When NASA’s Perseverance rover lands on Mars in February next year, it will begin its search for evidence of ancient life on the planet. To do this, the rover will be collecting samples of the Martian soil and rocks for analysis using its onboard instruments. But there’s only so much science that can be done in a rover the size of a small car. To complete a full analysis, scientists need to get those samples off Mars and bring them back to Earth. And that means that they need to be preserved in tubes which will keep them safe over a journey of millions of miles, which may take more than a decade.
The sample tubes onboard Perseverance — 43 in all — are about the size and shape of a test tube but are made mostly of titanium rather than glass. That makes them lightweight and strong, and they are covered in a special coating to protect them from the heat of the sun. And they had to be made to extremely exact specifications to fit into the rover’s sample analysis system perfectly.
“They are less than 6 inches [15.2 centimeters] long, but we still found over 60 different dimensions to scrutinize,” Sample Tube Cognizant Engineer Pavlina Karafillis of NASA’s Jet Propulsion Laboratory said in a blog post. “Because of the intricacies of all the complex mechanisms they will pass through during the Mars Sample Return campaign, if any measurement was off by about the thickness of a human hair, the tube was deemed not suitable for flight.”
The real challenge though was keeping the tubes clean. They need to be not just free of dust from Earth, but free from any kind of Earth contaminants at all. If any kind of lifeform or evidence of a lifeform from Earth got into the tubes and was then mixed into the Martian sample, it would throw off the results of any search for life completely.
“Compared to Mars, Earth is filled with evidence of the life that covers our planet,” explained Ken Farley, Mars 2020 project scientist at Caltech, in the post. “We needed to remove those signs so thoroughly that any scant evidence remaining can be confidently detected and differentiated when these first samples are returned.”
This means that absolutely nothing should be in any of the tubes that were loaded into Perseverance. “And when they said ‘nothing,’ they meant it,” said Ian Clark, the mission’s assistant project systems engineer for sample tube cleanliness at JPL. “An example: To achieve the kind of science the mission is going after, we needed to limit the total amount of Earth-based organic compounds in a given sample to less than 150 nanograms. For a set of particular organic compounds — ones that are very indicative of life — we were limited to less than 15 nanograms in a sample.”
To achieve this, the team used a variety of cleaning techniques with multiple steps of decontamination. “We did all our assembly in a hyper-clean-room environment, which is essentially a clean room inside a clean room,” said Clark. “Between assembly steps, the sample tubes would be cleaned with filtered air blasts, rinsed with deionized water, and sonically cleaned with acetone, isopropyl alcohol, and other exotic cleaning agents.”
Finally, with all 43 tubes ready, they were loaded into the rover. Of these tubes, 38 will collect samples from Mars. The other five will act as “witness tubes,” providing one more layer of protection against contamination by sampling the atmosphere around a sample collection. If there are any impurities or contaminants coming from the rover itself or the spacecraft parts, these will be detected in the witness tubes and can be allowed for.
Perseverance is set to land on Mars and begin its mission to search for evidence of ancient life on February 18, 2020. NASA hopes to begin the process of collecting samples and returning them to Earth in the early 2030s.
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