U14A-01 INVITED
Geologic Setting and Soil Physical Properties of the Mars Phoenix Landing Site
The Phoenix Lander touched down ~30 km to the southwest (68.22 N, 234.25 E) of the Amazonian aged, 10 km wide, bowl-shaped Heimdall impact crater. The lander is sitting on ejecta deposits from the Heimdall event that were emplaced as a ground hugging, volatile rich flow, interpreted to be a consequence of impact into icy soil and bedrock. The ejecta deposits have been differentially eroded by aeolian activity and reworked by permafrost-related processes into polygonal ground. Rock abundances are low relative to most of Mars and rocks are concentrated in troughs in between polygons and tend to be evenly spaced, implying an on-going process of polygon formation. Rocks range from tabular to rounded in shape and massive to vesicular in texture. Very few aeolian features (e.g., ripples or ventifacted rock surfaces) are evident, in contrast to the other Mars landing sites. Based on analyses of Mars Reconnaissance Orbiter CRISM hyperspectral data (~0.4 to 4 micrometers) and Phoenix observations, the surface cover is dominated by basaltic soils (sandy silts) and ferric-rich dust, with only contribution from minerals formed under aqueous conditions. The soil is cloddy and adheres to spacecraft surfaces, probably because of electrostatic charging. Densely-cemented icy soil is found within a few centimeters of the surface and once exposed and allowed to warm in the sunlight the ice eventually sublimates into the atmosphere, leaving behind soil lag deposits. The Phoenix landing site is unique relative to the other five sites (two Viking Landers, Pathfinder, Spirit and Opportunity rovers) because of the high latitude, location on relatively young ejecta emplaced as a volatile-rich flow, and because the ice table depth is predicted to have varied from centimeters to as much as a meter beneath the surface during orbital shifts associated with Martian Milankovitch cycles and consequent insolation over the northern latitudes.
U14A-02 INVITED
Phoenix landing site and sample context images from the Surface Stereo Imager
Phoenix landed in the northern plains of Mars in an area with low rock abundance dominated by few-meter- scale polygonal patterned ground with decimeter scale troughs. The Phoenix Surface Stereo Imager (SSI) provides geomorphic and spectral information about the Phoenix landing site for scales that range from site- wide to context for samples analyzed by other Phoenix instruments. The SSI is a multispectral stereo camera with properties that are comparable to the Mars Exploration Rover Pancam. It has MER-heritage 1024x1024 pixel detectors, a 14-degree field of view for individual images, and resolution as high as 1-2 mm for near- field terrain (0.24 mrad/pixel). Images are taken through one of 24 filters, including 13 unique spectral bandpasses, 2 stereo bandpasses, 2 filters paired with lenses for best focus on the lander deck, 6 solar filters for atmospheric dust and water vapor and ice measurements, and 1 polarizer. The stereo separation of the eyes is 15 cm, and the focus and toe-in are optimized at 3 to 3.5 m to support Robotic Arm (RA) operations. SSI can image from the camera bar at -72 degrees to the zenith, and through 360 degrees of azimuth. As with Pancam, panoramic images are built on the ground from a number of individual frames. SSI provided geomorphic information through a set of campaigns. Three major site panoramas were acquired: on sols 1 and 3, a low-resolution monochromatic site panorama provided context for higher- resolution images in the RA workspace; a color-stereo panorama was completed on sol 43; a multispectral high-resolution panorama is currently underway. High resolution detail observations were conducted throughout the mission for high priority targets in and beyond the workspace. These campaigns show a landscape dominated by polygons with typical diameters of 2 to 4 meters. Troughs between the polygons have depths of typically 5-20 cm relative to the polygon centers. Phoenix landed with access to a trough and parts of two polygons within the RA workspace. Beyond the workspace, a nearby hill blocks the view of Heimdal crater, but hills to the south and west provide visual localization. Color information about the site came from the panoramic imaging and multispectral imaging of workspace and other targets. The site is nearly monochromatic, dominated by ferric absorptions in the visible except where the RA uncovered material with a substantially less red slope in the visible. This material was identified as water ice based on the red slope in the visible, a 1-micrometer downturn, and shrinkage and disappearance of small, bright particles on time scales consistent with sublimation of water ice.
U14A-03 INVITED
Preliminary identification of minerals at the Mars Phoenix Landing Site
The Phoenix Lander touched down on May 25, 2008 in an area of the north polar polygonal terrain of Mars identified by the Mars Odyssey GRS to have high concentrations of ice a few centimeters beneath the surface (68.2 N, 234.2 E). Phoenix carries several instruments in its payload that are well suited to the study of secondary minerals. Minerals identified to date include water ice, carbonates, and possible phyllosilicates. Ice was found in two forms: several centimeters beneath dry soil on the edge of the Humpty-Dumpty polygon (Dodo-Goldilocks trench) as a relatively pure ice deposit, and as soil-rich ice beneath the center of the Wonderland polygon (Snow White trench). Ice has been independently identified by its sublimation and formation of a lag deposit, its reflectance spectrum, and by scanning calorimetry in which an endothermic transition near 0 °C was observed accompanied by evolution of water vapor. Based on CRISM orbital hyperspectral data (0.4 to 2.5 micrometers) the surface soils are spectrally dominated by nanophase iron oxides and basaltic sandy silts, consistent with observations from the SSI and optical microscope images. Calcium carbonate has been identified in the soils by an endothermic transition beginning around 700 °C accompanied by evolution of CO2. Another endothermic transition around 700-800 °C accompanied by evolution of H2O and a subsequent exothermic transition around 880 °C suggest the presence of a phase that contains structural Al-OH. Phyllosilicates are one possible phase that may exhibit such a thermal and evolved gas behavior. The lack of SO2 releases from ambient to 1000°C TEGA runs suggests that Mg-, Ca-, and Fe-sulfates are not present in the soils around the landing site. The presence of carbonates in the soils at the Phoenix landing site is supported by the alkaline pH measured by the MECA Wet Chemistry Laboratory. The carbonates appear to be more concentrated at the ice-soil boundary and may have formed by the interaction of CO2-charged liquid water (thin films ) on particle surfaces. The presence of a phase that contains structural Al-OH may also indicate that liquid water interacted with materials in the soil and formed secondary aluminosilicate phases, perhaps phyllosilicates. Carbonates and possible phyllosilicates are strong indicators for aqueous processes and provide support that the patterned ground near the Phoenix landing site may have had conditions favorable for habitability.
U14A-04 INVITED
Discovery of Perchlorate at the Phoenix Landing Site
One of several payload components on the Phoenix Lander, the Microscopy, Electrochemistry, and
Conductivity Analyzer (MECA) is a suite of instruments that includes a microscopy station (optical and atomic
force), four wet chemistry laboratories (WCL), and a soil probe. After the addition of up to 1 cm3 of martian
soil into 25 ml of an aqueous calibration solution, the WCL measures solution cation and anion
concentration, including pH, as well as total conductivity and cyclic voltammetry. With the exception of a
redundant coulombic titration of halides, all cation and anion measurements are made with ion selective
electrodes (ISE). Among the species not directly measured are sulfate and carbonate, which can be inferred
indirectly by the response to acid and Ba additions, and soluble Fe, which can sometimes be detected with
cyclic voltammetry.
Responses from several cation and anion sensors were observed almost immediately upon addition of soil to
the solution. Most striking was a three order-of-magnitude increase of the Hofmeister series sensor, which
could only be explained by a large concentration of the perchlorate ion, ClO4-. Perchlorates are highly water
soluble oxidants, often deliquescent, and some are powerful freezing-point depressors that can form
aqueous brines at mean Martian temperatures appropriate to this region, as low as -70 deg C. This
combination of properties has implications that span the disciplines of geochemistry, atmospheric sciences,
astrobiology, and the potential for future human exploration. An important qualification of any such
discussion, however, is uncertainty about how widespread the distribution of perchlorate may be.
Other WCL findings, including alkaline pH and buffered response to purposeful addition of acid consistent
with the presence of carbonates, will also be summarized.
http://phoenix.lpl.arizona.edu/
U14A-05
The Aqueous Chemistry of the Soils at the Phoenix Landing Site
The MECA Wet Chemistry Laboratory (WCL) analyses on the Phoenix Mars Lander have provided the first direct evidence of the soluble ionic components of the Martian soil. The analyses were performed on samples acquired from the surface (Rosy Red) and at the soil/ice interface approximately 4-5 cm under the surface (Sorceress). Even though the samples are from a rather unique site because of the high polar latitude and the polygon-patterned ground, they present a picture of a geochemical environment different from some previously hypothesized. Addition of 25mL of a water/calibrant solution to approximately 1cc of each of the soil samples resulted in the detection of a variety of ionic species, increased solution conductivity, and a slightly alkaline pH. The major constituent cations identified and quantified to date include Na+, K+, Mg2+, and Ca2+, while the anions included Cl- and ClO4-. Sulfate analysis was performed using a Ba2+ titration method. Even though carbonate and bicarbonate were not directly measured, their presence and quantification is supported by the alkaline pH of the solution, its buffering capacity after the addition of an acid, common ion effects, conductivity, and the modeled equilibrium species distribution of the system. The species distribution resulting from the modeling and consideration of additional interactions; dissolution, precipitation, ion exchange, ads/desorption, charge balance, the behavior over the several hours of monitoring, provided constraints for carbonate speciation and concentration and was used to formulate and test soil simulants. Results from the Thermal and Evolved Gas Analyzer (TEGA) also support the presence of a significant amount of calcite in the soil.
U14A-06
Physical Properties of the Icy Soil at the Phoenix Landing Site
The geomorphological setting of the subpolar terrain at the landing site is characterized by polygonal structures. These structures are generated by long term and periodic cycles of contraction and expansion of the subsurface icy soil. The physical properties of the covering soil layer effectively control the details of this process that has its counterpart on earth in (sub) polar regions including the Siberian tundra and in Antartica. One of the prime science goals of the Phoenix mission is to investigate the physical properties of the icy soil, how these processes are influenced by water vapour diffusion in the regolith and exchange of the water vapour with the atmosphere. It is important to understand these processes on diurnal, seasonal, and climatic time scales. Phoenix landed in the middle of one of the polygons. Its retro rockets cleared the ice table of the polygon underneath the jet assemblies from ca. 5 to 10 cm of loose cloddy regolith. Soil was piled up in the centre. The fact that the soil looked still cloddy similar to that in undisturbed areas suggests strong cohesiveness of the matrix material. The clumps were not destroyed by the blast. Excavated regolith material imaged in the scoop was made up of agglomerates of grains smaller than the best resolution of the Robotic Arm Camera (20 micron). Higher resolution images (4 micron) of the microscope corroborate that the soil is predominantly composed of agglomerates of very small particles with a mean size comparable to those observed in the Martian atmosphere. The Atomic Force Microscope reveals micron sized particles and smaller, partly of plate-like shape, indicating clay like particles. The matrix material of the soil is of reddish colour probably due to iron oxideadmixture. Only about 10% by volume of the soil are most often rounded grains between 40 to 100 micrometers of diameter. Some are glassy resembling micro tektites, and most of these are magnetic. The cohesiveness of the clumps and clods of matrix material is most likely caused by interfacial water, but electrostatic and van der Waals forces could also play a part. The soil also sticks readily to the scoop. Once desiccated in the scoop clumps fall apart further indicating that water was a major agent responsible for the cohesiveness of the soil.
U14A-07
Microscopic Investigation of Martian Soil Samples at the Phoenix Site
We have used the optical and atomic force microscopes (OM and AFM) of the MECA microscopy station on Phoenix (M. Hecht et al., Microscopy Capabilities of the Microscopy, Electrochemistry, and Conductivity Analyzer , JGR accepted for publication) to image samples within reach of the robot arm and delivered to sets of substrates mounted in a sample wheel. For loading the sample, the wheel was pushed out of the MECA enclosure, exposing only one set of substrates: strong and weak magnets, micro-buckets, silicone and silicon featuring grids of micromachined small holes and posts to capture particles. A thickness of up to 200 micrometers of material can be brought into the microscopy station under a leveling blade before the samples are rotated into the field of view of the microscopes as the substrates are tilted from horizontal to vertical. This tilt can cause the loss of a portion of the material depending on the relative strength of the adhesion forces compared to Martian gravity. The time constraints of sample delivery have so far ensured that any ice would have sublimed prior to delivery. From OM images of fully loaded substrates the particles found so far can be very coarsely grouped into three different categories: 1. subrounded strongly magnetic grains, of both a rough and glassy appearance with different shades of yellow, red, brown and black color in a size range of 50 to 100 micrometers, comprising about 10% of the sample volume; 2. small white flecks of a few micrometers in size, about 0.5% of the sample volume; 3. a majority component of a fine, uniformly coloured orange-reddish dust forming agglomerations from a few tens of microns in diameter to below the resolution of the OM with less magnetic attraction than the larger grains. Using populations on more sparsely populated substrates a size distribution could be estimated. The particle size distribution increases with decreasing size until cut off by the 4-micrometer resolution limit of the OM. The AFM confirmed the presence of these smaller particles, down to the submicrometer scale. These often appeared flat and angular. It is hypothesized that the soil observed so far consists of magnetic minerals at various stages of degradation, with the most degraded including a proportion of flattened micrometer-sized clay particles, together with a distinct but small proportion of pale mineral or salt grains.
U14A-08
The Periglacial Landscape and Ground Ice at the Mars Phoenix Landing Site
The Phoenix spacecraft landed in a high-latitude region of Mars rich in subsurface water ice and geologic landforms that evolved in the presence of this ice. Ground ice (subsurface ice and icy soil) at depths of a few centimeters has been found by excavation at the landing site. These ice-table depths are largely in line with pre-landing predictions. As expected, the ice-table depth exhibits some variability due to topographic and surface material effects, though other processes may play a role. The frozen ground is generally densely cemented by ice and widespread under and around the spacecraft. These ice-table characteristics and the current martian climate are ideal for formation of polygonally-patterned ground by seasonal thermal- contraction cracking. Indeed, polygonal ground is ubiquitous throughout the region and northern plains of Mars as seen from orbit. Surface imaging by the Phoenix spacecraft provides the first close up view of these well developed features. These polygons are characterized by perimeter troughs and central mounds. The trough depths range from centimeter to a couple-of-decimeter scale. The dominant polygons outlined by these troughs are small relative to typical terrestrial forms, only a few meters across. As on Earth, small centimeter-scale furrows created by surface fines infiltrating into sub-surface cracks, strongly suggest that the polygon-forming processes are currently active. Variability of the depth along individual troughs and small furrows cross cutting polygon mounds indicate a complex history of polygon formation on more than one size scale. While rock abundances are low, heterogeneously-scattered small rocks are common. Rock distributions suggest that rocks are preferentially concentrated in polygon troughs, consistent with thermal- contraction-based cryoturbation. Few ventifacts are observed suggesting that aeolian erosion of the surface is slower than cryoturbation. The size, morphological characteristics, and development of polygons are consistent with the presence of shallow ground ice and permanently subfreezing temperatures throughout geologically recent times.
U14A-09
Observations of Dust, Ice Water Clouds, and Precipitation in the Atmosphere of Mars
The Phoenix mission has included several instruments for observing the atmosphere of Mars. The measurements include atmospheric temperature, pressure, wind, humidity, optical depth, composition, and imaging. A unique instrument on the Phoenix mission was a lidar that measured the backscatter of pulsed laser light emitted upward into the atmosphere. The lidar measurements of dust provided a view of the structure of the boundary layer, the changes throughout each sol, and over longer time scales with passing weather systems and seasonal progression. The depth of the boundary layer followed a daily cycle with a peak height in the late afternoon. The vertical structure of the atmospheric dust is often remarkably homogeneous, in agreement with modeling, but on occasion there are layers of enhanced scattering that are more difficult to explain. Water ice clouds were detected by the lidar as layers of enhanced signal. The ratio of the extinction and backscatter coefficients was similar to that associated with cirrus clouds on earth. The extinction coefficient derived from the lidar signals was used to estimate the ice water content. Around the time of summer solstice the most prominent clouds were detected at heights above 10 km. As the season progressed and the polar atmosphere started to cool (50 sols past solstice) clouds started to form in the boundary layer. Two separate cloud layers formed each sol around midnight. One cloud at ground level and another at the top of the residual boundary layer (4-6 km). Fall streaks could be clearly seen in the lidar observations, where ice crystals were precipitating toward the ground. The precipitating ice crystals sublimated in the air below the upper level cloud to form virga. The precipitation within the ground level cloud likely reached the surface. Analysis of these observations has involved the use of numerical modeling of the dynamics, radiative transfer, and microphysics in the atmosphere of Mars. The simulations are compared directly to the lidar dust and cloud measurements.
U14A-10
Physical and Thermodynamical Evidence of Liquid Water on Mars
The objective of the Phoenix Mars mission is to determine if Mars' polar region can support life. Since liquid
water is a basic ingredient for life, as we know, an important goal of the mission is to determine if liquid water
exists at the landing site. It is believed that a layer of martian soil preserves ice by forming a barrier against
sublimation, but that exposed ice sublimates without the formation of the liquid phase. Here we show physical
and thermodynamical evidence that besides ice, liquid saline-water exists in areas disturbed by the Phoenix
lander. Moreover, we show that the thermodynamics of freezing/thaw cycles ranging from diurnal to climatic
time-scales leads to the formation of saline solutions with freezing temperatures much higher than current
summer ground temperatures where surface ice is believed to exist near the surface. Thus, we hypothesize
that liquid saline-water is common on Mars. This discovery has important implications for the stability of water,
weathering, glaciology, mineralogy, geochemistry and the habitability of Mars.
http://phoenix.lpl.arizona.edu