A Sodium-based Comet (Planetary Science)

Considering minor bodies as “asteroids” or “comets” can be an understatement. We are taught this by the asteroids Phaethon and Bennu, whose activities are not yet understood in all their aspects. For Phaethon, however, a window has opened: the ejection activity of meteoroids from its surface could be due to the sublimation of sodium, as indicated by both the theoretical model and the laboratory experiments recently conducted by a team from NASA’s Jpl.

The minor bodies of the inner solar system are generally grouped into two phenomenological classes: comets and asteroids . The main difference that distinguishes these two types is that the first shows some kind of activity, resulting in the emission of gas and dust into space. For comets, the cause of the emission is usually attributed to the presence of volatile materials frozen on the surface of the body such as water, carbon dioxide or carbon monoxide, in turn indicators of the region of formation of the body in the protoplanetary disk.

This rather sharp distinction has been called into question by the discovery of active asteroids (such as asteroid 6478 Gault ). In this case we are dealing with heterogeneous bodies that show activity for different causes: impact events, overcoming of the spin-barrier or activity due to the presence of water ice below the surface.

However, there are some minor bodies that do not fit into any of these broad categories. The most notable example of an “out-of-the-box body” is the asteroid (3200) Phaethon : a near-Earth (Neo) object that is only active when it is close to the perihelion – about 21 million km from the Sun. Phaethon is associated with the meteor current responsible for the Geminid swarm , which has its maximum on December 14 of each year. Another asteroid out of the chorus is (101955) Bennu due to its activity of ejection of stones in space whose origin is not well understood.

Considering that the surface temperature of Phaethon reaches about 750 ° C at perihelion, it is clear that every volatile element thermally connected with the surface has long been lost in space and cannot be responsible for the emission of meteoroids that go to supply the Geminid current, as would happen for a classic comet. Moreover, Phaethon’s activity occurs only in the vicinity of the perihelion, if the cause were the sublimation of the ice it would instead be extended along almost the entire orbit. Spectral observations conducted in 2020 indicated a complete absence of hydrated minerals on Phaethon’s surface, further supporting the conclusion that water is not the source of the activity.

In the past, the effects of the extreme temperatures that the asteroid experiences at perihelion had been examined to justify Phaethon’s activity. These studies had shown that the thermal excursion cycle can efficiently fragment surface boulders which would thus become a source of dust and debris that could be ejected from the surface to fuel the observed activity and associated meteor flow. Because of this peculiar activity Phaethon was considered to be a ” rock comet “. However, thermal fracturing alone is unlikely to produce enough acceleration to allow dust and debris to break through and leave Phaethon. For this reason, an alternative element has recently been considered:sodium . The basic idea is that Phaethon’s activity is due to the sublimation of the sodium present in the rocky matrix of the body which replaces the water vapor in a traditional comet. This mechanism would also explain the reason for the scarcity of sodium in the Geminid meteors: the sodium would be lost in space allowing dust and fragments to leave the surface of the asteroid.

The total sodium production rate calculated for Phaethon as a function of the heliocentric distance within approximately one astronomical unit from the Sun (45 days before perihelion). Credits: Joseph R. Masiero et al. 2021 Planet. Sci. J. 2 165

To investigate the plausibility of this hypothesis, a team of astrophysicists led by Joseph Masiero of the Jet Propulsion Laboratory built a thermophysical model of Phaethon, considering it as a spherical, rotating and orbiting body around the Sun made up of a porous mixture of silicates and grains of sodium. The team also performed heating experiments in the laboratory on samples of the Allende meteorite , a carbonaceous chondrite that could come from a Phaethon-like body, to look for the release of sodium gas.

The theoretical model shows that, within about an astronomical unit (AU) of the Sun, the sodium production rate increases by six orders of magnitude, briefly reaching a molar production rate similar to the water production rate of Comet 67P / Churyumov. –Gerasimenko at 2.1 AU. If pure sodium were present on Phaethon as in the model and such strong outgassing occurred, it would easily cause dust to eject into space in a manner similar to a classical comet.

In the laboratory, the sodium content was determined over a range of temperatures on the individual minerals within each sample, which were also analyzed after heating to determine the changes in their chemical composition. The heating-cooling cycle was comparable in length to the Phaethon rotation period (3.6 hours), with temperatures compatible with the transition to perihelion. The result of the laboratory tests was a depletion of sodium compared to the other chemical elements present: this shows that, under the right conditions, sodium can be dispersed in space.

The theoretical-experimental work is not conclusive, but it makes us understand that classifying minor bodies as “asteroids” and “comets” is too simplifying: we must not only consider the quantity of ice they contain, but also which elements vaporize at high temperatures.

Featured image: Artist’s impression of the Sun-heated asteroid Phaethon. The surface becomes so hot that the sodium inside the rock can vaporize and vent into space, causing it to glow like a comet and removing dust and small rock debris. Credits: Nasa / Jpl-Caltech / Ipac


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