South Polar Cap Research Articles

Principal-plane BRDF of CO2 ice morphologies in controlled Mars polar conditions

Mars South Polar Residual Cap (SPRC) persists throughout the Southern hemisphere's warmest months. During the colder seasons, carbon dioxide (CO2) accumulates as ice on the surface of the planet, forming a seasonal cap during fall and winter and sublimating throughout spring. The seasonal cap and SPRC both exhibit unexpected reflectance properties that have not been explained. To better understand the Martian surface CO2 ice and how it affects Mars' geomorphology, atmosphere, and climate, it is imperative to corroborate laboratory experiments with orbital observations. This study uses a custom-designed goniometer for making reflectance measurements within the MARs Volatile and Ice evolutioN (MARVIN) environmental chamber, in which we can measure the Bidirectional Reflectance Distribution Function (BRDF) of different phases of CO2 under Mars polar conditions for the first time. This has the potential to explain present-day observations from orbit and enhance our understanding of the evolution of ice on Mars.

Seasonal cap dynamics at the South Pole of Mars in the 36th Martian year, observed by Tianwen-1

The seasonal variation of the Martian South Polar Seasonal Cap (SPSC) is a significant factor to influence Mars atmospheric cycle, which has been continuously observed in previous missions. The SPSC degrades asymmetrically throughout the Martian seasons. However, developing a general description for this asymmetric recession is challenging, and this is crucial for identifying key stages (timesteps) which represents moments of special attribute throughout the recession. Here, we present the Martian Year 36 (MY36) results from the continuous observation of the Martian SPSC by the Moderate Resolution Imaging Camera (MoRIC) onboard the Tianwen-1 orbiter. A time-series of digital orthophoto maps (DOMs) characterizing the south cap were collected ranging from the solar longitude (Ls) 210° to 332°. A seasonal recession model of the cap was established by incorporating a 5th-order polynomial fitting of the time-series on cap latitude, from which the first derivative (recession rate), second derivative (acceleration), and third derivative (jerk) can be obtained. This methodology enables the extraction of critical timesteps that facilitate the categorization of the recession into distinct phases, each characterized by its recession rate and other factors. By the visual interpretation of remote sensing images obtained by Tianwen-1 and the result of timesteps showed that the pause time of the South Polar Residual Cap (SPRC) is about Ls = 324.5°-328° with an area of 7.9 × 104 km2. The discrepancies between our results and previous investigations indicate that the Martian climate presents inter-annual fluctuations. The MoRIC image data can be used to quantitatively capture the fine recession dynamic of the southern polar cap.The continuous operation of the Tianwen-1 orbiter will enable further data analysis for the dynamics of the Martian polar caps. This will provide observational constraints for Martian climate models and crucial information for the research of the Martian modern climate.

Long-term Variation of the Solar Polar Magnetic Fields at Different Latitudes

The polar magnetic fields of the Sun play an important role in governing solar activity and powering fast solar wind. However, because our view of the Sun is limited in the ecliptic plane, the polar regions remain largely uncharted. Using the high spatial resolution and polarimetric precision vector magnetograms observed by Hinode from 2012 to 2021, we investigate the long-term variation of the magnetic fields in polar caps at different latitudes. The Hinode magnetic measurements show that the polarity reversal processes in the north and south polar caps are non-simultaneous. The variation of the averaged radial magnetic flux density reveals that, in each polar cap, the polarity reversal is completed successively from the 70° latitude to the pole, reflecting a poleward magnetic flux migration therein. These results clarify the polar magnetic polarity reversal process at different latitudes.

Tracking the south polar seasonal cap retreat of Mars using computer vision

We use daily polar stereographic mosaics obtained from the Mars Color Imager (MARCI) to autonomously track the recession of the South Polar Seasonal Cap (SPSC) during Mars Years (MYs) 28 to 31, for the period Ls = 178° to Ls = 345°. Utilizing an autonomous approach, we can delineate the SPSC by fitting an ellipse, offering the advantage of faster processing. This allows us to monitor the recession with enhanced time sampling capabilities. The SPSC recession is divided into two segments separated by a discontinuity at Ls = ∼260°, corresponding to the detachment of the Mountains of Mitchel seasonal cap from the main cap. Before the discontinuity, the SPSC exhibits variability where the recession rate can vary by up to 50%, with the fastest recession occurring at Ls = ∼235° and the slowest at Ls = ∼191°. After the discontinuity, the SPSC experiences reduced variability and undergoes a monotonic deceleration until Ls = ∼319°, gradually approaching the stable latitude of ∼86° S, consistent with the South Polar Residual Cap. MY 28 demonstrates distinct behavior in response to the Global Dust Storm (GDS), as the SPSC was smaller and sublimated faster before the onset of the storm compared to the 4-year average. Plain language summaryThe seasonal frost layer on Mars, consisting of frozen carbon dioxide, covers vast expanses during the winters. This frost forms as carbon dioxide gas in the atmosphere turns into solid ice on the surface, impacting the overall atmospheric pressure. We studied daily images captured by the Mars Color Imager (MARCI) to automatically monitor the retreat of the South Polar Seasonal Cap (SPSC) as it returns to the atmosphere during the spring over a span of four years. To achieve this, we employed a computer programming language called Python and a publicly available software library designed for visual analysis. By fitting an ellipse that most closely resembles the contour of the cap, we were able to trace and measure the change in the SPSC over time and determine how quickly SPSC shrinks. We discovered that our strategy produces results that are similar to those of previously published approaches, but in a lot less time and with increasing accuracy. We noticed new patterns in the seasonal retreat of the seasonal cap, including brief periods of faster shrinking, likely influenced by wind patterns. Additionally, we investigated a prominent icy area known as the Mountains of Mitchel seasonal cap and found that its behavior differed before and after a significant global dust storm in Mars Year 28.

Implications of MOLA topography on the presence of liquid water under the south polar cap of Mars

The existence of liquid water at the base of the south polar layered deposits (SPLD) on Mars has been proposed on the basis of radar data, but dry interpretations have been advanced as well. Here, we consider topography of the SPLD surface above the location of hypothesized water using individual altimetry tracks rather than gridded or interpolated data. Liquid water could create negative surface relief through drainage events and subsequent collapse, or positive surface relief through ice expansion. We find meters of positive topographic relief relative to the surrounding region, consistent with thermal expansion of the ice column from the added geothermal heat that would be required to maintain liquid water at the SPLD base. However, topographic analysis of randomly chosen areas throughout the SPLD suggest that positive topographic relief of this amplitude has a 20% chance of occurring through other processes or statistical noise. Therefore, we conclude that both wet and dry interpretations of the radar data remain viable.

Composition and thermal properties of Ganymede’s surface from JWST/NIRSpec and MIRI observations

Context. We present the first spectroscopic observations of Ganymede by the James Webb Space Telescope undertaken in August 2022 as part of the proposal “ERS observations of the Jovian system as a demonstration of JWST’s capabilities for Solar System science”. Aims. We aimed to investigate the composition and thermal properties of the surface, and to study the relationships of ice and non-water-ice materials and their distribution. Methods. NIRSpec IFU (2.9–5.3 μm) and MIRI MRS (4.9–28.5 μm) observations were performed on both the leading and trailing hemispheres of Ganymede, with a spectral resolution of ~2700 and a spatial sampling of 0.1 to 0.17″ (while the Ganymede size was ~1.68″). We characterized the spectral signatures and their spatial distribution on the surface. The distribution of brightness temperatures was analyzed with standard thermophysical modeling including surface roughness. Results. Reflectance spectra show signatures of water ice, CO2, and H2O2. An absorption feature at 5.9 μm, with a shoulder at 6.5 μm, is revealed, and is tentatively assigned to sulfuric acid hydrates. The CO2 4.26-μm band shows latitudinal and longitudinal variations in depth, shape, and position over the two hemispheres, unveiling different CO2 physical states. In the ice-rich polar regions, which are the most exposed to Jupiter’s plasma irradiation, the CO2 band is redshifted with respect to other terrains. In the boreal region of the leading hemisphere, the CO2 band is dominated by a high wavelength component at ~4.27 μm, consistent with CO2 trapped in amorphous water ice. At equatorial latitudes (and especially on dark terrains), the observed band is broader and shifted toward the blue, suggesting CO2 adsorbed on non-icy materials, such as minerals or salts. Maps of the H2O Fresnel peak area correlate with Bond albedo maps and follow the distribution of water ice inferred from H2O absorption bands. Amorphous ice is detected in the ice-rich polar regions, and is especially abundant on the northern polar cap of the leading hemisphere. Leading and trailing polar regions exhibit different H2O, CO2, and H2O2 spectral properties. However, in both hemispheres the north polar cap ice appears to be more processed than the south polar cap. A longitudinal modification of the H2O ice molecular structure and/or nanometer- and micrometer-scale texture, of diurnal or geographic origin, is observed in both hemispheres. Ice frost is tentatively observed on the morning limb of the trailing hemisphere, which possibly formed during the night from the recondensation of water subliming from the warmer subsurface. Reflectance spectra of the dark terrains are compatible with the presence of Na- and Mg-sulfate salts, sulfuric acid hydrates, and possibly phyllosilicates mixed with fine-grained opaque minerals, with a highly porous texture. Latitude and local time variations of the brightness temperatures indicate a rough surface with mean slope angles of 15°–25° and a low thermal inertia Γ = 20 − 40 J m−2 s−0.5 K−1, consistent with a porous surface, with no obvious difference between the leading and trailing sides.

Compositional Constraints of Ice Lobes at the Edge of Martian South Polar Cap and the Possibility for CO2 Ice

As one of the major ice reservoirs on Mars, the south polar layered deposits (SPLD) are mainly composed of water ice and dust except for the perennial CO2 ice deposits on the surface and within buried layers. At the edge of Planum Australe, the Shallow Radar detected the base of two ice lobes, which can be used to estimate their dielectric properties and analyze the components. These measurements, combined with topographic data, allow for the relative permittivity of the lobe materials to be estimated at 2.73 ± 0.67. Under the constraint of the SPLD density, the permittivity value translates to the existence of CO2 ice depending on the dust content. If the dust content is larger than 24%, the lobes are CO2 free and composed of water ice, dust, and pore space. Whereas, for the dust content of <12%, our results suggest that the lobes must contain a certain amount of CO2 ice, which could be an average volume of 30%. When the dust content is between 12% and 24%, the existence of CO2 ice is undetermined. This study improves the understanding of ice deposits at the Martian south polar.

Moats in the residual south polar cap of Mars: Ages, formation, and evolution

The residual south polar cap of Mars (RSPC) is a region comprising tabular deposits of solid CO2 averaging a few m thick resting on layered, water-ice rich deposits a few km thick. Estimates of the ages of morphologic features in the RSPC have relied on average rates of change obtained from geographically limited samples. In this work we apply image coverage of the whole RSPC at 6 m/pixel spanning up to seven Mars years to determine how well ages of RSPC forms can be modelled from recent observations, and to explore implications of the calculated chronology. Topographic troughs separating CO2 layers of different ages and thickness, termed moats, are widespread, changing, and dateable features of the RSPC. Although they all develop by the same phenomena, we divide moats into two groups based on their linear dimensions and the span of relevant spacecraft imaging: 1) moats separating large areas of CO2 ice of different age and thickness, termed sinuous boundary moats, that imaging data show are related to post-1972 CO2 deposition. 2) moats separating the walls of pits from thinner, younger, interior mounds, termed pit moats; these are not resolved in early spacecraft images. Our survey of the entire summer RSPC spanning seven Mars years yields modelled formation times for both kinds of moats that cluster largely about Mars Years (MY) 11–13. A population of smaller moats is calculated to have formed near MY 26–28. These times closely follow planet-encircling dust events in MY 9, MY 12, and MY 28 (Earth years 1971, 1977, 2007). High-resolution images show that both pit and sinuous moats are initiated by slow downward erosion of debris left by retreating scarps of CO2 ice that are flanked by brighter, accumulating CO2 ice. We find that the most probable role of the dust events in moat formation is to clear thin CO2 layers from underlying water-ice-rich materials through thermal effects of the atmospheric dust content. The age results severely limit the magnitude of variations in moat scarp retreat rates during the last 25 MY. Pit moats have limited lifetimes because scarp retreat eventually removes the interior mound that defines the inner moat wall. Pits can reach much greater ages than moats; some pits formed prior to MY -100 (Earth year 1765).

Possible Atmospheric Water Vapor Contribution from Martian Swiss Cheese Terrain

Mars’s south polar residual cap (SPRC) is a several-meters-thick CO2 ice cap with a variety of features, including quasi-circular depressions known as “Swiss cheese” that may expose underlying water ice. Swiss cheese pits have been suggested as a source for the observation of unusually high water vapor during the southern summer of Mars Year (MY) 8 (1969). To evaluate this hypothesis, we map the current extent of Swiss cheese pits to estimate the contribution to atmospheric water vapor from sublimation from the pits. We use data from the Mars Reconnaissance Orbiter Context Camera to map individual features and use the Mars Climate Sounder to obtain surface temperatures to estimate areas of exposed water ice to infer the amount of water vapor sublimated under typical south polar summer atmospheric conditions. We find that there is a negligible impact on atmospheric water vapor from sublimation with the current coverage and temperatures of Swiss cheese terrain (0.2% of the SPRC at an average of ∼161 K). At current typical temperatures, complete removal of residual CO2 from 99% of the SPRC would be required to sublimate enough water vapor to reproduce the MY 8 observation. However, a modest increase in temperature (∼20 K) could lead to a dramatic increase in sublimation rate, such that only water ice over 2.6% of the SPRC area would recreate the MY 8 observation. >180 K surface water ice has been observed on Mars, but such temperatures are likely transient at the south pole over the past ∼30 Mars years.

Spatio‐Temporal Level Variations of the Martian Seasonal North Polar Cap From Co‐Registration of MOLA Profiles

AbstractThe seasonal deposition and sublimation of CO2 constitute a major element in Martian volatile cycles. We reprocess the Mars Orbiter Laser Altimeter (MOLA) data and apply co‐registration procedures to obtain spatio‐temporal variations in levels of the Seasonal North Polar Cap (SNPC). The maximum level over the Residual North Polar Cap (RNPC) is 1.3 m, approximately half of that at the south pole (2.5 m). However, the maximum level in the dune fields at Olympia Undae can be up to 3.8 m. Furthermore, off‐season decreases up to 3 m during the northern winter at Olympia Undae are observed. These are likely due to metamorphism effects accentuated by the reduced snowfall at this period. Meanwhile, off‐season increases of up to 2 m during the northern spring are noted, the cause of which remains to be explored. The volume of the SNPC peaks at the end of northern winter and is estimated to be approximately 9.6 × 1012 m3, which is 2% more than that of the Seasonal South Polar Cap. The bulk density of the SNPC can go through phased decreases in accordance with phased accumulation at northern high‐latitudes. These findings can put important constraints on the Martian volatile cycling models.

Surface topographic impact of subglacial water beneath the south polar ice cap of Mars

Bright radar reflections observed in the Ultimi Scopuli region of Mars’ south polar layered deposits1,2,3 by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument have been interpreted as the signature of areas of subglacial water beneath it. However, other studies put forward alternative explanations, which do not imply the presence of liquid water4,5,6. Here we shed light on the issue by looking at the surface topography of the region. On Earth, reduced or absent basal friction, and consequent ice velocity changes, cause a distinct topographic signature over subglacial lakes7. Using Mars Orbiter Laser Altimeter data8, we identify and characterize an anomaly in the surface topography of the south polar layered deposits overlying the area of the putative lakes, similar to those found above terrestrial subglacial lakes of similar size. Ice flow model results suggest that comparable topographic anomalies form within 0.5–1.5 Myr with locally elevated geothermal heating9 or 2–5 Myr without elevated geothermal heating2. These findings offer independent support for the presence of basal water beneath Ultimi Scopuli and suggest that surface topography could supplement radar returns to help identify other potential subglacial water bodies.

Spatio‐Temporal Level Variations of the Martian Seasonal South Polar Cap From Co‐Registration of MOLA Profiles

AbstractThe seasonal deposition and sublimation of CO2 represents a major element in the Martian volatile cycle. Here, co‐registration strategies are applied to Mars Orbiter Laser Altimeter profiles to obtain spatio‐temporal variations in snow/ice level of the Seasonal South Polar Cap (SSPC), in grid elements of 0.5° in latitude from 60° to 87°S and 10° in longitude. The maximum snow/ice level in the range of 2–2.5 m is observed over the Residual South Polar Cap. Peak level at the Residual South Polar Cap in Martian Year 25 (MY25) are found to be typically ∼0.5 m higher than those in MY24. The total volume is estimated to peak at approximately 9.4 × 1012 m3. In addition, a map of average bulk density of the SSPC during its recession is derived. It implies much more snowfall‐like precipitation at the Residual South Polar Cap and its surroundings than elsewhere on Mars.

The stability of a liquid-water body below the south polar cap of Mars

Radar data from the Mars Express spacecraft show bright subsurface reflections in the Planum Australe area that could be due to liquid water located at a depth of 1.5 km (Orosei et al., 2018). If this interpretation of the data is correct, the presence of such water would have important implications for the present-day thermal state of the region. In this article, we recalculate the depth of the liquid water and we analyze the influence of the regional thermal properties in the surface heat flow and the subsurface temperatures. We have obtained a new depth to the bright reflector between 1.3 and 1.5 km by using a temperature dependent relative permittivity for the water ice and taking into account the dust content in the area. We show that regional properties in the SPLD moderately influence the thermal state of the area where the liquid water is located. A better knowledge of the porosity profile in the studied area is necessary to constrain surface heat flow and subsurface temperatures accurately. Our findings are in agreement with previous work that shows anomalously high local heat flows would be required to sustain liquid water at this location.

InSight Pressure Data Recalibration, and Its Application to the Study of Long-Term Pressure Changes on Mars.

Observations of the South Polar Residual Cap suggest a possible erosion of the cap, leading to an increase of the global mass of the atmosphere. We test this assumption by making the first comparison between Viking 1 and InSight surface pressure data, which were recorded 40 years apart. Such a comparison also allows us to determine changes in the dynamics of the seasonal ice caps between these two periods. To do so, we first had to recalibrate the InSight pressure data because of their unexpected sensitivity to the sensor temperature. Then, we had to design a procedure to compare distant pressure measurements. We propose two surface pressure interpolation methods at the local and global scale to do the comparison. The comparison of Viking and InSight seasonal surface pressure variations does not show changes larger than ±8 Pa in the CO2 cycle. Such conclusions are supported by an analysis of Mars Science Laboratory (MSL) pressure data. Further comparisons with images of the south seasonal cap taken by the Viking 2 orbiter and MARCI camera do not display significant changes in the dynamics of this cap over a 40 year period. Only a possible larger extension of the North Cap after the global storm of MY 34 is observed, but the physical mechanisms behind this anomaly are not well determined. Finally, the first comparison of MSL and InSight pressure data suggests a pressure deficit at Gale crater during southern summer, possibly resulting from a large presence of dust suspended within the crater.

Carbon Dioxide Ice Glaciers at the South Pole of Mars

AbstractMassive, kilometer thick deposits of carbon dioxide (CO2) ice have been detected at the south polar cap of Mars by radar investigations. These deposits are divided into several units that are separated by thin water ice bounding layers. Recent studies investigated the accumulation history of CO2 ice and found that the deposits most likely formed during several episodes in the past, when Martian obliquity was much lower than now. Those studies, while able to predict total volumes of CO2 ice consistent with those observed, did not attempt to explain the anomalous three‐dimensional distribution (thickness or extent) of CO2 ice or the ice's offset from the topographic high of the polar cap. In this paper we use a combination of feature analysis and numerical modeling to demonstrate that the CO2 deposits flow as glaciers and that glacial flow distributes the ice into its current position. Further, this distribution allows the ice to survive during high obliquity excursions.

The Bumpy Search for Liquid Water at the South Pole of Mars

Studies since 2018 have provided competing explanations of bright radar reflections from the base of the south polar ice cap.

Prospects for mapping temporal height variations of the seasonal CO2 snow/ice caps at the Martian poles by co-registration of MOLA profiles

We investigate the feasibility and demonstrate the merits of using Mars Orbiter Laser Altimeter (MOLA) profiles to retrieve seasonal height variations of CO2 snow/ice cap in Mars’ polar areas by applying a co-registration strategy. We present a prototype analysis on the research region of [85.75°S, 86.25°S, 300°E, 330°E] that is located on the residual south polar cap. Our method comprises the recomputation of MOLA footprint coordinates with an updated Mars Global Surveyor (MGS) ephemeris and a revised Mars rotation model. The reprocessed MOLA dataset at the South Pole of Mars (poleward of 78°S) is then self-registered to form a coherent reference digital terrain model (DTM). We co-register segments of reprocessed MOLA profiles to the self-registered MOLA reference DTM to obtain the temporal height differences at either footprints or cross-overs. Subsequently, a two-step Regional Pseudo Cross-over Adjustment (RPCA) procedure is proposed and applied to post-correct the aforementioned temporal height differences for a temporal systematic bias and other residual errors. These pseudo cross-overs are formed by profile pairs that do not necessarily intersect, but are connected through the underlying DTM. Finally, CO2 snow/ice temporal height variation is obtained by median-filtering those post-corrected temporal height differences. The precision of the derived height change time series is ∼4.7 ​cm. The peak-to-peak height variation is estimated to be ∼2 ​m. In addition, a pronounced ”pit” (transient height accumulation) of ∼0.5 ​m in magnitude centered at Ls ​= ​210° in southern spring is observed. The proposed method opens the possibility to map the seasonal CO2 snow/ice height variations at the entire North and South polar regions of Mars.

Alternatives to Liquid Water Beneath the South Polar Ice Cap of Mars

AbstractRadar sounding of Mars’ south polar ice cap has revealed portions of its base with stronger radar returns than its surface. These have been used to estimate high real permittivity values consistent with liquid water. However, the thermal, chemical, and geological conditions required to sustain such water bodies are challenging to achieve in the contemporary Martian environment. Three recent papers address this conflict. Khuller and Plaut (2021, https://doi.org/10.1029/2021GL093631) find similar radar signatures across the ice cap, expanding the range conditions associated with water candidates. Bierson et al. (2021, https://doi.org/10.1029/2021GL093880) show that, for non‐zero conductivity, a wider range of materials could produce the observed echoes. Similarly, Smith et al., (2021, https://doi.org/10.1029/2021GL093618) use modeling, laboratory tests, and remote sensing data to propose smectite deposits as their specific source. While not incompatible with bodies of liquid brines, these papers provide potential alternative sources for the bright radar observations.

The Composition of the South Polar Cap of Mars Derived From Orbital Data

AbstractThe flexure of the lithosphere under stresses imposed by the geologically young south polar cap is one of the few clues we have regarding the south polar cap composition and the present‐day thermal state of Mars. Here, we combine radar, gravity, and topography data with a flexural loading model to estimate the bulk density () and average real dielectric constant () of the south polar cap, and the elastic thickness of the lithosphere (). Given the uncertainties of the data, our results constrain to be 1,100–1,300 kg (best fit of 1,220 kg ), to be 2.5–3.4 (best fit of 3.3), and to be greater than 150 km (best fit of 360 km). Based on these results, the maximum lithospheric flexure is 770 m, and the polar cap volume could be up to 26% larger than previous estimates that did not account for lithospheric flexure. Our inferred compositions imply that the dust concentration would be at least 9 vol% if the ice content were negligible, and that the ice concentration would be more than the known 1 vol% if the dust concentration were less than 9 vol%. The 1‐ lower limit on implies a surface heat flow that is less than 23.5 mW . This lower limit is significantly less than the range of acceptable values at the north pole (330–450 km, heat flow of 11–16 mW ), and helps satisfy global thermal evolution simulations that predict hemispheric differences in surface heat flow.

Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals

AbstractRecent results from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument have been interpreted as evidence of subsurface brine pooled beneath 1.3 km‐thick South Polar Layered Deposit (SPLD). This interpretation is based on the assumption that the regionally high strength of MARSIS radar reflections from the base of the ice cap is due to a strong contrast in dielectric permittivity across the basal interface. Here, we demonstrate that the high‐power reflections could instead be the result of a contrast in electric conductivity. While not explicitly excluding a liquid brine, our results open new potential explanations for the observed strong radar reflections, some of which do not require liquid brine beneath SPLD. Potential basal materials with suitably high conductivity include clays, metal‐bearing minerals, or saline ice.