Perennial Cap Research Articles

The Detectability Limit of Organic Molecules Within Mars South Polar Laboratory Analogs

AbstractA series of laboratory experiments was carried out in order to generate a diagnostic spectrum for polycyclic aromatic hydrocarbons (PAHs) of astrobiological interest in the context of the Martian South Polar Residual Cap (SPRC), to establish PAH spectral features more easily detectable in CO2 ice (mixed with small amounts of H2O ice) than the previously reported absorption feature at 3.29 µm in order to constrain their detectability limit. There is currently no existing literature on PAH detection within SPRC features, making this work novel and impactful given the recent discovery of a possible subglacial lake beneath the Martian South Pole. Although they have been detected in Martian meteorites, PAHs have not been detected yet on Mars, possibly due to the deleterious effects of ultraviolet radiation on the surface of the planet. SPRC features may provide protection to fragile molecules, and this work seeks to provide laboratory data to improve interpretation of orbital remote sensing spectroscopic imaging data. We also ascertain the effect of CO2 ice sublimation on organic spectra, as well as provide PAH reference spectra in mixtures relevant to Mars. A detectability limit of ∼0.04% has been recorded for observing PAHs in CO2 ice using laboratory instrument parameters emulating those of the Compact Reconnaissance Imaging Spectrometer for Mars, with new spectral slope features revealed between 0.7 and 1.1 µm, and absorption features at 1.14 and most sensitively, at 1.685 µm. Mars regolith analog mixed with a concentration of 1.5% PAHs resulted in no discernible organic spectral features. These detectability limits measured in the laboratory are discussed and extrapolated to the effective conditions on the Mars South Polar Cap in terms of dust and water ice abundance and CO2 ice grain size for both the main perennial cap and the H2O ice‐dust sublimation lag deposit.

A comparison of Martian north and south polar cold spots and the long‐term effects of the 2001 global dust storm

In the 1970s, Viking and Mariner observed areas in the polar regions of Mars with winter brightness temperatures below the expected kinetic temperatures for CO2 ice sublimation. These areas have since been termed “cold spots” and have been identified as surface deposits of CO2 atmospheric condensates and, occasionally, active CO2 storms. Three Mars years of data from the Mars Global Surveyor Thermal Emission Spectrometer were used to observe autumn and winter cold spot activity. In this study, cold spots that occur near and on the southern perennial cap were compared to those found near or on the northern perennial cap. On the southern perennial cap, cold spots associated with topographic features (induced by orographic lifting) were less common than cold spots independent of topography, similar to the north. However, the cold spots in the south lasted longer than those observed in the north. There is also evidence that cold spot formation in the south was affected by the global dust storm of 2001, even though the dust storm occurred during the southern spring and summer seasons. Prior to the dust storm, the amount of overall cold spot activity closer to the perennial cap increased and the average CO2 grain size for most of the cold spots increased as well. Following the dust storm, the majority of cold spots in the south increased in size and duration but they did not form north of 62°S latitude, whereas, in other years, cold spots formed as far north as 48°S.

On the mystery of the perennial carbon dioxide cap at the south pole of Mars

A perennial ice cap has long been observed near the south pole of Mars. The surface of this cap is predominantly composed of carbon dioxide ice. The retention of a CO2 ice cap results from the surface energy balance of the latent heat, solar radiation, surface emission, subsurface conduction, and atmospheric sensible heat. While models conventionally treat surface CO2 ice using constant ice albedos and emissivities, such an approach fails to predict the existence of a perennial cap. Here we explore the role of the insolation‐dependent ice albedo, which agrees well with Viking, Mars Global Surveyor, and Mars Express albedo observations. Using a simple parameterization within a general circulation model, in which the albedo of CO2 ice responds linearly to the incident solar insolation, we are able to predict the existence of a perennial CO2 cap at the observed latitude and only in the southern hemisphere. Further experiments with different total CO2 inventories, planetary obliquities, and surface boundary conditions suggest that the location of the residual cap may exchange hemispheres favoring the pole with the highest peak insolation.

Sublimation kinetics of CO 2 ice on Mars

Dynamic models of the martian polar caps are in abundance, but most rely on the assumption that the rate of sublimation of CO 2 ice can be calculated from heat transfer and lack experimental verification. We experimentally measured the sublimation rate of pure CO 2 ice under simulated martian conditions as a test of this assumption, developed a model based on our experimental results, and compared our model's predictions with observations from several martian missions (MRO, MGS, Viking). We show that sun irradiance is the primary control for the sublimation of CO 2 ice on the martian poles with the amount of radiation penetrating the surface being controlled by variations in the optical depth, ensuring the formation and sublimation of the seasonal cap. Our model confirmed by comparison of MGS-MOC and MRO-HiRISE images, separated by 2–3 martian years, shows that ∼0.4 m are currently being lost from the south perennial cap per martian year. At this rate, the ∼2.4-m-thick south CO 2 perennial cap will disappear in about 6–7 martian years, unless a short-scale climatic cycle alters this rate of retreat.

Spatial and temporal distributions of Martian north polar cold spots before, during, and after the global dust storm of 2001

In the 1970s, Mariner and Viking observed features in the Mars northern polar region that were a few hundred kilometers in diameter with 20 μm brightness temperatures as low as 130 K (considerably below CO2 ice sublimation temperatures). Over the past decade, studies have shown that these areas (commonly called “cold spots”) are usually due to emissivity effects of frost deposits and occasionally to active CO2 snowstorms. Three Mars years of Mars Global Surveyor Thermal Emission Spectrometer data were used to observe autumn and wintertime cold spot activity within the polar regions. Many cold spots formed on or near scarps of the perennial cap, probably induced by adiabatic cooling due to orographic lifting. These topographically associated cold spots were often smaller than those that were not associated with topography. We determined that initial grain sizes within the cold spots were on the order of a few millimeters, assuming the snow was uncontaminated by dust or water ice. On average, the half‐life of the cold spots was 5 Julian days. The Mars global dust storm in 2001 significantly affected cold spot activity in the north polar region. Though overall perennial cap cold spot activity seemed unaffected, the distribution of cold spots did change by a decrease in the number of topographically associated cold spots and an increase in those not associated with topography. We propose that the global dust storm affected the processes that form cold spots and discuss how the global dust storm may have affected these processes.

Distribution of the ices exposed near the south pole of Mars using Thermal Emission Imaging System (THEMIS) temperature measurements

Understanding the present and past water cycle on Mars requires an accurate knowledge of the distribution and amount of H2O available near the surface. In this article, we present a map of the distribution of the surface material exposed between 87°S and 70°S in the summer (e.g., CO2 and H2O ices, dust) based on temperature measurements made by Thermal Emission Imaging System (THEMIS). Our compositional map (100 m per pixel) is in good agreement with spectral mapping returned by Observatoire pour la minéralogie, l'Eau, la Glace et l'activité (OMEGA) and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Exposed water ice covers a total surface area of approximately 40,000 km2. A large fraction of the water ice is exposed at the periphery of the CO2 cap. An approximately 25,000‐km2 large patch centered at 83.5°S and 345°E is discovered and represents the largest exposure of water ice in the southern hemisphere. It is not located on the south polar layered deposits but on the surrounding mantled terrains. THEMIS VIS, MOC and HiRISE images indicate that the surface roughness of exposed water ice terrains is typically lower than that of the surrounding dust. Polygonal patterns are observed on the water ice but not exclusively. There is a strong correlation between the surface albedo, the composition of the exposed material, and the timing of the initial seasonal CO2 frost deposition and final removal. These exposed water ice outcrops are not stable in the present environment and lose a vertical layer of tens to hundreds of micrometer a year to the atmosphere. The spike of water vapor above the south pole during the southern summer occurs while the water ice is still covered by a layer of seasonal CO2 frost, indicating that the sublimation of the exposed water ice is not the main contributor of vapor for the southern atmosphere. As water ice is not stable, it may indicate the past location of part of the CO2 perennial cap that has been eroded.

PFS/MEX observations of the condensing CO 2 south polar cap of Mars

The condensing CO 2 south polar cap of Mars and the mechanisms of the CO 2 ice accumulation have been studied through the analysis of spectra acquired by the Planetary Fourier Spectrometer (PFS) during the first two years of ESA's Mars Express (MEX) mission. This dataset spans more than half a martian year, from Ls ∼ 330 ° to Ls ∼ 194 ° , and includes the southern fall season which is found to be extremely important for the study of the residual south polar cap asymmetry. The cap expands symmetrically and with constant speed during the fall season. The maximum extension occurs sometime in the 80°–90° Ls range, when the cap edges are as low as −40° latitude. Inside Hellas and Argyre basins, frost can be stable at lower latitudes due to the higher pressure values, causing the seasonal cap to be asymmetric. Within the seasonal range considered in this paper, the cap edge recession rate is approximately half the rate at which the cap edge expanded. The longitudinal asymmetries reduce during the cap retreat, and disappear around Ls ∼ 145 ° . Two different mechanisms are responsible for CO 2 ice accumulation during the fall season, especially in the 50°–70° Ls range. Here, CO 2 condensation in the atmosphere, and thus precipitation, is allowed exclusively in the western hemisphere, and particularly in the longitudinal corridor of the perennial cap. In the eastern hemisphere, the cap consists mainly of CO 2 frost deposits, as a consequence of direct vapor deposition. The differences in the nature of the surface ice deposits are the main cause for the residual south polar cap asymmetry. Results from selected PFS orbits have also been compared with the results provided by the martian general circulation model (GCM) of the Laboratoire de Météorologie dynamique (LMD) in Paris, with the aim of putting the observations in the context of the global circulation. This first attempt of cross-validation between PFS measurements and the LMD GCM on the one hand confirms the interpretation of the observations, and on the other hand shows that the climate modeling during the southern polar night on Mars is extremely sensitive to the dynamical forcing.

Dust aerosols above the south polar cap of Mars as seen by OMEGA

The time evolution of atmospheric dust at high southern latitudes on Mars has been determined using observations of the south seasonal cap acquired in the near infrared (1–2.65 μm) by OMEGA/Mars Express in 2005. Observations at different solar zenith angles and one EPF sequence demonstrate that the reflectance in the 2.64 μm saturated absorption band of the surface CO 2 ice is mainly due to the light scattered by aerosols above most places of the seasonal cap. We have mapped the total optical depth of dust aerosols in the near-IR above the south seasonal cap of Mars from mid-spring to early summer with a time resolution ranging from one day to one week and a spatial resolution of a few kilometers. The optical depth above the south perennial cap is determined on a longer time range covering southern spring and summer. A constant set of optical properties of dust aerosols is consistent with OMEGA observations during the analyzed period. Strong variations of the optical depth are observed over small horizontal and temporal scales, corresponding in part to moving dust clouds. The late summer peak in dust opacity observed by Opportunity in 2005 propagated to the south pole contrarily to that observed in mid spring. This may be linked to evidence for dust scavenging by water ice-rich clouds circulating at high southern latitudes at this season.

Deposition of CO2and erosion of the Martian south perennial cap between 1972 and 2004: Implications for current climate change

We present a comparison of Mariner 9, Viking, Mars Orbiter Camera, and Mars Odyssey Thermal Emission Imaging System visible and infrared images of the south perennial cap taken during the summer between 1972 and 1977 and between 1999 and 2004. Between 1972 and 1977, the lateral expansion and the reduction of the patchiness of the cap indicate that CO2ice was deposited. During subsequent years, the distribution of the CO2ice has been modified by the erosion and deposition of CO2on the cap at the kilometer scale. Because vertical deposition of CO2on the cap is only detected when the patchiness decreases as observed between 1972 and 1977, present deposition on the cap would not be detectable and cannot be ruled out. Therefore the current lateral erosion of the walls of the CO2cap at the meter scale is not a sufficient observation to conclude that the climate is changing on Mars. The present mass balance of the cap cannot be estimated from available south polar cap images, and it is not possible to determine with the existing data if the climate of Mars is changing.

Albedo models for the residual south polar cap on Mars: Implications for the stability of the cap under near-perihelion global dust storm conditions

It is uncertain whether the residual (perennial) south polar cap on Mars is a transitory or a permanent feature in the current Martian climate. While there is no firm evidence for complete disappearance of the cap in the past, clearly observable changes have been documented. Observations suggest that the perennial cap lost more CO 2 material in the spring/summer season prior to the Mariner 9 mission than in those same seasons monitored by Viking and Mars Global Surveyor. In this paper we examine one process that may contribute to these changes—the radiative effects of a planet encircling dust storm that starts during late Martian southern spring on the stability of the perennial south polar cap. To approach this, we model the radiative transfer through a dusty planetary atmosphere bounded by a sublimating CO 2 surface. A critical parameter for this modeling is the surface albedo spectrum from the near-UV to the thermal-IR, which was determined from both space-craft and Earth-based observations covering multiple wavelength regimes. Such a multi-wavelength approach is highly desirable since one spectral band by itself cannot tightly constrain the three-parameter space for polar surface albedo models, namely photon “scattering length” in the CO 2 ice and the amounts of intermixed water and dust. Our results suggest that a planet-encircling dust storm with onset near solstice can affect the perennial cap's stability, leading to advanced sublimation in a “dusty” year. Since the total amount of solid CO 2 removed by a single storm may be less than the total CO 2 thickness, a series of dust storms would be required to remove the entire residual CO 2 ice layer from the south perennial cap.

Mars Surface Diversity as Revealed by the OMEGA/Mars Express Observations

The Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) investigation, on board the European Space Agency Mars Express mission, is mapping the surface composition of Mars at a 0.3- to 5-kilometer resolution by means of visible-near-infrared hyperspectral reflectance imagery. The data acquired during the first 9 months of the mission already reveal a diverse and complex surface mineralogy, offering key insights into the evolution of Mars. OMEGA has identified and mapped mafic iron-bearing silicates of both the northern and southern crust, localized concentrations of hydrated phyllosilicates and sulfates but no carbonates, and ices and frosts with a water-ice composition of the north polar perennial cap, as for the south cap, covered by a thin carbon dioxide-ice veneer.

Evolution of Martian polar landscapes: Interplay of long‐term variations in perennial ice cover and dust storm intensity

Recent Viking Orbiter observations of the Martian polar layered deposits, thought to be thick accumulations of dust, have provided new insights into the mechanisms of topographic evolution of these terrains. Here we report the discovery of wavelike topographic relief on north polar surfaces previously supposed to be flat plains. These undulations have wavelengths of a few kilometers and amplitudes of probably only a few tens of meters. The crests and troughs extend laterally up to hundreds of kilometers and have radii of curvature in plan of several hundred kilometers. Much steeper slopes, exhibiting fine terracing due to layering in the deposits, occur within troughs that separate areas of undulating plains and that parallel in strike the topographic grain of the undulations. In the model presented here, both the undulations on the plains surfaces and the layers that appear in cross section on steeper slopes are attributed to the same basic cause—periodic episodes of dust accumulation. These are probably associated with variations in global dust storm intensity and perennial polar ice cap size, brought about by periodic changes in the eccentricity of the Martian orbit and the orientation of its spin axis with respect to the ecliptic. In earlier work it has been proposed that each layer formed in a single cycle of deposition. Specifically, it was argued that dust from global dust storms is swept from the atmosphere by the accumulating annual ice and retained in the place of deposition by a matrix of perennial ice. Here we propose a mechanism for generating the undulating topography at the edge of the advancing or receding perennial ice cap in an environment of periodic deposition of dust. We have recognized several families of topographic undulations with markedly different orientations. In regions where two differently oriented sets of undulations overlap, the wave crests and troughs break up into mounds and shallow basins. We attribute this type of landscape to the occurrence of two passages of the perennial ice cap edge across the same area with the ice cap margin oriented differently on the two occasions. This implies past variations in both the size of the perennial polar cap and the location of its center if we assume that the shape of the perennial cap has not varied drastically from circular. Also reported here is evidence for a genetic relationship between undulating plains and the curvilinear defrosted bands which are the dominant physiographic features of the summer residual polar cap. We establish here that most of these curvilinear features are topographic troughs, and we propose that they are derived from a primary undulating landscape by an insolation‐controlled process. First, areas of steeper slopes within the undulating terrain lose their perennial ice cover because of a change in insolation conditions. Then, these slopes evolve into topographic troughs because in the absence of perennial ice cover they are unable to trap deposited dust. We attribute the planimetric complexity of the troughs—sharp changes in orientation, lateral offsets, and branching—to derivation from intersecting sets of primary topographic undulations with different orientations. In summary, undulating landscapes and trough complexes of the Martian polar layered deposits appear to record both lateral migration and expansion and contraction of perennial ice caps over geological history. Detailed mapping of the planimetric and stratigraphic relationships of the polar landscapes may ultimately provide the basis for a detailed model of landscape evolution and a historical record of variations on the size and location of the perennial ice cap.

Wind erosion in the Martian polar regions

Photographic evidence suggests that many topographic features of the polar regions of Mars may have been sculptured by wind action. In the case of the pitted terrain, this is established by relating the orientations of elongate basins and grooves to patterns of albedo markings that are generally accepted to be wind-formed. Small-scale flutes and other textures of laminated terrain recognized outside the perennial ice cap in the south polar region are also attributed to eolian action. Winds flowing off the polar cap and spiraling toward the west seem to be responsible. More speculatively the large-scale circumpolar features which underlie the residual polar caps may be related to wind erosion. Rotational motion localized on circular segments of the perennial frost deposits are proposed as a mechanism.