  Chemical compositions of planets reveal much about their formation
environments. Such information is well sought-after in studies of Solar System
bodies and extra-solar ones. Here, we investigate the composition of
planetesimals in the beta Pic debris disk, by way of its secondary gas disk. We
are stimulated by the recent JWST detection of an Ar II emission line, and aim
to reproduce extensive measurements from the past four decades. Our
photo-ionization model reveals that the gas has to be heavily enriched in C, N,
O, and Ar (but not S and P), by a uniform factor of about 100 relative to other
metals. Such an abundance pattern is both reminiscent of, and different from,
that of Jupiter's atmosphere. The fact that Ar, the most volatile and therefore
the hardest to capture into solids, is equally enriched as C/N/O suggests that
the planetesimals were formed in a very cold region (T < 35K) with abundant
water ice. In the debris disk phase, these volatile are preferentially
outgassed from the dust grains, likely via photo-desorption. The debris grains
must be 'dirty' aggregates of icy and refractory clusters. Lastly, the observed
strength of the Ar II line can only be explained if the star beta Pic (a young
A6V star) has sizable chromospheric and coronal emissions, on par with those
from the modern Sun. In summary, observations of the beta Pic gas disk rewind
the clock to reveal the formation environment of planetesimals.
