  We present a photometric and astrometric study of the split active asteroid
P/2016 J1 (PANSTARRS). The two components (hereafter J1-A and J1-B) separated
either $\sim$1500 days (2012 May to June) or 2300 days (2010 April) prior to
the current epoch, with a separation speed $V_{\mathrm{sep}} = 0.70 \pm 0.02$ m
s$^{-1}$ for the former scenario, or $0.83 \pm 0.06$ m s$^{-1}$ for the latter.
Keck photometry reveals that the two fragments have similar, Sun-like colors
which are comparable to the colors of primitive C- and G-type asteroids. With a
nominal comet-like albedo, $p_{R} = 0.04$, the effective, dust-contaminated
cross sections are estimated to be 2.4 km$^{2}$ for J1-A, and 0.5 km$^{2}$ for
J1-B. We estimate that the nucleus radii lie in the range $140 \lesssim
R_{\mathrm{N}} \lesssim 900$ m for J1-A and $40 \lesssim R_{\mathrm{N}}
\lesssim 400$ m, for J1-B. A syndyne-synchrone simulation shows that both
components have been active for 3 to 6 months, by ejecting dust grains at
speeds $\sim$0.5 m s$^{-1}$ with rates $\sim$1 kg s$^{-1}$ for J1-A and 0.1 kg
s$^{-1}$ for J1-B. In its present orbit, the rotational spin-up and
devolatilization times of 2016 J1 are very small compared to the age of the
solar system, raising the question of why this object still exists. We suggest
that ice that was formerly buried within this asteroid became exposed at the
surface, perhaps via a small impact, and that sublimation torques then rapidly
drove it to break-up. Further disintegration events are anticipated due to the
rotational instability.
