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Phoenix science instruments

The Phoenix Mars Lander carries seven science instruments, three of which are suites of multiple tools.

The Robotic Arm will allow Phoenix to explore vertically and to use instruments on the spacecraft deck to analyze samples of Martian soil and ice. The arm will dig trenches, position some arm-mounted tools for studying the soil in place, and deliver scooped-up samples to other instruments.

The aluminum and titanium arm is 2.35 meters (7.7 feet) long. One end is attached to the lander's deck. An elbow joint is in the middle. The other end has a scoop with blades for digging into the soil and a powered rasp for breaking up frozen soil. The arm moves like a backhoe, using four types of motion: up-and-down, side-to-side, back-and-forth and rotating.

The arm can reach far enough to dig about half a meter (20 inches) deep. However, the subsurface ice layer expected at the landing site may not lie that deep. Once the arm reaches the icy-soil layer, the powered rasp will be used to acquire samples.

Because the arm will be making direct contact with icy soil that is conceivably a habitat where microbes could survive, extra precaution has been taken with it to prevent introducing life from Earth. Before the arm was given a sterilizing heat treatment in March 2007, it was enclosed in a biological barrier wrap. This barrier is keeping microbes off the arm during the subsequent months before launch. It will not open until after Phoenix has landed on Mars.

The robotic arm uses design work for a similar arm flown on the 1999 Mars Polar Lander mission, with refinements including enhanced capability for collecting an icy sample.

A team led by Robert Bonitz at NASA's Jet Propulsion Laboratory, Pasadena, Calif., engineered and tested the Phoenix Robotic Arm. Alliance Spacesystems Inc., Pasadena, built it. Ray Arvidson of Washington University in St. Louis is the lead scientist for this instrument.

The Robotic Arm Camera rides fastened to the arm just above the scoop. It will provide closeup color images of Martian soil at the landing site, of the floor and walls of trenches dug by the arm, and of soil and ice samples before and after they are in the scoop.

Information the camera reveals about soil textures will aid in selecting what to pick up as samples for analysis. Observations of trench walls will determine whether they show fine-scale layering, which could result from changes in the Martian climate.

The camera has a double Gauss lens system, a design commonly used in 35-millimeter cameras. Images are recorded by a charge-coupled device (CCD) similar to those in consumer digital cameras. The instrument includes sets of red, green and blue light-emitting diodes (LEDs) for illuminating the target area.

The focus can be adjusted by a motor, which is a first for a camera on an interplanetary spacecraft. The focus can be set as close as about 11 millimeters (half an inch) and out to infinity. With a resolution of 23 microns per pixel at the closest focus, this camera can show details much finer than the width of a human hair.

A team led by H. Uwe Keller at the Max Planck Institute for Solar System Research, Katlenburg- Lindau, Germany, and by Peter Smith at the University of Arizona originally built this camera for the Mars Surveyor 2001 Lander mission, which was canceled in 2000. It is similar to a camera on the robotic arm of the unsuccessful Mars Polar Lander spacecraft, though with an improved illumination system. For the Phoenix Robotic Arm Camera, Keller is the lead scientist and Chris Shinohara of the University of Arizona is the lead engineer.

The Surface Stereoscopic Imager will record panoramic views of the surroundings from atop a mast on the lander. Its images from two cameras situated about as far apart as a pair of human eyes will provide three-dimensional information that the Phoenix team will use in choosing where to dig and in operating the robotic arm.

A choice of 12 different filters for each eye enables the instrument to produce images not only in full color, but in a several specific visual and infrared frequencies useful for interpreting geological and atmospheric properties. The multispectral and three-dimensional information will help scientists understand the geology of the landing area.

The twin cameras will be able to look in all directions from a perch about 2 meters (7 feet) above Martian ground level. They will see with about the same resolution as human eyes, capturing each view onto 1-megapixel charge-coupled devices (a 1,024-by-1,024-pixel CCD for each eye).

The instrument will sometimes point upward to assess how much dust and water vapor is in the atmosphere. When the robotic arm delivers soil and ice samples to deck-mounted instruments, the Surface Stereoscopic Imager will be able to look downward to inspect the samples. Views of the spacecraft's deck will also monitor dust accumulation, which is of scientific interest for inferences about Martian winds and of engineering interest for effects of the dust buildup on solar panels.

A University of Arizona team led by Chris Shinohara built the Phoenix Surface Stereoscopic Imager. Mark Lemmon of Texas A&M University, College Station, is lead scientist for this camera. The instrument closely resembles a stereo imager on Mars Polar Lander, which in turn used design features from the imager on Mars Pathfinder, which provided stereo views from the surface of Mars in July 1997.

The Thermal and Evolved-Gas Analyzer will study substances that are converted to gases by heating samples delivered to the instrument by the robotic arm. It provides two types of information.

One of its tools, called a differential scanning calorimeter, monitors how much power is required to increase the temperature of the sample at a constant rate. This reveals which temperatures are the transition points from solid to liquid to gas for ingredients in the sample. The gases that are released, or "evolved," by this heating then go to a mass spectrometer, a tool that can identify the chemicals and measure their composition.

The mass spectrometer will determine whether the samples of soil and ice contain any organic compounds. It would be used to identify the types and amounts if any are present. Finding any would be an important result for interpreting the habitability of the site.

The instrument will also give information about water and carbon dioxide present as ices or bound to minerals. The amount of heat needed to drive off water or carbon dioxide that is bound to minerals is characteristically different for different minerals. The calorimeter's information from that process can help identify minerals in the soil, including carbonates if they are present.

The mass spectrometer will measure the ratios of different isotopes of carbon, oxygen, hydrogen, argon and some other elements in the Martian samples. Isotopes are alternate forms of the same element with different atomic weights due to different numbers of neutrons. Ratios can be changed by the effects of long-term processes that act preferentially on lighter or heavier isotopes of the same element. For example, some of Mars' original water was lost from the planet by processes at the top of the atmosphere, favoring the removal of lighter isotopes of hydrogen and oxygen and leaving modern Mars water with a raised ratio of heavier isotopes.

The instrument has eight tiny ovens for samples, each to be used only once. The ovens are about 1 centimeter (about half an inch) long and 2 millimeters (one-eighth inch) in diameter. At the start of an analysis, sample material is dropped into the oven through a screen. The oven closes after a light-beam detector senses that it is full. The experiment gradually heats samples to temperatures as high as 1,000 degrees Celsius (1,800 degrees Fahrenheit).The heating process drives off water and any other volatile ingredients as a stream of gases. Those gases are directed to the mass spectrometer.

One of the samples that the instrument will analyze will be a special material that the lander carries from Earth, specially prepared to be as free of carbon as possible. This will serve as an experimental control as the instrument analyzes samples excavated at Mars. The control material is made of a machinable glass ceramic substance named Macor, from Corning Inc. The arm will scrape some of it up and deliver it to the analyzer to get a reading showing how well the experiment can eliminate carbon carried from Earth. Carbon detected in assessments of Martian samples might be unavoidable traces of Earth carbon if the readings are no higher than the amount in this control sample.

The mass spectrometer part of the analyzer will examine samples of atmosphere at the landing site, in addition to the evolved gases from scooped-up samples. The atmospheric measurements will add information about humidity to the weather data monitored by the spacecraft's Meteorological Station.

The Thermal and Evolved-Gas Analyzer was built by teams at the University of Arizona, led by William Boynton (science lead) and Heather Enos (project manager), and at the University of Texas, Dallas, led by John Hoffman. It is adapted from a similar instrument with the same name that flew on the Mars Polar Lander mission in 1999.

The Microscopy, Electrochemistry and Conductivity Analyzer will use four tools to examine soil. It will assess characteristics that a gardener or farmer would learn from a soil test, plus several more. Three of the tools will analyze samples of soil scooped and delivered by the robotic arm -- a wet chemistry laboratory and two types of microscopes. The fourth tool is mounted near the end of the arm, and has a row of four small spikes that the arm will push into the ground to examine electrical conductivity and other properties of the soil.

The wet chemistry laboratory has four teacup-size beakers. Each will be used only once. Samples from Mars' surface and three lower depths may be analyzed and compared. The instrument will study soluble chemicals in the soil by mixing water with the sample to a soupy consistency and keeping it warm enough to remain liquid during the analysis.

On the inner surfaces of each beaker are 26 sensors, mostly electrodes behind selectively permeable membranes or gels. Some sensors will give information about the pH of the soil -- the degree to which it is acidic or alkaline. Soil pH is an important factor in what types of chemical reactions, or perhaps what types of microbes, a soil habitat would favor, and it is has never been measured on Mars. Other sensors will gauge concentrations of such ions as chlorides, bromides, magnesium, calcium and potassium, which form soluble salts in soil, and will record the level of the sample's oxidizing potential. One chemically important ion in soil, sulfate, cannot be directly sensed, so the analysis of each sample will end with a special process that determines the amount of sulfate by observing its reaction with barium. Comparisons of the concentrations of water-soluble ions in samples from different depths may provide clues to the history of water in the soil.

The wet chemistry setup has a built-in robotic laboratory technician that adds potions to each beaker in a choreographed two-day sequence. Before each soil sample goes in, about 25 cubic centimeters (nearly two tablespoons) of ice with dilute concentrations of several ions is slowly melted in a special container, a process that takes one to two hours. After the water is released into the beaker, the sensors make baseline measurements at this starting point. The first of five pill-size crucibles of prepared chemicals is then added to increase ion concentrations by a known amount in order to calibrate the measurements. Next, a drawer above the beaker extends to receive the soil sample, and the scoop on the robotic arm drops up to one cubic centimeter (one-fifth of a teaspoon) of soil into the drawer, which then retracts and dumps the sample into the beaker. A paddle stirs the soup for hours while the sensors take measurements. The next day, the second crucible adds nitrobenzoic acid to the beaker to test how ions from the soil react to increased acidity. The last three crucibles hold barium chloride. As they are added, one at a time, any sulfate from the soil reacts with the barium to make an insoluble compound, taking both the barium and the sulfate out of solution. The amount of sulfate in the soil sample is determined by measuring the amount of unreacted barium left behind.

The "microscopy" part of the Microscopy, Electrochemistry and Conductivity Analyzer will examine soil particles and possibly ice particles with both an optical microscope and an atomic force microscope. The robotic arm delivers soil samples to a wheel that rotates to present the samples to the microscopes. Along the perimeter of the wheel are substrates with different types of surfaces, such as magnets and sticky silicone. This allows the experiment to get information from the particles' interaction with the various surfaces, as well as from the sizes, shapes and colors of the particles themselves.

The biggest particles the optical microscope can view are about as long across as the thickness of a dime, just over a millimeter. The smallest it can see are about 500 times smaller -- about 2 microns across. That would be the smallest scale ever seen on Mars, except that the atomic force microscope will image details down to another 20 times smaller than that -- as small as about 100 nanometers, one one-hundredth the width of a human hair.

The optical microscope obtains color information by illuminating the sample with any combination of four different light sources. The illumination comes from 12 light-emitting diodes shining in red, blue, green or ultraviolet parts of the spectrum. The atomic force microscope assembles an image of the surface shape of a particle by sensing it with a sharp tip at the end of a spring, which has a strain gauge indicating how far the spring flexes to follow the contour of the surface. The process is like a much smaller version of a phonograph needle tracking the bumpiness inside the groove of a vinyl record.

The shapes and the size distributions of soil particles may tell scientists about environmental conditions the material has experienced. Tumbling rounds the edges. Repeated wetting and freezing causes cracking. Clay minerals formed during long exposure to water have distinctive, plate-shaped particle shapes.

The "conductivity" part of the Microscopy, Electrochemistry and Conductivity Analyzer will assess how heat and electricity move through the soil from one spike to another of a four-spike electronic fork that will be pushed into the soil at different stages of digging by the arm. For example, a pulse of heat will be put onto one spike, and the rate at which the temperature rises on the nearby spike will be recorded, along with the rate at which the heated needle cools off. A little bit of ice in the soil can make a big difference in how well the soil conducts heat. Similarly, soil's electrical conductivity is a sensitive indicator of moisture in the soil. Soil moisture may have subtle stages intermediate between frozen solid and liquid, including warm ice and water films, which may be biologically available. The device, called the thermal and electrical conductivity probe, adapts technology used in commercial soil-moisture gauges for irrigation control systems.

The conductivity probe has an additional role besides soil analysis. It will serve as a humidity sensor when held in the air. Also, slight temperature changes from one spike to the next can allow it to estimate wind speed.

The Microscopy, Electrochemistry and Conductivity Analyzer is based on an instrument developed for the Mars Surveyor 2001 Lander mission, which was canceled in 2000. The instrument for Phoenix inherited many of the original electronic and structural components. The conductivity probe and other improvements have been added to the earlier design.

A team led by Michael Hecht at NASA's Jet Propulsion Laboratory, Pasadena, Calif., designed and built the analyzer. A consortium led by Urs Staufer of the University of Neuchatel, Switzerland, provided the atomic force microscope. The University of Arizona provided the optical microscope, equipped with an electronic detector (the same as in the Robotic Arm Camera) from the Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany. John Marshall of the SETI Institute, Mountain View, Calif., is lead scientist for the optical microscope. Transfer Engineering and Manufacturing Inc., Fremont, Calif. (formerly Surface/Interface Inc. of Mountain View, Calif.) designed the sample wheel for the microscopes. Aaron Zent of NASA Ames Research Center, Moffett Field, Calif., is science lead for the thermal and electrical conductivity probe, built by Decagon Devices Inc., Pullman, Wash. For the wet chemistry experiment, Thermo Fisher Scientific (formerly the Water Analysis Division of Thermo Corp., Beverly, Mass.) provided the chemical beakers; Starsys Research Corp., Boulder, Colo. provided the chemistry actuator assemblies and Tufts University, Medford, Mass., prepared the crucibles of reagents for mixing with the soil samples. Sam Kounaves of Tufts is lead scientist for the wet chemistry investigation.

The Meteorological Station will track daily weather and seasonal changes using temperature and pressure sensors plus a laser-reflection instrument. The information collected by this first high-latitude weather station on Mars will aid understanding of how water is cycled seasonally between ice on the ground and vapor in the atmosphere.

The laser tool, called a lidar for "light detection and ranging," uses powerful laser pulses in a way comparable to radio pulses emitted by a radar instrument. The laser beam is emitted vertically into the atmosphere. Atmospheric dust and ice particles in the beam's path reflect the light, sending it in all directions, including straight downward. A telescope integrated into the instrument detects the downward-reflected light. Analysis of the strength and time-delay of the reflections reveals information about the sizes and altitudes of the particles. Tracking changes in these atmospheric particles' abundances and locations over time will help researchers study how clouds and dust plumes form and move.

The weather station includes a 1.2-meter (4-foot) mast bearing sensors at three heights to monitor how temperature varies with height near the surface. The temperature sensors are thin-wire thermocouples; they measure temperature by its effect on the flow of an electrical current through a closed circuit of two metals with different thermal properties. The thermocouples use the metals chromel (a nickel and chromium alloy) and constantan (a copper and nickel alloy). Also, hanging from the top of the mast is a wind telltale. This is a small tube that will be deflected by the wind. The science payload's stereo camera will record images of the telltale that will be used to determine wind direction and speed. The top of the meteorology mast, at 1.14 meters (3.75 feet) above the deck, is the highest point on the lander.

The Canadian Space Agency provided the Meteorological Station for Phoenix. Jim Whiteway of York University, Toronto, Ontario, leads the Canadian science team. The instrument construction was led by the Space Missions Group of MDA Ltd., Brampton, Ontario, with contributions from Optech Inc., Toronto, for the lidar. The Finnish Meteorological Institute provided the instrument for measuring atmospheric pressure. Aarhus University, Denmark, constructed the wind telltale.

The Mars Descent Imager will take a downward-looking picture during the final moments before the spacecraft lands on Mars. This image will provide a bridge between orbiter-scale and lander-scale images. It is expected to show geological context helpful for planning the lander's activities and for interpreting other science instruments' observations and measurements. Conditions at the landing site have different implications if the site appears to be typical of a much broader area than if the site happens to be an unusual patch of ground unlike its surroundings.

This camera is mounted on the outer edge of the payload deck of the lander. It was designed to take several images, but the plan was altered to just one image after testing showed that a data-handling component elsewhere on the lander had a small possibility of triggering loss of some vital engineering data if it receives imaging data during a critical phase of final descent. The timing of the image will be planned to yield higher resolution of the landing area than currently possible from orbit.

The imager weighs just 480 grams (1 pound). The optics provide a field of view of 75.3 degrees. Exposure time is 4 milliseconds.. At that speed, some blurring may occur as the descent engines vibrate the spacecraft while the camera takes its image.

Future spacecraft to the surface of Mars may need capability for steering themselves to avoid hazards or to reach specific landing sites. Descent imaging could be an important component of the technology for accomplishing that.

A tiny microphone is riding on the descent camera. It might catch sounds around the spacecraft while the camera is taking its image. There are no plans to power the camera to take pictures or to record sounds after the landing.

The Mars Descent Imager that will fly on Phoenix was originally built for the Mars Surveyor 2001 Lander mission, by a team led by Michael Malin at Malin Space Science Systems, San Diego. A similar camera flew on Mars Polar Lander.

Research Strategy

The planned operational life of the Phoenix Mars lander after it reaches Mars is 90 Martian days. Each Martian day, often called a "sol," lasts about 40 minutes longer than an Earth day. That gives the Phoenix team three months to use the lander's instruments to address the water and habitat questions of the mission's science objectives. Planning and practice simulations before landing will prepare the team to make the best possible advantage of that time.

Observations made by orbiter spacecraft during the evaluation of candidate landing sites for Phoenix provide a starting-point base of knowledge about the area. After Phoenix has landed, views from the Surface Stereoscopic Imager, with added context from the Mars Descent Camera and closer looks with the Robotic Arm Camera, will be used for choosing where to collect the first soil sample for analysis.

The first samples fed into the lander's analyzers will come from the surface. Decisions about how much deeper to go before analyzing another sample will depend on results from the surface material and on what the robotic arm camera and stereo imager see in the soil. The Thermal and Evolved-Gas Analyzer can check for organics and other volatiles in up to eight samples. Researchers must be choosier with samples for the wet chemistry laboratory of the Microscopy, Electrochemistry and Conductivity Analyzer, which can examine four different samples. The microscopes and conductivity probe can analyze soil more frequently during the 90 sols. Meanwhile, the weather station and stereo imager will monitor changes in water and dust in the atmosphere throughout the mission. If the spacecraft remains functional longer than the 90-day prime mission, weather information might be collected during the approach of the northern hemisphere autumn on Mars, when dwindling sunlight will eventually make operations impossible, and buildup of carbon-dioxide frost will coat the spacecraft.

What types of findings would help answer questions about the history of water? If the microscopes find fine silty sediments or clay textures, that would be evidence supporting the hypothesis that the northern highlands of Mars once held an ocean. The presence of carbonates or other minerals that form in liquid water would be another. Rounded sand grains in the soil could suggest a history of flowing water.

The mass spectrometer will measure isotopic ratios. If there are differences between those ratios in subsurface ice and in atmospheric water, that could suggest the subsurface ice is ancient. A gradient in the concentrations of salts at different depths in the soil would support the hypothesis that climate cycles periodically thaw some subsurface ice. The conductivity probe's findings about thermal properties of the soil, combined with determination of the depth to an icy layer, could strengthen estimates for how much change in climate would be needed to melt the ice. The same ground-truth information will refine models of the ice's depth and of atmosphereice interactions in widespread areas of Mars that contain subsurface ice.

Using the microscopes and arm camera to learn about how porous and layered the soil is will help assess whether liquid water has come and gone. The conductivity probe will assess whether, even today, the soil may have thin films of unfrozen water. Atmospheric measurements as the season progresses through the Martian summer will aid the understanding of how water is seasonally cycled between solid and gas phases in the current Martian climate.

What types of findings would help answer questions about whether this site could formerly or still support microbial life? Three important factors in the suitability of a habitat for life are the availability of liquid water, the presence of carbon and access to energy. So evidence about the history of water will be one part of evaluating the habitat.

The Thermal and Evolved-Gas Analyzer has the dramatic job of checking for organic carbon molecules layer by layer as samples come from farther below the surface. Soluble sulfate minerals would be a possible source of energy that could sustain life. The wet chemistry lab of the Microscopy, Electrochemistry and Conductivity Analyzer can identify potential chemical-energy sources if they are present in soil samples. While the science payload of Phoenix is not designed for life detection, this mission is an important stepping stone in the search for whether Mars has life. Orbital observations indicate that areas with shallow subsurface ice make up at least a quarter of the red globe. Phoenix will be the first mission to visit such a site. Its findings about habitability could guide where a future spacecraft searching for life is sent.

Researchers have equipped Phoenix to look for answers to many questions posed in advance about water and habitat. However, if previous interplanetary missions are an indicator, some of the most important results from Phoenix may be surprises that raise new questions.