Comet C2017 K2 PanSTARRS provided a rare opportunity to investigate the evolution of coma composition and outgassing patterns over a transitional heliocentric distance Rh range where activity drivers in comets are thought to change from hypervolatile CO CH4 C2H6 andor CO2dominated to H2Odominated. We performed highresolution crossdispersed nearinfrared spectroscopy of C2017 K2 with iSHELL at the NASA Infrared Telescope Facility IRTF and NIRSPEC at the W. M. Keck 2 Observatory. We report gas rotational temperatures Trot and molecular production rates Q mols or upper limits for the hypervolatile species CH4 CO and C2H6 together with less volatile ices CH3OH H2O HCN C2H2 NH3 and OCS over a range of preperihelion distances Rh 3.15  2.35 au. We also report or stringently constrain abundance ratios mixing ratios of the targeted species with respect to CO C2H6 and when detected H2O. All volatiles were enriched relative to water in C2017 K2 when compared to their mean values among Oort Cloud comets whereas abundances relative to C2H6 were consistent with their average values from other longperiod comets.
['Astrophysics - Earth and Planetary Astrophysics', 'A.0']
Infrared Compositional Measurements in Comet C2017 K2 PanSTARRS at Heliocentric Distances Beyond 2.3 AU
{'Heliocentric Distances Beyond 2.3 AU': 'Chemeda Ejeta (1) , Erika Gibb (1) , Michael A. DiSanti (2) , Hideyo Kawakita (3) , Boncho P. Bonev (4) ,  \nNeil Dello Russo (5) , Nathan Roth (2,6) , Younas Khan (7) , Adam J. McKay (8) , Michael R. Combi (9) ,  \nLori Feaga (10) , Mohammad Saki (1) , Ronald J. Vervack Jr. (5) , Yinsi Shou (9)  \n- (1) Department of Mathematics, Physics, Astronomy, and Statistics, University of Missouri-St.  \nLouis, Saint Louis, MO 63121, USA  \n- (2) Solar System Exploration Division, Planetary Systems Laboratory, MS 693, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA\n- (3) National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan\n- (4) Department of Physics, American University, Washington, DC 20016, USA\n- (5) Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723,', 'USA': '- (6) Catholic University of America, Washington, DC\n- (7) University of Alabama at Birmingham, Birmingham, AL, USA\n- (8) Department of Physics and Astronomy, Appalachian State University, Boone, NC 28608-2106\n- (9) Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA\n- (10) University of Maryland, Department of Astronomy, College Park, MD 20742', 'Abstract': 'Comet  C/2017  K2  (Pan-STARRS)  provided  a  rare  opportunity  to investigate  the  evolution  of  coma  composition  and  outgassing  patterns  over  a transitional heliocentric distance (R h )  range where activity drivers in comets are thought to change from "hypervolatile" (CO, CH 4 , C 2 H 6 , and/or CO 2 )-dominated to H 2 O-dominated. We performed high-resolution ( 𝜆 / 𝛥𝜆 » 25,000 - 42,000), crossdispersed, near-infrared spectroscopy of C/2017 K2 with iSHELL at the NASA Infrared  Telescope  Facility  (IRTF)  and  NIRSPEC  at  the  W.  M.  Keck  2 Observatory. We report gas rotational temperatures (T rot ) and molecular production rates (Q; mol/s) or upper limits for the \'hypervolatile\' species CH 4 , CO, and C 2 H 6 , together with less volatile ices (CH 3 OH, H 2 O, HCN, C 2 H 2 , NH 3 , and OCS) over a range of pre-perihelion distances, R h = 3.15 - 2.35 au. We also report (or stringently constrain) abundance ratios (mixing ratios) of the targeted species with respect to CO, C 2 H 6, and (when detected) H 2 O.  All volatiles were enriched relative to water in  C/2017 K2 when compared to their mean values among Oort Cloud comets, whereas abundances relative to C 2 H 6 were consistent with their average values from other long-period comets.', '1. INTRODUCTION': "Because of their ~ 4.5 Gyr residence in the far-out, cold Oort cloud region, long-period comets are thought to be least affected by solar radiation, making them the most primitive small icy bodies in the solar system. They can, therefore, be used to constrain conditions present in the protoplanetary disk when (and where) they formed and, thus, the subsequent evolution of the early solar system (Dello Russo et al. 2016). Although coma volatile abundances relative to H 2 O (the most abundant ice in most comets) provide an excellent proxy for bulk nucleus composition when comets are close to the Sun, more distant comets may be driven by preferential outgassing of hypervolatile  species,  such  as  CO  and  CO 2 (A'Hearn  et  al.  2012;  Ootsubo  et  al.  2012)  or alternatively  exothermic  crystallization  of  amorphous  H 2 O  ice  (e.g.,  Jewitt  2009;  GuilbertLepoutre et al. 2012; Prialnik & Bar-Nun, 1992). For comets that have been observed to be active at large distances post-perihelion, heat acquired near perihelion could be slowly transferred into the interior of the nucleus through conduction. This could cause subsurface volatiles to activate later and fuel distant outbursts (e.g., Prialnik, 1992).  \nThe critical region for the transition to water-driven sublimation for an inbound comet is thought to be between ~2.5-3 au (e.g., Ootsubo et al. 2012, Meech and Svoren 2004), which is supported by Biver et al. (2002) who conducted an investigation of the outgassing behavior of various molecular species (including OH, CO, HCN, CH 3 OH, and H 2 CO) at radio wavelengths for comet Hale-Bopp spanning a wide heliocentric distance (R h ) range of 0.9-14 au and found that the OH production rate (a proxy for H 2 O production rate) surpassed that of CO within about 3 au. Thus,  to  more  accurately  determine  the  volatile  abundances  of  comets  for  comparison  to protoplanetary disk models, it is crucial to understand when volatile species in comets are fully activated.  \nThe discovery of long-period comet C/2017 K2 (hereafter K2), by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) on UT 2017 May 21 at a heliocentric distance R h = 15.9 au (Wainscoat et al. 2017), provided a rare opportunity for a detailed study of the evolution of cometary activity at larger R h . Analysis of prediscovery archival data later showed that K2 was active even at R h = 23.7 au (Jewitt et al. 2017; Meech et al. 2017; Hui et al. 2018). As this was well beyond the distance at which water crystallization should occur, the activity was likely due to the sublimation of hypervolatile ices (Meech et al. 2017; Jewitt et al. 2017), which is supported  by  the  detection  of  CO  in  K2  at  R h =  6.72  au  (Yang  et  al.,  2021).  Atomic  oxygen emission at R h = 2.8 au also suggested significant CO 2 -driven activity (Cambianica et al. 2023).  \nIn this paper, we report high-resolution, near-infrared spectroscopic observations of K2 on nine distinct dates spanning R h = 3.15 - 2.35 au pre-perihelion with the goal of investigating the distance at which water and other less volatile species begin sublimating. In section 2, we discuss observations of K2. In section 3, we present data reduction and analysis. In section 4, we present a detailed discussion, and in section 5, we summarize our observational results.", '2. OBSERVATIONS': 'Observations of K2 were carried out on nine dates from UT 2022 May 18-Aug 21, spanning R h ~ 3.15 - 2.35 au pre-perihelion. Comet K2 had a perihelion distance of ~ 1.80 au on UT  2022 December 20. The observations were performed using both iSHELL (Rayner et al. 2022) at the NASA Infrared Telescope Facility (IRTF) and NIRSPEC (Martin et al. 2016, 2018) at Keck II, to sample (primarily) the \'hypervolatile\' species CO, CH 4 , and C 2 H 6 , and to test the extent to which H 2 O was activated in K2 over this range of R h .  \nThe observations were acquired using a 0.75" wide x 15" long slit for iSHELL and a 0.43" x  24"  slit  for  NIRSPEC,  resulting  in  a  resolving  power  ( 𝜆 / 𝛥𝜆 )  ~  4.2 × 10 4 and  ~  2.5 × 10 4 , respectively.  The comet frames were obtained using an exposure sequence A-B-B-A, with A and B beams placed equidistant to either side of the slit center and separated by half  its length (e.g., DiSanti et al. 2006; Bonev et al. 2021).  The arithmetic operation A - B - B + A isolates the net comet signal within each 2048x2048-pixel frame (e.g., see Figure 1of DiSanti et al. 2017).  \nWe acquired spectra using three different iSHELL settings: Lp1 ranging from ~3.3 - 3.6 µm to sample C 2 H 6 , CH 4 , CH 3 OH, and OH* (or prompt OH emission), M2 covering ~4.5 -5.2 µm to sample CO, OCS, and H 2 O, and a customized L-band setting (L-custom) spanning ~2.8-3.1 𝜇 m, to sample H 2 O, HCN, C 2 H 2 , and NH 3 . The NIRSPEC observations were performed using the KL1 setting to sample H 2 O, C 2 H 6 , and CH 3 OH and the KL2 setting to sample H 2 O, HCN, C 2 H 2 , CH 4 , C 2 H 6 , and CH 3 OH (see Table 1). CO and CH 4 have the lowest vacuum sublimation temperatures of any cometary ices targeted using IR spectroscopy. Both require sufficiently large geocentric velocities (d 𝛥 /dt), to Doppler-shift cometary emission lines from the (opaque) cores of their strong terrestrial counterpart absorptions and into regions of higher atmospheric transmittance. Table 1 includes dates on which d 𝛥 /dt ~ 12 km s -1 or higher, thereby enabling measurement of these two molecules species. Together with only slightly less volatile C 2 H 6 ,  we  refer  to  CO  and  CH 4 as \'hypervolatiles.\'  \nTable 1 Observing Log for K2  \nNotes. R h , Δ, and d Δ/ dt are the heliocentric distance, geocentric distance, and geocentric velocity, respectively of comet K2, and T int is the total on-source integration time. The slit position angle (PA) was oriented along the projected Sun-comet line on all dates; those for NIRSPEC observations are marked \'*\'; iSHELL was used on all other dates.  \n- a Spectral setting used to sample molecular lines.', '3.1 Initial Processing': "We followed a data reduction procedure tailored to iSHELL and NIRSPEC spectra (e.g.,  \nDiSanti et al. 2017, 2021; Dello Russo et al. 2020; Bonev et al. 2021, 2023). Each echelle order was  flat-fielded,  dark  subtracted,  cleaned  for  cosmic  ray  hits,  and  spectrally  and  spatially straightened such that each row corresponded to a unique position along the slit and each column to a unique wavelength. For iSHELL (NIRSPEC) observations, comet spectra were extracted from the processed orders by summing signal over 15 (9) rows centered on the nucleus, defined as the  \nposition  of  peak  emission  intensity.  We  refer  to  these  as  'nucleus-centered'  spectral  extracts (nucleus-centered  spectra),  from  which  nucleus-centered  production  rates  (see  §3.3)  were calculated. The nucleus-centered aperture consisted of a central square box (defined by the slit width),  plus  equivalent  boxes  to  either  side  of  the  nucleus,  hence  a  1x3  rectangular  nucleuscentered aperture.  \nContributions  from  continuum  and  gaseous  emissions  (e.g.,  as  shown  in  Fig.  1)  were determined  as  outlined  in  DiSanti  et  al.  (2017)  and  Bonev  et  al.  (2021).  Column  burdens  of absorbing species in the telluric atmosphere were retrieved using the NASA Planetary Spectrum Generator (Villanueva et al. 2018), by fitting a synthetic spectrum of atmospheric transmittance to IR flux standard star spectra obtained in each instrument setting using the widest available slit (4.0'' for iSHELL and 0.72'' for NIRSPEC). These column burdens were then applied to the comet spectra:  the  fully  resolved  ( 𝜆 / ∆𝜆 ~10 6 )  synthetic  transmittance  function  was  convolved  to  the resolving power of the comet data and scaled to the level of the comet continuum. The cometary emission lines were isolated by subtracting this modeled continuum.  \nFinally,  each  modeled  g-factor  (i.e.,  modeled  line  intensity)  was  corrected  for  the monochromatic  atmospheric  transmittance  at  its  Doppler-shifted  wavelength,  based  on  the geocentric velocity of the comet at the time of the observation. Synthetic emission models for each targeted species were then compared to the observed line intensities. The g-factors used in this study were generated for H 2 O (Villanueva et al. 2012a), OH* (Bonev et al. 2006), HCN and NH 3 (Villanueva et al. 2013), C 2 H 2 (see appendix C of Villanueva et al. 2011a,), C 2 H 6 (Villanueva et al. 2011a), CH 3 OH (Villanueva et al. 2012b; DiSanti et al. 2013; Bonev et al., in prep), CH 4 (Gibb et al. 2003), and CO (DiSanti et al. 2001).  \nFigure 1. Sample spectra and best-fit fluorescent model of K2 on UT 2022 August 10 (a), August 19 [(b) and (c)], and August 20 (d). The observed spectra (black) are superimposed with the telluric model  (yellow).    Continuum-subtracted  spectra  with  individual  fluorescence  emission  models (color-coded by species) are shown below the observed spectrum in each panel. The bottom trace indicates  the  residual  spectrum  (after  subtracting  the  modeled  continuum  and  total  molecular contributions), with the superimposed ± 1σ stochastic noise envelope shown in orange.  \n<!-- image -->  \nFigure 2. Sample spectra of H 2 O, OH*, HCN, and C 2 H 2 acquired using the NIRSPEC KL1 (top panel) and KL2 (bottom panel) settings on UT 2022 July 28  with  56 and 60 minutes on the source, respectively (Table 1). The observed spectra are in black, with the telluric model overplotted in yellow  (for  KL1)  and  green  (for  KL2).  The  individual  fluorescence  models  are  indicated  in different colors, with red showing the total fluorescence and telluric models. The residual spectra are displayed at the bottom of each panel with 1σ stochastic noise envelope superimposed.  \n<!-- image -->", '3.2 Determination of Rotational Temperatures': 'The  excitation  conditions  in  cometary  comae  can  be  quantified  through  the  rotational temperature (T rot ). Obtaining T rot involves analysis of F line /g vs. upper state rotational energy (E u ),  \nfor  emission  lines  spanning  a  sufficient  range  of  E u ,  where  F line is  the  transmittance-corrected molecular line flux, and g is the fluorescence efficiency (or g-factor) of observed ro-vibrational lines (see Eq. 1 and related discussion). At the optimum T rot , the quantity F line /g is constant (within uncertainity) for all E u ; this T rot corresponds to the zero-slope best fit of F line /g vs. E u (Dello Russo et al. 2004; DiSanti et al. 2006).  \nFor  each  date,  we  obtained  consistent  rotational  temperatures  for  our  nucleus  centered apertures (see § 3.3), for all species having sufficient signal-to-noise ratio (SNR; see Table 2). When the SNR for a given species was insufficient, we assumed a T rot determined from species measured  on  the  same  night.  This  assumption  is  reasonable  given  that  T rot obtained  from contemporaneously measured volatile species (both within the same setting and across settings) are generally found to be consistent (e.g., see § 3.1 in Gibb et al. 2012), and thermodynamic models for the inner (collisional) coma also show roughly the same effective temperature for all species observed within a given field of view (Combi et al. 2004; Fougere et al. 2012). In particular, for the UT July 28 NIRSPEC data, production rates are listed for both 40 K and 50 K temperatures (see Table 2), as these are representative of those measured for H 2 O, HCN, C 2 H 6 , and CH 3 OH on this date. We note that for each of these species, the production rates at 40 and 50 K agree within their 1-sigma uncertainties.', '3.3. Molecular Production Rates and Mixing Ratios': 'The nucleus-centered molecular production rate, Q nc (molecules s -1 ), is given as (DiSanti et al. 2014):  \nQ nc = !"# ! $ " %(\')) # * # , [1]  \nwhere F i (Wm -2 ) is the flux of emission line \' i \' contained in a 1x3 rectangular aperture centered on the peak emission intensity, and corrected for telluric transmittance at its Doppler-shifted line frequency, 𝑔 + (W  molecule -1 )  and 𝜏 + (s)  are  the  fluorescence  efficiency  (g-factor)  and  photodissociation lifetime, respectively at R h = 1 au (both are R h -dependent through the relations 𝑔 = 𝑔 + 𝑅 , -. and 𝜏 = 𝜏 + 𝑅 , . ), f(x) is  the  fraction of molecules contained in the nucleus-centered beam assuming entirely native release (i.e., release directly from the nucleus), and ∆ (m) is the geocentric distance.  \nThe technique of obtaining production rates is based on the Q-curve analysis (e.g., Bonev 2005). The square aperture (as defined in § 3.1) was stepped along the spatial profile of each measured  emission  (SNR-permitting),  and  Q  was  computed  at  each  step,  assuming  uniform symmetric outflow. A symmetrized Q-curve was produced by averaging the production rates at diametrically opposite positions about the nucleus, to average over potential asymmetric outflow and thereby obtain the most reliable production rate (Xie and Mumma 1996; also, see § 2.5 of Villanueva et al. 2011b) and thereby obtain the most reliable production rate. These symmetrized production  rates  invariably  increase  with  distance  from  the  nucleus,  then  level  off  (within uncertainty) to a terminal (i.e., total) production rate, Q term .  For species released directly from the nucleus (this is termed native release), the leveling-off distance depends on atmospheric seeing, which redistributes flux from the innermost coma into the wings of the profile. The increase of production  rate  from  the  nucleus-centered  position  (Q nc )  to  the  terminal  (off-nucleus)  region (defining Q term ) establishes a \'growth factor\' (GF ≡ Q term /Q nc ), which accounts for the \'lost\' (i.e., redistributed)  flux  (this  phenomenon  is  also  referred  to  as  slit  loss).    Generally,  the  same atmospheric seeing conditions, as well as positioning in the slit affects molecules similarly within the same instrument setting (Villanueva et al. 2011b), assuming purely native release.  Therefore,  \nwhen low SNR precluded determining a reliable GF, we assumed the value determined from one or  more  simultaneously  (or,  if  not,  contemporaneously;  see  §4.2)  measured  species.  Relevant production rates and growth factors are indicated in Table 2.  \nThe  chemical  taxonomy  of  comets  is  based  on  the  chemical  abundances  of  molecular species, expressed as abundance ratios (i.e., mixing ratios), traditionally defined as the ratio of the production rate of  molecule \'X\' to that of simultaneously or contemporaneously measured H 2 O [i.e., Q(X)/Q(H 2 O)], usually the most abundant molecule in a cometary comae.  Table 2 also lists mixing ratios with respect to simultaneously or contemporaneously measured CO and C 2 H 6 ; these hypervolatiles serve as useful compositional baselines for comets at larger R h , particularly when H 2 O is not securely detected (or when its sublimation is not fully activated). Even at smaller R h , where water production is fully activated, the value alterative baselines for expressing mixing ratios has been utilized (e.g., Bonev et al. 2021, DiSanti et al. 2021).  \nTable 2 Volatile Composition of Comet K2 obtained using iSHELL and NIRSPEC  \nNotes.  \n- (a) Rotational temperature determined using 8-15 lines for CO, 4 lines for CH 4 , and 16-48 lines for\n- C 2 H 6 . Values in parentheses are assumed.\n- (b) Nucleus centered production rate (Q nc ).\n- (c) Growth factor. Values in brackets are assumed.\n- (d) Global production rate.\n- (e) Molecular abundance with respect to H 2 O. For Aug 19 and 21, Q(H 2 O) from Aug 20 was assumed.\n- (f) Molecular  abundance  with  respect  to  C 2 H 6 . For  Aug  20  and  21,  Q(C 2 H 6 )  from  Aug  19  was assumed.\n- (g) Molecular abundance with respect to CO. For Aug 19 and 20, Q(CO) from Aug 21 was assumed; for July 28 and Aug 4, Q(CO) from Aug 10 was assumed.  \n**NIRSPEC dates. Both C 2 H 6 & CH 3 OH are reported for July 28 (from both KL1 & KL2 settings) and Aug 04 (from KL2 only). CH 3 OH in KL2 on the 2 dates was determined using an  emission model  in the 2920 cm -1 region ( Bonev et al., in prep ).', '4.1 Heliocentric dependence of Production Rates in C/2017 K2 (PanSTARRS)': 'We investigated pre-perihelion volatile activity in comet K2 between R h = 3.15 and 2.35 au. Only the hypervolatiles CO, CH 4 , and C 2 H 6 were detected beyond ~2.8 au, however, K2 was sufficiently bright and active that H 2 O, HCN, C 2 H 2 , and CH 3 OH were subsequently also detected  \n(or were constrained to meaningful levels) at smaller R h . The production rates of CO, CH 4 , and C 2 H 6 were relatively constant (within uncertainty) beyond ~2.6 au. The overall gas production increased  at  smaller  R h (owing  to  increased  solar  flux)  as  expected  (see  Figure  3),  but  with hypervolatile  production  rates  remaining  relatively  constant  (albeit  with  a  modest  increase  in Q(CH 4 ) between 2.40 and 2.36 au). The CO production rate showed a moderate increase between 3.15 and 2.35 au inbound, with power-law slope  R h -1.8 ,  which is generally consistent with the slope measured for Hale-Bopp at R h ~ 6.7- 0.9 au pre-perihelion (Biver et al. 2002). Combi et al. (2023) reported post-perihelion H 2 O production rates  for K2, using the Solar Wind ANisotropies (SWAN) instrument onboard the Solar and Heliospheric Observer (SOHO) satellite. The values measured by SWAN are about a factor of two or more higher than our pre-perihelion values for K2 at similar R h .  Cometary gas production rates are often asymmetric with respect to perihelion, but even absent this, higher Q(H 2 O) measured by SWAN might be expected given its larger field of  view  compared  with  ground-based  IR  spectrographs,  in  particular,  if  additional  H 2 O  was released from icy grains in regions of the coma not encompassed by the IR slit. As an example, this was inferred for comet C/2012 S1 (ISON) over a similar range in pre-perihelion R h (Combi et al. 2014; DiSanti et al. 2016, § 4.1.2).  \nFigure 3. Production rates of CO (blue), C 2 H 6 (red), CH 4 (green), H 2 O (magenta), CH 3 OH (orange), C 2 H 2 (olive), and HCN (turquoise) in K2 as a function of heliocentric distance.  \n<!-- image -->', '4.2 Comparison with other OCCs within R h = 2 au': 'We also compared the molecular abundances of volatile species, measured with respect to H 2 O (as well as to C 2 H 6 ) in K2 (from Table 2), to their mean values among OCCs within R h = 2 au measured at high spectral resolution between l ~ 2.8 - 5.0 µm [Fig. 4. (a) and (b)]. Once water (the least volatile ice measured in comets) is fully activated, the abundances of the volatile species relative to H 2 O are expected to reflect the actual composition of ices in the nucleus. The molecular abundances in K2 relative to H 2 O changed significantly with R h between 2.54 and 2.35 au with the level of enrichment dropping as the comet approached the Sun, which would be consistent with  \na scenario in which water was beginning to activate over this heliocentric range. Thus, because H 2 O production in K2 was possibly not yet fully activated at the heliocentric distances of our measurements, we compared the molecular abundances of volatile species in K2 (CH 4 , CH 3 OH, CO, C 2 H 2 and HCN)  with respect to H 2 O (as well as to C 2 H 6 ) in K2 to their mean values among OCCs that were observed within R h = 2 au at high spectral resolution between l ~ 2.8 - 5.0 µm [Table 3, Fig. 4. (a) and (b)]. Table 3 clearly shows that the overall abundances relative to water were significantly higher than their mean values from OCCs, with the enhancements being most pronounced for the hypervolatiles CO and CH 4 . We note that with iSHELL these hypervolatiles were acquired using different instrument settings from that of H 2 O (water was not detected in the M2 setting, even at R h = 2.35 au; see Table 2). Thus, the effect of rotation over the short time intervals between observations in different settings, as for example different surface regions of the nucleus become illuminated (and activated), cannot be ruled out.   On the other hand, it is important to note that the abundances (including those of the hypervolatiles) relative to C 2 H 6 were generally within the range of values from OCCs measured to date within R h ~ 2 au or even slightly depleted (as for CH 3 OH). The enriched abundances of volatile species with respect to H 2 O suggest that either H 2 O was not fully activated even at 2.35 au and/or that K2 was unusually enriched in these volatiles, as was the case for comets C/2001 A2 (Gibb et al. 2007; Magee-Sauer et al. 2008), or that  a significant amount of H 2 O was released by icy grains outside the slit (e.g., Dello Russo et al. 2008).  The mixing ratios of hypervolatiles as well as less volatile ices such as HCN decreased with  decreasing  R h relative  to  H 2 O,  consistent  with  the  former  possibility.  Even  though  K2 presented a relatively rare opportunity to conduct a detailed IR study of cometary activity at larger Rh,  this  discussion  emphasizes  the  need  to  measure  additional  comets    within  the  critical heliocentric distance range R h ~2-3 au (or larger).  \nIt is worth noting the mixing ratios for C 2 H 6 , CH 3 OH, and HCN relative H 2 O at 2.54 au, 2.48 au and 2.36 au (see Table 3). The abundances of these volatiles decreased between 2.54 and 2.48 au, and then remained more nearly constant from 2.48 to 2.36 au. We note that H 2 O was comeasured within the same instrument settings used for the (Keck/NIRSPEC) measurements at R h = 2.54 and 2.48 au. On the other hand, for the C 2 H 6 and CH 3 OH abundances relative to H 2 O at R h = 2.36 au, the H 2 O measurement was obtained a day later (at 2.35 au), so (as mentioned previously) the possibility of nucleus rotation influencing the abundances of these volatiles relative to H 2 O (or overall diurnal changes in gas production, regardless of the cause) cannot be ruled out.  \nTable 3. Summary of volatile abundances in K2 and comparisons with OCCs within 2 au  \nNotes: The C 2 H 6 /H 2 O, CH 3 OH/H 2 O, and CH 3 OH/C 2 H 6 abundance values indicated at 2.54 au are the weighted mean of the abundances for KL1 and KL2 indicated in Table 2.  \n- (a) Mean abundances relative to simultaneously (or contemporaneously) measured C 2 H 6 and H 2 O in comet K2.\n- (b) Mean abundances among OCCs, taken from Dello Russo et al. (2016) and  results for comets observed (published) subsequently: C/2020 S3 (Ejeta et al. 2024), C/2017 E4 (Faggi et al. 2018), C/2013 V5 (DiSanti et al. 2018),  C/2021  \nA1 (Faggi et al. 2023; Lippi et al. 2023), C/2002 T7 and C/2015 ER61 (Saki et al. 2021), C/2018 Y1 (DiSanti et al. 2021), C/2014 Q2 (Dello Russo et al. 2022), and C/2012 K1 (Roth et al. 2017).', '4.3 Comparison with other OCCs beyond R h = 2 AU': 'To put K2 in context, we also compare its volatile abundances to those of other comets  \nobserved beyond ~2.0 au [Fig. 4 (c), (d), (e) and (f)].  The CH 4 abundance relative to CO in K2 was constant (within uncertainties) between R h = 3.15 and 2.35 au (Fig. 4(c)); however, it was significantly higher than observed for C/2006 W3 (Christensen) and C/2016 R2 (Pan-STARRS) at similar R h (Bonev et al. 2017; Bockéele-Morvan et al. 2010; McKay et al. 2019; Wierzchos & Womack 2018), comets for which CO dominated the outgassing activity [these are also shown in Fig. 4 (c)]. In addition, as shown in Fig.4 (e), the CO abundance relative to C 2 H 6 in K2 was lower than that for comets C/1995 O1 (Hale-Bopp) and C/2006 W3 at similar R h (see Bonev et al. 2017; Bockéele-Morvan et al. 2010; DiSanti et al. 2001; Dello Russo et al. 2001). Again, this is due to these comets being significantly CO-rich compared to K2. On the other hand, Outsubo et al. (2012) observed a number of comets spanning R h = 2.42 - 3.29 au and obtained only upper limits for CO production that were of order ~10 26 molecules/s, giving only 3𝜎 upper limits (ranging from ~410 %) for the CO abundance (relative to H 2 O) for some of these comets [Fig. 4 (f)]. Thus, more measurements of CO and C 2 H 6 in  comets  beyond  ~2  au  are  required  to  draw  more  definitive conclusions. We also note that even though comet 17P/Holmes had an outburst few days prior to its post-perihelion observations at heliocentric distances 2.45 - 2.47 au, the measurements show enhanced abundances for volatiles C 2 H 6 , CH 3 OH, HCN, and C 2 H 2 relative to H 2 O (Dello Russo et al. 2008). Similarly, post-perihelion observations of comet C/2010 G2 (Hill) at R h ~2.5au, about a week following its outburst, show higher abundances of volatiles CO, CH 4 ,  C 2 H 6 ,  HCN, and CH 3 OH relative to H 2 O (Kawakita et al. 2014).  \nApart  from  R h -dependence,  the  overall  production  rates  depend  on  the  radius  of  the nucleus, the active fractional surface area, whether the comet is observed pre- or post-perihelion (e.g., DiSanti et al. 2014, Roth et al. 2018), and the dust-to-gas ratio. Yang et al. (2021) measured a  3.6 𝜎 CO  detection  at  R h =  6.72  au  and  dust/gas  ratio  >  1  in  comet  K2.  Using  imaging  \nobservations, Jewitt et al. (2017) determined the radius of the nucleus of K2 to be ≤ 9 km, smaller by a at least a factor of ~2-5 compared to Hale-Bopp, which was estimated to have a radius of 30 ± 10 km (Fernández 2002). Even though the size of C/2016 R2 is unknown, Wierzchos et al. (2018) suggested its radius to be ≤ ~15 km assuming its CO production rate is proportional to the surface area of its nucleus. Even though there were no accurate size measurements for comets C/2006 W3 and C/2016 R2, it is possible that the significantly lower CO production rates we measured for K2 compared to those measured for these comets at similar R h could be due to, among other factors, the dusty nature of K2 and/or its smaller inferred size.  \n2  \n<!-- image -->  \nFigure 4. The abundances of volatiles in comet K2 [panels (a) and (b)], abundances in K2 and OCCs measured at comparable heliocentric distances [panel (c)], and comparison of abundances of volatiles in K2 in comparison with OCCs measured to date in the infrared wavelength [panels (d),  (e)  and  (f)].  The  vertical  dotted  lines  indicate  measurements  within  2  au,  where  most measurements are performed.', '5. SUMMARY': "We obtained  pre-perihelion  observations  of  comet  K2  covering  a  heliocentric  distance range R h ~ 3.15 - 2.35 au, to measure the evolution of hypervolatiles CO, CH 4 , and C 2 H 6 , and to test  the  activity,  especially  the  onset  of  H 2 O sublimation at detectable levels, and to track the development of other volatiles (C 2 H 2 , NH 3 , HCN, CH 3 OH, CN, and OCS).  \nAbundances  of  volatile  species  with  respect  to  simultaneously  or  contemporaneously observed H 2 O in K2 were significantly higher than the mean values from OCCs measured to date, most of which were measured within 2.0 au from the Sun. On the other hand, the abundances of these  volatiles  relative  to  C 2 H 6 remained  more  consistent  with  R h and  were  generally  also consistent with their mean values from OCCs (see Table 3).  This could imply that comet K2 was either enriched in these (normally 'trace') volatiles (or was depleted in water), or that H 2 O was not yet fully activated beyond 2.0 au. The increase in water production rate near 2.40 au that was not  seen  to  the  same  degree  in  other  molecules  suggests  the  latter  possibility.  Clearly,  more observations of comets are needed over the critical range R h ~  2  -  3  au,  to  more  conclusively establish the heliocentric distance at which (or the range of R h over which) cometary H 2 O becomes fully activated, as well as to test the behavior of ice sublimation in comets beyond 2 au.", 'Acknowledgments': 'Data  for  this  study  were  obtained  at  the  NASA  Infrared  Telescope  Facility  (IRTF), operated by the University of Hawai՛i under contract NNH14CK55B with NASA, and the W.M.  \nKeck  Observatory,  operated  as  a  scientific  partnership  among  the  California  Institute  of Technology, the University of California, and NASA. The W.M. Keck Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors gratefully acknowledge the support of the IRTF and Keck 2 staff for their help in acquiring the K2 data. E.G. and C.E. acknowledge support from NSF under award 2009910. M.D. and N.X.R.  acknowledge support through the NASA Solar System Observations Program under award 22-SSO22\\_0013. B.P.B acknowledges the support of  NSF grant under the award: AST-2009398.', 'References': 'A՛Hearn, M., Feaga, L. M., Keller, H. 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