C/2016 R2 (PanSTARRS)

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 (N
₂), but extremely poor in water (H
₂O). The high abundance of CO and N
₂ 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. In terms of number of molecules, the volatile composition of C/2016 R2's coma comprises over 80% CO gas, roughly 12–17% carbon dioxide (CO
₂) gas, roughly 4–6% N
₂ gas, and trace amounts (≤1%) of water vapor and other compounds. 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, and virtually no N
₂.

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 comparable to those seen in the exceptionally active comets Comet Hale–Bopp and 17P/Holmes. 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, 200 kilograms (0.2 long tons; 0.2 short tons) of N
₂ per second, and 9 kilograms (20 pounds) of water per second during January–February 2018. Observations suggest that these gases were mainly emitted directly from the surface of C/2016 R2's nucleus, though some CO was also observed being produced outside the nucleus due to sublimation of ejected ice grains.

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. 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). Methylidyne radicals (CH), cyano radicals (CN), dicarbon (C
₂), and tricarbon (C
₃) have also been detected in C/2016 R2's coma. 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. 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. 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.

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. 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. Cations of N⁺
₂, CO⁺
, and CO⁺
₂ have been detected in C/2016 R2's coma. 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), CO⁺
₂ becomes more abundant as CO⁺
becomes neutralized by frequent collisions between molecules in the denser gas environment. The dominant blue spectral emission of CO⁺
(and N⁺
₂ by a small part) gives C/2016 R2 its deep blue color. This contrasts with typical comet colors, which range from gray ("neutral") to yellow or green due to C
₂ spectral emission and sunlight scattering by dust particles.

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. The majority of elemental carbon and oxygen in C/2016 R2's gas composition is contained in CO and CO
₂. 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. 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. 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. 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.

Dust and metals

Telescope observations have shown very little amounts of dust emitting from C/2016 R2, which indicates the comet is dust-poor. 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. 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. 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, which is one of the highest iron and nickel emission rates seen among comets. 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.

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). 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. 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.

Coma and tail morphology

Observations of C/2016 R2 during the leadup to its perihelion in 2018 showed that it had a diffuse and spherical coma, due to its predominantly gaseous emissions. The appearance of C/2016 R2's coma has been described as diffuse in dates as early as 2016. The coma of C/2016 R2 grew as large as 350,000 km (220,000 mi) (angular diameter 3.7 arcminutes) in March 2018. 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 and a transient shell or ring-shaped feature about 140,000 km (87,000 mi) in diameter. The shell or ring-shaped feature might have been caused by an impulsive outgassing event, as it was only seen on 21 February 2018. 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.

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. During this time, C/2016 R2's sun-facing hemisphere was outgassing CO with an amount and speed (0.51 km/s or 0.32 mi/s) twice as high as its nightside (0.25 km/s or 0.16 mi/s). Spectroscopic observations were able to identify this enhanced sunward outgassing by detecting a slight blueshift in the spectral emissions of CO.

C/2016 R2 had a deep blue ion tail that was visible from November 2017 to the end of April 2018. The tail spanned over 3 million km (1.9 million mi) in length (angular length 15 arcminutes) during January and February 2018. The tail was primarily made of CO⁺
ions that were being blown away from the Sun by the solar wind. Like the coma, the ion tail of C/2016 R2 owes its blue color to the spectral emission of CO⁺
. During January and February 2018, the ion tail of C/2016 R2 exhibited complex structures and streamers that visibly changed within hours. 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. Cometary outbursts and fluctuations in the solar wind may have additionally contributed to the complex structure of C/2016 R2's tail.

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. 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. 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). 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. The upper end of this diameter range would be considered larger than an average comet. 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.

Observational history

Discovery

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

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

Origin

Although the highly distant and eccentric orbit of C/2016 R2 suggests that it originated from the Oort cloud, the comet's unusual volatile composition makes its origin unclear. If C/2016 R2 formed in the Solar System, it could have either accreted from the protosolar disk 4.6 billion years ago or it could have formed as an icy fragment ejected from a planetary collision. 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) for these substances to condense into solid grains.

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. At these distances, condensed CO and N
₂ grains would be pure and would outnumber water ice grains. However, the outcome of this formation process is highly dependent on various unknown properties of the protosolar nebula, such as its gas viscosity. 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. 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.

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. 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. 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. 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.

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. A similar scenario has been proposed for the interstellar object 1I/ʻOumuamua. 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). 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, and that the collisional dynamics and interiors of icy bodies beyond Neptune are poorly understood. Various studies have shown that it is difficult to retain large amounts of N
₂ in energetic impacts, which further complicates this hypothesis.

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. For example, the interstellar comet 2I/Borisov is known to have a CO-rich and water-poor composition similar to C/2016 R2. 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. The ¹²C/¹³C isotope ratio of C/2016 R2 could potentially match that of the interstellar medium, which might support an interstellar origin. However, the ¹²C/¹³C ratio could alternatively match that of the Solar System due to uncertainties and the orbit of C/2016 R2 resembles those of many known Oort cloud comets, so the hypothesis of an interstellar origin is considered unlikely.