It was one of those "big, by the way" stories. In September, scientists reported that Venus' atmosphere appears to be bound to phosphine, a possible sign of life.
Now more and more emphasis is placed on the “if”. While scientists are re-analyzing Venus announcement data and adding other data sets to the mix, the original claim of unexplained amounts of phosphine is being questioned. And that’s good, many scientists say.
“That’s exactly how science should work,” says planetary scientist Paul Byrne of North Carolina State University in Raleigh, who studies Venus but has not participated in any of the phosphine roles. "It's too early to say in one way or another what this detection means for Venus."
Here's a closer look at the efforts to move from "if" to "true:"
The great claim
On 14 September, astronomer Jane Greaves of Cardiff University in Wales and colleagues reported seeing phosphine signals in the clouds of Venus using two different telescopes (SN: 14/09/20). Phosphine seemed to be too abundant to exist without some kind of source replenishing it. That source could be strange microbes living in the clouds or some strange unknown Venusian chemistry, the team said.
Greaves and colleagues first saw phosphine with the James Clerk Maxwell telescope in Hawaii and followed with the powerful range of ALMA telescopes in Chile. But these ALMA data, and particularly how they were manipulated, are now being questioned.
© ALMA (ESO, NAOJ, NRAO)
Reading the data: real molecules or random swings?
Key observations of Venus were spectra or plots of light coming from the planet in a series of wavelengths. Different molecules block or absorb light at specific wavelengths, so the search for dives in a spectrum can reveal the chemicals in the planet’s atmosphere.
Phosphine appeared as a dip in the spectrum of Venus at about 1.12 millimeters, a wavelength of light that the molecule was thought to absorb. If the spectrum of Venus could be drawn as a straight line at all wavelengths of light, phosphine would make a deep valley at that wavelength.
But real data is never so easy to read. In real life, other sources, from the Earth's atmosphere to the internal operation of the telescope, introduce rocking or "noise" in that pleasant straight line. The larger the motions, the less scientists believe that dives represent interesting molecules. Any particular dive can be just a random, extra large move.
That problem is further exacerbated when looking at a bright object like Venus with a powerful telescope like ALMA, says Martin Cordiner, an astrochemist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Cordiner uses ALMA to observe other objects in the solar system. like Saturn's moon Titan, but did not participate in the work of Venus.
“The reason these blows and swings are due to the intrinsic brightness of Venus, which makes it difficult to get a reliable measurement,” Cordiner says. "You may think you're dazzled by a bright light: if there's a bright light in your vision, then it diminishes your ability to pick out fainter details."
So astronomers do a few different things to smooth out the data and let the actual signals shine. One strategy is to write an equation that describes the swings caused by noise. Scientists can subtract that equation from the data to highlight the signal that interests them, such as erasing the background of a photo to let a portrait subject appear. That’s standard practice, Cordiner says.
But it is possible to write an equation that fits the noise too well. The simplest equation that could be used is just a straight line, also known as a first order polynomial, described by the equation y = mx + b. A second-order polynomial adds a term with x squared, a third with x in cubes, and so on.
Greaves and colleagues used a twelfth-order polynomial or an equation with twelve terms (plus a constant, the + b of the equation), to describe the noise in their ALMA data.
“That was a red flag that needed to be examined in more detail and that the results of that polynomial fit could not be reliable,” Cordiner says. Going up to the power of 12 could mean that a researcher subtracts more noise than is actually random, allowing them to find things in the data that they really aren’t.
To see if the researchers were a little excessive in their polynomial fit, astrophysicist Ignas Snellen of Leiden University in the Netherlands and colleagues re-applied the same noise reduction recipe to ALMA data on Venus and found no statistically significant signs of phosphine. in a paper published on arXiv.org on October 19th.
Researchers then tested the same noise filter in other parts of the Venus spectrum, where no interesting molecules should be found. They found five different signs of molecules that are not really there.
“Our analysis … shows that at least a handful of false characteristics can be obtained with his method, and therefore (we conclude) that the analysis presented does not provide a solid basis for inferring the presence of (phosphine) in the atmosphere of Venus ". wrote the team.
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Looking for other data and still not getting help
Meanwhile, ALMA scientists discovered a separate, unspecified problem in the data used to detect phosphine and pulled that data from the observatory's public archive to examine and reprocess it, according to a statement from the Southern European Observatory, of which ALMA is part.
“This doesn’t happen very often,” says Martin Zwaan of the ESO ALMA Regional Center in Garching, Germany, but this is not the first. When problems are discovered, it is standard practice to reprocess the data. “In many cases, it doesn’t significantly affect the outcome of science,” Zwaan says. "In the case of phosphine in Venus, this (result) has not yet been established."
What can scientists do while they wait? One of the best ways to confirm phosphine is to see a signal equivalent to a different wavelength in the Venus spectrum. Unfortunately, the news is not very good there either. In an article in Astronomy & Astrophysics, Paris Observatory astronomer Thérèse Encrenaz and colleagues (including Greaves and some other authors of the original article) looked at archived data from an infrared spectrograph called TEXES operating in Hawaii. Those observations could see phosphine at the tops of the clouds of Venus, a lower part of the sky than ALMA could see.
Greaves and colleagues approached Encrenaz to search for phosphine at infrared wavelengths before the original paper came out, but those observations were canceled by the COVID-19 pandemic. So Encrenaz analyzed the data he had collected between 2012 and 2015 and found nothing.
“At the level of the cloud tops, there is none (phosphine),” says Encrenaz. That doesn’t necessarily mean there isn’t phosphine higher in the sky; there is simply no clear explanation of how it would come about. “The reasoning for Jane Greaves’ diary was that phosphine came from the clouds, ”Encrenaz says. "So there's a big problem."
"That's what science looks like."
There are still ways that Venus phosphine can pass through. If it varies over time, for example, there may be times when astronomers observe and not at others. However, it is too early to invoke that scenario, Cordiner says. "It doesn't make sense to talk about the hourly variability of a signal if it's not."
But this is not a crisis, says Clara Sousa-Silva, an astrochemist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and co-author of the original article. Other groups challenging the finding “are completely normal and what I hoped (or didn’t expect) would happen,” he wrote in an email. "It's usually a phase of a project that I like, and I hope people realize that this is what science looks like."
The silver lining on all of this is that it thrilled people with Venus, says Byrne, who is a member of NASA’s Venus Exploration Analysis Group.
“These papers provide a lot of value and a necessary assessment of these extraordinary claims,” he says. "If nothing else, it shone the little we understood about Venus. And the only way to get those answers is if we go to Venus."