Potential signs of life on Venus are fading fast

doi:10.1126/science.370.6520.1021

GSTDTAP > 气候变化

DOI	10.1126/science.370.6520.1021
	Potential signs of life on Venus are fading fast
	Paul Voosen
	2020-11-27
发表期刊	Science
出版年	2020
英文摘要	The announcement in September took the world by storm: In radio emissions from Venus's atmosphere, researchers found signs of phosphine, a toxic compound that on Earth is made in significant amounts only by microbes and chemists. The unexpected detection could point to a microbial biosphere floating in the venusian clouds, the researchers suggested in Nature Astronomy . But almost immediately, other astronomers began to point out questionable methods or said they couldn't reproduce results. Now, after reanalyzing their data, the original proponents are downgrading their claims. Phosphine levels are at least seven times lower than first claimed, the authors reported in a preprint posted on 17 November to arXiv. But the team still believes the gas is there, Jane Greaves, an astronomer at Cardiff University who led the work, said in a talk last week to a NASA Venus science group. “We have again a phosphine line.” The original observations were made in 2017 at the James Clerk Maxwell Telescope (JCMT) in Hawaii, and in 2019 at the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. In Venus's radio spectrum, Greaves and her colleagues detected an absorption line they attributed to phosphine. The researchers went to great lengths to remove confounding effects such as absorption by Earth's own atmosphere. But critics said such aggressive fixes made the discovery of a false positive more likely. ALMA scientists have since found a new noise source: telescope calibration errors. After reanalyzing the ALMA data, Greaves said her team now finds phosphine at just 1 part per billion (ppb). That's still above levels that can be explained by natural processes such as volcanic eruptions or lightning strikes, Greaves said. A study published last month in Astronomy & Astrophysics , led by Therese Encrenaz, an astronomer at the Paris Observatory, ruled out higher phosphine levels. Her team analyzed observations made in 2015 by NASA's Infrared Telescope Facility in Hawaii. Phosphine should have popped out if it had existed at levels above 5 ppb. “It's easy to see there's no phosphine line,” Encrenaz says. If the line does exist, it might not be due to phosphine, according to a critique submitted to Nature Astronomy . It argues that the dip in the JCMT spectrum can be explained by an overlapping absorption line from sulfur dioxide (SO2), the gas that makes up most venusian clouds. The Greaves team concedes the point in its reanalysis. “We emphasize that there could be a contribution from SO2,” they write. But the width of the absorption line in the ALMA data suggests the feature isn't “solely SO2,” they write. Just where any signal is coming from is also in dispute. ALMA is only sensitive to absorption from substances at altitudes above 70 kilometers (km), Encrenaz says. But the Nature Astronomy paper suggested the signal originated some 55 km up, in warmer, more hospitable cloud layers. “This is very difficult to conceive,” Encrenaz says. Greaves and her co-authors argue in their reanalysis that ALMA is unable to capture the full width—and therefore depth—of the signal. “There is no empirical evidence that [phosphine] lies only above 70 km.” Colin Wilson, a co-author of the Nature Astronomy critique, says it's too early to say where the “phosphine roller coaster will end up.” More observations at ALMA might settle the issue, he says. “Whether or not we find phosphine, we're likely to find something new.”
领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/304855
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Paul Voosen. Potential signs of life on Venus are fading fast[J]. Science,2020.
APA	Paul Voosen.(2020).Potential signs of life on Venus are fading fast.Science.
MLA	Paul Voosen."Potential signs of life on Venus are fading fast".Science (2020).