Biology Hanging By A Thread?

The Knoll Criterion on Mars

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Airbag marks near where the Sol 19 image was captured containing the thread seen in microscopic view in the banner image.
Credit:NASA/JPL

One engineering obstacle to overcome when landing on Mars is the treacherous descent and landing. From start to finish, this mission phase can last six minutes. Because of its nail-biting drama, it is often referred to as the six minutes of hell.

If horizontal winds blow the rover’s parachute sideways during descent, the precious payload might scrape rather than bounce. That possibility of shear against sandpaper-like soil prompted a relatively late addition to the mission planning. Stabilizing horizontal thrusters were added to compensate if any wind started to tilt the otherwise vertical path. This attention to detail proved invaluable during the first landing attempt. When the Spirit rover descended towards Gusev crater, just such unpredictable winds had to be corrected for. If all had not gone according to plans, the airbag fabric might have ripped catastrophically.

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Airbag Vectran fabric laid across simulated Mars soil from the JPL sandbox in Pasadena California .
Credit:NASA/JPL

On February 12th, the nineteenth day on the other side of the planet for the Opportunity rover, one curious image stood out. The picture was downloaded in the daily batch from a microscopic imager peering onto the pebbly surface. On Sol 19, a long, thin feature surprised the science team.

Measuring 6 millimeters long and 60 micrometers across, this thread was smaller than the size of an average human hair. At first glance, many speculated whether the thread might point towards some strange biological origin.

The lack of another microscopic image capturing such a thread in view, however, made the science and engineering team’s detective work difficult. But using their expertise from so many landing simulations, the rover team set out to test if they could reproduce this feature in the JPL sandbox.

A best first guess was that when the rover’s airbag hit the surface, tiny threads had been stripped from the fabric and laid out across the martian soil. Their experiment entailed a grab bag of starting materials: Mars soil simulant and airbag fabric made of Vectran (a synthetic material stronger than Kevlar, which is tough enough to qualify for bulletproof vests).

Placing Vectran threads against the backdrop of simulated Mars soil gave the team a first view of what the microscopic imager might have seen.

To recreate similar conditions, the team still needed to know exactly where the rover was on Sol 19. They also wanted to know how its robotic arm turret was positioned for such an extended camera view.

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Rover impact simulation video frame. Each rover could bounce twenty or more times before coming to rest, with the average impact of a golf cart being dropped from the roof of a five-story building.
Credit: D. Maas/NASA/JPL/Cornell

The rover’s navigation and front hazard avoidance cameras narrowed down their choices to the rim of Eagle Crater. Two airbag marks could be seen nearby. Suddenly two lines of forensic evidence came together: a location near bounce marks and a recreated microscopic scene on Earth with Vectran threads.

The threads in Pasadena’s sandbox closely resembled what had first surprised scientists nearly a month earlier at Eagle Crater on Mars.

The threads of this mystery seemed not to show martian biology in microscopic view, but another kind of throw-away terrestrial biology at work: the airbags had shed fabric and the camera showed human engineering in action.

What lesson can be learned from the thread mystery? How does shape itself guide a biological interpretation?

One answer is the Knoll criterion. Named after Harvard paleontologist Andrew Knoll, the methodology is cited as one example of not just how a shape might be similar to something biological, but whether a presumption is given to another explanation in the absence of biology. "You do your exploration," said Knoll, "and if, in the course of that exploration, you find a signal that is (a) not easily accounted for by physics and chemistry or (b) reminiscent of signals that are closely associated with biology on Earth, then you get excited. What will happen then, I can guarantee you, is that 100 enterprising scientists will go into the lab and see how, if at all, they can simulate what you see – without using biology."

This is an extension of Carl Sagan’s classic comment, that extraordinary claims require extraordinary evidence.


The Rover Science and Engineering Team contributors to the thread mystery included Joy Crisp, Jim Rice, Paul Bartlett, Ken Herkenhoff, Justin Maki, Tom Rivellini and Brad Jolit. MER flight planning chronicled in the diary of the principal investigator for the science packages, Dr. Steven Squyres: Parts 1 * 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 *12.

Related Web Pages

JPL Rovers
Spirit’s Sol images and slideshow
Opportunity image gallery and slideshow
Mars Berries Once Rich in Iron-Water
NASA’s RATs Go Roving on Mars

Water Signs
Microscopic Imager
Gusev Crater
Pancam- Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer