Study the history of water at the site.

The history of water is written in the soils. Chemical and geological evidence allows us to read this record to determine the history of water and the processes that control its distribution and phases. What is the origin of the ice layer and how does it currently interact with the atmosphere? Once emplaced, has it gone through freeze-thaw cycles? How do local processes link with the global water cycle?

Search for clues to the origin of the ice.

The existence of shallow subsurface ice could be the result of several processes:  diffusive transport into the regolith from the atmosphere; seepage of sub-polar liquid water (e.g., from basal melting) into the circumpolar regolith;  a remnant of an older, larger ice sheet; or the frozen residue of an ancient ocean. Two of those cases imply emplacement as liquid, the other two as ice.

Diffusive transport into and out of the regolith is a seasonal process that occurs on both Mars and Earth. As a result, volatiles are expected in the polar regions a few tens of cm below the surface. Finding this volatile layer was a primary goal of the Polar Lander mission. Yet the magnitude of the ice signature seen by Odyssey brings the diffusive model into question.

Another possibility is that a larger ice cap has retreated, subliming to a depth where it comes to equilibrium with the atmosphere. In this case, we might find solid ice intermixed with dust below a transition region. Neither scenario requires large amounts of liquid water.

The landing site for Phoenix is also hypothesized to have been a deep ocean basin in the planet’s distant past. The salinity of such an ancient ocean or the amount of dissolved CO₂ is unknown, but the eventual evaporation or sublimation of this water body might have left sedimentological evidence accessible to Phoenix. However, ice-rich regions also occur in the south where past oceans are not suspected.

Phoenix will search for several clues to the origin of the circumpolar ice. In the process of excavating the trench, we will be able to examine possible layering relationships between the ice and the soil, the depth at which the ice is encountered, the relationship between the ice volume and the soil porosity, the density and “solidity” of any ice layers (“fluffy” versus “compact”), and post depositional distortions to the ice/soil layers. Additionally, the relationship of the ice to the soil's chemistry will be assessed—Phoenix will determine the relationship of the ice to oxidant, organics, and other chemical/compositional gradients and horizons (such as concentrations of salts or hydrated minerals). These phenomena should provide insight into whether the ice is relict or currently interacting with the atmosphere, or if it has been liquid at periods in the past.

Search for evidence of past episodic liquid water

Despite its current dryness, Mars shows extensive signs of the past operation of a hydrological cycle. During the present epoch, perihelion occurs during southern summer, and the north-south difference in the normalized maximum insolation is nearly 50% because of the large eccentricity of the orbit. Consequently, high southern latitudes are now among the warmest places on Mars in the summer, and temperatures exceed the melting point of ice for several days each year.

Liquid water is not stable when its vapor pressure exceeds the local atmospheric pressure. Usually this corresponds to the pressure of its triple point (6.1 mb), which is never exceeded in the southern polar region, though the vapor pressure may be lower if the water is briny, capped by ice, or mixed with dust or soil

In high northern latitudes in summer, the atmospheric pressure is above the triple point (8-10 mb), but because northern summer occurs at apehelion, temperatures are far below the water melting point except under unique circumstances [Hecht 2002]. However, conditions for melting may occur during periods when insolation is higher.

Mars’ rotational dynamics force periodic soil-warming phases as the longitude of perihelion precesses, completing a full cycle in 51,000 years. Thus, cyclically, perihelion occurs during northern summer. Unpublished runs of the GCM show that it is warm enough at high northern latitudes for several days each summer to allow near-surface ground ice to melt. In addition to changes in peak insolation resulting from orbital precession, obliquity variations can dramatically increase average insolation at high latitudes over timescales of 10⁵ and 10⁶ years, perhaps leading to greater warming.

Insolation changes will transition over hundreds or thousands of years, perhaps causing freeze-thaw cycles, and therefore have parallels to terrestrial permafrost. These cyclic transitions may drive periglacial processes that could shape surface features and subsurface layering and affect the chemistry and composition of soil. The depth of freeze-thaw effects in the regolith will have some correlation to the periodicity, e.g., diurnal cycles will penetrate only to a few centimeters, multi-year cycles to much deeper. Thus, the history of water and its phase transitions may be written into the upper meter of the surface.

In terrestrial permafrost/periglacial regions and high altitude cold regions of earth, freeze-thaw cycling and the general proximity of ice-water layering leads to phenomena such as patterned ground (soil clast sorting) cryoturbation (distortion of soil layers), and solifluction (soil creep by water saturation of a soil layer overlying an ice substrate). Imaging the trench may reveal such soil disturbances.

Wetted regolith would likely equilibrate with the atmosphere by upward percolation of water via capillary action between the soil particles. As the water moved through the soil, it would leach compounds and leave them in concentrated soil horizons as the saturation level changed or as the water evaporated. Thus, vertical compositional fractionation of the soil (as occurs in many terrestrial soils) is expected. Soil horizons might exhibit concentrations of salts, carbonates, iron compounds, and soluble materials.

The concentration of compounds at certain horizons should be evident from the chemistry, from spectroscopic (color) variations with depth, and from changes in soil induration – hardpans, duricrusts, and nodules are typical products of cementation of soil horizon minerals (e.g., terrestrial caliche). This implies that they can be detected by imaging the trench and monitoring soil resistance during digging, and confirmed by chemical measurements.

Indications of abundant liquid water on early Mars might come from fine clastic sediments like layers of indurated mud and silt. Coarser clastic sediments such as sandy material might indicate water transport, particularly if sediments are well sorted and grains are rounded. Phoenix measurements will be able to distinguish between subaqueous and aeolian grains. The techniques of determining the provenance, transport history, and diagenetic history of sedimentary clasts are well established using grain sorting, shape, and surface textures.

For standing bodies of liquid water to have been stable on ancient Mars, a dense CO₂ atmosphere would have been required to bring average temperatures above freezing. Much of this early atmosphere should have chemically combined with the surface, forming carbonates. Phoenix will be able to detect two classes of minerals that are indicative of past water. The first class is water-soluble minerals such as salts, carbonates, and evaporitic compounds. The second class includes hydrous mineral species: clays and hydrous iron oxides, for example. Microscopy can detect clay minerals by their platy shape when scrutinized by an the Atomic Force Microscope. Clays have been predicted to occur in Martian soil from chemical models , but so far they have eluded detection. Recent spectroscopic data could imply clays in the soil, or conversely, andesitic lavas underlying the northern basin. We do not rule out the possibility that al least some of the hydrogen signal seen by the Odyssey GRS may be derived from such minerals, since we would expect to find clays in a region that may have been subject to wetting.

Comparative mass-spectroscopy measurements of atmospheric gases and soil volatiles will determine the isotopic ratios, indicative of the interaction between the surface and atmosphere over long time scales. The D/H ratio of atmospheric water vapor increases rapidly over time as H preferentially escapes to space, and decreases when “new” water is injected into the atmosphere from a reservoir. Differences between the D/H ratio of atmosphere and subsurface ice reflect the degree of mixing between them. The same will be true for isotopes of O and C. Isotopic ratios will therefore help determine the origin of ice deposits—an important goal for Phoenix.

Phoenix does not detect episodic past liquid water directly. Measurements (thermal conductivity, diffusivity, soluble ion concentration, depth to the ice layer, and ice-water ratio) characterize the relevant physical properties of the soil, and allow calculations to show whether melting can occur during high insolation periods.

Characterize Mars’ present climate
and climatic processes.

The Phoenix mission begins with the retreat of the polar cap and the exposure of the cold soil to sunlight after a long winter. Understanding the near-surface meteorological conditions is important in providing a lower boundary condition for orbital measurements and models, even though global climate models cannot be fully constrained from one site. There has never been a weather station in Martian polar regions and none are planned.

The surface-atmosphere interaction, particularly the exchange of volatiles, is key to understanding the present and past climate. The Phoenix mission will test the hypothesis that diffusive transport into the regolith could have caused substantial subsurface layers of ice. We propose to correlate the humidity with wind direction to understand the rates at which the vapor is moved northward and southward near the seasonal cap boundary. Jones et al. [1979] and Wall [1981] argue that water is deposited at the ground while mixed with dust. Phoenix will test this idea by quantifying the water and dust transport and mixing at the landing site.

An important climatic question for Mars is whether or not the global water cycle is closed on an annual basis. The question stems from the hemispheric asymmetry in the polar caps: the northern cap (water-ice) is three times larger than the southern counterpart (CO₂ ice). Measurements of the column water-vapor abundance as a function of season and latitude indicate that the seasonal maximum is near L_s 120, which occurs during the Phoenix mission. It has also been suggested that water-ice clouds may play a significant role in the climate by retaining water in, and scavenging water to, the northern hemisphere. This mechanism may also drive the polar caps’ hemispheric asymmetry.

Water-vapor transport and water-ice clouds [Tamppari and Bass, 2000] in the north polar region of Mars are not well understood. While interannual variations in the appearance of the north polar caphave been interpreted as being caused by dust storms, these variations occur seasonally, indicating possible late summer water-ice deposits.

To verify models of polar climate, Phoenix will measure pressure, temperature, and winds with frequent sampling, column water abundance to several ppm accuracy, cloud and dust opacity to a few hundredths, dust radiative properties , and cloud movement. Phoenix will also provide “ground-truth” verificaction for recently developed mesoscale models designed to more accurately model these local and regional events.

Diurnal changes in the local atmosphere (and surface) are unknown at high latitudes. Colburn et al. [1989] and Smith and Lemmon [1999] found that opacity differences between morning and mid-day were consistent with water-ice clouds forming at night and their subsequent evaporation as the day warmed. The cloud layer height is uncertain, and could be resolved by comparing Phoenix’s meteorological station measurements to its lidar observations of the boundary layer.

Search for Habitable Zones

Phoenix will assess the biological potential of near-surface ice and determine its habitability. A habitable environment is one that allows life to grow and reproduce, even if such conditions occur infrequently. Does liquid water occur on Mars that might sustain life at the landing site? Are there energy sources that can sustain life? Is the environment hazardous to life?

Identify possible energy sources for life.

The case for episodic melting of near surface ice at the Phoenix landing site was discussed above.  Many terrestrial organisms survive in a dormant state for long periods of time, returning to a growing state when conditions allow. Dormant live bacteria have been found frozen in permafrost, encased in salt crystals or amber drops for up to 40 million years.  An environment in which liquid water occurs briefly and infrequently might still be habitable.

In addition to liquid water and biogenic elements (C, H, N, O, P, S), which are already known to be present on Mars and which will be measured by Phoenix, life requires an energy source. At the Martian surface, sunlight provides a readily available fuel, but its use requires a chromophore, such as chlorophyll, to convert it to a usable form. Deeper underground, where sunlight does not penetrate, life must rely on other energy sources.

Organic compounds are a plausible energy source for life underground, and these form the basis for the bulk of the terrestrial subsurface biosphere. In addition, subsurface life forms exist on Earth that derive energy by reacting hydrogen produced in the decomposition of basalt with CO₂. Metabolic energy under oxygen free conditions from other mineral substrates may also be possible.

Phoenix will search for organic compounds and minerals that could provide energy for subsurface metabolism. Redox pairs, identified by chemical analysis, may determine whether the chemical potential energy of the soil is capable of sustaining life. Gradients in redox potential, moreover, are often indicative of actual biological activity. Phoenix will also characterize key fundamental soil properties such as pH and eH which are relevant to habitability.

Determine whether the subsurface environment is hostile to life.

Even with energy sources and liquid water, other factors may inhibit biological activity. The surface is bathed in ultraviolet radiation that is hostile to life. In the dry martian environment, that same radiation can induce the photochemical formation of powerful oxidants that can break apart organic molecules. Such oxidants have been invoked as the most likely explanation of the Viking biology experiment results, and several plausible oxidants were proposed. But just a few millimeters beneath the surface organisms would be protected from direct radiation, and oxidants would not be expected to form. Moreover, oxidants that migrated from the surface would be expected to disappear because of degradation processes.

Using the MECA chemistry lab, Phoenix will characterize the gradient of oxidants with depth to assess whether the subsurface environment is hospitable to life. Oxygen produced by the reaction of superoxides and similar species with water will be detected with the dissolved oxygen electrode. Weaker oxidants such as peroxides will determine the redox potential of the system, as measured by the platinum ORP electrode. Oxidants that convert iodide to iodine may be measured by adding solid potassium iodide.  The iodide electrode measures the iodide added, while the ORP electrode measures the ratio of iodide to iodine. Cyclic voltammetry provides additional information about specific oxidation and reduction processes in the solution.

Even in the absence of life on Mars, meteorites bring organic molecules to the surface. As such, the discovery of organic matter is an ambiguous biomarker, but it would provide definitive confirmation that the subsurface environment is non-destructive. Phoenix will search for organic molecules with 50 ppb sensitivity, 20X more sensitive than Viking.

Finally, a focus on habitability does not rule out a direct search for signatures of life produced in previous epochs. Phoenix will look for organic deposits or other biomarkers relict of life in an ancient ocean or living in the permafrost and metabolizing when liquid water is present. A mass spectrometer will detect isotopic fractionation in minerals such as carbonates and in organic compounds. Terrestrial biological systems preferentially incorporate lighter isotopes (e.g., ¹²C over ¹³C) into organic matter. Minerals produced in biologically mediated reactions similarly may have fractionated isotopes. Thus, isotopic ratios may provide evidence that biological processes took place at some point in the past [McKay et al. 1992]. Phoenix will carry a mass spectrometer to detect isotopic fractionation in minerals, and in organic compounds.