AbstractIron oxide and hydroxide minerals, likely responsible for Mars' distinctive red color, offer critical insights into the planet's ancient and current climate, as well as its potential habitability. Several previous studies attributed Mars' reddish hue to anhydrous hematite (Fe2O3) and suggested that its formation is a geologically young process. Recent analyses by the Mars Science Laboratory (MSL) rover revealed the presence of volatiles and amorphous materials in the surface fines and dust, but mineralogy remained unresolved. Here, we present evidence that poorly crystalline ferrihydrite (Fe5O8H · nH2O) is responsible for the red color of the Martian dust, as identified through a combination of orbital (CRISM & OMEGA), in-situ (MSL ChemCam, MER Pancam and Pathfinder IMP), and laboratory visible near-infrared spectra. We employ quantitative spectral analyses, which demonstrate that among various iron oxyhydroxides, ferrihydrite is most consistent with the observed Martian dust spectra. In addition, our dehydration experiments show that ferrihydrite does not transform into other more crystalline iron oxide phases when exposed to present-day Martian conditions. The preservation of ferrihydrite until present time is inconsistent with a sustained warm climate after it was formed, since warm conditions would favor transformation into more crystalline hematite and/or goethite. We propose that the formation of abundant ferrihydrite indicates a cool, wet environment in the last stages of early Mars, favorable to oxidative conditions, followed by a transition to a hyper-arid erosional environment that has persisted to the present day.IntroductionIdentifying the dominant iron oxide phases in Martian dust can provide quantitative constraints on the planet’s ancient chemical environments and climate conditions. On Earth iron oxides form under specific environmental conditions including pH, temperature, redox state, and water availability (Cornell & Schwertmann, 2003). The reddish coloration of the Martian surface has been investigated since the early telescopic observations that hinted at the presence of impure iron ore known as limonite, which contains the crystalline iron (oxy)hydroxide mineral goethite (α-FeOOH) (Adams & McCord, 1969; Dollfus, 1957; Sagan et al., 1965; Sharonov, 1961). Subsequent ground-based telescopic and laboratory observations attributed the reddish hue to the presence of pigmentary anhydrous hematite (α-Fe2O3; termed “nanophase NpOx”) dispersed in the surface regolith and/or coating of rocks (Bell et al., 1990; Morris et al., 1989). Based on the lack of water absorption features at near infrared (NIR) wavelengths (1 – 2.5 μm)as determined by ESA’s Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) spectrometer, it was argued that the anhydrous and dusty regions contain ferric oxides, possibly hematite or maghemite (γ-Fe2O3)(Bibring et al., 2006). Furthermore, a widely used mineralogical model (Bibring et al., 2006) proposed that these anhydrous ferric oxides in Martian dust formed by continuous oxidation and weathering under water-poor surface conditions during the Amazonian period, which spans from approximately 3 billion years ago to the present.Early spacecraft observations revealed a distinctive 3 μm hydration feature in the Martian dust spectrum (Murchie et al., 1993; Pimentel et al., 1974) well before the weaker NIR spectral features associated with alteration minerals were identified (Bibring et al., 2006). Later evaluation of the OMEGA data noted that the large 3 μm absorption band is deeper in the observations of bright, dusty regions when compared to dark, less dusty terrains (Jouglet et al., 2007; Milliken et al., 2007). The increased strength of this absorption band in dusty regions was attributed to either higher abundances of water adsorbed on grain surfaces due to the large surface to volume ratio of the dust particles (e.g. Zent & Quinn, 1997) or H2O bound in hydrated minerals in the dust. Audouard et al. (2014), using ten years’ worth of OMEGA data, showed that the 3-µm band can be attributed to tightly bound H2O and/or hydroxyl groups in the mineral structure of the dust. NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) also indicated a deep absorption centered at 3 µm (Murchie et al., 2019) in bright, dusty regions. Finally, laboratory reflectance investigations of Martian meteorite ALH 84001 revealed a 3-μm hydration band, which was attributed to H2O, although no bands were observed at 1.4 or 1.9 µm (Bishop et al., 1998). Basaltic volcanic glasses also typically include a broad 3-µm band due to H2O without the weaker 1.4 or 1.9 µm (e.g. Bishop, 2019).Data collected by the MIMOSII Mössbauer instrument (MB) on the Mars Exploration Rovers (MER) showed the existence of coarse-grained hematite and goethite in specific rock outcrops as well as the ubiquitous presence of undetermined iron oxide phase (“nanophase NpOx”) in the fine dust (Morris et al., 2006). While MER MB data can be used to determine the Fe oxidation state (Fe3+/FeT) it is more difficult to distinguish the mineralogy of ferric iron present in the Martian dust (Morris & Klingelhöfer, 2008). This difficulty arises because in the microcrystal range, the distinct characteristics of different iron oxides gradually disappear as particle size and crystallinity decrease, resulting in broad and diffuse spectral lines (Coey, 1974; Murad & Schwertmann, 1980). Further, characterization of nanophase components is difficult in mixtures. However, data from MERs showed that the iron concentration in the fine dust is positively correlated with sulfur and chlorine abundances, while dark olivine-rich soils contained lower abundances of these elements, suggesting that iron in the dust is a product of chemical alteration (Ming et al., 2008; Morris et al., 2006; Yen et al., 2005). The MERs were also equipped with a series of magnet arrays designed to analyze airfall dust. The analysis of the magnetic targets using MB spectral and imaging systems identified two distinct ferric iron endmembers in the dust: one comprising strongly magnetic and dark-colored magnetite, and the other an unidentified bright-colored (oxy)hydroxide exhibiting weak magnetic properties (Goetz et al., 2005; Madsen et al., 2009). Earlier results from the Mars Pathfinder mission (Madsen et al., 1999), which utilized five magnets of varying strengths, indicated that the magnetic properties of Martian soil are likely due to small amounts of maghemite present in intimate association with silicate particles, suggesting that the dust particles are composites containing both magnetic and non-magnetic components.NASA’s Mars Science Laboratory (MSL) rover provided several key chemistry and mineralogy measurements of Martian dust and soils. The Chemistry and Camera (ChemCam) instrument utilized its laser-induced breakdown spectroscopy (LIBS) capability to analyze the composition of airfall dust. In each of the initial laser shots from a series of 50 shots on dusty rock surfaces and calibration targets that collected dust over the years, ChemCam consistently detected a hydrogen signal that exhibited no diurnal variation, suggesting that hydrogen is chemically bound within the dust particles (Lasue et al., 2018; Meslin et al., 2013). Samples from the dust covered sand dune known as “Rocknest” were measured with the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument. These measurements revealed that up to scooped soil is X-ray amorphous and that ~20 wt. % of the amorphous component consists of iron oxides (Bish et al., 2013; Blake et al., 2013). In addition, the Alpha Particle X-ray Spectrometer (APXS) instrument analyzed air fall dust on the science observation tray. These measurements (Berger et al., 2016) indicated that the dust is compositionally similar to the bulk basaltic Mars crust (Gellert & Yen, 2019; McLennan & Taylor, 2008), but is enriched in SO3, Cl and Fe, which is in agreement with MER observations (Goetz et al., 2005). Both APXS and ChemCam measurements suggested that the amorphous iron oxide component observed at “Rocknest” soils may be linked to dust (Berger et al., 2016; Lasue et al., 2018). The Sample Analysis at Mars (SAM) instrument, which includes a gas chromatograph and a quadrupole mass spectrometer, detected volatile species (H2O, SO2, CO2 & O2) when the ’Rocknest’ sample was heated to ~835 °C (Leshin et al., 2013). This finding  suggested that H2O is bound to the amorphous component of the sample, as the CheMin instrument did not detect any crystalline phyllosilicate minerals in this sample (Leshin et al., 2013).Here we report the spectral detection of ferrihydrite (Fe5O8H · nH2O) – a poorly crystalline X-ray amorphous and hydrated iron oxide mineral – using a combination of orbital, in-situ and laboratory visible near-infrared (VNIR) spectra. In addition, we show that ferrihydrite is stable under simulated present-day Martian conditions (UV irradiation, 6 mbar pressure, CO2 atmosphere). We then discuss its importance and implications for the past climate and habitability on Mars.

Recent observations from the ExoMars Trace Gas Orbiter (TGO) have revealed the presence of hydrogen chloride (HCl) in the martian atmosphere. HCl shows strong seasonality, primarily appearing during Mars’ perihelion period and rapidly decreasing afterwards faster than projected from photolysis and gas-phase chemistry. HCl also shows anti-correlation with atmospheric water ice. One candidate explanation is heterogeneous chemistry. We present the first results from a heterogeneous chlorine chemistry scheme incorporated into a Mars global climate model (GCM), with atmospheric dust/water ice parameterized as an HCl source/sink respectively. Results were compared against a Mars GCM with gas-phase only chlorine chemistry and observations from TGO’s Atmospheric Chemistry Suite (ACS). We found that the heterogeneous scheme significantly improved the modelled HCl seasonal, latitudinal, and vertical distribution, supporting a crucial role for heterogeneous chemistry in Mars’ chlorine cycle. Remaining discrepancies show that further work is needed to characterise the exact aerosol reactions involved.

Martian linear dune gullies are landforms consisting of parallel and often sinuous channels with distinct levees and pit-shaped endings that occur in the mid-latitudes on Mars. Recent observations of ongoing activity link their formation to sliding blocks of CO2 ice in early spring. Here we combine laboratory experiments in which we release CO2-ice blocks on sandy slopes under Martian atmospheric pressure with morphometric observations of linear dune gullies on Russell crater mega dune to test this hypothesis. Our work shows that linear dune gullies are carved by burrowing blocks of CO2 ice that induce explosive ballistic sediment transport, resulting in gullies with all morphological characteristics we observe on Mars; high levees, deep channels, and extreme channel sinuosity. Our findings highlight the specific climatological, topographical, and grain size conditions under which these landforms originate, making it possible to use them as a microclimate proxy on present-day and potentially ancient Mars.

Detecting trace gases such as hydrogen chloride (HCl) in Mars' atmosphere is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism/outgassing. However, after its discovery using the mid-infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl

Martian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid-latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present-day gully activity. However, due to a lack of in-situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2-driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2-driven granular flows shows that they are dynamically similar to terrestrial two-phase granular flows, i.e. debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2-driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far-reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully-like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation-driven flow processes.

Subtle mounds have been discovered in the source areas of martian kilometer-sized flows and on top of summit areas of domes. These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface. Two theories have been proposed: a) ascent of more viscous mud during the late stage of eruption and b) expansion of mud within the conduit due to the instability of water under martian conditions. Here we present experiments performed inside a low-pressure chamber, designed to investigate whether the volume of mud changes when exposed to a reduced atmospheric pressure. Depending on the mud viscosity, we observe volumetric increase of up to 30% at the martian average pressure of ~6 mbar. This is because the low pressure causes instability of the water within the mud, leading to the formation of bubbles that increase the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lavas or mud volcano eruptions caused by a rapid pressure drop. We conclude that the mounds associated with putative martian sedimentary volcanoes might indeed be explained by volumetric changes of the mud. We also show that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth, because mud dynamics are affected by the formation of bubbles in response to the low atmospheric pressure.

Super-rotation affects - and is affected by - the distribution of dust in the martian atmosphere. We modelled this interaction during the 2018 global dust storm (GDS) of Mars Year 34 using data assimilation. Super-rotation increased by a factor of two at the peak of the GDS, as compared to the same period in the previous year which did not feature a GDS. A strong westerly jet formed in the tropical lower atmosphere, with strong easterlies above 60 km, as a result of momentum transport by thermal tides. Enhanced super-rotation is shown to have commenced 40 sols before the onset of the GDS, due to equatorward advection of dust from southern mid-latitudes. The uniform distribution of dust in the tropics resulted in a symmetric Hadley cell with a tropical upwelling branch that could efficiently transport dust vertically; this may have significantly contributed to the rapid expansion of the storm.

The vertical opacity structure of the martian atmosphere is important for understanding the distribution of ice (water and carbon dioxide) and dust. We present a new dataset of extinction opacity profiles from the NOMAD/UVIS spectrometer aboard the ExoMars Trace Gas Orbiter, covering one and a half Mars Years (MY) including the MY 34 Global Dust Storm and several regional dust storms. We discuss specific mesospheric cloud features and compare with existing literature and a Mars Global Climate Model (MGCM) run with data assimilation. Mesospheric opacity features, interpreted to be water ice, were present during the global and regional dust events and correlate with an elevated hygropause in the MGCM, providing further evidence for the role of regional dust storms in driving atmospheric escape as reported elsewhere. The season of the dust storms also had an apparent impact on the resulting lifetime of the cloud features, with events earlier in the dusty season correlating with longer-lasting mesospheric cloud layers. Mesospheric opacity features were also present during the dusty season even in the absence of regional dust storms, and interpreted to be water ice based on previous literature. The assimilated MGCM temperature structure agreed well with the UVIS opacities, but the MGCM opacity field struggled to reproduce mesospheric ice features, suggesting a need for further development of water ice parameterizations. The UVIS opacity dataset offers opportunities for further research into the vertical aerosol structure of the martian atmosphere, and for validation of how this is represented in numerical models.

Lower atmosphere variations in the martian water vapour and hydrogen abundance during the Mars Year (MY) 34 C storm from LS=326.1◦-333.5◦ and their associated effect on hydrogen escape are investigated using a multi-spacecraft assimilation of atmospheric retrievals into a Martian global circulation model. Elevation of the hygropause and associated increase in middle atmosphere hydrogen at the peak of the MY 34 C storm led to a hydrogen escape rate of around 1.4×109 cm−2s−1 , meaning the MY 34 C storm enhanced water loss rates on Mars to levels observed during global-scale dust storms. The water loss rate during the MY 34 C storm (a loss of 15% of the total annual water loss during only 5% of the year) was three times stronger than the weak MY 30 C storm assimilation, demonstrating that interannual variations in C storm strength must be considered when calculating the integrated loss of water on Mars.

We show a positive vertical correlation between ozone and water ice using a vertical cross-correlation analysis with observations from the ExoMars Trace Gas Orbiter’s NOMAD instrument. We find this is particularly apparent during the first half of Mars Year 35 (LS=0-180) at high southern latitudes, when the water vapour abundance is low. This contradicts the current understanding that ozone and water are, in general, anti-correlated. However, our simulations with gas-phase-only chemistry using a 1-D model show that ozone concentration is not influenced by water ice. Heterogeneous chemistry has been proposed as a mechanism to explain the underprediction of ozone in global climate models (GCMs) through the removal of HOX. We find improving the heterogeneous chemical scheme causes ozone abundance to increase when water ice is present, better matching observed trends. When water vapour abundance is high, there is no consistent vertical correlation between observed ozone and water ice and, in simulated scenarios, the heterogeneous chemistry does not have a large influence on ozone. HOX, which are by-products of water vapour, dominate ozone abundance and mask the effects of heterogeneous chemistry on ozone. This is consistent with gas-phase-only modelled ozone, showing good agreement with observations when water vapour is abundant. High water vapour abundance masks the effect of heterogeneous reactions on ozone abundance and makes adsorption of HOX have a negligible impact on ozone. Overall, the inclusion of heterogeneous chemistry improves the ozone vertical structure in regions of low water vapour abundance, which may partially explain GCM ozone deficits.

We present ~1.5 Mars Years (MY) of ozone vertical profiles, covering Ls = 163deg; in MY34 to Ls = 320deg; in MY35, a period which includes the 2018 global dust storm. Since April 2018, the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) spectrometer aboard the ExoMars Trace Gas Orbiter has observed the vertical, latitudinal and seasonal distributions of ozone. Around perihelion, the relative abundance of ozone (and water from coincident NOMAD measurements) increases strongly together below ~40 km. Around aphelion, decreases in ozone abundance exist between 25-35 km coincident with the location of modelled peak water abundances. We report high latitude (above 55deg;), high altitude (40-55 km) equinoctial ozone enhancements in both hemispheres. The northern equinoctial high altitude enhancement is previously unobserved and forms prior to vernal equinox lasting for almost 100 sols (Ls ~350‑40deg), whereas the southern enhancement persists for over twice as long (Ls = ~5-140deg;). Both layers reform at autumnal equinox, with the northern layer at a lower abundance. These layers likely form through a combination of anti-correlation with water and the equinoctial meridional transport of O and H atoms to high-latitude regions. The descending branch of the main Hadley cell shapes the ozone distribution at Ls = 40-60deg;, with the possible signature of a northern hemisphere thermally indirect cell identifiable from Ls = 40-80deg;. The ozone retrievals presented here provide the most complete global description of Mars ozone vertical distributions as a function of season and latitude.

The local redistribution of granular material by sublimation of the southern seasonal CO2 ice deposit is one of the most active surface shaping processes on Mars today. This unique geomorphic mechanism has been linked to the dendritic, branching, ‘spider’-like araneiform terrain and associated fans and spots - features which are native to Mars and have no Earth analogues. However, there is a paucity of empirical data to test the validity of this genetic hypothesis. Additionally, it is unclear whether the organised radial patterns of araneiforms require a singular or multiple seasonal events to form. Here we present the results of a suite of laboratory experiments undertaken to investigate if the interaction between a sublimating CO2 ice overburden with central vents and a porous, mobile regolith will mobilise grains from beneath the ice in the form of a plume to generate araneiform patterns. We investigate the physical constraints on the level of branching of these features and the area that they cover. We provide the first observations of plume activity via CO2 sublimation and consequent erosion of araneiform features. Based on calculations of the mass flux sublimation rate of CO2 ice blocks buried under sediment and consequent volume transport, we show that CO2 sublimation can be a highly efficient agent of sediment transport under Martian pressure.