U11B-0016
Atmospheric Dynamics at the Phoenix Landing site as seen by the Surface Stereo Imager
The Surface Stereo Imager has been used to observe the sky at the Phoenix Landing Site in Green Valley, Mars over the course of the primary and extended mission. Over this time period blowing dust aloft and clouds near the horizon have been observed. These subtle features are apparent due to the high signal to noise ratio of the camera which allows for the removal of a mean frame from multiple images captured in rapid succession and the ability to conduct simultaneous capture through different filters in each camera eye. The velocity of the features observed near the zenith is not directly inferred by this method due to a lack of information about the height of the features (other then that they are far beyond the camera cross-over point). However, the direction can typically be extracted, except in cases where features do not track from frame to frame due to high winds aloft. This direction varies mainly by time of day and is consistent with the near surface winds as measured by the Wind Telltale, located on the meteorological mast. As for the horizon data set, features remain subtle until trains of cloud-like features become visible between sols 65 and 75 and remain a daily feature. However, these are not the only features revealed as structure within the blowing dust is visible in nearly all data sets processed to date. Examples of both types of features will be presented and discussed. In addition to these data sets aloft, the rate of accumulation of air-fall dust over the course of the mission can also be determined based on observations of the Wind Telltale mirror. To date, nearly 3000 observations of this reflective surface have been taken in many lighting conditions throughout the day, as well as in shadow. These show varying degrees of dust and frost accumulation. As such, we will report on a high-temporal resolution data set derived from photometric calculations of the mirror.
U11B-0017
Atmospheric structure from Phoenix atmospheric entry data
The atmospheric structure at the time of landing of NASA's Phoenix probe has been derived from
measurements of the aerodynamic drag of the spacecraft during atmospheric entry and descent. The result
provides the first atmospheric structure in Mars' polar environment obtained from in situ measurements.
Phoenix was equipped with an inertial measurement unit (IMU) that used accelerometers for linear
acceleration measurement in three Cartesian axes and ring-laser gyroscopes to measure the three-
dimensional orientation of the probe (Taylor et al., 2008). The temperature structure of the atmosphere
along the flight path was calculated via a four-step process: (i) integrating forward the IMU data to obtain the
time history of the spacecraft velocity vector relative to the atmosphere as a function of altitude; (ii)
calculating atmospheric density from drag, with iteration for aerodynamic coefficient dependence on density;
(iii) integrating the hydrostatic equation to derive the vertical pressure; and (iv) calculating atmospheric
temperature from the equation of state. Initial profile reconstruction shows reasonable agreement with
predictions in the middle atmosphere for the given season and time of day (landing occurred at 16h 33min
37sec in local solar time expressed as a 24-hour clock). However, the derived lower atmospheric structure
below ~0.1 mbar is generally warmer than predicted. A possible explanation could be a shallower vertical
distribution of dust that usually assumed.
References: P. A. Taylor, D. C. Catling, M. Daly, C. S. Dickinson, H. O. Gunnlaugsson, A-M. Harri, C. F.
Lange, Temperature, pressure and wind instrumentation on the Phoenix meteorological package, J.
Geophys. Res., 113, EA0A10, doi:10.1029/2007JE003015, 2008.
U11B-0018
Possible Segregated Ice at the Phoenix Landing Site: Was Liquid Water Involved?
Lander cameras on the Phoenix mission revealed polygonal terrain at the landing site. Areas identified by topography within the work area of the arm included a polygon and a surrounding trough. Two trenches were dug, the first (Goldilocks) on the shoulder of a trough area exposed a bright, hard material and the second (Snow white) in the center of the polygon exposed hard material, but with multispectral properties indistinguishable from soil. Visibile-NIR spectra of the Goldilocks bright material are consistent with slightly dusty ice. When first exposed, a 2 cm chunk of material broke off and was observed to completely disappear in 3 sols, an implied sublmation rate of 100 micrometers per hour. We hypothesize that the Goldilocks bright material is segregated ice. The material is hard, localized, has distinct edges, and was initially covered with only 3 cm of soil, thus was 2cm shallower than the hard layer in the Snow white trench in spite of a more south-facing exposure. A trench dug 40 cm further south of Goldilocks, with similar orientation, reached 18 cm depth without encountering hard material. Plausible mechanisms for emplacement of segregated ice include liquid water pooled into a thermally-produced crack analogous to terrestrial ice wedge polygon formation, snowparticles depositing preferentially in the troughs, and vapor deposition preferentially into cracks (D. Fisher, Icarus 179, 387, 2005). Mission observations were performed relevant to evaluating these formation mechanisms. Wet chemistry analyses of soils suggest they contain Mg(ClO4)2, a soluble hygroscopic salt with a eutectic freezing point of /– 68C. If liquid water moved though the soil and formed the bright deposit in Goldilocks trench, a higher concentration of perchlorate would be expected in the area of the ice. Mg(ClO4)2. 6 H2O would crystallize when the salty water froze, forming white rhombohedral crystals. After scraping away the surface soil, approximately 500 cm2of bright material was exposed in Goldilocks trench and left undisturbed for 79 sols. During this time, the brightness of the material slowly faded and, by sol 99, a sublimation lag covered the bright deposit with nearly the same spectral properties as soil. It was not possible to obtain a large enough sample of the lag to directly measure salt concentration with a wet chemistry cell. Instead, a small sample of the lag was examined with the Optical Microscope to look for morphological evidence of salts. The material was stickier and more cohesive than previous soil samples examined with the microscope, and a population of light colored particles up to 30 microns in diameter with evidence of angularity consistent with microcrystallinity was found. This observation is suggestive of possible salts more concentrated in this area. In conclusion, the microscopy results are consistent with a liquid water formation mechanism but inconclusive without a direct measurement of the composition of the material.
U11B-0019
Effects of deliquescent salts in soils of polar Mars on the flow of the Northern Ice Cap
The discovery of substantial amounts of magnesium and perchlorate by Phoenix' "Wet Chemistry Lab" (WCL) in the soil of Polar Mars suggests that magnesium perchlorate could be the dominant salt in the polar region's soils. This prospect opens some unexpected doors for moving liquid water around at temperatures as low as -68C. In its fully hydrated form ,this salt water mixture has a high density (~ 1700 kgm /cubic meter) (Besley and Bottomley,1969) and a freezing point of -68C (Pestova et al., 2005).This perchlorate is very deliquescent and gives off heat as it melts ice. About 1.8 gram of ice can be 'melted' by 1 gm of pure magnesium perchlorate . If the reported 1 percent perchlorate is typical of polar soils and if 5 percent of the Northern Permanent Ice Cap is soil then the perchorate , makes up about 0.0005 the of the ice cap. Given the average thickness of the ice cap is about 2000 meters,this suggests there enough perchorate in the ice cap to generate about 2m of salty water at the bed. Because of its density the perclorate salty water would pool over impervious layers and make the bed into a perchorate sludge that could be mobilized and deformed by the overburden of ice. The deformation of mobile beds is a well known phenomenon on some terrestrial glaciers presently and was thought to have played a major role during the Wisconsinan ice age (Fisher et al., 1985) . The perchorate sludge would be deformed and moved outwards possibly resulting its re-introduction to the polar environment. Having a deliquescent salt sludge at the bed whose melting point is -68C would mean that the ice cap could slide on its deformable bed while the ice itself was still very cold and stiff . This possibility has been modeled with a 2D time varying model . Adding the deformable bed material allows ice cap motion even at ice temperatures cold enough to generate and preserve the scarp/trough features. When the perchlorate formation mechanisms and rates are known the ultimate importance of it in the water cycle of Mars will be clearer. The ice cap has long been thought of as a possible re-charge area for the deep water return flow (Clifford , 1987) . If perchlorate is formed sufficiently quickly, this view would be strengthened in spite of the low temperatures. Clifford S.M. 1987. Polar basal melting. JGR. Vol. 92, No. B9, pp 9135-9152. Besley L. M. and G.A. Bottomley. 1969. The water vapour equilibria over magnesium perchlorate hydrates. Journal of Chemical Thermodynamics. 1, pp13-19. Fisher, D.A., Reeh, N., and Langley, K. 1985. Objective reconstructions of the late Wisconsinan Laurentide Ice Sheet and the significance of deformable beds. Géographie physique et Quaternaire, v. 39, no. 3, p. 229-238. Pestova O. N.,Myund L.A.,Khripun M.K. and A.V. Prigaro. 2005. Polythermal study of systems M(ClO4)2-H2O (M2+=Mg2+, Ca2+, Sr2+, Ba2+). Russian Journal of Applied Chemistry , Vol.78.No.3,pp409-413. class="ab'>
U11B-0020
Size-Frequency Distribution of Rock Clasts at the Phoenix Landing Site
Rock populations on the plains surrounding the Phoenix landing site were analyzed using a combination of ground-based and orbital data. We determined the size-frequency distribution of rocks larger than 1.5 meters in diameter using images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE). Surface images taken by the Phoenix Lander Surface Stereo Imager camera were used to characterize the size-frequency distribution of rocks as small as 2 centimeters. Comparison of the size-frequency distribution of rocks for the Phoenix landing site with model curves shows that the rock population is characterized by significantly more pebble-sized rocks (>10 centimeters) than simple crushing models predict. Additionally, comparison with rock counts from Mars Exploration Rover Spirit rover images show that the Phoenix landing site is depleted in rocks relative to the Gusev plains. The depletion of rocks of all sizes at the Phoenix landing site is consistent with the proposed hypothesis that rocks were removed from the surface during fluidized ejecta emplacement by nearby Heimdall crater. We also characterized rock populations on a detailed scale within eight meters of the Phoenix Lander. Results indicate that more rocks are located in polygon troughs than in polygon interiors, although the biggest rocks are found within polygon interiors. Nearest neighbor statistics show that rocks with diameters between 2cm and 30cm on polygon interiors are clumped or less uniformly dispersed, whereas rocks in polygon troughs are uniformly spaced. The differences observed between rock distributions within the polygon interior and polygon trough units suggest that polygons are actively redistributing rocks in a manner consistent with thermal-contraction-based cryoturbation.
U11B-0021
Soil Properties Analysis of the Phoenix Landing Site Based on Trench Characteristics and Robotic Arm Forces
The Phoenix Mars lander has had access to polygonal terrain; specifically, two polygons and a trough. Slopes in the trenches and dump piles created from the interaction of the Phoenix robotic arm (RA) with the soil around its landing site are similar to those seen on previous missions, such as the MER and Viking missions. This indicates similar cohesion and angle of internal friction to previous landing sites. For example, trench slopes typically range from 44-72° and dump pile slopes range from 20-30°. There are at least two very different types of materials at the site: a layer of soil which goes down to several centimeters below the surface and, below that, a layer of icy soil. The RA can easily dig through the top layer of soil, often using 20-30N force. However, when it encounters icy soil, the RA requires tens of scrapes with the lower tungsten carbide blade on its scoop to progress even a few millimeters. To verify soil property parameters, we analyze the normal and shear stresses exerted on the soil by digging, scraping, and rasping with the RA.
U11B-0022
Subsurface Materials Exposed at the Phoenix Landing Site
The Phoenix Mission excavated materials at the Phoenix Landing site using the Robotic Arm (RA) while materials in the trenches and in the talus piles were documented with the Surface Stereo Imager (SSI) using 15 filters with bands from 485-1005 nm. Two polygons (Humpty Dumpty and Wonderland) are in the workspace and have frozen ice/soils mixtures that were exposed and monitored. Samples collected from the trenches were also delivered by the MECA Optical Microscope (OM) providing information on the optical properties and grain size/texture of these materials at microscopic scales. Excavations in Humpty Dumpty (Dodo-Goldilocks and Upper Cupboard trenches) revealed a high albedo deposit consistent with a relatively pure ice spectrally. Dodo-Goldilocks was first exposed on Sol 9 with digging stopping on Sol 21. The ice in Dodo-Goldilocks was left undisturbed till Sol 99. The sublimation process monitored using 15 filter observations. Over time the ice signature decreased in the deposit and became consistent with the surface soils seen at the landing site. Sublimation was not uniform with some high albedo patches prominent on Sol 102. On Sol 99 the robotic arm scraped the sublimation lag from Dodo-Goldilocks and delivered the sample to the MECA OM. Collection of the sample exposed fresh high albedo ice. High albedo materials at the Dodo-Goldilocks and Upper Cupboard trenches were generally hard based on interactions with the Robotic Arm. During the initial excavation, clods of ice/soil mixtures were produced that sublimated away over within 4 sols. These ice clods could have been formed by brittle fracture of very cold pure ice or be a small deposit of weaker ice soil/layer on top of the Dodo-Goldilocks deposit. Excavation in Wonderland (Snow White trench) revealed an ice/soil mixture with significantly more soil than at Dodo-Goldilocks based on spectral characteristics. After exposing a harder, spectrally different layer with the RA secondary blade, sublimation of these materials rapidly produced a lag deposit. The Snow White area was selected to be the site where TEGA would collect an icy sample which required periodic scraping and rasping. This produced visible amounts of material that could be measured spectrally from the soil/ice boundary and from inside the ice layer. These soils are consistent spectrally with other soils at the site. This talk will present key observations of these two deposits and contrast their characteristics. Possible modes of formation for these deposits will be discussed with evidence both for and against presented. For the Wonderland deposits, vapor diffusion is the leading candidate. For Humpty Dumpty various mechanisms are under investigation including burial of snow/ice by geologic processes, vapor diffusion, and processes involving salts.
U11B-0023
Temperature and Pressure at the Phoenix Landing Site
During the Phoenix lander mission air temperatures were measured at three levels on a 1-m mast and pressure was measured on the deck of the lander, itself about 1 m above ground level. Measurements were made at 0.5 Hz and has run almost continuously through the landed mission apart from short daily breaks for data transfers. The diurnal temperature data were generally very similar from one day to the next throughout the first 60 sols of the mission with daily maxima of around -30C and minima of -80C. These match our boundary-layer model predictions. In mid-sol, highly convective periods there are large, relatively rapid turbulent fluctuations in temperature. Coupled with wind data and Monin-Obukhov similarity theory we can estimate sensible heat fluxes in this period. At night (although the sun remains above the horizon) there are generally smaller turbulent fluctuations although bursts of turbulence occur on most nights between 2200 and 0200. Pressure data still need some post-processing but can be used to clearly identify the passage of vortex features (usually several per sol, near mid-day and early afternoon) with or without dust.
U11B-0024
The Role of a Winter CO2 Ice Mantle in the Formation of Patterned Ground at the Phoenix Landing Site
The Phoenix lander rests on a landscape of meter-scale polygonal patterned ground strewn with pebble and cobble-size stones that exhibit a tendency to concentrate in the troughs at polygon boundaries. It is postulated that movement of these stones to the troughs is assisted by the seasonal cover of carbon dioxide ice; during the winter, the thin soil layer is sandwiched between an underlying mass of water ice (ice- cemented soil) and the overlying CO2 ice layer. As the CO2 ice layer forms, it encases surface stones and prevents their movement while the underlying water ice contracts. A shear is therefore set up in the soil layer that moves the lower boundary of the soil towards the polygon center. In summer when the CO2 ice disappears, the water ice expands again, but now carries the soil with it uniformly. Thus, stones are able to move radially away from polygon centers, but not in the reverse direction. The presence of many stones still in the center of polygons may suggest that they are being resupplied from polygon centers, emerging from the bulging ice beneath. The fact that the Phoenix site appears to be affected by aeolian processes yet is largely devoid of ventifacts lends support to the notion of a mobile and replenished stone supply. Polygonal recycling of soil materials would also explain how stones of pebble and cobble-size became embedded in fine-grained soil that appears (from microscopy) to be largely aeolian in origin. By cryoturbational action, the stones, possible from a nearby impact site, are moved from depth to become continually mixed with the products of aeolian mantling. Recycling also has important implications for soil chemistry because the soil is not in static equilibrium with the immediate weathering environment. Rather, the soil is a dynamic product that drags surface products downwards, while releasing new minerals from beneath.
U11B-0025
Winds at the Phoenix Landing Site
Local wind speeds and directions have been measured at the Phoenix landing site using the Telltale wind indicator. The Telltale is mounted on top of the meteorological mast at roughly 2 meters height above the surface. The Telltale is a mechanical anemometer consisting of a lightweight cylinder suspended by Kevlar fibers that are deflected under the action of wind. Images taken with the Surface Stereo Imager (SSI) of the Telltale deflection allows the wind speed and direction to be quantified. Winds aloft have been estimated using image series (10 images ~ 50 s apart) taken of the Zenith (Zenith Movies). In contrast enhanced images cloud like features are seen to move through the image field and give indication of directions and angular speed. Wind speeds depend on the height of where these features originate while directions are unambiguously determined. The wind data shows dominant wind directions and diurnal variations, likely caused by slope winds. Recent night time measurements show frost formation on the Telltale mirror. The results will be discussed in terms of global and slope wind modeling and the current calibration of the data is discussed. It will also be illustrated how wind data can aid in interpreting temperature fluctuations seen on the lander.
U11B-0026
Sublimation of Exposed Snow Queen Surface Water Ice as Observed by the Phoenix Mars Lander
One of the first images obtained by the Robotic Arm Camera on the Mars Phoenix Lander was that of the surface beneath the spacecraft. This image, taken on sol 4 (Martian day) of the mission, was intended to check the stability of the footpads of the lander and to document the effect the retro-rockets had on the Martian surface. Not completely unexpected the image revealed an oval shaped, relatively bright and apparently smooth object, later named Snow Queen, surrounded by the regolith similar to that already seen throughout the landscape of the landing site. The object was suspected to be the surface of the ice table uncovered by the blast of the retro-rockets during touchdown. High resolution HiRISE images of the landing site from orbit, show a roughly circular dark region of about 40 m diameter with the lander in the center. A plausible explanation for this region being darker than the rest of the visible Martian Northern Planes (here polygonal patterns) is that a thin layer of the material ejected by the retro-rockets covered the original surface. Alternatively the thrusters may have removed the fine surface dust during the last stages of the descent. A simple estimate requires that about 10 cm of the surface material underneath the lander is needed to be ejected and redistributed to create the observed dark circular region. 10 cm is comparable to 4-5 cm predicted depth at which the ice table was expected to be found at the latitude of the Phoenix landing site. The models also predicted that exposed water ice should sublimate at a rate not faster but probably close to 1 mm per sol. Snow Queen was further documented on sols 5, 6 and 21 with no obvious changes detected. The following time it was imaged was on sol 45, 24 sols after the previous observation. This time some clear changes were obvious. Several small cracks, most likely due to thermal cycling and sublimation of water ice appeared. Nevertheless, the bulk of Snow Queen surface remained smooth. The next image of Snow Queen was taken on sol 73. This time its appearance was dramatically different. The surface had become much rougher and many cracks of at least 1 mm depth and decimeter scale length had appeared. The surface colour of Snow Queen was now no longer different from that of the surrounding regolith. This observation is compatible with the ice table sublimating away, leaving behind a lag deposit of thickness of the order of 1 mm. We will present these data as well as thermal models, including the diurnal cycle of the interaction with the atmosphere, which may explain the observed evolution of Snow Queen.
U11B-0027
Spectral Modeling of Ground Ices Exposed by Trenching at the Phoenix Mars Landing Site
The Phoenix Lander, which landed on the northern plains of Mars on 25 May 2008, used its Robotic Arm (RA) to dig six trenches during its nominal 90-sol mission: Dodo-Goldilocks, Snow White, Cupboard, Neverland, Burn Alive, and Stone Soup. During excavation of the first five of these, the RA encountered hard material interpreted to be the ice table, and the Stereo Surface Imager (SSI) imaged the exposed materials using 15 filters spanning a wavelength range from 445 to 1001 nm. Materials exposed in the Dodo- Goldilocks and Snow White trenches are spectroscopically dissimilar: Dodo-Goldilocks hard material is brighter relative to the surrounding soil, and has a distinct downturn around 800 nm resulting from a dusty ice with low soil-to-ice ratio. Snow White hard stuff varies in brightness and spectral shape depending on the phase angle, with low-phase angle images showing dark material and higher phase angles showing more soil-like material. The Snow White material does not have the strong 800-nm downturn seen in Dodo- Goldilocks, because the soil-to-ice ratio is high as inferred by the rapid development of a sublimation lag; however, the albedo variation with phase angle could be due to strong forward-scattering at low phase angles, consistent with icy material. A modified Hapke model is used to estimate the relative abundances of water ice and dust in the Dodo- Goldilocks and Snow White materials, with dehydrated palagonite as an analogue for dust . The ice exposed at Dodo-Goldilocks must be relatively dust-free, since only a small amount of dust is needed to obscure water ice absorptions. In our modeling, we find that as little as 5 wt% 20-um dust is enough to completely mask the 1001 nm absorption in 1-mm grain size water ice. Dodo-Goldilocks spectra can have up to a 20% drop in reflectance from 800 nm to 1001 nm, which is best-matched in our Hapke model by water ice with path lengths on the order of 2-3 mm. The Snow White dark materials typically have a small downturn at approximately 900 nm, with a depth on the order of a few percent. This could be the result of finer-grained ice or a higher dust:ice ratio. Further modeling is needed to understand the behavior of the dark and bright material at the Snow White trench.
U11B-0028
Erosion Dynamics during Phoenix Landing on Mars
Unique from past planetary surface missions, the Phoenix spacecraft used pulsed retro-rockets to land on the northern polar region of Mars. Mainly viscous shear erosion caused by descent jets had minimally altered previous landing sites. Here we report novel simulations of surface modification by pulsed thruster plumes, and assess the erosion processes leading to the first exposure of ice below the Martian regolith. At Mars atmospheric pressure, we find that the repetitive injection of high pressure gas into porous soil by the pulsed engines leads to the propagation of cyclic radial shock waves within the soil. We show that these shock waves cause 'explosive erosion' and excavate the regolith down to the ice table in a radius of ~75 cm under the lander. Moreover, coarse and fine particles are ejected outward to a radius of 3 m and ~20 m from the thrusters, respectively. The results of our simulations are confirmed by images of the Phoenix landing site and provide important insights into the geology, glaciology and geomorphology of the landing site. These erosion dynamics may lead to ammonia hydrates and ammonium salts, but may demonstrate limited soil contamination. By comparing results from the landing site and our simulations, we come to the initial conclusions that the Martian arctic regolith has high porosity and permeability, mixture of fines with coarse particles, and exhibit cohesive stresses greater than 0.9 kPa.
U11B-0029
Time-Dependent SSI Multispectral Properties for Rock, Soil, Ice, and Sublimation Lags at the Phoenix Landing Site on Mars
The Surface Stereo Imager (SSI) on the Phoenix Lander is a 15 band multispectral imager covering the spectral range from 0.45 to 1.00 micrometers. More than 250 15-filter spectral image cubes have been obtained for surface targets at the Phoenix landing site in the north polar region of Mars. The spectra of surface soils and rocks are dominated by a ferric absorption edge from nanophase ferric oxide, and they are broadly similar to most multispectral data obtained during the Pathfinder and MER missions. Negative spectral slopes between about 0.70 and 1.00 micrometers, indicative of high concentrations of olivine in the El Dorado sand sheet at Gusev crater, were not detected. The albedo (cos(i) corrected) of Phoenix surface spectra is highly dependent on the time of sol (albedo at 0.80 micrometers varies by a factor of 2), consistent with opposition and phase function effects. Subsurface layers bearing water ice were exposed at a depth of about 4 cm by digging with the robotic arm scoop. The SSI spectra of icy materials are highly variable, ranging from typical ice (spectrally neutral and high albedo near 0.7) at the Dodo-Goldilocks trench to low albedo spectra (about 0.3 at 0.80 micrometers) with a ferric absorption edge at the Snow White trench. The differences are attributed, respectively, to low and high concentrations of fine-grained and ferric-rich material dispersed throughout the ice. The spectra of the icy surfaces are dependent on time as the ice sublimes. At Snow White, an optically thick (about 300 micrometers) sublimate lag develops within two sols. At Dodo- Goldilocks, the time scale for development of an optically thick sublimate lag is 5 to greater than 60 sols, depending on location within the trench. The spectra of sublimate lag are equivalent to those for fine-grained soil.
U11B-0030
Phoenix Magnetic Properties Experiments Using the Surface Stereo Imager and the MECA Microscopy Station
Phoenix carries as part of its scientific payload a series of magnetic properties experiments designed to utilize onboard instruments for the investigation of airborne dust, air-fall samples stirred by the retro-rockets of the lander, and sampled surface and sub-surface material from the northern plains of Mars. One of the aims of these experiments on Phoenix is to investigate any possible differences between airborne dust and soils found on the northern plains from similar samples in the equatorial region of Mars. The magnetic properties experiments are designed to control the pattern of dust attracted to or accumulated on the surfaces to enable interpretation of these patterns in terms of certain magnetic properties of the dust forming the patterns. The Surface Stereo Imager (SSI) provides multi-spectral information about dust accumulated on three iSweep targets on the lander instrument deck. The iSweeps utilize built in permanent magnets and 6 different background colors for the dust compared to only 1 for the MER sweep magnet. Simultaneously these iSweep targets are used as in-situ radiometric calibration targets for the SSI. The visible/near-infrared spectra acquired so far are similar to typical Martian dust and soil spectra. Because of the multiple background colors of the iSweeps the effect of the translucence of thin dust layers can be estimated. High resolution images (4 micrometers/px) acquired by the Optical Microscope (OM) showed subtle differences between different soil samples in particle size distribution, color and morphology. Most samples contain (typically 50 micrometer) large, subrounded particles that are substantially magnetic. The colors of these particles range from red, brown to (almost) black. Based on results from the Mars Exploration Rovers, these dark particles are believed to be enriched in magnetite. Occasionally, also very bright, whitish particles were found on the magnet substrates, likely held by cohesion forces to the magnet surface and/or to other (magnetic) particles.
U11B-0031
Regolith-Atmosphere H2O Exchange and Surface Energy Balances at the Phoenix Landing Site
The Phoenix lander touched down on May 25, 2008 at 68.16°N 233.35°E, an area that is fairly typical of the Vastitas Borealis plains. The high latitude regolith is a reservoir for substantial near-surface ground ice, and appears to play a significant role in the seasonal H2O cycle. The Phoenix landing site is within the circumference of the seasonal CO2 cap, and surface energy balance at the site is key to characterize in understanding the seasonal volatile cycles. Nighttime soil temperatures reach lows of ~ 203K at Ls = 98 -100°, dropping to 199K by Ls 107°. Maximum daytime temperatures are not measured directly, due to self-shadowing by the instrumentation on the end of the Robotic Arm. Numerical analysis suggests that peak temperatures at the visible surface may reach 265 K. The thermal inertia of the materials measured by Phoenix is considerably higher than observed from orbit, and appears to be out of thermal equilibrium with ground ice, which was excavated at depths of a few centimeters. Along with the detection of a visible dark halo around the Phoenix lander from HiRise, these results indicate that the pre-existing surface was heavily disturbed during landing. Atmospheric H2O is variable during the day, averaging just under 2 Pa. During the evening hours, the variability disappears, and the abundance of H2O tracks the falling temperatures. The temperature- dependent behavior begins long before either the soil or atmosphere reaches the frost point, which argues against H2O ice as the buffer. In addition, a plot of PH2O against the soil temperature yields an estimate of ΔH H2O = 30 kJ/mole, which is too low for the enthalpy of ice sublimation, but is characteristic of hydrogen bonds. Electrical conductivity measurements in the regolith detect no measureable changes associated with the exchange of H2O, but simultaneous dielectric permittivity measurements record abrupt discontinuities associated with the onset of adsorptive H2O scavenging. These data support the hypothesis that the Martian boundary layer collapses in the evening, and that the regolith adsorptively buffers the H2O content of the atmosphere overnight.
U11B-0032
Phoenix Mars Lander: Vortices and Dust Devils at the Landing Site
Near continuous measurements of temperatures and pressure on the Phoenix Mars Lander are used to identify the passage of vertically oriented vortex structures at the Phoenix landing site (126W, 68N) on Mars. Observations: During the Phoenix mission the pressure and temperature sensors frequently detected features passing over or close to the lander. Short duration (order 20 s) pressure drops of order 1-2 Pa, and often less, were observed relatively frequently, accompanied by increases in temperature. Similar features were observed from the Pathfinder mission, although in that case the reported pressure drops were often larger [1]. Statistics of the pressure drop features over the first 102 sols of the Phoenix mission shows that most of the events occur between noon and 15:00 LMST – the hottest part of the sol. Dust Raising: By assuming the concept of a vortex in cyclostrophic flow as well as various assumptions about the atmosphere, we obtain a pressure drop of 1.9 - 3.2 Pa if dust is to be raised. We only saw few pressure drops this large in Sols 0-102. However, the features do not need to pass directly over the lander and the pressures could be lower than the minima we measure. Furthermore, the response time of the pressure sensor is of order 3-5 s so it may not capture peak pressure perturbations. Thus, more dust devils may have occurred near the Phoenix site, but most of our detected vortices would be ghostly, dustless devils. Modelling: Using a Large Eddy Simulation model, we can simulate highly convective boundary layers on Mars [2]. The typical vortex has a diameter of 150 m, and extends up to 1 km. Further calculations give an incidence of 11 vortex events per day that could be compatible with the LES simulations. Deeper investigation of this is planned -but the numbers are roughly compatible. If the significant pressure signatures are limited to the center of the vortex then 5 per sol might be appropriate. The Phoenix mission has collected a unique set of in situ meteorological data from the Arctic regions on Mars. Modelling work shows that vertically oriented vortices with low pressure, warm cores, can develop on internal boundaries, such as those associated with cellular convection, and this is supported by observations. Simple cyclostrophic estimates of vortex wind speeds suggest that dust devils will form, but that most vortices will not be capable of lifting dust from the surface. So, at least in the first 102 sols, most of the Phoenix devils are dustless. References [1] F Ferri, PH Smith, M Lemmon, NO Renno; (2003) Dust devils as observed by Mars Pathfinder. JGR,108, NO. E12, 5133, doi:10.1029/2000JE001421. [2] Gheynani, B.T. and Taylor, P.A., (2008), Large Eddy Simulation of vertical vortices in highly convective Martian boundary layer, Paper 10 B.6, 18th Symposium on Boundary Layers and Turbulence, June 2008, Stockholm, Sweden
U11B-0033
Voltammetric and Chronopotentiometric Soil Analysis at the Phoenix Landing Site
Two electrochemical methods, cyclic voltammetry (CV) and chronopotentiometry (CP) have been employed to search for soluble electroactive species in martian soils collected by the Phoenix lander. The CV and CP measurement capabilities are included as part of the wet chemical laboratory (WCL) component of the Phoenix Microscopy, Electrochemistry, and Conductivity Analyzer (MECA). In a CV experiment, the electrochemical cell current is measured as the potential is swept between two set voltages, with a reversal in scan direction occurring at a set switching potential. As is typical in laboratory experiments, a triangular waveform with a constant scan rate (forward and reverse scans) is used for the WCL CV implementation. A three-electrode configuration is employed, with the WCL platinum ORP electrode serving as the counter electrode, a gold electrode serving as the working electrode, and one of the WCL chloride ion selective electrodes serving as the reference electrode. In a CV measurement, the level of current that flows between the working and counter electrodes at a given potential depends on a number of factors including the concentrations of soluble redox active species in the samples. In a CP measurement, the electrochemical cell potential is measured as the current is stepped (or ramped) from zero to a set current. As is typically done in laboratory experiments, either a current step or linear current ramp is used in the WCL CP implementation. Similar to the case with the WCL CV measurements, a three-electrode configuration is employed; however, a platinum electrodes serves as a cathodic working electrode. As current is applied in a CP measurement, electrochemical reactions take place at characteristic electrochemical cell potentials and proceed until reactants are depleted at the surface of the electrode. When the reactant concentration drops to zero at the electrode surface, the transition time is reached and electrode potential rapidly shifts. The transition time is a function of the concentration of a reactant in solution and the potential at which a reaction occurs depends, in part, on the redox potential of the process.