Sediments Within the Icy North Polar Deposits of Mars Record Recent Impacts and Volcanism

The North Polar Layered Deposits (NPLD) of Mars are ice‐rich sedimentary layers that formed under the influence of Mars' modern climate and thus record the recent climatic history of Mars, analogous to terrestrial ice sheets. The 2013–2023 Planetary Science Decadal Survey recommends a lander mission to sample the NPLD for climatic records; however, linking the geologic record to the climatic history will require quantitative dating of the NPLD. In this study we use orbital reflectance spectroscopy to show for the first time that dateable mafic lithics are present throughout the NPLD. We find significant glass as well as diverse crystalline minerals, which suggests that surface processes like impacts and volcanism were active during the late Amazonian and transported sediments from across the planet to the north pole. In situ investigation of the NPLD will thus provide critical quantitative constraints on both the recent geologic and climatic histories of Mars.

exist within the NPLD (Bouška & Bell III, 1993;Kerber et al., 2012), orbital investigations of the composition of the NPLD have focused on hydrated minerals (B. H. Horgan et al., 2009;Massé et al., 2010Massé et al., , 2012 and ice grain size (Calvin et al., 2009), so it is unknown whether or not the NPLD contain primary mafic silicates that could indicate dateable volcanic or impact deposits. This study utilizes CRISM high resolution orbital visible/near-infrared hyperspectral images of exposed strata at peripheral scarps and interior troughs in the NPLD (Brown et al., 2012), to constrain for the first time the composition, origin, and distribution of dark non-ice materials in the NPLD and to test whether or not the NPLD contain suitable materials for quantitative geochronology.

Spectral Diversity in the NPLD
CRISM spectra are typically analyzed after ratioing to a bland part of the scene to suppress atmospheric effects, instrumental artifacts, and surface dust. Here we have implemented a modified version of this technique that also suppresses surface ice ( Figure S4 in Supporting Information S1). For details on spectral analysis, see methods section in supporting information (Tables S3 and S4 in Supporting Information S1 contain spectral data), and see Figure 1a and Table S1 in Supporting Information S1 for location of CRISM sites. Figures 2, S2, and S3 in Supporting Information S1 show CRISM images and sampled spectra locations on their RGB color-composite maps generated using standard CRISM spectral parameters (Viviano-Beck et al., 2014) as listed in Table S2 in Supporting Information S1.
CRISM spectra of NPLD outcrops exhibit signatures due to both water ice and lithic materials, including airfall dust and mafic minerals. NPLD lithics fall into five distinct spectral classes, as shown in Figure 3. Additional spectra are shown in Figures 2 and 4, and spectra from all figures are referred to based on their unique spectrum number (e.g., s1). After ratioing to bright icy and dusty regions in the same scene ( Figure S4 in Supporting Information S1: spectral processing approach), spectra from regions dominated by nanophase ferric oxide in martian dust (Type 1) are spectrally featureless and exhibit strong red spectral slopes (R. v. Morris et al., 1997). In contrast, ratio spectra of most dark-toned sediments show broad absorption bands at 1 and 2 μm, indicative of the presence of iron in mafic minerals. Spectra with broad absorption bands centered between 0.95 and 1.05 μm and near 2.1 μm are consistent with high-Ca pyroxene (HCP; Type 2; Cloutis & Gaffey, 1991;B. H. N. Horgan et al., 2014), and are similar to regions within the Olympia Undae portion of the circumpolar dune field (B. Horgan & Bell, 2012). Spectra with a more symmetric band centered beyond 1.08 μm and a second band near 2.1 μm are consistent with iron-bearing (e.g., basaltic) glass, possibly mixed with some HCP (Type 3; B. Horgan & Bell, 2012).
Other dark sediments exhibit a broad 1 μm absorption band but show negligible absorption at 2 μm (Type 4). These spectra could either be consistent with olivine or some glasses (e.g., tektites or obsidian), which show little to no 2 μm band ( Figure 3). Olivine displays a broad iron absorption band centered beyond 1 μm that can usually be distinguished from glass based on its strong shoulder near 1.25 μm, but the absorption band can become rounded to resemble glass upon substitution by other metals, mostly magnesium (King & Ridley, 1987). Glass spectra are sensitive to composition and cooling history (Minitti et al., 2002), and while glasses containing oxidized iron exhibit broad bands at both 1 and 2 μm, the 2 μm band is significantly diminished in glasses dominated by reduced iron (Cannon et al., 2017). In the circumpolar erg, spectra interpreted as glass exhibit weak or no 2 μm bands (B. Horgan & Bell, 2012). Thus, we hypothesize that spectra with a rounded (U-shaped) or narrow (V-shaped) symmetrical 1 μm absorption centered beyond 1.07 μm without a 1.3 μm shoulder are more consistent with glass, but may also be due to olivine or an olivine/glass mixture. Spectra with a relatively asymmetric 1 μm absorption that are often more V-shaped with a shoulder at 1.3 μm and centered at shorter wavelengths (∼1.05 μm) are more consistent with olivine (s12).
Many spectra exhibit strong blue-slopes superposed over these mafic bands (s5, s10-s12, s20) but sometimes the mafic bands can be entirely absent (Type 5; s6, s15). We interpret these dark and blue-sloped spectra as consistent with weathered (leached) glass, as previously detected in Siton Undae and other areas throughout the north polar sand sea (Figure 3; B. Horgan & Bell, 2012;Minitti et al., 2007). Within the NPLD, all of the mafic spectral signatures can be attributed to these endmembers or their mixtures.

Composition of Geologic Units Within the NPLD
Geologic studies using satellite imagery have mapped various ice-rich lithic units within the NPLD (Figures 1b  and 1c) and have constrained their stratigraphic relationships and possible origins based on morphology (Tanaka et al., 2012). The uppermost unit, Planum Boreum 3 (ABb3), represents the ongoing deposition of volatiles in the current low obliquity period . Lithics within ABb3 include lag deposits in ablation zones and wind-blown sediments eroding from underlying unit ABb1. Reflectance spectra from ABb3 are characterized as moderately dusty and dominated by water ice, however, ratio spectra either show weak mafic absorption bands (band depths less than 2%; s7, s8, s13, s14) or steep blue-slope with no absorption band consistent with weathered glass (Type 5; s15). Absorption band centers for these spectra vary between 1.00 and 1.12 μm with little to no 2 μm absorption, indicating glass or olivine (Type 4) possibly mixed with some pyroxene (Type 2).
Planum Boreum 2 (ABb2) is a dark-toned sedimentary unit also known as the north polar "veneers", which may represent an unconformity within the NPLD. ABb2 is interpreted as either a distinct mantling deposit or an ablation lag whose modern extent is likely due to wind transport toward the periphery of the polar cap (Rodriguez et al., 2007). Spectra from ABb2 lack water ice absorptions and have low reflectance values indicating low dust cover. Upon ratioing, ABb2 spectra generally exhibit a strong blue-sloped and concave-up shape, with a broad and symmetric 1 μm band centered at wavelengths longer than 1.07 μm and a shallow 2 μm band (Type 4; s10, s12). These properties are consistent with a mixture dominated by weathered glass. Occasionally, spectra consistent with pyroxene (Type 2) and unweathered glass (Type 3) are detected within the ABb2 unit (s3, s9, s11).
The unit forming most of the NPLD is the Planum Boreum 1 unit (ABb1) which is composed of thousands of meter-scale, horizontally stacked, and ice-rich layers, totaling up to 1,500 m thick. Reflectance spectra from ABb1 contain the strongest dust signatures, possibly because each layer is a result of cyclic deposition and 4 of 10 ablation processes, concentrating the seasonal dust as well as any deposited lithics, which makes them ideal targets for extracting climate records (Hvidberg et al., 2012). While dust (Type 1) dominates ABb1, ratio spectra primarily show V-shaped absorptions centered beyond 1.08 μm without 2 μm bands consistent with glass or olivine (Type 4; s5). Other spectra with 2 μm absorption bands exhibit glass-like absorptions (Type 3; s17).
At lower elevations, the NPLD grades into the basal Cavi (ABbc) unit that is hypothesized to have formed during the Middle to Late Amazonian period (Nerozzi et al., 2022;Tanaka et al., 2012). ABbc is marked by low albedo aeolian cross-strata of water ice-cemented sandy material, hypothesized to be an ancient polar dune field, which are inter-bedded with bright pure ice layers that may be remnants of ancient polar caps (Fishbaugh & Head, 2005;Nerozzi & Holt, 2019;Tanaka et al., 2008). Spectrally, ABbc is less dusty and icy than overlying units, and displays the strongest mafic signatures. However, the signatures vary significantly even within local outcrops. Strong HCP (Type 2) absorption bands (s2, s18) are observed, as well as both V-and U-shaped 1 μm absorptions beyond 1.075 μm (Type 4; s19, s20) with little to no 2 μm absorption band indicating the presence of glass or olivine (s15-s17). Finally, strongly blue-sloped spectra with no mafic bands consistent with weathered glass are also detected (Type 5; s6).
The Rupes (ABbr) unit, once a 'paleo-plateau', forms most of the basal unit and contains the oldest north polar sediments (Tanaka et al., 2008). Early to Late Hesperian in age, Rupes is over a kilometer in thickness and composed of layers of ice and fine-grained material that is hypothesized to have been eroded and transported from the Scandia region (Nerozzi et al., 2022;Tanaka et al., 2008Tanaka et al., , 2012. Lacking dark sandy surficial mantle, the sampled surface is likely a sublimation lag of fine-grained materials comprising Rupes, whose spectra appear dust and ice free, with a broad and symmetric absorption band at 1.1 μm and negligible absorption at 2 μm, most consistent with glass (Type 4; s4).

Volcanic and Impact Sediments Within the NPLD
While glass and weathered glass signatures are the most widespread, mafic minerals like pyroxene, and likely olivine, are also common within the NPLD which suggests diverse sources and depositional processes. Mafic materials within the NPLD are broadly similar to those detected in the surrounding sand sea, but the mineralogy of lithics within the NPLD is spatially heterogenous, unlike the surrounding sand seas (B. Horgan & Bell, 2012). Diverse lithics eroding from NPLD troughs and scarps become well-mixed during aeolian transport into the sand seas, making it appear spectrally homogenous over vast expanse (Ewing et al., 2010;Tanaka et al., 2008). However, it is unlikely that the reverse case is true, that the NPLD lithics are aeolian sediments sourced from these dune fields or the surrounding plains. Wind models suggest that strong katabatic winds drive transport down off the cap and not up onto it (Smith & Spiga, 2018), so sediment is more likely to be emplaced onto the NPLD either ballistically or via atmospheric fallout. Compositional similarities to the surrounding plains may thus be due to both deposition of the same materials regionally and because the NPLD is a source of sediment for the plains. The heterogeneity of the NPLD lithics also supports diverse origins unrelated to the sand sea. In particular, detection of olivine-like signatures within the NPLD suggests that at least some materials are sourced from  Figure 1a for location), labeled with NPLD stratigraphic units, are paired with RGB colorcomposites of CRISM summary products BD530 (red), BDI1000VIS (green), and HCPINDEX2 (blue). The spectral parameters measure absorptions at 0.53, 1, and 2 μm which here are indicative of ferric-dust, glass, and HCP, respectively. (b) When projected over MOLA elevation model, HRL0000B69E shows a trough whose equatorward facing slope (s16) is dust-dominated whereas the poleward facing slope (s13-s15) displays strong mafic signatures. far distances, since olivine signatures are only detected in bedrock or sediments on the surrounding plains at distances >1,000 km (Salvatore et al., 2010).
We hypothesize that impact ejecta are a major source for mafic sediments in the NPLD. Impacts are frequent on Mars even today and are expected to ballistically deposit sediments locally and globally (Daubar et al., 2013). Proximal impact ejecta would contain a large fraction of diverse local country rock (e.g., pyroxene) and variable amounts of impact glass, with a grain-size decreasing with distance from the impact. Proximal ejecta could thus form locally thick impactite mantling layers over the NPLD, and we hypothesize that the ABb2 dark sedimentary mantle could be such an impactite deposit. Distal impacts would produce globally distributed mm to tens of cm thick layers of sand-sized glass (microtektites) or partially crystalline (microcrystites) spherules (Bouška & Bell, 1993). For example, a 15 km diameter crater can produce a global average of 40 microtektites per square centimeter (Lorenz, 2000). The iron in distal impact spherules is reduced during vapourization, causing a weaker 2 μm band consistent with Type 4 glassy spectra (Figures 2b, 3 and 4) observed within the NPLD. Larger tektites can be either glassy (Types 3 or 4) or crystalline (Type 2, pyroxene; Type 4, olivine) depending on their cooling history (Johnson & Melosh, 2014;Schultz & Mustard, 2004), therefore, distal ejecta could thus produce a mix of glassy and crystalline sand-sized mafic grains throughout the NPLD, consistent with the mafic sedimentary lags that we observe at the NPLD outcrops.
Some mafic sediments at the NPLD could also be ash deposits sourced from volcanic eruptions. While the existence of volcanic edifices in the northern lowlands is controversial (Garvin et al., 2000), more distant sources could also have deposited ash, transported by atmospheric suspension and dispersed to distant locations by global atmospheric circulation (Kerber et al., 2012). Volcanic tephra can be crystalline or glass-rich, where glass abundances are significantly enhanced by water/ice interactions during eruption (Henderson et al., 2021;Wall et al., 2014). However, coarse (sand and larger) tephra is only deposited close to the source. Climate models suggest that only the finestgrained volcanic ash (particle size ∼1 μm) can be latitudinally transported from the mid-latitudes and beyond to the poles, especially at low modern atmospheric densities (Kerber et al., 2012). Since no known major volcanoes are located at northern high-latitudes, any volcanic ash in the NPLD will account for a very small volume of sediment relative to the full stratigraphy (Kerber et al., 2012). However, the most recent large volcanic eruptions on Mars are thought to have occurred only 1 to a few 10s of million years ago (Hartmann & Berman, 2000;Márquez et al., 2004;Neukum et al., 2004) so evidence of their eruption may exist within the NPLD.
In either case, NPLD would contain a unique record of the eruption and impact history of Mars during the late Amazonian, and sediments from these sources could provide both compositional markers for establishing stratigraphy across the region, as well as dateable minerals within the layers for quantitative geochronology using techniques like 40 Ar-39 Ar dating (Basile et al., 2001;Buizert et al., 2014). However, any landed mission analyzing the sediments for geochronology must consider the impact of local reworking of NPLD layers and sediments resulting from trough migration (Smith et al., 2013). Therefore, landing sites for these missions must target least impacted stratigraphic horizons. Extracting ice cores and constraining dates for climate records will provide better constraints on the effects of astronomical forcing (Levrard et al., 2007) and stochastic processes like impacts (Perron & Huybers, 2009) on planetary climatology, as well as substantially improve our understanding of modern sediment sources on Mars, evolution of Mars' interior, and recent impact flux at Mars (Banfield et al., 2020).

Significance of Dust and Non-Dust Lithics in the NPLD
The NPLD are typically modeled as a simple mixture of ice and martian airfall dust (containing nanophase ferric oxides with other amorphous and crystalline minerals; Ehlmann et al., 2017), with individual layers varying from ∼95% ice to up to ∼50% dust based on radar models and spectral observations (Calvin et al., 2009;Lalich et al., 2019). However, this simple model is inconsistent with detections of coarse-grained mafic sediment, which suggests that lithics are entrained within the NPLD through a combination of dust falling on the surface, deposits of lithic sediments sourced from impacts and volcanic eruptions, and ablation, leading to stratigraphic variations in the concentration of both types of lithics.
Airfall martian dust is the largest and most continuous source of refractory lithic material to the NPLD. Atmospheric dust changes energy flux in the atmosphere which has the potential to be a major internal driver of climate variations on Mars (Kahre et al., 2017). These climatic variations can alter the atmospheric state and thereby its ability to access different reservoirs of dust across the planet. Hence, the concentration and composition of dust in the NPLD may provide records of major dust events or cycles, and may reflect temporal variations in dust composition and characteristics sourced from different regions of Mars under changing climatic conditions (Hvidberg et al., 2012).
Although the NPLD contains mafic materials, the nature of their occurrence varies. The RGB spectral parameter color-composite images in Figure 2 show that the NPLD (unit ABb1 in red) forming upper Planum Boreum is comprised of a stack of icy layers and thin ablation lags that are ferric-dust rich with minor and spatially variable components of both crystalline and glass-rich mafic sediments that we interpret as sourced from impacts and volcanism. The underlying Cavi unit (ABbc in blue/green) is sand-rich and dominated by locally heterogeneous mafic sediments. The heterogeneity suggests that the Cavi is not just well-mixed sands similar to and perhaps sourced from the surrounding dune fields, and that it may contain materials from ballistic sources as well. The polar veneers (ABb2 in green) that mantle the NPLD are dominated by much more homogeneous glass-rich materials. The homogeneity suggests that this unit could either represent a deposit from a distinct mantling event or an ablation lag reworked by aeolian processes to concentrate sand-sized glassy materials.
Concentration of coarse glassy sediments during ablation of the NPLD appears to be actively occurring today in the north polar troughs (Figure 2b). Solar insolation and katabatic winds erode the equatorward-facing slope (magenta in Figure 2b), blowing away fine-grained ferric-dust and lithic fragments and concentrating reworked coarse glass-rich sediments (green/blue in Figure 2b) at the trough bottom and over the poleward facing slope (Smith et al., 2013). This observation strongly supports an ablation lag origin for the similarly glass-rich and sand-size sediments of the veneers/ABb2 unit, which we hypothesize contains significant abundances of coarse glassy grains originating as distal impact spherules, possibly from a large number of impact events. Determining the range of ages of sediments in this unit could thus provide both an important stratigraphic marker within the NPLD and a critical constraint on the impact flux at Mars during the Amazonian.
Past mantling by impact ejecta, ash, a dust event, or an ablation lag has major implications for the thermal stability, strength of the stratified structure, and the age of ice at the NPLD (Levrard et al., 2007;Sori et al., 2016). The presence of lithics other than dust may increase the thickness and insulating properties of sublimation lag deposits, which controls ablation rates of underlying ice, and hence, the time taken to build or remove the layers forming the NPLD. For example, the ABb2 dark sedimentary unit could represent a major lag that once covered the NPLD, which may have increased the stability of the NPLD during the most recent periods of high obliquity.
Lithics in the NPLD also record the recent history of water activity at the north pole. Detection of weathered glass (s15), which if altered in situ, suggests water activity in the recent past of the NPLD. At the low temperatures of the NPLD, liquid water brines can persist as thin film surrounding lithic sediments which causes acid leaching of cations and forming weathering rinds on the surface of glassy sediments (Chemtob et al., 2010;Minitti et al., 2007). The NPLD and surrounding regions are known to contain sulfates (B. H. Horgan et al., 2009;Massé et al., 2010Massé et al., , 2012 and other salts (Toner et al., 2014), that may have formed by alteration of lithics surrounded by ice-brine slush or episodic freeze-thaw cycle (Bishop et al., 2021). The degree of aqueous activity is perhaps controlled by climate change caused by astronomical forcing or due to discrete stochastic events related to impacts or volcanic eruptions, which would have influenced the habitability of Mars' polar terrains.

Conclusions
Our analysis shows a significant concentration of lithics other than airfall ferric-dust throughout the NPLD, and detections include both glass and crystalline mafic minerals. Glasses are a dominant material throughout the NPLD, suggesting that distal impact spherules has been a major source of sand-sized sediment. Volcanic ash is likely to be present but primarily as very fine-grained sediments. Thus, along with martian dust cycles, geologic processes like impacts and volcanic eruptions during the late Amazonian influenced formation of the NPLD by depositing material from regional and global sources. The role of these stochastic geological processes and subsequent reworking over the polar cap, due to ablation and aeolian activity, must be factored into radar and climate models to improve estimates of the NPLD's physical properties and timing of accumulation. The presence of dateable sediments within the NPLD make it an excellent site for assessing Mars' recent rates of impacts and volcanic activity, and carrying out a first paleoclimate study on another planet using a landed robotic asset.