Seasonal Ice Cap Research Articles

Planet Four: A Neural Network’s search for polar spring-time fans on Mars

Dark deposits visible from orbit appear in the Martian south polar region during the springtime. These are thought to form from explosive jets of carbon dioxide gas breaking through the thawing seasonal ice cap, carrying dust and dirt which is then deposited onto the ice as dark ‘blotches’, or blown by the surface winds into streaks or ‘fans’. We investigate machine learning (ML) methods for automatically identifying these seasonal features in High Resolution Imaging Science Experiment (HiRISE) satellite imagery. We designed deep Convolutional Neural Networks (CNNs) that were trained and tested using the catalog generated by Planet Four, an online citizen science project mapping the south polar seasonal deposits. We validated the CNNs by comparing their results with those of ISODATA (Iterative Self-Organizing Data Analysis Technique) clustering and as expected, the CNNs were significantly better at predicting the results found by Planet Four, in both the area of predicted seasonal deposits and in delineating their boundaries. We found neither the CNNs or ISODATA were suited to predicting the source point and directions of seasonal fans, which is a strength of the citizen science approach. The CNNs showed good agreement with Planet Four in cross-validation metrics and detected some seasonal deposits in the HiRISE images missed in the Planet Four catalog; the total area of seasonal deposits predicted by the CNNs was 27% larger than that of the Planet Four catalog, but this aspect varied considerably on a per-image basis.

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.

Asymmetries in Snowfall, Emissivity, and Albedo of Mars' Seasonal Polar Caps: Mars Climate Sounder Observations

AbstractThe stability of the residual carbon dioxide cap near the south pole of Mars is currently not well understood. The cap's survival depends on its radiation budget, controlled by the visible albedo and infrared emissivity. We investigated the role of CO2 snowfall in altering the albedo and emissivity, leading to the observed asymmetry in the net CO2 accumulation at the two poles. Uncontaminated snowfall increases albedo, and lowers emissivity, due to scattering by optically thick clouds and granular surface deposits. Data from the Mars Climate Sounder (MCS) show that fall and winter snowfall is correlated with higher springtime albedo at both poles. For the seasonal CO2 deposits in each polar region >60° latitude, we find mean albedo values of 0.39 in the north and 0.51 in the south, and winter 32‐μm emissivity values of 0.84 in the north and 0.87 in the south. Using a radiative transfer model and the MCS data, we find that the north polar deposits have ∼10× higher dust content than those in the south, explaining the ∼31% lower albedo of the north seasonal cap during spring. Our model shows that greater amounts of snowfall can explain the ∼4% lower emissivity of the north polar seasonal cap. These findings demonstrate that winter snowfall and dust transport affect the composition of Mars' seasonal ice caps and polar energy balance. Snowfall and dust loading are therefore important in modeling the CO2 cycle on Mars, as well as the planet's long‐term climate variations.

Planet Four: Probing springtime winds on Mars by mapping the southern polar CO2 jet deposits

The springtime sublimation process of Mars’ southern seasonal polar CO2 ice cap features dark fan-shaped deposits appearing on the top of the thawing ice sheet. The fan material likely originates from the surface below the ice sheet, brought up via CO2 jets breaking through the seasonal ice cap. Once the dust and dirt is released into the atmosphere, the material may be blown by the surface winds into the dark streaks visible from orbit. The location, size and direction of these fans record a number of parameters important to quantifying seasonal winds and sublimation activity, the most important agent of geological change extant on Mars. We present results of a systematic mapping of these south polar seasonal fans with the Planet Four online citizen science project. Planet Four enlists the general public to map the shapes, directions, and sizes of the seasonal fans visible in orbital images. Over 80,000 volunteers have contributed to the Planet Four project, reviewing 221 images, from Mars Reconnaissance Orbiter’s HiRISE (High Resolution Imaging Science Experiment) camera, taken in southern spring during Mars Years 29 and 30. We provide an overview of Planet Four and detail the processes of combining multiple volunteer assessments together to generate a high fidelity catalog of ∼ 400000 south polar seasonal fans. We present the results from analyzing the wind directions at several locations monitored by HiRISE over two Mars years, providing new insights into polar surface winds.

The role of snowfall in forming the seasonal ice caps of Mars: Models and constraints from the Mars Climate Sounder

Wintertime observations of the martian polar regions by orbiting spacecraft have provided evidence for carbon dioxide clouds, which measurably alter the polar energy budget and the annual CO2 cycle. However, it has remained unclear whether snowfall contributes a substantial quantity to the accumulating seasonal ice caps. We develop models to constrain precipitation rates based on observations of south polar CO2 clouds by the Mars Climate Sounder (MCS), and show that snowfall contributes between 3% and 20% by mass to the seasonal deposits at latitudes 70–90°S. The lower bound on this estimate depends on a minimum effective cloud particle size of ∼50μm, derived by comparing the short lifetimes (less than a few hours) of some clouds with calculated sedimentation velocities. Separate constraints from infrared spectra measured by MCS suggest CO2 cloud particles in the size range 10–100μm. Snow particles are not likely to re-sublime before reaching the surface, because the lower atmosphere in this region remains near saturation with respect to CO2. Based on cooling rate calculations, snowfall originating below 4km altitude likely contributes a comparable or greater amount to the seasonal deposits than the rest of the atmosphere. Due to the positive feedback between cloud particle number density and radiative cooling, CO2 snow clouds should propagate until they become limited by the availability of condensation nuclei or CO2 gas. Over the south polar residual cap, where cloud activity is greatest, atmospheric radiative cooling rates are high enough to offset heat advected into the polar regions and maintain consistent snowfall. At latitudes of 60–80°S the lower atmosphere tends to be slightly sub-saturated and rapid cooling by mechanical lift driven by orography or convergent flow may be required to initiate a snowstorm, consistent with the more sporadic clouds observed by MCS in this region, and their correlation with topographic features. Snowfall and accumulation at the surface are found to be inevitable consequences of the polar energy budget, unless advection redistributes heat from lower latitudes in much greater quantities than expected.

Carbon dioxide ice clouds, snowfalls, and baroclinic waves in the northern winter polar atmosphere of Mars

The formation of CO2 ice clouds in the northern winter polar atmosphere of Mars and their relation to baroclinic planetary waves, which dominate local dynamics, are studied using a general circulation model. The simulation shows that clouds are formed at altitudes of up to ∼ 40 km, and their occurrence correlates to a large degree with the cold phases of transient planetary waves. Ice particles formed up to ∼ 20 km can reach the surface in the form of snowfall in certain longitude regions, while in others, these particles likely sublimate in the lower warmer atmospheric layers. The simulation suggests that about a half of the seasonal ice cap is created by CO2 snow, while the remaining half by direct condensation on the surface. Thus, the occurrences of ice clouds and rates of deposition are closely linked to traveling planetary waves, indicating the possibility for the reliable forecasts of CO2 snow storms.

Observations of the northern seasonal polar cap on Mars III: CRISM/HiRISE observations of spring sublimation

We analyze a series of targeted CRISM and HiRISE observations of seven regions of interest at high latitudes in the Northern polar regions of Mars. These data allow us to investigate the temporal evolution of the composition of the seasonal ice cap during spring, with a special emphasis on peculiar phenomena occurring in the dune fields and in the vicinity of the scarps of the North Polar Layered Deposits (NPLDs). The strength of the spectral signature of CO2 ice continuously decreases during spring whereas the one of H2O ice first shows a strong increase until Ls=50°. This evolution is consistent with a scenario previously established from analysis of OMEGA data, in which a thin layer of pure H2O ice progressively develops at the surface of the volatile layer. During early spring (Ls<10°), widespread jet activity is observed by HiRISE while strong spectral signatures of CO2 ice are detected by CRISM. Later, around Ls=20–40°, activity concentrates at the dune fields where CRISM also detects a spectral enrichment in CO2 ice, consistent with “Kieffer’s model” (Kieffer, H.H. [2007]. J. Geophys. Res. 112, E08005. doi:10.1029/2006JE002816) for jet activity. Effects of wind are prominent across the dune fields and seem to strongly influence the sublimation of the volatile layer. Strong winds blowing down the scarps could also be responsible for the significant spatial and temporal variability of the surface ice composition observed close to the NPLD.

Locations of thin liquid water layers on present-day Mars

CRISM indicates the presence of water ice patches in Richardson crater, located on Mars’ southern polar region at the area of the seasonal ice cap. Numerical simulations suggest that the maximum daytime temperature of the ice at these locations is between 195 and 220K during local spring. Previous studies suggest that at these temperatures liquid interfacial water could be present. Here, for the first time, we provide an example where the environmental conditions allow for the formation of such liquid films on present day Mars at the southern hemisphere. The upper bound estimated H2O loss during the presence of these water ice patches is approximately 30μm between Ls=200 and 220, though it may be as low as 0.1μm depending on the ambient water vapor. The upper bound value is larger than the expected condensation thickness in autumn; however, it may still be realistic due to CO2 gas jet generated deposition and possible subsequent accumulation on mineral grains. The presence of this interfacial water may have impact on local chemical processes along with astrobiological importance.

Carbon dioxide snow clouds on Mars: South polar winter observations by the Mars Climate Sounder

We present south polar winter infrared observations from the Mars Climate Sounder (MCS) and test three hypotheses concerning the origins of “cold spots”: regions of anomalously low infrared brightness temperatures, which could be due to enrichment in non‐condensable gases, low‐emissivity surface frost, or optically thick CO2 clouds. Clouds and surface frosts have been historically difficult to distinguish, but the unique limb sounding capability of MCS reveals extensive tropospheric CO2clouds over the cold spots. We find that both clouds and surface deposits play a significant role in lowering the infrared emissivity of the seasonal ice cap, and the granular surface deposits are likely emplaced by snowfall. Surface temperatures indicate the polar winter atmosphere is enriched by a factor ∼5–7 in non‐condensable gases relative to the annual average, consistent with earlier gamma ray spectrometer observations, but not enough to account for the low brightness temperatures. A large ∼500‐km diameter cloud with visible optical depth ∼0.1–1.0 persists throughout winter over the south polar residual cap (SPRC). At latitudes 70–80°S, clouds and low emission regions are smaller and shorter‐lived, probably corresponding to large‐grained “channel 1” clouds observed by the Mars Orbiter Laser Altimeter. Snowfall over the SPRC imparts the lowest emissivity in the south polar region, which paradoxically tends to reduce net accumulation of seasonal CO2 by backscattering infrared radiation. This could be compensated by the observed anomalously high summertime albedo of the SPRC, which may be related to small grains preserved in a rapidly formed snow deposit.

Climate of Extraterrestrial Planets with Oceans and Carbonate-Silicate Geochemical Cycle Under Various Obliquities

AbstractWe systematically investigated the climate of water-rich terrestrial planets with a negative feedback mechanism of carbonate-silicate geochemical cycle against the climate under various obliquities and semi-major axes. We found that, while the permanent ice-cap mode (partially ice-covered throughout the year) and the seasonal ice-cap mode (partially ice-covered seasonally) exist stably at low obliquity conditions, the ranges of semi-major axis for these climate modes shrink and finally disappear with an increase of obliquity. When carbonate-silicate geochemical cycle is taken into account, the ranges of semi-major axis for all the climate modes expand at any obliquities, compared with the cases without carbonate-silicate geochemical cycle, indicating that the carbonate-silicate geochemical cycle strongly stabilizes the climate for the planets with any obliquities inside the habitable zone.

Spatial distributions and seasonal variations of features related to a venting process at high southern latitudes observed by the MOC camera

A distinctive terrain named cryptic region which is characterized by regions of low albedo and low temperature has been identified on the Martian south polar cap. In this zone, many fan- and spider-shaped features of km-scale appeared following the sublimation of the CO 2 frost layer. These peculiar features were apparently caused by a wind-blown system of dust-laden jets. During the warming period starting at L s∼180°, the seasonal ice cap regresses and fans and spiders appear in sequence. These surface features are repeatable events that tend to occupy the same areas from year to year. In this study, we use the Mars Orbiter camera (MOC) narrow-angle images to produce a statistical study of the time distributions of the fans and spiders as functions of L s and as functions of the topography. The time variations and spatial distributions of these features are further correlated with the CO 2 ice coverage measured by the Mars Orbiter laser altimeter (MOLA) instrument. We have documented that most of the fans are found in the early spring with L s<230° and the fans and spiders coexist at L s=250°±20°. It is also found that there is a strong dependence on latitude and altitude with fans and spiders most often observed at high latitude (>83°S) and high altitude (>2500 m). Our statistical result also indicates that the occurrence of fans is highly correlated with the thickness of the CO 2 frost thus providing support for the venting model.

Surface dust redistribution on Mars as observed by the Mars Global Surveyor and Viking orbiters

The variation of surface dust coverage on Mars is mapped using Mars Global Surveyor (MGS) and Viking albedo data. Albedo is shown to correlate well with spectrally derived measurements of surface dust abundance and is subsequently used to gauge dust coverage. Atmospheric aerosols modify the albedo observed from orbit, complicating this analysis. However, opacity cycles are highly repeatable, and simultaneous, independent records of aerosol opacities are available to isolate their impact. The MGS albedo and imaging data contain global coverage on a daily basis, allowing the relationship between dust cover and specific meteorological events to be elucidated. The 2001 global dust storm produced the largest changes in surface dust coverage during the MGS mission. Other processes yielding significant changes include seasonal cap‐edge winds, seasonally varying regional winds, local/regional dust storms, and extratropical cyclones. Dust devils and ongoing, small‐scale dust lifting do not appear to significantly modify the global patterns of dust cover. Finally, we show that the apparent long‐term darkening of the southern mid and high latitudes between the Viking and MGS eras is largely a consequence of the timing of image acquisition relative to global dust storms and surface dust “cleaning” by the seasonal ice cap; it does not represent a steady decadal‐scale, secular change. In fact, following the 2001 global dust storm, in late southern spring, the southern hemisphere was brighter in MGS than in Viking data. This study reveals albedo to be a dynamic, climatological variable for Mars, similar to sea‐surface temperature for terrestrial meteorology and climate.

CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap

The martian polar caps are among the most dynamic regions on Mars, growing substantially in winter as a significant fraction of the atmosphere freezes out in the form of CO2 ice. Unusual dark spots, fans and blotches form as the south-polar seasonal CO2 ice cap retreats during spring and summer. Small radial channel networks are often associated with the location of spots once the ice disappears. The spots have been proposed to be simply bare, defrosted ground; the formation of the channels has remained uncertain. Here we report infrared and visible observations that show that the spots and fans remain at CO2 ice temperatures well into summer, and must be granular materials that have been brought up to the surface of the ice, requiring a complex suite of processes to get them there. We propose that the seasonal ice cap forms an impermeable, translucent slab of CO2 ice that sublimates from the base, building up high-pressure gas beneath the slab. This gas levitates the ice, which eventually ruptures, producing high-velocity CO2 vents that erupt sand-sized grains in jets to form the spots and erode the channels. These processes are unlike any observed on Earth.

Surface temperature of Martian regolith with polygonal features: influence of the subsurface water ice

This work is an extension of our previous paper (Icarus 160 (2002) 73), where we analyzed seasonal cycle of condensation and sublimation of CO 2 ice in the polygonal features. Such structures are visible in several images of the southern polar region, obtained by Mars Orbiter Camera on board the Mars Global Surveyor. In images taken during the end of the southern spring, when the surrounding surface is free of the seasonal frost, CO 2 ice still appears to be present within the polygonal troughs. In the previous paper we described how the thickness of CO 2 ice in the troughs evolves throughout the seasons. Here, we consider the influence of this cycle on the spatially averaged surface temperature, which can be determined from the orbital IR measurements. We model data from the Thermal Emission Spectrometer (TES) on Mars Global Surveyor to constrain the physical properties of the regolith in general and water ice content in particular. We have found that the spring evolution of the spatially averaged surface temperature of the polygonal regions occurs in several distinctive steps. After recession of the seasonal ice cap, temperature increases rapidly no longer being controlled by the condensation point of the surface CO 2 ice. During this first phase it reaches a value of about 20 K lower than that expected for smooth surface. This effect is observed in TES data and is due to the fact that CO 2 ice within the polygonal troughs is still present at this stage. The sublimation of CO 2 ice within the troughs and the resultant increase of the surface temperature to the expected value of about 275 K takes another 20 sols. The exact time when and in which way this CO 2 ice sublimes completely away depends on several parameters. Our simulations suggest that the most important parameter is the thermal conductivity of the regolith which in turn depends on the amount of the subsurface water ice. Thus, measurements of the surface temperature of regions covered with polygonal features can be used to deduce the content of water ice in the regolith.

Investigation of the nature and stability of the Martian seasonal water cycle with a general circulation model

We describe the first use of a general circulation model to study the Martian water cycle. Water is treated as a passive tracer, except for ice‐albedo coupling. The model is used to assess which mechanisms and water reservoirs are critical to the seasonal evolution of water and specifically the attainment of an interannually repeatable steady state. The model comes to a reasonable steady state with active surface ice and atmospheric vapor and ice reservoirs. A regolith is not necessary. The mechanism of equilibration results from independent parameters controlling the transport of water between the northern polar and the extratropical atmospheres at different seasons. Water export from the northern summer pole results from weak mixing across a strong vapor gradient, dependent upon northern cap temperatures. Import at other seasons depends on stronger mixing and weak vapor gradients, which are history dependent. Equilibration is achieved when the fluxes balance, minus a small net loss to the south. We find that with a southern residual CO2 cap, the water cycle cannot be completely closed. We conclude that the northern summer cap temperature determines the bulk humidity of the atmosphere, all else being equal. We proceed to show that a water cap exposed in southern summer would be unstable with respect to the north for dynamical as well as thermal reasons. At high obliquity (45°), much higher vapor abundances result in more widespread surface ice with seasonal ice caps overlapping in the equinoctial subtropics, producing year‐round stability of water ice just north of the equator.

Modelling of the combined late-winter ice cap edge and slope winds in Mars Hellas and Argyre regions

Towards the end of southern hemisphere winter (Ls ≈ 180°) the Martian southern polar cap extends equatorward to 40°S and covers at least, the southern slopes of the Hellas and Argyre impact basins. Subsequently, during retreat of the seasonal ice cap, varying configurations of ice coverage on these slopes occur. Since both sloping topography and ice-edge effects can independently drive mesoscale circulations, the superposition of these two processes may then generate interesting wind patterns. A set of numerical experiments has been performed with the University of Helsinki 2-D Mars Mesoscale Circulation Model (MMCM) in order to study the characteristics of circulations driven by these combined forcings. A model-centre latitude of 57°S and a slope angle of 0.6°, both representative of Hellas southern slope, are used. When compared with the winds arising in the ice-free slope case, ice coverage in the upper extent of the slope results in diminished upslope (daytime) winds, while the down-slope (nighttime) flow is enhanced. Ice coverage in the lower section of the slope in turn causes enhanced upslope (daytime) and attenuated downslope (nocturnal) flows. This arises due to the daytime off-ice near-surface flow induced by the thermal contrast at the ice cap edge. The surface winds are persistently downslope over a fully ice-covered slope. Inclusion of atmospheric dust (τ = 0.3) reduces the ice-edge forcing. In comparison with the dust-free situation, the resulting circulation is almost unchanged in the case of ice-covered upper part of the slope, in the opposite case the daytime flow is attenuated and the nocturnal downslope flow enhanced. When the entire slope is ice-covered, the flow is amplified due to the increased direct atmospheric heating. Inclusion of a large scale circulation component (7 m⧸s southerly wind) in conjunction with an ice-covered slope top results in the generation of a downslope windstorm (föhn, or bora-type of event) with near surface winds exceeding 30 m⧸s. Winds of this magnitude, not realised in any of the other experiments, approach speeds deemed capable of lifting dust from the surface.