The NASA equivalent of duct tape could leak enough methane to confuse the next Mars rover’s life-detecting sensors.
Astrobiologists found evidence for three distinct plumes of methane flowing from beneath the planet’s surface, like swamp gas or a burp, in January 2009. The gas could simply mean that Mars is more geologically active than previously thought. But because much of Earth’s methane is a byproduct of life, the plumes could point to something living, eating or breathing methane beneath the Martian surface.
To settle the question of the methane’s origin, the next Mars rover, called Mars Science Laboratory or Curiosity, will launch in late 2011 equipped with a suite of instruments capable of sniffing out one molecule of methane in a billion other molecules.
But some of the materials in the rover itself could also release methane and confuse the sensors. In a paper in press in the journal Icarus, microbiologist and veteran Mars simulator Andrew Schuerger of the University of Florida and colleagues show that the tape used to hold the rover’s joints together could release enough methane to be a problem.
“I think it’s a valid concern,”said planetary scientist Adam Johnson of Indiana University, who has investigated which Earth microbes could hitchhike to Mars but was not involved in the new work. “We’re sending a very very sensitive instrument, and we are able to produce concentrations of methane that are orders of magnitude above the detection limits for that instrument.”
Schuerger and colleagues placed 18 materials in the Mars Simulation Chamber, a stainless steel cylinder whose interior mimics the atmosphere, dustiness, sunlight, temperature and pressure at the Martian surface.
“Andrew’s simulation setup in his chamber is state of the art, the best simulation chamber in the world,” Johnson said. “As far as simulation of the Mars conditions, you can’t ask for much better.”
The researchers tested a variety of biological materials, including amino acids, DNA and spores of a common soil-dwelling bacterium. They also checked several materials used to build the rover itself, including vacuum grease, a small sundial like the one rovers Spirit and Opportunity use to calibrate colored images, and kapton tape, the space industry equivalent of duct tape.
“I kind of think of it as electrical tape on Mars,” Johnson said. “It’s used for everything on there.”
After eight hours in the chamber, all the organic materials tested emitted some amount of methane, though not enough to worry about in most cases. The methane comes from the interaction of sunlight with materials that contain a methyl group, one carbon atom attached to three hydrogen atoms. Ultraviolet radiation from the sun (or, in the simulation chamber, a special lamp) could rip methyl groups from the materials. The charged methyl groups could then steal an extra hydrogen atom from a neighboring molecule to form stable molecules of methane, which has one carbon and four hydrogens.
Surprisingly, the bacterial spores they tested leaked noticeable amounts of methane, even after they had been irradiated to death. But the standards for cleaning the rover before launch are so stringent that there probably won’t be enough spores left on the rover by launch time to pose much of a problem.
The most trouble could come from kapton tape, which is ubiquitous and unavoidable on the rover. Schuerger’s team found that in the first few Martian days of the mission, the sensors in Curiosity’sTunable Laser Spectrometer could pick up a few tens of methane molecules per million other molecules, about 100 times above the instrument’s detection limits.
This is especially worrisome given that Curiosity uses about 3 square meters of kapton tape, more than any previous rover.
“It’s a big monster rover,” said NASA planetary scientist Paul Mahaffy, who is in charge of MSL’s Sample Analysis at Mars (SAM) instrument. “They use the appropriate level of tape to secure that stuff down. There’s just more of it than there might have been on [Spirit and Opportunity].”
The rover team already has a few low-tech solutions in mind to find the true Martian methane, Mahaffy said. First, they’ll take measurements at night, when ultraviolet radiation will be at a low.
“My best guess is, once you rotate into the dark, methane production stops pretty fast,” Mahaffy said. “By sampling at night we’d get a much cleaner sniff of the Martian atmosphere.”
The rover will also rotate the sensors into the wind to get the strongest whiff of the Martian atmosphere. Schuerger and colleagues suggest coming up with more detailed models of how much methane kapton tape will produce, and where on the rover it’s likely to show up. They also note that kapton tape gives off less and less methane as time goes on, so methane detections in the later parts of the mission should be more reliable.
“By no means does is nullify the measurement we’re trying to do on [Mars Science Laboratory],” Mahaffy said.
Still, the study is “very useful,” Mahaffy said. “It will help us do a better job of sorting out what’s really there on Mars, and what we might bring along from Earth. The last thing we want to do is have a false positive.”
Images: 1. Engineers assemble the Mars Science Laboratory (“Curiosity”), using rolls of shiny kapton tape. (NASA/JPL-Caltech) 2. The Mars Simulation Chamber. (Schuerger et al./Icarus)
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