C/2016 R2 (PanSTARRS)
DiscovererPan-STARRS
Discovery siteHaleakalā Observatory
Discovery date7 September 2016
Epoch29 May 2018 (JD 2458267.5)
Observation arc1,762 days (4.82 years)
Obs4,319
Perihelion2.602 AU
Aphelion≈1,600 AU (inbound)[1]
≈1,100 AU (outbound)
Semimajor≈780 AU (inbound)
≈530 AU (outbound)
Period≈22,000 years (inbound)
≈12,000 years (outbound)
Eccentricity0.99631
Inclination58.224°
Asc node80.569°
Arg peri33.192°
Mean0.001°
Tjup1.060
Earth moid1.720 AU
Jupiter moid2.117 AU
Mean diameter5-30 km mi 0[2]: 20[3]: 5
M 17.3
M 211.2
Magnitude9.8 (2018 apparition)
Last p9 May 2018

C/2016 R2 (PanSTARRS), or simply C/2016 R2, is an unusual long-period comet that is extremely rich in carbon monoxide and nitrogen, but contains very little water.[4] It was discovered on 7 September 2016 by the Pan-STARRS astronomical survey at Haleakalā Observatory in Hawaii. The comet attracted attention from many astronomers as it approached its closest point to the Sun in May 2018 when it was inside of the asteroid belt at 2.6 AU.[5] It has been observed to have a very complex tail. The comet nucleus is estimated to be 5-30 km mi 0 in diameter.[2]: 20[3]: 5

Inbound, the comet orbited the Sun on a 22,000 year orbit, which took it out about 1600 AU.[1] It was found to differ from typical comets, and was found to be rich in carbon monoxide (CO) but depleted in hydrogen cyanide (HCN), resulting in a blue coma.[6][3] The blue color is thought to come from the rich amounts of carbon monoxide being ionized.[7] The comet made its closest approach to the Sun in May 2018.[7]

Orbit

C/2016 R2 is a long-period comet that follows a highly distant and eccentric orbit around the Sun. It is classified as a dynamically old Oort cloud comet, because it has passed close to the Sun many times in the past.[8]: 1 C/2016 R2 passed perihelion on 9 May 2018 and will return in about 12,000 years.[1] Before 2018, C/2016 R2's last perihelion was about 21,600 years ago.[8]: 1[1]

Composition and gas emissions

Volatiles

C/2016 R2 is remarkable for its unusual gas composition, which is extremely rich in carbon monoxide (CO) and molecular nitrogen (N2), but extremely poor in water (H2O).[4] The high abundance of CO and N2 in a comet is unusual because they are hypervolatile compounds, which means they easily sublimate at low temperatures and should become severely depleted over the age of the Solar System.[2]: 21 In terms of number of molecules, the volatile composition of C/2016 R2's coma comprises over 80% CO gas,[2]: 19 roughly 12–17% carbon dioxide (CO2) gas,[a] roughly 4–6% N2 gas,[b] and trace amounts (≤1%) of water vapor and other compounds.[2]: 19, 22 This is completely different from typical Solar System comets, whose comas are primarily made of water vapor, 1%–30% of CO2 and CO, trace amounts (few percent) of miscellaneous organic compounds,[2]: 1 and virtually no N2.[4]: 1

During January–February 2018 (3 months before perihelion), when C/2016 R2 was 2.8 AU from the Sun, it was estimated that C/2016 R2 was outgassing roughly 4,600 kilograms (5.1 short tons) of CO per second (~1×1029 CO molecules per second)—this is a very large outgassing rate comparable to those seen in the exceptionally active comets Comet Hale–Bopp and 17P/Holmes.[8]: 8[c] In addition, C/2016 R2 was estimated to be outgassing roughly 700 kilograms (0.8 short tons) of CO2 per second,[d] 200 kilograms (0.2 short tons) of N2 per second,[e] and 9 kilograms (20 pounds) of water per second[f] during January–February 2018.[2]: 18 Observations suggest that these gases were mainly emitted directly from the surface of C/2016 R2's nucleus,[2]: 20 though some CO was also observed being produced outside the nucleus due to sublimation of ejected ice grains.[9]

Spectroscopic observations by various telescopes on Earth and in space have identified at least 12 different chemical species emitting from C/2016 R2 during January–February 2018.[2]: 1 Besides CO, CO2, water, and N2, these include methane (CH4), ethane (C2H6), hydrogen cyanide (HCN), methanol (CH3OH), formaldehyde (H2CO), carbonyl sulfide (OCS), acetylene (C2H2), and ammonia (NH3).[2]: 1 Methylidyne radicals (CH), cyano radicals (CN), dicarbon (C2), and tricarbon (C3) have also been detected in C/2016 R2's coma.[10]: 1 When compared to typical comets, all of these chemical species in C/2016 R2 (except N2) are heavily depleted relative to CO, but are enriched relative to water.[2]: 1, 17 The sulfur-containing compounds hydrogen sulfide (H2S) and carbon monosulfide (CS) were not detected in C/2016 R2, which suggests that they are also highly depleted.[8]: 10 Only the mixing ratios of methanol to CO2 (CH3OH/CO2) and methanol to methane (CH3OH/CH4) in C/2016 R2 are considered typical of Solar System comets.[2]: 1

Radiation from the Sun can ionize gases and trigger photochemical reactions within the coma of C/2016 R2, which causes it to glow with spectral emission lines.[11][12][10]: 9 For example, sunlight can break down CO and CO2 into atomic oxygen, which has been detected emitting green and red light in C/2016 R2's coma.[10]: 7 Cations of N2+, CO+, and CO2+ have been detected in C/2016 R2's coma.[10] CO+ is the most dominant ion in C/2016 R2's coma, although at distances close to the comet's nucleus (within 1,000 kilometres or 620 miles), CO2+ becomes more abundant as CO+ becomes neutralized by frequent collisions between molecules in the denser gas environment.[12]: 4048 The dominant blue spectral emission of CO+ (and N2+ by a small part[3]: 1) gives C/2016 R2 its deep blue color.[13][8]: 1 This contrasts with typical comet colors, which range from gray ("neutral") to yellow or green[11] due to C2 spectral emission and sunlight scattering by dust particles.[14][8]: 1

More than 99% of elemental nitrogen in C/2016 R2's gas composition is contained in the form N2, while the remaining amount is contained in the trace species NH3 and HCN.[2]: 18 The majority of elemental carbon and oxygen in C/2016 R2's gas composition is contained in CO and CO2.[2]: 21 Whereas C/2016 R2's primary carbon content matches those of typical comets, C/2016 R2's primary oxygen content does not—for typical comets, oxygen is mainly stored in water.[2]: 21 The presence of molecular oxygen (O2) in C/2016 R2 has not been ruled out, however—if O2 exists in the comet, it might be abundant and it could account for a sizable fraction of C/2016 R2's oxygen content instead.[2]: 21 Analysis of nitrogen spectral emission in C/2016 R2's coma suggests that its 14N/15N isotope ratio is at least 100, which is consistent with 14N/15N ratios seen in other comets.[10]: 12 Likewise, analysis of CO+ spectral emission in C/2016 R2's coma suggests that its 12C/13C isotope ratio is 73 ± 20, which is consistent with the 12C/13C ratios of either the Solar System (89 ± 2) or the interstellar medium (68 ± 15) within error bars.[15]

Dust and metals

Telescope observations have shown very little amounts of dust emitting from C/2016 R2, which indicates the comet is dust-poor.[8]: 10[3]: 3 Little is known about C/2016 R2's dust and non-volatile (refractory) composition, so the atomic abundances in the comet's overall composition are unknown.[2]: 22 Equally small amounts of atomic iron (Fe I) and nickel (Ni I) vapor where detected in C/2016 R2's coma during February 2018; the concentration of nickel relative to iron is close to 1, similar to other typical comets.[16][17]: 373 It is estimated that C/2016 R2 was emitting roughly 4×1023 atoms of iron and nickel per second (or up to ~40 grams of iron and nickel per second) during this time,[g] which is one of the highest iron and nickel emission rates seen among comets.[18]: 12 A 2021 study led by Manfroid et al. suggested that space weathering of iron- and nickel-bearing compounds in comets could lead to their observed iron and nickel vapor emissions, although the nature of these compounds is unknown.[16][17]: 373–374

Similar objects

As of 2025, only two other long-period comets have been identified as analogues of C/2016 R2: C/1908 R1 (Morehouse) and C/1961 R1 (Humason).[8][19] These two comets share C/2016 R2's blue color, low dust emission, CO- and N2-rich composition, and relatively high N2/CO mixing ratios of a few percent.[19]: 11 All three may belong to a distinct and rare group of comets, although further discoveries and measurements of their water abundances are needed to confirm this.[19]: 8

Coma and tail morphology

Observations of C/2016 R2 during the leadup to its perihelion in 2018 showed that it had a diffuse[9]: 4 and spherical coma, due to its predominantly gaseous emissions.[2]: 8 The appearance of C/2016 R2's coma has been described as diffuse in dates as early as 2016.[20] The coma of C/2016 R2 grew as large as 350,000 km (220,000 mi) (angular diameter 3.7 arcminutes) in March 2018.[20] Infrared imaging by the Spitzer Space Telescope in February 2018 revealed subtle structures within the inner coma of C/2016 R2, including a spiral feature possibly caused by a gas jet rotating with the nucleus[2]: 8 and a transient shell or ring-shaped feature about 140,000 km (87,000 mi) in diameter.[9]: 14 The shell or ring-shaped feature might have been caused by an impulsive outgassing event, as it was only seen on 21 February 2018.[2]: 8 If this shell or ring was made of CO ice grains, the sublimation of these grains might explain the observed production of CO gas outside the nucleus.[9]: 14

Imaging observations by Spitzer and radio telescopes in January–February 2018 have shown that the comet's inner coma appeared brighter and elongated toward the direction of the Sun, due to intensified outgassing from the heated, sunlit surface of C/2016 R2's nucleus.[2]: 8[9]: 5, 14 During this time, C/2016 R2's sun-facing hemisphere was outgassing CO with an amount and speed (0.51 kilometres per second or 0.32 miles per second) twice as high as its nightside (0.25 kilometres per second or 0.16 miles per second).[9]: 15 Spectroscopic observations were able to identify this enhanced sunward outgassing by detecting a slight blueshift in the spectral emissions of CO.[3]: 2[9]: 5

C/2016 R2 had a deep blue ion tail[8]: 1 that was visible from November 2017 to the end of April 2018.[21][20] The tail spanned over 3 e6km in length (angular length 15 arcminutes) during January and February 2018.[20] The tail was primarily made of CO+ ions[2]: 8 that were being blown away from the Sun by the solar wind.[11] Like the coma, the ion tail of C/2016 R2 owes its blue color to the spectral emission of CO+.[5][11][3]: 1 During January and February 2018, the ion tail of C/2016 R2 exhibited complex structures and streamers that visibly changed within hours.[20][11] Images from January 2018 showed that the ion tail appeared to twist and wave, possibly because of a concentrated gas jet that was rotating with the comet's nucleus.[20][11] Cometary outbursts and fluctuations in the solar wind may have additionally contributed to the complex structure of C/2016 R2's tail.[11]

Nucleus

Little is known about the properties of C/2016 R2's nucleus, because it was heavily obscured by intense outgassing during its 2017–2018 apparition.[citation needed] Because different volatile substances have different volatilities (e.g. CO sublimates more easily than water) and could be trapped within non-volatile substances, it is possible that the observed volatile composition of the coma may not match the intrinsic volatile composition of the nucleus.[2]: 19[9]: 15 The coma surrounding C/2016 R2's nucleus makes it appear brighter and larger than it actually is—for example, if one assumes the nucleus's apparent brightness is entirely due to sunlight reflecting from a dark, solid surface, then the result would be an overestimated diameter of 38 km (24 mi).[22]: 5183–5184 One method of estimating the diameter of C/2016 R2's nucleus is by measuring its CO outgassing rate—if the CO outgassing is proportional to the surface area of its nucleus, then its nucleus must be between 5 and 30 km mi in diameter.[2]: 20[3]: 5 The upper end of this diameter range would be considered larger than an average comet.[3]: 5 If the diameter of C/2016 R2's nucleus truly lies within this range, then it would suggest that its areal water outgassing rate is indeed below average, which would mean that the nucleus's intrinsic volatile composition should match that of the coma.[2]: 20

Observational history

Discovery

C/2016 R2 was discovered on 7 September 2016 by the Pan-STARRS astronomical survey at Haleakalā Observatory in Hawaii.[23]

Astronomers first noticed the unusual nature of C/2016 R2 in 2017 when it began to develop a tail.[21][24] C/2016 R2 was notably bluer than any other comet seen in the past 50 years.[24] C/2016 R2 did not come close enough to Earth to be seen with binoculars.[24]

Origin

Although the highly distant and eccentric orbit of C/2016 R2 suggests that it originated from the Oort cloud,[8] the comet's unusual volatile composition makes its origin unclear.[2] If C/2016 R2 formed in the Solar System, it could have either accreted from the protosolar disk 4.6 billion years ago[4]: 5 or it could have formed as an icy fragment ejected from a planetary collision.[8]: 11 In both scenarios, C/2016 R2 must have formed far from the Sun, beyond the CO and N2 frost lines where temperatures are cold enough (less than 50 K[3][2]: 21) for these substances to condense into solid grains.[8]: 11

Protosolar disk hypothesis

For the protosolar disk accretion scenario, a 2021 study led by Mousis et al. suggested that C/2016 R2 could accumulate its high concentration of CO and N2 if it formed between 10 and 15 AU from the proto-Sun, where the CO and N2 frost lines reside.[4]: 1, 5 At these distances, condensed CO and N2 grains would be pure and would outnumber water ice grains.[4]: 5 However, the outcome of this formation process is highly dependent on various unknown properties of the protosolar nebula, such as its gas viscosity.[4]: 5 C/2016 R2's overabundance of simple molecules like N2 and CO suggests that its formation environment was chemically inactive and protected from photodissociation by solar radiation.[2]: 18, 21 It is believed that the lack of photodissociation prevented simple molecules from chemically reacting, which limited the production of more complex molecules like NH3 and HCN in C/2016 R2.[2]: 21

If C/2016 R2 formed in the protosolar disk, then it must have been ejected via gravitational interactions with the giant planets, as that is the most plausible pathway to its present-day eccentric and distant orbit.[25]: 1 A 2022 study led by Andersen et al. showed 90% of objects that formed near the N2 and CO frost lines were ejected within 10 million years after their formation, with 1–10% of these ejected objects ending up in the Oort cloud.[25]: 1 The early ejection of C/2016 R2 into the Oort cloud would allow it to retain most of its original hypervolatiles for billions of years.[25]: 5 While this scenario could explain both C/2016 R2's hypervolatile abundance and the apparent rarity of hypervolatile-rich Oort cloud comets, its chronology with respect to other early Solar System events (e.g. the Sun's escape from its birth cluster, the jumping-Jupiter scenario, etc.) is uncertain.[25]: 5

Collisional fragment hypothesis

If C/2016 R2 formed from a collision event, then it should come from a differentiated icy dwarf planet like Pluto, as such objects are known to be abundant in hypervolatiles.[8]: 11[2]: 22 A similar scenario has been proposed for the interstellar object 1I/Oumuamua.[22]: 5194 For C/2016 R2, the collision event would have likely taken place in the Kuiper belt billion years ago, when it was being gravitationally perturbed by the 2:1 orbital resonance between Jupiter and Saturn (see grand tack hypothesis).[25]: 6[22]: 5194 However, this hypothesis is complicated by the fact that the relative abundances of CO, CH4, and N2 seen in C/2016 R2 do not match the surface composition of Pluto,[2]: 22[22]: 5192 and that the collisional dynamics and interiors of icy bodies beyond Neptune are poorly understood.[25]: 6[22]: 5194 Various studies have shown that it is difficult to retain large amounts of N2 in energetic impacts, which further complicates this hypothesis.[22]: 5194

Captured interstellar object hypothesis

It is possible that C/2016 R2 did not originally form in the Solar System and was instead captured from another star system, which would explain its unique volatile composition.[2]: 22 For example, the interstellar comet 2I/Borisov is known to have a CO-rich and water-poor composition similar to C/2016 R2.[25]: 1[22]: 5192 The possibility of an interstellar origin for C/2016 R2 was first considered by McKay et al. in 2019, who suggested that the Sun might have exchanged Oort cloud comets with other closely passing stars when it was still forming in its birth cluster.[2]: 22 The 12C/13C isotope ratio of C/2016 R2 could potentially match that of the interstellar medium, which might support an interstellar origin.[15]: 8 However, the 12C/13C ratio could alternatively match that of the Solar System due to uncertainties[15]: 8 and the orbit of C/2016 R2 resembles those of many known Oort cloud comets,[10]: 12 so the hypothesis of an interstellar origin is considered unlikely.[2]: 22[15]: 8

See also

  • 3I/ATLAS – An interstellar comet with a very high CO2/H2O mixing ratio, like C/2016 R2
  • (LINEAR) – A large long-period comet with a relatively high concentration of N2 (N2/CO = 6%})[10]
  • 29P/Schwassmann–Wachmann – A highly active centaur exhibiting frequent outbursts with high amounts of CO and CO2 outgassing[10]

Notes

  1. ^ McKay et al. (2019) reported a CO2/CO mixing ratio of 18.2 ± 3.5 %.[2]: 22 Given the percentage of CO in C/2016 R2's volatile composition (80% CO in terms of number of molecules),[2]: 19 multiplying that percentage by the CO2/CO mixing ratio gives roughly 14.6 ± 2.8 % CO2 (full rounded range 12–17%).
  2. ^ Mousis et al. (2021) cite reported N2/CO mixing ratio of 0.06 ± 0.01 and 0.08.[4]: 1 Given the percentage of CO in C/2016 R2's volatile composition (80% CO in terms of number of molecules),[2]: 19 multiplying that percentage by the N2/CO mixing ratio gives roughly 5 ± 1 % N2 (full range 4–6%).
  3. ^ Carbon monoxide or CO has a molar mass of 28.010 grams/mole, where 1 mole is equivalent to 6.022×1023 molecules (Avogadro's number). The January–February 2018 CO emission rate of ~1×1029 molecules/second can be divided by 6.022×1023 molecules/mole to give ≈1.66×105 moles of CO/second. Dividing the moles of CO by the molar mass of CO gives a CO mass emission rate of ≈4.65×106 grams/second, or ≈4,650 kilograms/second when multiplying by 1 kilogram/1000 gram. Note that different estimates for C/2016 R2's CO molecule emission during January–February 2018 range from 9.54 ± 0.91×1028 molecules/second (McKay et al. 2019) to 1.44×1029 molecules/second (Biver et al. 2018)
  4. ^ Carbon dioxide or CO2 has a molar mass of 44.009 grams/mole, where 1 mole is equivalent to 6.022×1023 molecules (Avogadro's number). The January–February 2018 CO2 emission rate of 4.8 ± 1.1×1028 molecules/second given in McKay et al. (2019) can be divided by 6.022×1023 molecules/mole to give ≈16,600 moles of CO2/second. Dividing the moles of CO2 by the molar mass of CO2 gives a CO2 mass emission rate of ≈7.31×105 grams/second, or ≈731 kilograms/second when multiplying by 1 kilogram/1000 gram. For simplicity, the result may be rounded down to one significant figure: ≈700 kilograms/second.
  5. ^ Molecular nitrogen or N2 has a molar mass of 28.014 grams/mole, where 1 mole is equivalent to 6.022×1023 molecules (Avogadro's number). The January–February 2018 N2 emission rate of 4.8 ± 1.1×1028 molecules/second given in McKay et al. (2019) can be divided by 6.022×1023 molecules/mole to give ≈7,970 moles of N2/second. Dividing the moles of N2 by the molar mass of N2 gives a N2 mass emission rate of ≈2.23×105 grams/second, or ≈223 kilograms/second when multiplying by 1 kilogram/1000 gram. For simplicity, the result may be rounded down to one significant figure: ≈200 kilograms/second.
  6. ^ Water or H2O has a molar mass of 18.015 grams/mole, where 1 mole is equivalent to 6.022×1023 molecules (Avogadro's number). The January–February 2018 H2O emission rate of 3.1 ± 0.2×1026 molecules/second given in McKay et al. (2019) can be divided by 6.022×1023 molecules/mole to give ≈515 moles of H2O/second. Dividing the moles of H2O by the molar mass of H2O gives a H2O mass emission rate of ≈9.27×103 grams/second, or ≈9.27 kilograms/second when multiplying by 1 kilogram/1000 gram. For simplicity, the result may be rounded down to one significant figure: ≈9 kilograms/second.
  7. ^ Extended Data Figure 2 of Manfroid et al. (2021) give a logarithmic iron (Fe) and nickel (Ni) vapor emission rate of log Q(Fe+Ni) ≈ 23.6. This value can be plugged in as an exponent of 10 to give the actual Fe+Ni emission rate of 1023.6 ≈ 3.98×1023 atoms/second. The molar masses of Fe and Ni are 55.845 grams/mole and 58.693 grams/mole, respectively. Since the Ni/Fe mixing ratio in C/2016 R2 is close to 1 within an order of magnitude, it is reasonable to take the arithmetic average of the Fe and Ni molar masses: 57.269 grams/mole. To convert the Fe+Ni emission rate to moles/second, divide the atoms/second by 6.022×1023 atoms/mole (Avogadro's number); this gives roughly ≈0.661 moles of Fe+Ni. Following the assumption that Ni and Fe are in equal proportions, we can simply multiply moles by the average Fe+Ni molar mass (57.269 grams/mole) to get ≈37.9 grams of Fe+Ni. The result may be rounded down to 1 significant figure, which would give ≈40 grams.

References

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