects sampling the core area material of the parent body to the outer layers. In this study, we developed a method to determine the petrologic type of ordinary chondrite parent bodies based on reflectance spectroscopy. We also calculated the petrologic type of asteroid (25143) Itokawa, which is consistent with the returned samples from the JAXA Hayabusa mission. Finally, we calculate the petrologic type of 28 LL near-Earth asteroids. Our results show that the surface material of most LL chondrite near-Earth asteroids is of petrologic grade higher than 4. The ratio of LL chondrite near-Earth asteroids with high petrologic type (5 and 6) to LL chondrite near-Earth asteroids with low petrologic type is 0.79. This also means that LL chondrite near-Earth asteroids may originate primarily from the core area of the main belt parent body or bodies.

While phyllosilicates were the first water-bearing minerals inferred to be present on Mars from analysis of Earth-based telescopic spectra (McCord et al. 1982), it took recent spacecraft missions to enable mapping the global distribution and determining the types of specific species that are present on the surface and their geological context.

In near‐infrared spectral studies, the relationship between the 1‐μm absorption (Band I) center and the band area ratio (BAR, the area ratio of 2–1‐μm absorption features) is useful in compositional and mineralogical analyses of ferrous mineral‐bearing mixtures. Zhang and Cloutis (2020), https://doi.org/10.1029/2020ea001153, investigated various lunar ferrous iron‐bearing rocks and minerals and found that pyroxene‐bearing materials rich in ilmenite (Ilm), plagioclase (Pl), or glass are offset from the lunar olivine‐clinopyroxene‐orthopyroxene (Ol‐Cpx‐Opx) mixing line in the plot of the BAR versus the Band I center. To analyze the variation trends of the spectral properties of these mixtures with different components, this study presents a systematic evaluation of laboratory spectra of terrestrial and synthetic ferrous iron‐bearing mineral mixtures based on published databases. In general, the mixing trends of the Pl‐pyroxene mixtures, the glass‐pyroxene mixtures, and the Ilm‐basalt mixtures are consistent with the findings of Zhang and Cloutis (2020), https://doi.org/10.1029/2020ea001153. Moreover, this study also finds that the BAR of the Pl‐pyroxene mixtures varies nonlinearly with different relative abundances and that the BAR is generally not sensitive to Pl abundances below 60%. For glass‐pyroxene mixtures, the corresponding data points are usually appreciably offset from the Ol‐Cpx‐Opx mixing line at glass abundances above 20%. The BAR of the Ilm‐basalt mixtures increases with increasing Ilm content, mainly due to the weakening of 1‐μm absorption generally being greater than that of 2‐μm absorption. Plain Language Summary Reflectance spectroscopy is a powerful remote‐sensing technique for determining the composition of the Moon's surface. The relationship between two specific spectral characteristics of minerals commonly found on the Moon is useful for analyzing the mineralogical components of iron‐bearing mineral mixtures. These are due to the presence of iron in various phases, and are an important probe into lunar geology. Zhang and Cloutis (2020), https://doi.org/10.1029/2020ea001153, investigated various lunar iron‐bearing rocks and minerals and found that pyroxene‐bearing lunar materials rich in ilmenite (Ilm), plagioclase (Pl), or glass are offset from the lunar olivine‐clinopyroxene‐orthopyroxene (Ol‐Cpx‐Opx) mixing line in the plot of these two spectral characteristics. This study presents a systematic evaluation of laboratory spectra of terrestrial and synthetic iron‐bearing mineral mixtures based on published databases to explain how these mixtures (glass‐pyroxene, Pl‐pyroxene, and Ilm‐pyroxene mixtures) are offset from the lunar Ol‐Cpx‐Opx mixing line. Moreover, this study finds some relationships between the spectral characteristics and the relative abundance of these mixtures. These results expand the range of rock types that are present on the Moon that can be characterized by reflectance spectroscopy. Key Points The 1‐μm absorption center and band area ratio (BAR) of various natural and synthetic ferrous mineral‐bearing mixtures were analyzed The BAR of the plagioclase‐pyroxene mixtures varies nonlinearly with different relative abundances With increasing ilmenite (Ilm) content, the BAR of the Ilm‐basalt mixtures gradually increases