About two years ago I got a call from Professor Ben Weiss of MIT asking about SuperCam’s laser marking capabilities. Ben had just joined Perseverance as part of Return Sample Science, a group focused on collecting samples for return to Earth, with the goal of ensuring samples are collected under the right conditions to improve their scientific value once again on Earth. Ben’s specialty is ancient magnetism. In terrestrial rocks, this is the study of magnetism caused by the Earth’s magnetic field at the time of rock formation. Mars currently has a very weak magnetic field, but the strength of the Martian field in the past is largely unknown. It has important implications for the retention or loss of the Martian atmosphere over time, among other things. Suffice it to say that we would like to use the samples returned from the persistence task to fill in that knowledge gap.
To do this, for each sample of Martian rock that is returned, we need to know its original orientation. If the surfaces of those core samples have easily identifiable features, there is no problem. This was the case with the cores collected so far. However, if the surface is fine-grained, its rotational direction may not be distinguished. In this case, we need to make artificial marks on the surface.
We don’t have a dark marker, but we do have a pulsed laser. So Ben’s call to my lab two years ago got us thinking about how to identify samples, and we started some tests. JPL shipped several rocks of varying hardness to Los Alamos National Laboratory where they were marked with craters made with different numbers of laser shots. The rocks were sent back to the Jet Propulsion Laboratory for later drilling.
Fast forward to summer 2022. Team SuperCam was asked to be ready to mark a rock for drilling with only a few days’ notice. I was on SuperCam processes, and seeing when we might need tags soon, we decided to switch from the normal observation to the basic tagging sequence as a dry process. We have prepared different styles for the tag. The basic principle is to understand the rotational direction of the core after it is removed from the rock and placed in the sample tube. So, any asymmetrical pattern like the arrow will do the trick. However, in order to achieve efficiency, we decided to use the simplest such pattern, consisting of three points (or laser etching) with an unequal distance between them, like the capital letter “L.” SuperCam usually scans line (single row) or network patterns. To produce the “L” shape, we took a 2×2 grid pattern and removed one point from the sequence, so the laser only made three pits. Using 125 laser shots per hole, the result is shown in the “Pinefield Gap” target image. The sample cores are 13 mm (0.5 in) in diameter, so the L patterns should fit well on their top surfaces. With the dry run successful, we are ready to use the procedure to select future samples.
Over the past week, Perseverance has completed the second of two samples from the Jezero crater delta formation, from the Skinner Ridge block in the Hugwalloo Flats. Over the weekend, persistence drove about 25 meters to Wildcat Ridge, located slightly higher in Hogwallow, for further exploration.