Activity of fresh Phaethon-like regolith close to the Sun

The posterchild of near-Sun asteroids, 3200 Phaeton, exemplifies the poorly understood mechanisms driving their activity during perihelion passages. We study the nature of the activity of near-Sun asteroids by simulating their environment using a vacuum chamber and a high-power solar simulator (Tsirvoulis et al., 2022). Here we report on experiments with a CM carbonaceous chondrite simulant (Britt et al. 2019), because it appears to be the closest match for the fresh material on Phaeton, that is, the material that has not lost its volatiles due to recent perihelion passages (MacLennan & Granvik 2024, Schrader et al. 2025). 

In a set of roughly 50 experiments, we subjected CM regolith simulant to conditions experienced at heliocentric distances of 0.1–0.2 au. We conducted the experiments for two different grain sizes based on estimates from infrared observations (MacLennan et al. 2022): large grains with sizes between 1000 and 355 µm and small grains with sizes below 355 µm. The experiments were recorded, and the resulting videos were evaluated by comparing each frame to the next. We use two different image difference metrics: the mean square error (MSE), and the structural similarity index measure (SSIM; Wang et al, 2004), both of which quantify changes in images. Applying these metrics to the entire sequence of frames provides us with an objective way to evaluate the change in the sample over time, that is, measure the level of activity.  

We observe explosive events and jumping particles that can result in the complete sample moving away from the experiment region at the highest irradiances considered, that is, distances closest to the Sun. In our experiments roughly 3 grams of material are evaporated in about 100 seconds on a 2 by 2 cm area. If we assume this happens on the surface of a 1 km radius asteroid we get a mass loss of about 9000 kilograms per second, which is comparable to values of up to 2000 kilograms per second as measured by Knight et al. (2016) for comet 322P. 

There appears to be an activity breakpoint between 0.13 and 0.15 au, where above that distance no significant activity can be detected in our experiments. This coincides with the perihelion distance of Phaeton of 0.14 au and could potentially explain why Phaeton’s activity is only observed in the days surrounding its perihelion passage: the temperature threshold might only be reached at its perihelion.  

We found evidence that the observed activity is driven by outgassing of pyrite grains, contained in the CM simulant. The repeated outgassing of iron sulfides has been proposed by Suttle et al. (2024) to explain how repeated activity can be possible over long timescales. Furthermore, there are spectral links between Phaeton and the CY group of meteorites (MacLennan & Granvik 2024). These meteorites are known to have sulfide contents of up to 20% (King et al, 2015), which could imply that Phaeton indeed contains substantial amounts of sulfides. 

In our experiments, large grains show stronger levels of activity than small grains. However, this might not be due to grain size but to sulfide content: there seem to be more pyrite grains in the larger grains leading to stronger activity. On the other hand, stronger levels of activity could also be due to the way we measure activity: if a larger particle moves, more pixels in the video change, leading to higher activity measured by the software. If, however, the grain size actively influences the level of activity, this may have implications for the near-Sun asteroid population: asteroids with fine grained surfaces could survive longer in the extreme near-Sun environment.  

As a key finding, we identify an activation threshold near 0.14 au for CM material, above which no significant activity can be observed. Interestingly this threshold broadly coincides with Phaeton’s perihelion distance. The observed destruction could lead to a mass loss of up to 9000 kg per second, which aligns with observations of up to 2000 kg per second by Knight et al. (2015) for comet 322P.  

Furthermore, we find evidence that the observed activity could be driven by outgassing of iron sulfides in the material, which has been previously studied on a smaller scale by Suttle et al. (2024). We speculate that the observed mechanism can, in the more extreme radiation environments closer to the Sun, lead to the destruction of entire asteroids (Granvik et al. 2016). 

 

 

 

References: 

Georgios Tsirvoulis, Mikael Granvik, Athanasia Toliou: SHINeS: Space and High-Irradiance Near-Sun Simulator, Planetary and Space Science, 2022. 

https://doi.org/10.1016/j.pss.2022.105490. 

Britt, D.T., et al. (2019), Simulated asteroid materials based on carbonaceous chondrite mineralogies. Meteorit Planet Sci, 54: 2067-2082. 

 https://doi.org/10.1111/maps.13345 

MacLennan, E., Granvik, M. Thermal decomposition as the activity driver of near-Earth asteroid (3200) Phaethon. Nat Astron 8, 60–68 (2024).  

https://doi.org/10.1038/s41550-023-02091-w 

Devin L. Schrader et al., Geochimica et Cosmochimica Acta, 2025. 

https://doi.org/10.1016/j.gca.2024.12.021. 

Knight, Matthew M. and Fitzsimmons, Alan and Kelley, Michael S. P. and Snodgrass, Colin: COMET 322P/SOHO 1: An asteroid with the smallest perihelion distance? The Astrophysical Journal Letters, 2015. 

https://dx.doi.org/10.3847/2041-8205/823/1/L6 

Suttle, M.D., Olbrich, L.F., Bays, C.L. et al. Rapid heating rates define the volatile emission and regolith composition of (3200) Phaethon. Nat Commun 15, 7178 (2024). 

https://doi.org/10.1038/s41467-024-51054-w 

A.J. King, P.F. Schofield, K.T. Howard, S.S. Russell, Modal mineralogy of CI and CI-like chondrites by X-ray diffraction, Geochimica et Cosmochimica Acta, 2015. 

https://doi.org/10.1016/j.gca.2015.05.038. 

Granvik, M., Morbidelli, A., Jedicke, R. et al. Super-catastrophic disruption of asteroids at small perihelion distances. Nature 530, 303–306 (2016).  

https://doi.org/10.1038/nature16934