Understanding the enigmatic Comet-Like Activity of Asteroid (3200) Phaethon

Asteroid (3200) Phaethon has the closest orbital approach to the Sun (or ‘perihelion’) of any named asteroid in our solar system, at a distance of about 0.14 astronomical units (AU). This ‘Sun-grazing’ trajectory causes intense heating up to ~730°C at the asteroid's surface, leading to comet-like activity, including volatile gas emissions. However, the mechanism behind this phenomenon has long perplexed planetary scientists. Given Phaethon's asteroidal composition, these gases should logically derive from structurally bound water in hydrated minerals like phyllosilicates, in the absence of volatile ices which play this role on cometary bodies. However, we would expect such intensive heating to have dehydrated these surface minerals long ago, and with little/no re-surfacing activity observed, this question of this sustained activity remains. In our recent study published in Nature Communications, we experimentally heated carbon-rich meteorites, analogues for these asteroids, to provide insights into the processes which drive Phaethon's unique behaviour.

A Closer Look at asteroid Phaethon

With a diameter of roughly 5.6 km, Phaethon belongs to the population of Apollo near-Sun asteroids and is the parent body of the annual Geminids meteor shower. Phaethon is classified within the Pallas asteroid family, known for their highly eccentric orbits. Phaethon’s spanning distances as close as 1.4 AU and as far as 2.4 AU, with their accompanying temperature fluctuations. This family is observed to have a composition that closely resembles that of carbonaceous chondrites, specifically the CM or ‘Mighei’ type, which are rich in organic compounds and water-bearing minerals. Dynamical modelling suggests that Phaethon was ejected from its original location in the main belt to its current orbit.

However, unlike typical asteroids, Phaethon displays ‘comet-like’ activity in the form of a pseudo-coma (or poorly defined ‘tail’), comprising gas or perhaps very fine dust (<1 µm), despite lacking an essential component: an icy cometary nucleus. The compositional variations between icy comets and rocky asteroids mean that during these extreme heating events, we would expect to see one of the following scenarios: 1) asteroidal resurfacing and loss of huge volumes of dust and gas at every perihelion. This would expose a fresh surface and enable continued effusion but the activity would not be sustainable. 2) Cessation of activity through the formation of a refractory, less permeable crust, preventing dust and gas release. We are seeing evidence of neither of these processes. So, how is Phaethon’s crust still capable of releasing gasses without such huge mass loss? This paradox has puzzled scientists for years, raising questions about the fundamental mechanisms behind this type of activity, and has led to Phaethon being described as an ‘active asteroid’.

Understanding the Heating Mechanism

Through a series of laboratory experiments on carbonaceous chondrites (of the CM group), chosen as the closest analogues for Phaethon’s surface material, we simulated the extreme conditions of radiant solar heating experienced by Phaethon as it approaches perihelion. By subjecting these meteorite samples to rapid heating and cooling cycles, we mimicked the asteroid’s temperature fluctuations over diurnal cycles at perihelion, to study the composition of the volatile gasses produced during the mass loss.

This mass loss we saw, was found to be consistent with previous studies detailing the volatile budget of the CM chondrites, but this only raised further questions! So, why is Phaethon unique? The explanation lies in the chemistry and physical qualities of its exterior. In our study, we demonstrated that rapid heating rates, combined with the low permeability of Phaethon’s surface regolith, can lead to specific reactions between volatile gases emitted and decomposing minerals.

Notably, our experiments reveal that sulphur is the likely major volatile species behind the outgassing activity observed on Phaethon, but that there may also be small contributions from sodium. A perpetual cycle of thermally-driven gas-mineral reactions generates new sulphur-bearing (sulphide and sulphate) mineral species as a result of back reactions, reformation and slow release of the retained sulphur-bearing gasses at the asteroid’s rigid surface. This mechanism allows Phaethon to exhibit comet-like activity in the absence of constant surface renewal—a significant discovery that challenges previous assumptions.

Implications for JAXA's DESTINY+ Mission

Understanding the behaviour of Phaethon has direct implications for future space exploration. In 2029, the Japanese Aerospace Exploration Agency (JAXA) will launch the DESTINY⁺ mission whose primary goal is as a technological demonstration of future deep-space exploration technologies. This spacecraft will perform a fly-by of Phaethon to collect data on the asteroid's surface composition, structure, and the processes driving its activity.

The insights provided by our study will prove critical in interpreting the data gathered as part of this mission’s science objectives. Through laboratory-based observations we hope to provide a framework for understanding how Phaethon's surface reacts to solar heating, allowing us to anticipate the types of materials and emissions they might encounter during the mission. Collectively, this will provide a broader understanding of near-Sun objects and their chemical and physical evolution.

What Lies Beneath Phaethon’s Surface?

Another intriguing aspect prior to the returned data from the DESTINY⁺ mission is the prediction of Phaethon’s regolith composition, which has been suggested by previous studies to be of the lesser-known CY (‘Yamato’) chondrite group, primarily defined by their high sulphide abundance and heated nature. Through mid-infrared (IR) spectroscopic observations, it has been suggested that Phaethon's surface comprises anhydrous mineral phases including olivine and lesser quantities of Ca-sulphates, but also sharing this high enrichment in Fe-sulphides, making it comparably similar to the CY chondrites.

Despite these similarities, our findings suggest that Phaethon's composition is more likely to derive from CM or CI (‘Ivuna’) chondritic material, rather than from CY chondrites. However, there is an important distinction: although CY and CM chondrites have similar mineralogy, the processes which produce sulfur-bearing species in these chondrites are the result of different processes. Therefore, the mineralogical similarities do not necessarily indicate that they belong to the same meteorite group.

The Bigger Picture: Understanding Near-Sun Asteroids

Although Phaethon exhibits unique activity, it is just one of many near-Sun asteroids that experience intense solar heating. Our study marks a step forward in our understanding of (3200) Phaethon and of other near-Sun asteroids. As the launch date of the DESTINY⁺ mission draws nearer, laboratory-based studies such as ours in conjunction with remote sensing techniques will allow us to unravel the mysteries of these fascinating celestial bodies.

For more detailed information, you can read the full paper here.