Mars Pathfinder Instrument Descriptions

Imager For Mars Pathfinder (IMP)

The Imager For Mars Pathfinder is a stereo imaging system with color capability provided by a set of selectable filters for each of the two camera channels. It has been developed by a team lead by the University Of Arizona with contributions from the Lockheed Martin Group, Max Planck Institute For Aeronomy in Lindau, Germany, the Technical University Of Braunschweig in Germany and the Ørsted Laboratory, Niels Bohr Institute for Astronomy, Physics and Geophysics in Copenhagen, Denmark. It consists of three physical subassemblies: (1) camera head (with stereo optics, filter wheel, CCD and pre-amp, mechanisms and stepper motors); (2) extendable mast with electronic cabling; and (3) two plug-in electronics cards (CCD data card and power supply/motor drive card) which plug into slots in the Warm Electronics Box within the lander.

Figure 1. Close-Up of the IMP Camera Head

Azimuth and elevation drives for the camera head are provided by stepper motors with gear heads, providing a field of regard of ±180 degrees in azimuth and +83 degrees to -72 degrees in elevation, relative to lander coordinates. The camera system is mounted at the top of a deployable mast, a continuous longeron, open-lattice type provided by Able Manufacturing, Inc. When deployed, the mast provides an elevation of 1.0 m above the lander mounting surface.

The focal plane consists of a CCD mounted at the foci of two optical paths where it is bonded to a small printed wiring board, which in turn is attached by a short flex cable to the preamplifier board. The CCD is a front-illuminated frame transfer array with 23 micrometer square pixels. Its image section is divided into two square frames, one for each half of the stereo FOV's. Each has 256x256 active elements. A 256x512 storage section (identical to the imaging section) is located under a metal mask. The imp focal plane and electronics are nearly identical copies of the comparable subsystem employed in the Huygens Probe Descent Imaging Spectroradiometer (DISR), using the Loral 512X512 CCD.

The stereoscopic imager includes two imaging triplets, two fold mirrors separated by 150 mm for stereo viewing, a 12-space filter wheel in each path, and a fold prism to place the images side-by-side on the CCD focal plane. Fused silica windows at each path entrance prevent dust intrusion. the optical triplets are an f/10 design, stopped down to f/18 with 23-mm effective focal lengths and a 14.4 degree field of view. The pixel instantaneous field of view is one milliradian. The filter wheel four pairs of atmospheric filters, two pairs of stero filters, eleven individual geologic filters (which, when combined with the two pairs of stereo filters, result in thirteen distinct geologic filters) and one diopter or close-up lens, designed to acquire images of magnetic, wind-blown dust which adheres to a small magnet located on the IMP tip plate.

Full panoramas of the landing site are acquired during the mission using the stereo baseline provided by the camera optics. Additionally, monoscopic panoramas are acquired both prior and subsequent to the mast deployment, yielding vertically displaced stereo pairs with approximately 80 cm baseline. Images of a substantial portion of the visible surface are acquired in multispectral images with as many as eight spectral bands.

Figure 2. IMP Deployed On The Able Mast

A number of atmospheric imvestigations are carried out using imp images. aerosol opacity is measured periodically by imaging the Sun through two narrow-band filters. Dust particles in the atmosphere are characterized by observing Phobos at night. Water vapor abundance is measured by imaging the Sun through filters in the water vapor absorption band and in the spectrally adjacent continuum. Images of wind socks located at several heights above the surrounding terrain are used to assess wind speed and direction.

A magnetic properties investigation is included as part of the IMP investigation. A set of magnets of differing field strengths will be mounted to a plate and attached to the lander. Images taken over the duration of the landed mission are used to determine the accumulation of magnetic species in the wind-blown dust. Multispectral images of these accumulations may be used to differentiate among likely magnetic minerals.

The IMP investigation also includes the observation of wind direction using a small wind sock mounted above a reference grid, and a calibration and reference target mounted to the lander.

Alpha Proton X-Ray Spectrometer

This instrument is a foreign-provided derivative of instruments flown on the Russian Vega and Phobos missions and identical to the unit that was planned for flight on the Russian Mars '96 missions. Accordingly, the instrument has extensive, applicable flight heritage. With the mobility provided by the microrover and a deployment mechanism, the APX Spectrometer not only acquires spectra from the ubiquitous martian dust, but more importantly, is deployed to distinct rock outcroppings, thereby analyzing the native rock composition for the first time. The alpha and proton spectrometer portions are provided by the Max Planck Institute, Department of Chemistry, Mainz Germany. The x-ray spectrometer portion is provided by the University Of Chicago.

Figure 3. Alpha Proton X-Ray Spectrometer Front View On Rover

This elemental composition instrument consists of alpha particle sources and detectors for back-scattered alpha particles, protons and X-Rays. The APX Spectrometer will determine elemental chemistry of surface materials for most major elements except hydrogen. The analytical process is based on three interactions of alpha particles with matter: elastic scattering of alpha particles by nuclei, alpha-proton nuclear reactions with certain light elements, and excitation of the atomic structure of atoms by alpha particles, leading to the emission of characteristic X-Rays. The approach used is to expose material to a radioactive source that produces alpha particles with a known energy, and to acquire energy spectra of the alpha particles, protons and X-Rays returned from the sample. Such an instrument can identify and determine the amounts of most chemical elements.

The basis of the alpha mode of the instrument is the dependence of the energy spectrum of alpha particles scattered from a surface on the composition of the surface material. The method has the best resolving power for the lighter elements.

Figure 4. Alpha Proton X-Ray Spectrometer Sensor Head

The proton spectra for alpha particles interacting with elements with atomic numbers from 9 to 14 are very characteristic of the individual elements, reflecting the resonance nature of the nuclear interactions involved. The proton mode allows their detection and measurement.

The addition of a third detector for X-Rays Results in a significant extension of the accuracy and sensitivity of the instrument, particularly for the heavier, less abundant elements. Alpha sources produce characteristic X-Rays for a range of elements, giving an instrument sensitivity that can approach the ppm level.

The APXS sensor head is mounted external to the Rover chassis on a deployment mechanism (described below). This mechanism places the APXS in contact with rock and soil surfaces. The APXS electronics are mounted within the rover, in a temperature-controlled environment.

APXS Deployment Mechanism

The deployment mechanism supports the APXS under launch and landing loads and provides a means for positioning the APXS with a single actuator. The linkage allows the APXS to be placed at a variety of elevations above nominal ground level and at a variety of rotational orientations. The mounting of the APXS to the deployment mechanism permits about 20 degrees of compliance motion as the APXS is placed in contact with the sample. A set of contact closures on the APXS front aperture ring indicate to the Rover that the positioning is complete, thereby terminating the positioning motions.

Figure 5. Alpha Proton X-Ray Spectrometer Deployment Mechanism

Atmospheric Structure Instrument/Meterology Package

The ASI/MET is an engineering subsystem which acquires atmospheric information during the descent of the lander through the atmosphere and during the entire landed mission. It is implemented by JPL as a facility experiment, taking advantage of the heritage provided by the Viking mission experiments.

Data acquired during the entry and descent of the lander permits the reconstruction of profiles of atmospheric density, temperature and pressure from altitudes in excess of 100 km to the surface.

The accelerometer portion of the ASI is provided by the Attitude And Information Management subsystem of the lander. It consists of x-, y- and z-axis sensors. Several gain states are provided to cover the wide dynamic range from the micro-g accelerations experienced upon entering the atmosphere to the peak deceleration and landing events in the range of 30 to 50 g's.

The ASI/MET instrument hardware consists of a set of temperature, pressure and wind sensors and an electronics board for operating the sensors and digitizing their output signals. Temperature is measured by thin wire thermocouples mounted on a meteorological mast that is deployed after landing. The location of one thermocouple is chosed to measure atmospheric temperature during descent, and three more monitor atmospheric temperatures 25, 50, and 100 cm above the surface during the landed mission. Pressure is measured by a Tavis magnetic reluctance diaphragm sensor similar to that used by Viking, both during descent and after landing. The wind sensor employs six hot wire elements distributed uniformly around the top of the mast. Wind speed and direction 100 cm above the surface are derived from the temperatures of these elements.

Figure 6. ASI/MET Temperature And Wind Sensors

During entry and descent, the sampling of acceleration, pressure and temperature data is optimized to the vertical rate of decent through the atmosphere. After landing, considerable flexibility in data averaging and sampling is provided to measure the variation of temperature, surface pressure and wind on short, diurnal, and seasonal time scales over the life of the mission.

Three wind socks are located at various heights on the meterology mast to determine the speed and direction of winds at the Pathfinder landing site. The wind socks will be imaged repeatedly by the IMP. The orientations of the wind socks will be measured in the images to determine the wind velocity at three heights above the surface. This information can then be used to estimate the aerodynamic roughness of the surface in the vicinity of the lander, and to determine the variation in wind speed with height. Because the Viking landers had wind sensors at only one height, such a vertical wind profile has never been measured on Mars. This new knowledge will help to develop and modify theories for how dust and sand particles are lifted into the martian atmosphere by winds, for example. Because erosion and deposition of wind-blown materials has been such an important geologic process on the surface of Mars, the results of the wind sock experiment will be of interest to geologists as well as atmospheric scientists.

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