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For 50 years, scientists have debated whether the components of DNA - the molecule central to all life on Earth - could spontaneously form in space. A new analysis of a dozen meteorites found in Antarctica and elsewhere presents the strongest evidence yet that the answer is yes.
Meteorites are space rocks that have fallen to the ground, and the new report bolsters the notion that heavy meteorite bombardment of the early Earth may have seeded the planet with the stuff of life.
"Meteorites may have served as a molecular kit providing essential ingredients for the origin of life on Earth and possibly elsewhere," wrote the authors of the recent report in the Proceedings of the National Academy of Sciences.
While life has not been found beyond Earth, all earthly plants and animals rely on DNA to store genetic information.
It's these tiny rings that scientists at NASA and the Carnegie Institution for Science in Washington found in 11 of 12 meteorites they scrutinized.
Two of the meteorites in particular, called Murchison and Lonewolf Nunataks 94102, contained a trove of nucleobases, including those also found in DNA. But these meteorites also held an extraterrestrial secret: related but exotic nucleobases never seen before, said Michael Callahan, the NASA scientist who analyzed the space rocks.
Since the 1960s, other scientists have reported nucleobases in meteorites, but concerns about contamination always hung over those findings, said Max Bernstein, a NASA scientist who has studied organic molecules in meteorites but was not involved in the current study.
The detection of the exotic nucleobases, and their absence from surrounding material, helped rule out contamination in this study, Callahan said.
Bernstein said the study's thoroughness gave him confidence in its conclusions. "I don't think it's contamination," he said.
In laboratory experiments, Callahan and colleagues showed how the nucleobases could have formed inside meteorites.
Simple chemical reactions involving ammonia, water and hydrogen cyanide - all ingredients found in meteorites - produced the wide range of nucleobases the scientists found in the space rocks.
"It would be awfully coincidental if our laboratory chemistry produced the same things we saw in the meteorites," Callahan said.
Scientists have also found other building blocks of life - most notably amino acids, the links that form proteins - inside meteorites. While Bernstein said that it's impossible to discern whether the first life on Earth was built on chemicals that fell from the sky, that possibility is now stronger.
"These molecules are at the core of (life's) blueprints," Bernstein said of the nucleobases. "It's possible that the presence of these molecules in meteorites made us what we are today."
Compressed sensing was introduced to the field of magnetic resonance imaging in recent years as a promising method to significantly reduce scan time. The performance of CS depends on the sparsity of the image in the sparse domain, such as wavelet transform domain. In this report, we proposed a method to increase the sparsity of CS MRI by taking exponent of wavelet transform normalized to the range [0 1]. The method was tested on a digital phantom and in vivo MRI data, and the results show that EXP-WT can improve the quality of the reconstructed image or significantly speed up the reconstruction.
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1Medical Biophysics, University of Western Ontario, London, ON, Canada; 2Medical Imaging, Lawson Health Research Institute, London, ON, Canada; 3Psychiatry, University of Western Ontario, London, ON, Canada
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