The goal of the National Aeronautics and Space Administration's (NASA) missions to Mars is not just to look for signs of life. It is to look for signs of a second genesis--life that formed independently of life on Earth. In a plenary talk at the 2012 SciX conference, Chris McKay, a planetary scientist at the NASA Ames Laboratory, explained how this search is unfolding, including the important role of spectroscopic instruments, such as the laser-induced breakdown spectroscopy (LIBS) instrument on the Curiosity rover--the current mission on Mars.

A key advantage of LIBS for planetary science, said McKay, is that it is a noncontact method and thus does not require collecting samples. On Earth, scooping up dirt for samples is easy. But on other planets, with all of the equipment being operated remotely from Earth, it's a multistep process. NASA scientists have to position the rover--which usually takes two command cycles--then position the scoop, take a scoop of soil, confirm there is soil in the scoop, and bring it back to the instrument. All of that takes about a week. Also, the dirt is often sticky and clumpy. "On the Phoenix mission, the soil clogged up and wouldn't go through the filters into the mass spectrometer," said McKay. "Operators shook the rover to try to get the soil into the filter, and that created an electrical short."

Because LIBS has this advantage of being a noncontact method, it was the first analytical instrument used on Mars, before the rover even moved. "It showed the power of being able to conduct analyses while sitting still and shooting a laser," said McKay. Given this capability, the main purpose of the LIBS instrument will be to select samples for the more sophisticated and difficult-to-operate instruments that do organic analysis and mineralogy.

In spite of the excitement over the LIBS debut on Mars, however, McKay said there are still questions to be worked out. "The atmosphere on Mars is different from the atmosphere on Earth in terms of pressure and composition, and that affects the LIBS spectra," he explained. "So we are comparing the LIBS spectra with spectra from traditional Mars instruments like X-ray fluorescence [XRF], trying to get a handle on how best to use this instrument."

Sampling differences between LIBS and XRF must also be considered. The LIBS instrument generates a spectrum from five tiny sample spots on a rock surface. The XRF instrument used previously analyzed a much larger area, and a raster of the XRF sample revealed variations in the rock sample. "So our first task is to integrate this new instrument into our understanding of the mineralogy of rocks on Mars," said McKay. "And it's not trivial."

Life and Second Genesis

So once spectroscopic methods help choose samples and ultimately detect organic materials on Mars, how will NASA scientists know if the materials are biological or have a nonbiological source, such as meteorites or Miller-Urey experiments? And if they are biological, how will they know if the materials have a different origin from life on Earth?

The answer, McKay explained, is that biology is choosy--biology uses certain molecules over and over again--whereas chemistry is not. The classical example is amino acids. The amino acids used in life on Earth are all left-handed; those seen in Miller-Urey synthesis experiments or in meteorites are racemic. "If on another planet or moon, we find a set of amino acids that all share a common chiral symmetry, that would be pretty good evidence of life," he said. "And if that chirality were all right-handed, it would be good evidence of life that is different from life on Earth. It would be good evidence of a second genesis."

Such a discovery would be exciting for two reasons, said McKay. First, if we find an example of life that is not on Earth's tree of life, we could do comparative biochemistry, and would likely learn a lot of important information. "The second reason is that if we found that life started twice in our own solar system, it would be compelling evidence that life is common in our universe," he concluded.

Why Mars?

Of course, Mars is not the only place to look for a second genesis. Other top choices include Europa--a moon of Jupiter--and two moons of Saturn: Enceladus and Titan. So why start with Mars?

The answer, said McKay, is that Mars has various qualities that make it a good place to start. It has evidence of past liquid water, and an atmosphere with carbon and nitrogen that is also cold and dry, which makes for good preservation of evidence.

It's also close to us. The life cycle of a mission to Mars is four to five years, from conception to final data on Earth, he explained. Missions to the outer solar system such as Jupiter and Saturn have a life cycle of 20 years. "Thus, we've crashed more missions to Mars than we have sent to all other planets put together," joked McKay.

Spectroscopy in Future Planetary Missions

Yet the potential to use spectroscopic methods in space extends far beyond Mars. For example, spectroscopic methods could be used to survey the surface of Europa, from orbit, to detect a landing site that has organics preserved. It would be ideal, said McKay, if we could also characterize the organics, to learn roughly what kind of organics they are and what their level of preservation is in the high-radiation environment of Europa.

Spectroscopy could also be used to search for amino acids in the plume of Enceladus. Currently, there is a big debate at NASA over whether it's necessary to bring samples from the plume back to Earth to understand the nature of the organics and search for their possible biological origin, or if this can be done with in situ methods. "Right now, the state of the technology is such that I would argue that the only way we could do it is to bring the samples back to Earth," said McKay. "We don't have the analytical capability to do the measurements we want there."

But bringing samples back adds almost 15 years to the mission, and increases risk. As a result, there is a push to develop methods that can do the analysis in situ, both to survey the plume to decide where to sample, and then to sample. "So that leads to a desire for noncontact, remote spectroscopy" said McKay.

Missions to Europa and Enceladus will launch in the next 10 to 15 years. "These missions have a potential role for active spectroscopy, if the technology can be developed and proven in time to be included," said McKay. "The case of LIBS on the current Mars mission is a good example of how that can be done."