It was one of those “great, if true” stories. In September, scientists reported that the atmosphere of Venus appears to be littered with phosphine, a possible sign of life.
Now the “if” is becoming more and more important. As scientists re-examine the data behind the Venus announcement and add more data sets to the mix, the original claim of inexplicable levels of phosphine is being questioned. And that’s a good thing, say many scientists.
“This is exactly how science should work,” says Paul Byrne, a planetary scientist from North Carolina State University in Raleigh, who studies Venus but was not involved in any of the phosphine papers. “It is too early to say, one way or another, what this recognition means for Venus.”
Here’s a closer look at the effort to go from “if” to “true”:
The big claim
On September 14, astronomer Jane Greaves of Cardiff University in Wales and colleagues reported using two different telescopes to see signs of phosphine in the clouds of Venus (SN: 9/14/20). The phosphine seemed too abundant to exist without any source to replenish it. That source could be strange microbes living in the clouds or some strange unknown Venusian chemistry, the team said.
Greaves and colleagues discovered phosphine first with the James Clerk Maxwell telescope in Hawaii and then with the powerful ALMA telescope array in Chile. But this ALMA data, and especially the way it was treated, is now being called into question.
Reading data: real molecules or random wobbling?
The most important Venus observations were spectra or diagrams of the light coming from the planet in a range of wavelengths. Different molecules block or absorb light at certain wavelengths. So if you look for dips in a spectrum, you can spot the chemicals in a planet’s atmosphere.
Phosphine showed up as a dip in the Venus spectrum at about 1.12 millimeters, a wavelength of light that the molecule presumably absorbed. If the spectrum of Venus could be drawn as a straight line across all wavelengths of light, phosphine would form a deep valley at that wavelength.
But real data has never been so easy to read. In real life, other sources – from the earth’s atmosphere to the inner workings of the telescope itself – introduce wobbling or “noise” into this beautiful straight line. The greater the wobble, the fewer scientists believe the dips are interesting molecules. A given immersion may instead just be a random, extra-large jiggle.
That problem gets worse when you look at a bright object like Venus with a powerful telescope like ALMA, says Martin Cordiner, astrochemist at NASA’s Goddard Space Flight Center in Greenbelt, Md. Cordiner uses ALMA to observe other objects in the solar system . like Saturn’s moon Titan, but was not involved in the Venus work.
“The reason why these bumps and wobbles are here in the first place is because of the inherent brightness of Venus, which makes it difficult to get a reliable measurement,” says Cordiner. “You can imagine that it is blinded by a bright light: if your vision contains a bright light, your ability to see fainter details will be affected.”
So, astronomers do a few different things to smooth the data and let real signals shine through. One strategy is to write an equation that describes the wobble caused by the noise. Scientists can then subtract this equation from the data to highlight the signal they’re interested in, e.g. B. to hide the background of a photo so that a portrait appears. That’s standard practice, says Cordiner.
However, it is possible to write an equation that goes too well with the noise. The simplest equation to use is just a straight line, also known as a first order polynomial, which is described by the equation y = mx + b. A second order polynomial adds a term x square, third order with x diced and so on.
Greaves and colleagues used a twelfth order polynomial or an equation with twelve terms (plus a constant, the + b in the equation) to describe the noise in their ALMA data.
“That was a red flag that needed further investigation and the results of this polynomial fit may not be reliable,” says Cordiner. When a researcher reaches the power of 12, it can mean that a researcher is subtracting more noise than is really random, so they can find things in the data that aren’t actually there.
To see if the researchers were a little overzealous in their polynomial fitting, astrophysicist Ignas Snellen of the University of Leiden in the Netherlands and colleagues reapplied the same noise cancellation recipe to the ALMA data on Venus and found no statistically significant signs of phosphine in an article posted on arXiv.org on October 19.
Then the researchers tried the same noise filtering in other parts of the Venus spectrum where no interesting molecules should be found. They found five different signals from molecules that are not actually there.
“Our analysis… shows that at least a handful of spurious features can be obtained with their method, and therefore [we] conclude that the presented analysis does not provide a solid basis for inferring the presence of [phosphine] in the Venus atmosphere, ”wrote the team.
Look for other dates – and get no help yet
Meanwhile, ALMA scientists discovered a separate, unspecified problem in the data used to detect the phosphine and took that data from the observatory’s public archives for investigation and reprocessing. This emerges from a statement by the European Southern Observatory, of which ALMA is a part.
“That doesn’t happen very often,” says Martin Zwaan from the ESO ALMA Regional Center in Garching, Germany, but this is not a premiere. When problems are discovered, the common practice is to reprocess the data. “In many cases this has no significant influence on the scientific result,” says Zwaan. “In the case of the phosphine on Venus, this is it [outcome] has not yet been established. “
What can scientists do while they wait? One of the best ways to confirm the phosphine is to see an equivalent signal at a different wavelength in the Venus spectrum. Unfortunately, the news there isn’t great either. Appear in a paper in Astronomy & AstrophysicsAstronomer Thérèse Encrenaz of the Paris Observatory and colleagues (including Greaves and several other authors of the original paper) examined archived data from an infrared spectrograph called TEXES, which operates in Hawaii. These observations may have discovered phosphine in the cloud cover of Venus, a deeper part of the sky than what ALMA could see.
Greaves and colleagues had reached out to Encrenaz to look for phosphine in infrared wavelengths before the original paper came out, but those observations were undone by the COVID-19 pandemic. So Encrenaz went through data she had collected between 2012 and 2015 – and found nothing.
“There are none at the level of the cloud cover [phosphine] at all, ”says Encrenaz. That doesn’t necessarily mean that phosphine isn’t higher up in the sky – there just isn’t a clear explanation of how it would get there. “The reasoning in Jane Greaves’ paper was that phosphine comes from the clouds,” says Encrenaz. “So there is a big problem.”
“This is what science looks like.”
There are still ways that the phosphine can penetrate Venus. For example, if it varies over time, astronomers may sometimes look and others don’t. It’s too early to invoke this scenario, however, says Cordiner. “There is no point in talking about the temporal variability of a signal if it is not there.”
This isn’t a crisis, however, says Clara Sousa-Silva, an astrochemist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts and co-author of the original paper. Other groups who questioned the finding “are perfectly normal and what I expected (no, hoped) would happen,” she wrote in an email. “This is usually a phase of a project that I enjoy, and I hope people will see that this is what science is like.”
The silver lining in all of this is that people are excited about Venus, says Byrne, a member of NASA’s Venus Exploration Analysis Group.
“These papers offer great value and a necessary assessment of these extraordinary claims,” he says. “If nothing else, it has shed some light on how little we understand about Venus. And the only way to get those answers is to go to Venus. “