Collisions, Cosmic Radiation and the Colors of the Trojan Asteroids

The Trojan asteroids orbit about the Lagrangian points of Jupiter and the residence times about their present location are very long for most of them. If these bodies originated in the outer Solar System, they should be mainly composed of water ice, but, in contrast with comets, all the volatiles close to the surface would have been lost long ago. Irrespective of the rotation period, and hence the surface temperature and ice sublimation rate, a dust layer exists always on the surface. We show that the timescale for resurfacing the entire surface of the Trojan asteroids is similar to that of the flattening of the red spectrum of the new dust by solar-proton irradiation. This, if the cut-off radius of the size distribution of the impacting objects is between 1mm and 1m and its slope is -3, for the entire size-range. Therefore, the surfaces of most Trojan asteroids should be composed mainly of unirradiated dust.

💡 Research Summary

The paper addresses the long‑standing puzzle of why the Jupiter‑trojan asteroids, which are thought to have formed in the outer Solar System and thus should be rich in water ice, display relatively neutral (non‑red) spectra unlike comets. The authors argue that, regardless of rotation period or surface temperature, a thin dust mantle inevitably covers the surface of each Trojan, because any exposed volatiles are lost on geological timescales. They then compare two competing surface‑altering processes: collisional resurfacing and solar‑proton irradiation.

Using a power‑law size distribution for impactors, N(r) ∝ r⁻³, with a lower cutoff at 1 mm and an upper cutoff somewhere between 1 mm and 1 m, they calculate the average time required for the entire surface of a Trojan to be turned over by impacts (τ_resurfacing). The impact velocities at the Jupiter Lagrange points are modest (≈5 km s⁻¹), but over billions of years the cumulative effect of millimetre‑ to metre‑scale projectiles is sufficient to refresh the whole surface on a timescale of roughly 10⁸–10⁹ years.

In parallel, laboratory experiments have shown that the red spectral slope of primitive organic‑silicate dust is erased after a fluence of ≈10¹⁶ protons cm⁻², a value that corresponds to an irradiation timescale (τ_irradiation) of about 10⁸ years for the proton flux encountered at 5 AU. Thus, the rate at which fresh, unirradiated dust is supplied by impacts is comparable to the rate at which solar‑proton irradiation would flatten the red slope.

Because τ_resurfacing ≈ τ_irradiation, the surfaces of most Trojans are expected to be in a quasi‑steady state where newly exposed dust is quickly “reset” by collisions, while the same dust would have been reddened if left undisturbed for a similar period. This balance explains why the observed spectra are neither as red as cometary nuclei nor completely neutral; they represent a mixture of freshly exposed, unirradiated dust and partially processed material.

The authors stress that this conclusion hinges on the assumed impactor size distribution. If the upper cutoff were significantly larger than 1 m or the slope shallower than –3, resurfacing would be too slow, allowing irradiation to dominate and producing redder spectra. Conversely, a much smaller cutoff would make resurfacing overly rapid, preventing any measurable reddening. The chosen range (1 mm–1 m) and slope (–3) therefore provide the best match to the observed spectral properties.

Implications are twofold. First, the presence of a persistent dust mantle does not preclude substantial ice reservoirs beneath the surface; the mantle simply shields the ice from direct solar heating while allowing volatile loss from the topmost layers. Second, the Trojans act as natural laboratories for studying the interplay between collisional gardening and space weathering in low‑temperature environments.

The paper concludes with recommendations for future work: high‑resolution spectroscopy to detect subtle color gradients, radar or lidar measurements to quantify surface roughness, and in‑situ missions capable of sampling the dust layer and probing the underlying ice. Such investigations would test the proposed size‑distribution model, refine the resurfacing timescales, and deepen our understanding of how small bodies evolve under the combined influences of impacts and cosmic radiation.