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C/2016 R2 (PanSTARRS)
Carbon monoxide-rich comet From Wikipedia, the free encyclopedia
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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.[8] It was discovered on 7 September 2016 by the Pan-STARRS astronomical survey at Haleakalā Observatory in Hawaiʻi. 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.[9] It has been observed to have a very complex tail, which has been suggested to be due to a fast rotation period of the nucleus. The comet nucleus is estimated to be 5–30 km (3–19 mi) in diameter.[6]: 20 [7]: 5
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Inbound, the comet orbited the Sun on a 22,000 year orbit, which took it out about 1600 AU.[2] 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.[10][7] The blue color is thought to come from the rich amounts of carbon monoxide being ionized.[11] The comet made its closest approach to the Sun in May 2018.[11]
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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.[12]: 1 C/2016 R2 passed perihelion on 9 May 2018 and will return in about 12,000 years.[2] Before 2018, C/2016 R2's last perihelion was about 21,600 years ago.[12]: 1 [2]
Composition and gas emissions
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Volatiles
C/2016 R2 is remarkable for its unusual gas composition, which is extremely rich in carbon monoxide (CO) and molecular nitrogen (N
2), but extremely poor in water (H
2O).[8] The abundance of CO and N
2 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.[6]: 21 In terms of number of molecules, the volatile composition of C/2016 R2's coma comprises over 80% CO gas,[6]: 19 roughly 12–17% carbon dioxide (CO
2) gas,[a] roughly 4–6% N
2 gas,[b] and trace amounts (≤1%) of water vapor and other compounds.[6]: 19, 22 This is completely different from typical Solar System comets, whose comas are primarily made of water vapor, 1%–30% of CO
2 and CO, trace amounts (few percent) of miscellaneous organic compounds,[6]: 1 and virtually no N
2.[8]: 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 (4.5 long tons; 5.1 short tons) of CO per second (~1×1029 CO molecules per second)—this is a very large outgassing rate that has only been seen in exceptionally active comets like Comet Hale–Bopp and 17P/Holmes.[12]: 8 [c] In addition, C/2016 R2 was estimated to be outgassing roughly 700 kilograms (0.7 long tons; 0.8 short tons) of CO
2 per second,[d] 200 kilograms (0.2 long tons; 0.2 short tons) of N
2 per second,[e] and 9 kilograms (20 pounds) of water per second[f] during January–February 2018.[6]: 18 Observations suggest that these gases are mainly emitted directly from the surface of C/2016 R2's nucleus.[6]: 20
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.[6]: 1 Besides CO, CO
2, water, and N
2, these include methane (CH
4), ethane (C
2H
6), hydrogen cyanide (HCN), methanol (CH
3OH), formaldehyde (H
2CO), carbonyl sulfide (OCS), acetylene (C
2H
2), and ammonia (NH
3).[6]: 1 Methylidyne radicals (CH), cyano radicals (CN), dicarbon (C
2), and tricarbon (C
3) have also been detected in C/2016 R2's coma.[13]: 1 When compared to typical comets, all of these chemical species in C/2016 R2 (except N
2) are heavily depleted relative to CO, but are enriched relative to water.[6]: 1, 17 The sulfur-containing compounds hydrogen sulfide (H
2S) and carbon monosulfide (CS) were not detected in C/2016 R2, which suggests that they are also highly depleted.[12]: 10 Only the mixing ratios of methanol to CO
2 (CH
3OH/CO
2) and methanol to methane (CH
3OH/CH
4) in C/2016 R2 are considered typical of Solar System comets.[6]: 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.[14][13]: 9 For example, sunlight can break down CO and CO
2 into atomic oxygen, which has been detected emitting green and red light in C/2016 R2's coma.[13]: 7 Cations of N+
2, CO+
, and CO+
2 have been detected in C/2016 R2's coma.[13] CO+
is the most dominant ion in C/2016 R2's coma, although at distances close to the comet's nucleus (within 1,000 km or 620 mi) where the gas environment is denser, CO+
2 becomes more abundant as CO+
becomes neutralized by frequent collisions between molecules.[14]: 4048 The dominant blue spectral emission of CO+
(and N+
2 by a small part[7]: 1 ) gives C/2016 R2 its deep blue color.[15][12]: 1 This contrasts with typical comet colors, which range from gray ("neutral") to yellow or green[16] due to C
2 spectral emission and sunlight scattering by dust particles.[17][12]: 1
More than 99% of elemental nitrogen in C/2016 R2's gas composition is contained in the form N
2, while the remaining amount is contained in the trace species NH
3 and HCN.[6]: 18 The majority of elemental carbon and oxygen in C/2016 R2's gas composition is contained in CO and CO
2.[6]: 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.[6]: 21 The presence of molecular oxygen (O
2) in C/2016 R2 has not been ruled out, however—if O
2 exists in the comet, it might be abundant and it could account for a sizable fraction of C/2016 R2's oxygen content instead.[6]: 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.[13]: 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.[18]
Dust and metals
Telescope observations have shown very little amounts of dust emitting from C/2016 R2, which indicates the comet is dust-poor.[12]: 10 [7]: 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.[6]: 22 Equally small amounts of atomic iron (Fe I) and nickel (Ni I) vapor have been 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.[19][20]: 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.[21]: 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.[19][20]: 373–374
Visible spectrum of C/2016 R2 measured in February 2018. Emission lines caused by molecular nitrogen (N
2) are highlighted in blue, while emission lines caused by carbon monoxide (CO) are highlighted in green.
Ultraviolet spectrum of C/2016 R2 (top left) overlaid on top of a telescope photograph of the comet. Blue points in the spectrum highlight emission lines caused by iron vapor, while red points highlight emission lines caused by nickel vapor.[19]
Similar objects
As of 2025[update], only two other long-period comets have been identified as analogues of C/2016 R2: C/1908 R1 (Morehouse) and C/1961 R1 (Humason).[12][22] These two comets share C/2016 R2's blue color, low dust emission, CO- and N
2-rich composition, and relatively high N
2/CO mixing ratios of a few percent.[22]: 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.[22]: 8
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Nucleus
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Perspective
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), it is possible that the observed volatile composition of the coma may not match the intrinsic volatile composition of the nucleus.[6]: 19 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).[4]: 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 (3.1 and 18.6 mi) in diameter.[6]: 20 [7]: 5 The upper end of this diameter range would be considered larger than an average comet.[7]: 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.[6]: 20
Origin
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Perspective
Although the highly distant and eccentric orbit of C/2016 R2 suggests that it originated from the Oort cloud,[12] the comet's unusual volatile composition makes its origin unclear.[6] If C/2016 R2 formed in the Solar System, it could have either accreted from the protosolar disk 4.6 billion years ago[8]: 5 or it could have formed as an icy fragment ejected from a planetary collision.[12]: 11 In both scenarios, C/2016 R2 must have formed far from the Sun, beyond the CO and N
2 frost lines where temperatures are cold enough (less than 50 K[7][6]: 21 ) for these substances to condense into solid grains.[12]: 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 N
2 if it formed between 10 and 15 AU from the proto-Sun, where the CO and N
2 frost lines reside.[8]: 1, 5 At these distances, condensed CO and N
2 grains would be pure and would outnumber water ice grains.[8]: 5 However, the outcome of this formation process is highly dependent on various unknown properties of the protosolar nebula, such as its gas viscosity.[8]: 5 C/2016 R2's overabundance of simple molecules like N
2 and CO suggests that its formation environment was chemically inactive and protected from photodissociation by solar radiation.[6]: 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 NH
3 and HCN in C/2016 R2.[6]: 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.[23]: 1 A 2022 study led by Andersen et al. showed 90% of objects that formed near the N
2 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.[23]: 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.[23]: 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.[23]: 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.[12]: 11 [6]: 22 A similar scenario has been proposed for the interstellar object 1I/ʻOumuamua.[4]: 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).[23]: 6 [4]: 5194 However, this hypothesis is complicated by the fact that the relative abundances of CO, CH
4, and N
2 seen in C/2016 R2 do not match the surface composition of Pluto,[6]: 22 [4]: 5192 and that the collisional dynamics and interiors of icy bodies beyond Neptune are poorly understood.[23]: 6 [4]: 5194 Various studies have shown that it is difficult to retain large amounts of N
2 in energetic impacts, which further complicates this hypothesis.[4]: 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.[6]: 22 For example, the interstellar comet 2I/Borisov is known to have a CO-rich and water-poor composition similar to C/2016 R2.[23]: 1 [4]: 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.[6]: 22 The 12C/13C isotope ratio of C/2016 R2 could potentially match that of the interstellar medium, which might support an interstellar origin.[18]: 8 However, the 12C/13C ratio could alternatively match that of the Solar System due to uncertainties[18]: 8 and the orbit of C/2016 R2 resembles those of many known Oort cloud comets,[13]: 12 so the hypothesis of an interstellar origin is considered unlikely.[6]: 22 [18]: 8
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Gallery
See also
- 3I/ATLAS – An interstellar comet with a very high CO
2/H
2O mixing ratio, like C/2016 R2
Notes
- McKay et al. (2019) reported a CO
2/CO mixing ratio of 18.2%±3.5%.[6]: 22 Given the percentage of CO in C/2016 R2's volatile composition (80% CO in terms of number of molecules),[6]: 19 multiplying that percentage by the CO
2/CO mixing ratio gives roughly 14.6%±2.8% CO
2 (full rounded range 12–17%). - Mousis et al. (2021) cite reported N
2/CO mixing ratio of 0.06±0.01 and 0.08.[8]: 1 Given the percentage of CO in C/2016 R2's volatile composition (80% CO in terms of number of molecules),[6]: 19 multiplying that percentage by the N
2/CO mixing ratio gives roughly 5%±1% N
2 (full range 4–6%). - 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)
- Carbon dioxide or CO
2 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 CO
2 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 CO
2/second. Dividing the moles of CO
2 by the molar mass of CO
2 gives a CO
2 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. - Molecular nitrogen or N
2 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 N
2 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 N
2/second. Dividing the moles of N
2 by the molar mass of N
2 gives a N
2 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. - Water or H
2O 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 H
2O 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 H
2O/second. Dividing the moles of H
2O by the molar mass of H
2O gives a H
2O 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. - 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.
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References
External links
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