I have a few updates on some news stories I’ve been following for quite some time, both of which caused quite a stir when they were first announced: phosphine in Venus’ atmosphere, and dust causing Betelgeuse in late 2019 / early 2020.
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You may recall that a team of astronomers announced in September 2020 that they might have found evidence of the molecule phosphine in Venus’ atmosphere. Ordinarily, that would be a pretty esoteric discovery, but the point is, you wouldn’t expect to find that particular molecule there, because it’s pretty easily destroyed in Venus’ hellish environment … and on Earth, phosphine is mainly made by anaerobes bacteria as they dine on dead things that used to live.
So yes, quite a problem. But then doubts arose, with other scientists saying that the data used was not calibrated with the correct files, which could make it look like phosphine was being seen when it’s not actually there. There were some other problems too.
The original team responded, saying detection persisted when they used updated calibration files, although they were weaker. But then it got interesting.
The astronomers who made the original discovery looked at a spectrum and broke the light from Venus into separate colors. Different molecules absorb light with different specific colors, allowing them to be identified. However, sometimes molecules absorb very similar, if not overlapping, colors, confusing the problem. A new article has appeared that emphasizes this correctly, stating that sulfur dioxide (SO2) is mistaken for phosphine because it absorbs light with the same wavelength as phosphine.
It’s an interesting argument. Sulfur dioxide is known to occur in the atmosphere of Venus, and they claim (using models of the planet’s atmosphere) that the signature seen in the data can be explained by SO2 existing in a layer about 80 km above the surface of Venus. It was claimed that phosphine could be seen about 50 km away, but the new document claims that phosphine would soon be destroyed there.
The argument is compelling and could very well be correct. Phosphine may not be what was seen in the first place. The problem here is that this data was on the edge of what could be seen, so without going any further, deeper observations, the problem may not be solved. I expect we’ll be hearing more from the original team soon too.
Let’s talk about Betelgeuse, from a planet 40 million kilometers away to an inflated star 640 (or possibly 530) light years away.
The iconic star shocked everyone in late 2019 when its brightness plummeted like a rock and dimmed by about 50%. It was easy to notice by the eye, and quite bizarre how quickly it darkened.
Betelgeuse is a well-known variable star, the brightness of which varies by a few percent in a number of different cycles. But this deep dive was unprecedented and weird. Astronomers immediately came up with ideas to explain it. One of them was giant star spots, which turned out to be unlikely. Another was that the temperature might drop. A third, and the one that I think was most likely due to support from various sources, is that it erupted a huge cloud of dust blocking some of the light.
But a new article has just appeared that is raising the temperature again. Or down, I suppose: they show that part of the upper atmosphere of Betelgeuse could have cooled down quite a bit, which explains the decrease in light.
Stars emit light because they are hot. When they cool down, they get weaker. Betelgeuse, however, is a red supergiant, an enormous pocket of gas that is more massive and much larger than the sun. The physics of the outer layers is very complex and not very well understood.
The upper parts of the star physically expand and contract over a period of months to years, making the star brighter and fainter, slightly changing both color and temperature. In the new work, the authors show that parts of the upper atmosphere of Betelgeuse cooled several hundred degrees, which explains the dimming.
They looked at the molecule titanium oxide (TiO), which is often seen in very cool stars. It absorbs light with very specific colors in a characteristic way, and what they found is that the absorption by TiO changed as Betelgeuse weakened, indicating that it was cooler than previously thought. The exact temperature drop is difficult to determine, but at one point they show a clear drop of 150 Kelvin (one degree Celsius = 1 Kelvin). They claim that if the temperature drops by 250 K, then no substance is needed at all to explain the dimming.
Complicating this is that extremely high-resolution images of the star show that only the Southern Hemisphere faded, so it’s likely (or so reasoning) that the temperature drop took place there. If the temperature were to drop in only part of the atmosphere, it would be difficult to figure out how much, as the Northern Hemisphere remained the same, making the measurement confusing. So a drop of 250,000 is not necessarily unreasonable.
I wonder if there is more than one cause behind the dimming, both dust and a temperature drop. That is not out of the question; when something extreme happens in the universe, it is usually because two or more phenomena unite to increase their effect. I speculate here, but I certainly do not rule that out.
Funny: Venus is the brightest planet in our sky and the closest to Earth, and Betelgeuse is one of the brightest stars in the sky and also relatively close when stars go. But for both, mysteries abound.
There is much we know and understand well about the cosmos in which we live, but there is also much we do not know, not even our neighbors. And these back-and-forth arguments of scientists about data and cause and physics are normal to science; when we push the boundaries of knowledge, it takes time to figure out what we see. I expect both mysteries to be solved to everyone’s satisfaction, and then we move on to the next weird thing that Venus and Betelgeuse will do. That’s the way the universe works.