The Robotic Arm (RA) is critical to the operations of the Phoenix lander and is designed to dig trenches, scoop up soil and water ice samples, and deliver these samples to the TEGA and MECA instruments for detailed chemical and geological analysis. Designed similar to a back hoe, the RA can operate with four degrees of freedom: (1) up and down, (2) side to side, (3) back and forth, and (4) rotate around.
The RA will be 2.35 meters long with an elbow joint in the middle, allowing the arm to trench about 0.5 meters below the martian surface, deep enough to where scientists believe the water-ice soil interface lies. At the end of the RA is a moveable scoop, which includes ripper tines (sharp prongs) and serrated blades. Once icy soil is encountered, the ripper tines will be used to first tear the exposed materials, followed by applying the serrated blades to scrape the fractured soil. The scoop will then be run through the furrows to capture the fragmented samples, ensuring enough sample mass for scientific study on the lander platform.
Thermal and Evolved Gas Analyzer (TEGA) is a combination high-temperature furnace and mass spectrometer instrument that scientists will use to analyze martian ice and soil samples. The robotic arm will deliver samples to a hopper designed to feed a small amount of soil and ice into eight tiny ovens about the size of an ink cartridge in a ballpoint pen. Each of these ovens will be used only once to analyze eight unique ice and soil samples.
Once a sample is successfully received and sealed in an oven, the temperature is slowly increased at a constant rate, and the power required for heating is carefully and continuously monitored. This process, called scanning calorimetry, shows the transitions from solid to liquid to gas of the different materials in the sample: important information needed by scientists to understand the chemical character of the soil and ice.
As the temperature of the furnace increases up to 1000°C, the ice and other volatile materials in the sample are vaporized into a stream of gases. These are called evolved gases and are transported via an inert carrier to a mass spectrometer. The mass spectrometer is sensitive to detection levels down to 10 parts per billion, a level that may detect minute quantities of organic molecules potentially existing in the ice and soil.
The Microscopy, Electrochemistry, and Conductivity Analyzer, MECA's wet chemistry lab contains four single-use beakers, each of which can accept one sample of martian soil. Phoenix's Robotic Arm will initiate each experiment by delivering a small soil sample to one beaker, which is ready and waiting with a pre-warmed and calibrated soaking solution. Alternating soaking, stirring, and measuring, the experiment continues for the entire day. It concludes with the addition of two chemical pellets. The first contains an acid to tease out carbonates and other constituents that are only soluble in acidic solutions. The second contains specific reagents to test for sulfates and soil oxidants.
The optical and atomic-force microscopes complement MECA's wet chemistry experiments. With images from these microscopes, scientists will examine the fine detail structure of soil and water ice samples. The optical microscope will have a resolution of 4 microns per pixel, allowing detection of particles ranging from about 10 micrometers up to the size of the field of view (about 1 mm by 2 mm). Red, green, blue, and ultraviolet LEDs will illuminate samples in differing color combinations to enhance the soil and water-ice structure and texture at these scales. The atomic force microscope will provide sample images down to 10 nanometers - the smallest scale ever examined on Mars. Using its sensors, the AFM creates a very small-scale "topographic" map showing the detailed structure of soil and ice grains.
Prior to observation by each of the microscopes, samples are delivered by the Robotic Arm to a wheel containing sixty-nine different substrates. The substrates are designed to distinguish between different adhesion mechanisms and include magnets, sticky polymers, and "buckets" for bulk sampling. The wheel is rotated allowing different substrate-sample interactions to be examined by the microscopes.
MECA's final instrument, the thermal and electrical conductivity probe, will be attached at the "knuckle" of the RA. The probe will probably consist of three small spikes that will be inserted into the ends of an excavated trench. In addition to measuring temperature, the probe will measure thermal properties of the soil that affect how heat is transferred, providing scientists with better understanding of surface and atmospheric interactions. Using the same spikes, the electrical conductivity will be measured to indicate any transient wetness that might result from the excavation. Most likely, the thermal measurement will reflect ice content and the electrical, unfrozen water content.
Meteorological Station (MET) will record the daily weather of the martian northern plains using temperature and pressure sensors, as well as a light detection and ranging (LIDAR) instrument. With these instruments, MET will play an important role by providing information on the current state of the polar atmosphere and how water is cycled between the solid and gas phases in the martian arctic.
The MET's lidar is an instrument that operates on the same basic principle as RADAR, using powerful laser light pulses rather than radio waves. The lidar transmits light vertically into the atmosphere, which is reflected off dust and ice particles. These reflected light pulses and their time of return to the lidar instrument are analyzed, revealing information about the size of atmospheric particles and their location.
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