Prestellar grain-surface origins of deuterated methanol in comet
  67P/Churyumov-Gerasimenko
  Deuterated methanol is one of the most robust windows astrochemists have on
the individual chemical reactions forming deuterium-bearing molecules and the
physicochemical history of the regions where they reside. The first-time
detection of mono- and di-deuterated methanol in a cometary coma is presented
for comet 67P/Churyumov-Gerasimenko using Rosetta-ROSINA data. D-methanol
(CH3OD and CH2DOH combined) and D2-methanol (CH2DOD and CHD2OH combined) have
an abundance of 5.5+/-0.46 and 0.00069+/-0.00014 per cent relative to normal
methanol. The data span a methanol deuteration fraction (D/H ratio) in the
0.71-6.6 per cent range, accounting for statistical corrections for the
location of D in the molecule and including statistical error propagation in
the ROSINA measurements. It is argued that cometary CH2DOH forms from CO
hydrogenation to CH3OH and subsequent H-D substitution reactions in CH3-R.
CHD2OH is likely produced from deuterated formaldehyde. Meanwhile, CH3OD and
CH2DOD, could form via H-D exchange reactions in OH-R in the presence of
deuterated water ice. Methanol formation and deuteration is argued to occur at
the same epoch as D2O formation from HDO, with formation of mono-deuterated
water, hydrogen sulfide, and ammonia occurring prior to that. The cometary
D-methanol/methanol ratio is demonstrated to agree most closely with that in
prestellar cores and low-mass protostellar regions. The results suggest that
cometary methanol stems from the innate cold (10-20 K) prestellar core that
birthed our Solar System. Cometary volatiles individually reflect the
evolutionary phases of star formation from cloud to core to protostar.
