Gossans are surficial deposits that form in host bedrock by the alteration of sulphides by acidic and oxidizing fluids. These deposits are typically a few meters to kilometers in size and they constitute important vectors to buried ore deposits. Hundreds of gossans have been mapped by field geologists in sparsely vegetated areas of the Canadian Arctic. However, due to Canada’s vast northern landmass, it is highly probable that many existing occurrences have been missed. In contrast, a variety of remote sensing data has been acquired in recent years, allowing for a broader survey of gossans from orbit. These include band ratioing or methods based on principal component analysis. Spectrally, the 809 gossans used in this study show no significant difference from randomly placed points on the Landsat 8 imageries. To overcome this major issue, we propose a deep learning method based on convolutional neural networks and relying on geo big data (Landsat-8, Arctic digital elevation model lithological maps) that can be used for the detection of gossans. Its application in different regions in the Canadian Arctic shows great promise, with precisions reaching 77%. This first order approach could provide a useful precursor tool to identify gossans prior to more detailed surveys using hyperspectral imaging.

Due to a lack of pre‐Nectarian samples associated with specific impacts in the Apollo suite, the early Solar System's impact chronology is poorly constrained. We investigate where Artemis and other south polar lunar missions could sample impactites to determine the ages of pre‐Nectarian craters, focusing on the ancient craters Haworth, Shoemaker, and Faustini. We examine similarly sized non‐polar analogue craters containing mappable impact melt features and find no patterns that could be used to predict where similar deposits may have once existed in the south polar region. We also create a model of the ejecta stratigraphy of craters that post‐date the South Pole‐Aitken basin, from which we predict how younger, smaller craters may excavate ancient, buried impact melt to the surface. We find that, at most, primary impact melt from a source crater will comprise no more than 2%–3% of material at the surface above its buried rim, and <1% elsewhere. Furthermore, disruption of the ejecta stratigraphy by ballistic sedimentation and surface mixing due to impact gardening will make sampling specific sources more difficult. We conclude that determining the provenance of ancient impact melt samples will be difficult, but that sampling near the rim of a given source—such as in the “Haworth” Artemis candidate landing region—will maximize the chances of success.

IntroductionMartian analogue environments provide valuable opportunities to investigate the preservation of biosignatures and to refine strategies for their detection on Mars. Reactive gossans in permafrost - such as those found in the Canadian Arctic - constitute potential Martian analogues because of mineralogical similarities observed between terrestrial gossans and orbital or rover-based observations on Mars. Gossans are the superficial expression of sulfide-bearing rocks that have undergone chemical weathering through interaction with oxygenated groundwater. Gossans constitute one of the few iron-rich aqueous environments known to provide a habitable setting on Earth and potentially on Mars [1]. Previous studies have proposed that gossans, or related processes involving the chemical alteration of sulfide minerals, may have occurred on Mars in the past and that some of these reactions could persist today within the Martian permafrost [e.g. 2, 3, 4, 5].The Terrestrial Mineral Analysis by Remote Sensing (T-MARS) project investigated Arctic gossans located at the head of Expedition Fiord, Axel Heiberg Island, Nunavut, Canada (Fig. 1) [6]. It included the detection and study of gossans from orbit to the ground using instrumentation relevant to present and upcoming Mars exploration missions. The scientific objectives were (1) to characterize their geochemistry and mineralogy; and (2) understand their formation processes. The operational objectives were (3) to develop strategies for detecting and studying them on Earth and Mars; and (4) to detect potential biosignatures.MethodsPrior to field work Before the field campaigns, target gossans were selected according to both their accessibility and their scientific relevance. A principal component analysis (PCA) was applied to a WorldView-2 image. A colour composite was then generated using the principal components showing the strongest contrast between the eigenvector values associated with the original red and blue bands referring to the red/blue ratio often used to detect iron oxides (Fig. 1). Previously identified gossan sites helped identify additional potential outcrops for field investigation on the colour composite. A slope map was used to assess the accessibility of the selected sites.Field work Two field campaigns to Expedition Fiord on Axel Heiberg Island, NU were carried out in July 2022 and July 2023 in the vicinity of the McGill Arctic Research Station. The investigated gossans were sampled both at the surface and at depth through vertical trenches. Both sterile and non-sterile sampling procedures were applied depending on the selected follow up laboratory protocols. A HySpex SWIR-384 hyperspectral camera was used to image gossan occurrences and evaluate the potential of this technology as a reconnaissance tool for mineral mapping. A drone was also deployed to examine hard‑to‑reach areas and to capture broader views of outcrops that were not visible from the ground (Fig. 2).Laboratory analyses Samples were characterized using elemental analyses for sulfur, nitrogen and carbon, together with X-ray diffraction and X-ray fluorescence techniques [7, 8, 9]. Additional measurements included loss on ignition, pH, and SEM-EDX analyses. Sterile samples were also investigated for lipid biosignatures through n-alkane quantification by gas chromatography–mass spectrometry [7], as well as for organic carbon content using Raman spectroscopy [8]. Spectroscopic measurements were acquired with rover-mountable instruments, including visible to near-infrared reflectance spectroscopy and mid- to thermal-infrared diffuse reflectance spectroscopy.Remote sensing The VNIR reflectance spectra were used as endmembers for the Spectral Feature Fitting algorithm in ENVI to identify gossan occurrences in WorldView-2 and PRISMA imagery [9].Results and discussionElemental and spectroscopic analyses revealed mineral assemblages and oxidation gradients characteristic of gossans [10, 11, 12], with twenty-four mineral phases identified. Gypsum and quartz were common across all sites. Variations in primary sulfides and secondary sulfates indicate different oxidation stages, from actively weathering gossans to fully oxidized systems dominated by secondary sulfates. Organic carbon and potential lipid biosignatures were detected at depth. Classic gossan stratigraphy, with an oxidized surface layer over an unoxidized horizon, was observed at several sites, alongside additional stratification patterns ranging from distinct layering to strong heterogeneity (Fig. 3).The results suggest that these gossans formed through alteration of primary sulfides in diabase/gabbro intrusions or from mobilized evaporitic sulfates, likely driven by paleohydrothermal activity or fluid circulation associated with the emplacement of sills in the host sedimentary rocks.Field and laboratory observations made in this study indicate that successful identification of gossans on Mars will likely depend on targeting faulted regions where fracturing may have increased permeability, focusing on erosion-resistant ridges, using high-resolution orbital data and detecting co-located iron sulfides, oxides and sulfates.Figure 1. Geology of the Expedition Fiord area, Nunavut. A) Location of Axel Heiberg Island in the Canadian Arctic Islands. B) Location of the McGill Arctic Research Station on Axel Heiberg Island. C) Footprint of the study area. Basemap for insets A-B-C: Esri World Imagery (2025). D) PCA colour composite. Figure modified after [6].Figure 2: Multi-scale field images. A) Contextual overview of a sampling location acquired by drone. The white arrow indicates the gossan center shown in B (people present in the image for scale). B) Drone image of the gossan center, where a trench was excavated. C) Image captured by a team member on the ground showing gossan sampling. Figure from [6].Figure 3: Stratification patterns observed in trenches. A) Classical gossan stratigraphy...

We propose a simple method for the determination of the mass absorption coefficient of light-absorbing impurities in snow. It is based on the spectral albedo/reflectance measurements in combination with standard gravimetry techniques for the measurements of the impurity mass concentration in snow. The technique has been applied for the estimation of the dust mass absorption coefficient in snow close to the mining facilities in Kiruna (Sweden).

IntroductionHematite has been detected and characterized at several locations on Mars, using data acquired by orbit- and rover-based instruments. Global remote sensing studies were made using low spatial resolution datasets such as MGS/TES and MEX/OMEGA (both ~3–10 km/px), leading to the detection of large-scale hematite-rich regions such as Meridiani Planum, Aram Chaos and Aureum Chaos [e.g., 1–3]. Local remote sensing studies were made using higher spectral and spatial resolution datasets, such as oversampled MRO/CRISM data (6 to 18 m/px) [e.g., 4]. These local high-resolution orbital studies have mostly focused on rover exploration regions, leading to the detection of small-scale hematite occurrences such as the Vera Rubin Ridge in Gale Crater [4]. On the ground, hematite was identified at several locations by the Opportunity and Curiosity rovers [5, 6]. The presence of hematite is suggested as evidence of ancient aqueous environments on Mars [1, 2, 4, 7], since most formation mechanisms involve water [8, 9]. These mechanisms may also involve microbial activity, which could be preserved in the rocks [8]. Hematite occurrences may thus hint at past potential habitable environments.In this study, we aim to refine global-scale mapping of hematite occurrences on Mars using a combination of high spatial and spectral resolution orbital images acquired by the TGO/CaSSIS, MRO/CRISM and MRO/HiRISE instruments. Methods include (1) laboratory samples characterisation, (2) known hematite locations survey, (3) CaSSIS spectral parameters assessment and (4) CaSSIS-CRISM-HiRISE combined analysis to spectrally and morphologically characterise hematite-rich locations in detail. DatasetsThe CaSSIS instrument is a 4-band (BLU: 495, PAN: 678, RED: 836, NIR: 939 nm) visible to near-infrared (VNIR) stereo camera (~4 m/px) onboard the ExoMars 2016 Trace Gas Orbiter that allows for near-unique identification of hematite in NIR-RED-PAN colour composite images [10]. Its distinctive magenta colour in such colour composite is caused by the sensitivity of the RED filter (836 nm) to a prominent hematite absorption feature at ~860 nm, which causes a slight deflection relative to the PAN and NIR filters on either side [10] (Fig. 1). The CRISM instrument is a VNIR+IR camera which acquired hyperspectral targeted images (18 or 36 m/px) and multispectral mapping images at coarser spatial resolution [11]. The CRISM Multispectral Reduced Data Records (MRDR) and newly released VNIR Hyperspectral Data Records (VRDR) products are two near-global mosaics of mapping strips. MRDRs combine VNIR+IR detectors (410–3924 nm) spatially and spectrally downsampled to 180 m/px and 72 bands; while the VRDRs only use the VNIR detector (370–1020 nm), which is spatially and spectrally downsampled to 90 m/px and 90 bands. BD530, BD920 and BDI1000VIS spectral indices based on these products are used to locate regions that may contain crystalline hematite [11]. The HiRISE instrument is a 3-band, 25–50 cm/px camera which allows for detailed morphologic and textural observations at a higher resolution than CaSSIS and CRISM. Here, we use CaSSIS to map hematite and bridge the substantial spatial resolution gap between CRISM and HiRISE [10]. Fig. 1. Dark Subtraction corrected CaSSIS image MY37_029570_193_0 in Capri Chasma. A and B) NPB and NRP colour composite (0.1% linear stretch). Purple rectangles indicate the location of C and D insets. C and D) Hematite-rich locations exhibiting a dark magenta colour in NRP relative to the greyish background (linear stretch of 2% (C) and 1% (D)). E) CaSSIS-extracted spectra (purple circles), compared to hematite lab reference spectrum (USGS: FE2602) resampled to CaSSIS wavelengths (black squares). MethodsFirst, samples were created in the lab by mixing 0 to 15 wt.% of fine-grained red hematite with planetary simulant, to represent the abundance range observed on Mars [12]. Samples were characterized by VNIR spectroscopy to estimate the limit of detection of fine-grained hematite by hyperspectral sensors [13].Second, a survey of known hematite-rich locations was conducted by reviewing the literature and existing datasets to identify study areas for evaluating the hematite detection methods described below. CaSSIS, HiRISE and CRISM images and/or products overlapping the known locations are being corrected using a Dark Subtraction (DS) method to account for atmospheric scatter [14, 15], and a high-spatial-resolution spectral analysis and characterisation of known hematite locations is being conducted.Third, a comprehensive suite of CaSSIS spectral parameters was calculated on a CaSSIS-resampled spectral library consisting of a variety of minerals present on the Martian surface, sourced from reference reflectance spectral libraries (e.g., USGS) and orbital imagery. Spectral parameters include band ratios, spectral slopes and band depth and height relative to different 2-band continuums (Fig. 2). Statistical methods are being used to assess which spectral parameters are the most useful to distinguish between hematite and other minerals, and to investigate the distinction of other iron minerals (goethite, jarosite, ferrihydrite) with CaSSIS. This spectral parameter set will be applied to the CaSSIS data to produce a refined global scale mapping of hematite, and potentially other ferric iron minerals, at unprecedented spatial resolution. Fig. 2. Examples of spectral parameters used to distinguish hematite from other Mars minerals. On the figure, an example of a “classical” band depth (RED band depth on PAN-NIR continuum) and of a “modified” band height (PAN band height on RED-NIR continuum) are shown. Future work will include a combined analysis of DS-corrected CaSSIS-CRISM-HiRISE data at high spatial resolution to characterize the potential new hematite locations in the refined mapping. References[1] Christensen et al. (2001) JGR: Planets, 106(E10). [2] Glotch...