A Pregnancy Test for Mars
|The Mars Immunoassay Life Detection Instrument (MILDI) would use a tiny biochip to detect life on Mars based on the assumption that Mars has the same prebiotic chemicals as Earth.|
The test that tells women they are pregnant might also be able to find signs of living organisms on Mars. Dave McKay at the Johnson Space Center and British environmental microbiologist Andrew Steele – both are members of the NASA Astrobiology Institute (NAI) – together with their collaborators, are eager to test the feasibility of this method for finding life’s footprints on the Red Planet.
As presented by Steele and his colleagues at this year’s Lunar and Planetary Science Conference in Houston, the approach, which depends on the ability of the immune system to detect invaders, would involve a tiny biochip packed with thousands of antibodies that could be launched on a flight to Mars. The antibodies would recognize molecules of living organisms, the way that home pregnancy tests have harnessed antibodies to detect the hormones produced when a human embryo implants in the womb.
The proposal for the Mars Immunoassay Life Detection Instrument (MILDI) includes crosschecks to screen for potential contaminants from Earth or even from rocket fuels that could cloud the results.
It’s also conceivable that the proposed instrument could result in a false negative: It could miss forms of life different enough from terrestrial life that we simply can’t imagine how to design a detector for them. Steele thinks such a result is unlikely.
The project, he says, is based " on the assumption that Mars has the same prebiotic chemicals as Earth." Therefore, Steele contends, "you would expect that there would be some parallels in simple chemistry of any Martian organism" to that of terrestrial organisms.
The proposed instrument will use antibodies that recognize what astrobiologists propose are the most likely molecular signatures of life. This will include not only signatures of organisms alive today, but fossil remnants of past life as well. In experiments performed here on Earth, antibodies are able to sniff out the presence of extremely stable chemical fragments left from simple ancient life forms.
Antibodies are experts at detecting the shapes of molecules, says Steele. The match of the shapes of the foreign antigen to the body’s sentinel antibodies allows the two to stick together like two pieces of a jigsaw puzzle. In Steele’s experiment, the antibodies are designed to fluoresce when they encounter a matching antigen. Scientists can design custom, highly targeted antibodies, called monoclonals, to many molecules these days.
The key to finding out if Mars contains life now or did so in the past is to figure out what molecules are likely to be present in any living thing.
|Porphyrins are a ubiquitous class of naturally occurring compounds with many important biological representatives including hemes, chlorophylls, and several others.|
Porphyrins are one such group of molecules, says Steele. Life anywhere "is going to have some sort of electron transport or energy harnessing system. The common ones on Earth are based on porphyrins," which have very specific shapes.
"No life on this planet can do without porphyrins," Steele says, "they are an unambiguous marker for life and essential to metabolism. The carbon-based ring of a porphyrin carries oxygen in the bloodstream as part of the hemoglobin molecule, and also allows plants to capture sunlight’s energy with their chloroplasts. Even rudimentary bacteria swarming in the seas on Earth contain the porphyrin ring.
The specifics of porphyrin-like molecules could be different on Mars, he adds, but their shapes should be conserved and should be detectable by the appropriate panel of antibodies.
Citing another example, Steele says, "it does not matter what chemicals an extraterrestrial organism has chosen for its genetic makeup, as long as it forms a double helix of the same proportions we can potentially get a positive reaction with an Earth-based antibody."
Hopanes, a remnant of cell walls left behind by ancient bacteria that lived on Earth up to 2.7 billion years ago, would be another class of molecules to scan for with the antibody panel, Steele says.
"It’s a fossil" left in the petrochemical deposits within Earth, "a geological biomarker for life."
To perform the test, samples of Martian soil gathered by a lander could be extracted and applied to the chip, which would be illuminated by a UV laser light and read by a small camera. Numbered spots containing fluorescent antibodies would tell if sought-for substances were present.
|The MILDI may fly on one of the Mars missions planned for the end of this decade.|
Up to 10,000 test spots per square inch can be applied with the technology, which is presently being used to study the Earth’s genomes. The entire detector could be ten by ten centimeters. Other tests, some included on the spacecraft, some kept back on Earth, would use independent methods, including some not based on antibodies, to verify the MILDI’s reports.
Potential and Pitfalls
Greg Schmidt, head of the Astrobiology Integration Office at NASA Ames Research Center in Mountain View, California, says the approach "has enormous potential, provided that it is sensitive enough. They clearly have a plan of testing this in relevant environments. I think it is the right approach." Schmidt speculates that such an instrument might fly on the Mars suite planned for 2007.
Schmidt cautions that the samples might need to be drilled from below the Martian surface, and there is no agreement yet on how deeply to drill. "Oxidants in the upper part of Martian soil would likely destroy most organics that happen to be there," Schmidt notes.
Pamela Conrad, at the Jet Propulsion Laboratory’s Center for Life Detection in Pasadena, California, says that with the antibody strategy, "you can think about what life is, and what life does…what products life makes, and how you can recognize them." Yet there still is no consensus among astrobiologists on what molecules to look for to determine life, Conrad emphasizes. Antibody detectors are just one of a number of strategies under investigation.
"I advocate as many approaches as possible," Conrad says, and while she thinks the antibody approach is worth investigating, she emphasizes that NASA needs to be developing other methods to detect life as well.
"I’m interested in seeing that we avoid Earth-centric bias" as to what will constitute life elsewhere than on Earth, Conrad says. She is confident that astrobiologists can find "the least common denominator" of life, and it will be "something arranged some way in three dimensions." As compared to a rock, or water, or the gases of a planetary atmosphere, she says, "the chemistry that makes up life is going to look different."
Firouz Naderi, the Mars Exploration Program Manager at JPL, says that the Mars Immunoassay is part of NASA’s objectives to look for life on Mars in multiple ways, both on the planet itself, and in samples brought to Earth. "We may be looking for water, either ancient or present," or minerals as an indicator of water, and NASA also plans to bring Martian soil samples back to Earth by the end of the decade. "But also, we will invest some in in-situ life detection … [with] small enough laboratories that are flyable to the surface of Mars."
The Mars immunoassay project is now going to begin proof of concept. Steele is collaborating with Victor Parro at the Spanish astrobiology group, Centro de Astrobiologia, who have developed a DNA and protein array maker and reader. Along with Mary Schweitzer at Montana State University and at Portsmouth University in the UK, the team is putting proposed antibodies through testing against samples of pristine fossil bacterial biofilms already characterized by other methods. Oceaneering Space Systems in Houston is engineering the proposed instrument.
Once verified, the antibodies will be put into an array. The team then intends to test whether the assay can detect the life forms present in extreme environments on Earth. These tests would include microbes from the geothermal hot springs at Yellowstone and the frozen terrain of Antarctica.
Steele encourages interested collaborators to suggest available antibodies that would be appropriate to include on a mission to Mars.