Active Asteroid (6478) Gault: A Blue Q-type Surface below the Dust?

Abstract

We present near-infrared spectroscopy of the sporadically active asteroid (6478) Gault collected on the 3 m NASA/Infrared Telescope Facility observatory in late 2019 March/early April. Long-exposure imaging with the 0.5 m NEEMO T05 telescope and previously published data simultaneously monitored the asteroid activity, providing context for our measurements. We confirm Gault is a silicate-rich (Qor S-type) object likely linked to the (25) Phocaea collisional family. The asteroid exhibits substantial spectral variability over the 0.75-2.45 µm wavelength range, from unusual blue (s =-13.5±1.1% µm −1 ) to typical red (s =+9.1±1.2% µm −1 ) spectral slope, that does not seem to correlate with activity. Spectral comparisons with samples of ordinary chondrite meteorites suggest that the blue color relates to the partial loss of the asteroid dust regolith, exposing a fresh, dust-free material at its surface. The existence of asteroids rotating close to rotational break-up limit and having similar spectral properties as Gault further supports this interpretation. Future spectroscopic observations of Gault, when the tails dissipate, will help further testing of our proposed hypothesis.

A study of the physical properties of an active asteroid (6478 Gault)

“…After using the M4AST online tool, Gault can also be associated with Q-type asteroids (figure 5b). This taxonomic classification is consistent with the results from near-infrared spectra exhibiting deep absorption bands near 1 and 2 μm consistent with an S-or Q-type surface composition (Marsset et al 2019;Sanchez et al 2019) and from our visible photometric measurements (see sub-subsection 3.2.1). It is noteworthy that the other two possibilities (B-and C-class) have been reported by Lee (2019b) and Jewitt et al (2019), the spectrum in short wavelengths is very noisy (figure 5), and the chi-square values in table 1 for those three classes are very close to each other if we take the errors into account.…”

“…Gault is closer to S-complex. A silicate-rich object (S-or Q-type) is consistent with the result from the near-infrared spectrum (Marsset et al 2019;Sanchez et al 2019) and makes Gault a slight relative of the 25 Phocaea collisional family. Kleyna et al (2019) had an extensive series of observations but they found no convincing lightcurve, meaning the rotational period could only be estimated at ∼1 hr for one peak, and ∼2 hr for two peaks.…”

“…But, it is noteworthy that this value is close to the spin barrier of ∼2.2 hr estimated for a rubble pile asteroid, meaning its internal strength is too weak to sustain a fast rotation (e.g., Pravec & Harris 2000). As the archival data back in 2013 revealed an earlier phase of dust activity (Chandler et al 2019), these authors therefore ruled out a single-impact collision as the origin of the activity and suggested instead that the most likely scenario to explain the activity of this asteroid is breakup of the surface material or landslides resulting from a YORP-induced rotational disturbance (Chandler et al 2019;Hui et al 2019;Jewitt et al 2019;Kleyna et al 2019;Marsset et al 2019;Moreno et al 2019;Ye et al 2019a).…”

“…Although Gault is dynamically linked to the 25 Phocaea collisional family of S-type classification, its spectral and compositional similarity with this family had not been fully investigated (Sanchez et al 2019). For example, nearinfrared spectroscopy revealed that Gault is a silicate-rich (Q-or S-type) object (Marsset et al 2019), but broadband colors and some visible spectra revealed that this active asteroid is more similar to C-type asteroids than S-types (Ye et al 2019a;Jewitt et al 2019;Lee 2019b). The purpose of our work is to perform detailed timeseries photometric measurements with spectroscopic data obtained from the Lulin and Lijiang Observatories to determine the rotation period and exact taxonomic class of Gault.…”

Time-series and Phase-curve Photometry of the Episodically Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200, and ZTF

“…where ρ is the density of the ellipsoid and G is the Newtonian gravitational constant. Gault should have a density of roughly equal to 2.2 g cm −3 (Marsset et al 2019;Sanchez et al 2019), consistent with other S-type asteroids (Carry 2012). Figure 10 presents the critical period with which Gault would start shedding surface material as a function of the axial ratio and density.…”