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

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. Finally, comparison of the WAE results with ground test results makes possible a comparison of the Martian soil with mineral grain types and sizes found on Earth and show that the Martian dust is fine-grained and of limited hardness.