Evidence for Martian electrostatic charging and abrasive wheel wear from the Wheel Abrasion Experiment on the Pathfinder Sojourner rover

Abstract

Abstract. The Wheel Abrasion Experiment (WAE) on the Mars Pathfinder rover was designed to find out how abrasive the Martian dust would be on strips of pure metals attached to one of the wheels. A specially modified wheel, with 15 thin film samples (five each of three different metals), specularly reflected sunlight to a photovoltaic sensor. When the wheel was rotated to present the different sample surfaces to the sensor, the resulting signal was interpreted in terms of dust adhesion and abrasive wear. Many data sequences were obtained. Ground tests of similar wheels in a simulated Martian environment showed that static charging levels of 100-300 V could be expected. To prevent the possibility of Paschen discharge in the low-pressure Martian atmosphere, charge dissipation points were added to the Sojourner rover and were shown in ground tests to keep charging levels at 80 V or less. Nevertheless, significant dust accumulations on Sojourner's wheels may be interpreted as evidence for electrostatic charging. Simple considerations of the expected maximum level of charging and electrostatic dust adhesion lead to an estimate for the size of the adhering dust grains. From the WAE data, it is hypothesized that the photoelectric effect is the most important mechanism for slow discharge in Martian daylight. Sensor signals obtained late in the Pathfinder mission show that significant wheel wear was seen on the metal wheel strips, with the most wear on the thinnest aluminum samples and the least on the thickest nickel and platinum samples. An estimate is made of the reflectance of the adhering Martian dust. The depth of dig of the WAE wheel shows that the dust is in some places very loose and in others tightly packed. On Mars, the amount of wear for each metal coating was detected by a specially built photodetector, mounted on the wheel strut [see Wilt et al., 1997]. During each experiment, the WAE wheel was rotated backward through two revolutions, while Sojourner was held stationary by the other five nonrotating wheels, and the photodetector determined the amount of Martian sunlight reflected in turn by each metal coating on each WAE wheel strip. In some experiments the wheel was then rotated three revolutions without taking WAE data to help build up wear and, then, two more revolutions while taking data. Ideally, Sojourner was first pointed in the direction that would lead to the best specular reflection from the Sun, and the change in the amount of reflected signal indicated the amount of metal which had been removed by abrasion from each surface. In reality, inaccurate pointing and dust unevenly coating the wheel led to difficulties in interpreting the data. The wheel rotation was at a rate of 1 cm/s, and five readings were obtained from each coated sample in each wheel rotation. The central, or peak, reading from each sample is, by design, unaffected by light reflected from any adjacent samples.Complicating the interpretation of the results was a change in the reflectance due to dust which stuck to the WAE wheel.The...

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