Back in the earlier days of the internet, there was a viral video from a creator called Bill Wurtz called “the history of the entire world, i guess” which spawned a number of memorable memes, some of which are still in use to this day. One of those was a clip from the video where Wurtz states “The Sun is a deadly laser.” Apparently, that was more true than even he knew, as a new paper from Georgios Tsirvouils of the Luleå University of Technology in Sweden and his co-authors have shown experimental evidence that the Sun’s laser-like radiation is likely responsible for the death of a vast majority of closely-orbiting asteroids.
Scientists have known for a while that there is a notable lack of asteroids near the Sun. Notably, those with a low albedo (the darker ones) are particularly absent. Explanations ranged from tidal forcing tearing them apart over time, where asteroids on those orbits were eventually pulled into the Sun itself, or sublimation, whereby the asteroids would eventually evaporate entirely because of the Sun’s radiation.
In 2016, a paper from one of the paper’s co-authors - Mikael Granvik of the University of Helsinki - suggested that, instead of the previously suggested mechanisms, which would take millions or billions of years to destroy a single, moderately sized asteroid, asteroids underwent what the new paper calls "instantaneous thermally-driven erosion”. In layman’s terms, the Sun blows them up.
Here are some particle ejection events from the asteroid Bennu, similar to what might be expected on a grander scale closer to the Sun. Credit - AGU YouTube ChannelNow, almost ten years later, Dr. Granvik has some experimental evidence to back up his theory. The current study’s authors got some time in the Space and High-Irradiance Near-Sun Simulator (SHINeS) chamber, which is a specialized facility at the Luleå University of Technology designed to replicate the extreme vacuum and solar radiation akin to being next to the Sun.
They made samples of pellets of CI carbonaceous chondrite simulants and placed them in the chamber. And then they watched as the samples exploded. The process wasn’t necessarily “instantaneous”, at least not until you got much closer to the Sun than the orbit of Mercury. Some samples survived a few hours of radiation at around .22 AU, but at .1 AU, they almost instantaneously suffered from rapid thermal degradation.
Luckily, the team has a camera set up to watch the samples explode, and provide a detailed explanation of the three phases the samples seemed to go through. First is an initial heating phase, with minor dust release. Second is an “explosive ejection” phase where millimeter-sized fragments are forcefully pushed away from the surface of the asteroid. Finally, the third phase is Subsurface Degradation, where heat forces its way down into the structure of the asteroid, causing it to expand and then fall apart.
Video describing Phaethon, the “Rock Comet”. Credit - Sherlock Holmes YouTube ChannelThis process is exacerbated by the black surface of low-albedo asteroids, as they absorb more energy from the Sun, causing the heating process to progress faster. Which explains why there are even less of them than other types of asteroids floating near the Sun. It also offers some insight into some curious objects that have puzzled scientists for a while.
322P/SOHO 1 has been thought of as a comet/asteroid hybrid, as it passes extremely close to the Sun (.05 AU), and brightens like a comet, but doesn’t seem to have any noticeable tail. If 322P/SOHO 1 is, in fact, an asteroid, it might very well be a dark one undergoing this exact type of thermal erosion. The brightening could be explained by clouds of millimeter-sized dust being ejected from its surface, and the further thermal degradation of its interior is ongoing. In other words, we might be able to watch the Sun killing an asteroid in real time.
Another curious case is Phaethon, the rocky object that creates the Geminid meteor shower every year. It appears that the lab sample in the paper eroded at a speed 430,000 times faster than Phaethon appears to be losing mass. So why the massive difference? The authors think Phaethon itself is a CY chondrite asteroid, which has already been “baked” by numerous close encounters with the Sun, making it resistant to cracking. Another explanation is that the explosive force of the dust fragments in the lab might not be enough to escape the gravitational pull of a relatively large asteroid in space.
No matter the explanation, it seems the 2016 theory was correct - asteroids rapidly thermally disassemble when too close to the Sun. And we might even be able to watch one happening in stages. So Mr. Wurtz can take solace in (or maybe be upset by) the understanding that the Sun is deadly to things other than just life on Earth.
Learn More:
G. Tsirvouils et al. - Instantaneous thermally-driven erosion can explain dearth of dark near-Sun asteroids
