Surface and spectra The surface of Gonggong has an
albedo (reflectivity) of 0.14. The surface composition and spectrum of Gonggong is expected to be similar to that of , as both objects are red in color and display signs of water ice and possibly
methane in their spectra. The
reflectance spectrum of Gonggong was first measured in 2011 at
near-infrared wavelengths, with the Folded port InfraRed Echellette (FIRE)
spectrograph on the
Magellan Baade Telescope at the
Las Campanas Observatory in
Chile. Gonggong's spectrum exhibits a strong red
spectral slope along with broad
absorption bands at
wavelengths of 1.5
μm and 2 μm, meaning that Gonggong reflects more light at these wavelengths. Additional
photometric measurements from the
Hubble Space Telescope's
Wide Field Camera 3 instrument display similar absorption bands at 1.5 μm, which are characteristic features of water ice, a substance often found on large
Kuiper belt objects. The presence of water ice on the surface of Gonggong implies a brief period of
cryovolcanism in the distant past, when water erupted from its interior, deposited onto its surface, and subsequently froze. Gonggong is among the reddest trans-Neptunian objects known, especially in the visible and near-infrared. Its red color is unexpected for an object with a substantial amount of water ice on its surface, which are typically neutral in color, hence why Gonggong was initially nicknamed "Snow White". Gonggong's color implies that methane is present on its surface, although it was not directly detected in the spectrum of Gonggong due to the low
signal-to-noise ratio of the data. The presence of methane frost would account for its color, as a result of the
photolysis of methane by solar radiation and
cosmic rays producing reddish
organic compounds known as
tholins. Observations of Gonggong's near-infrared spectrum in 2015 revealed an absorption feature at 2.27 μm, indicating the presence of
methanol along with its irradiation products on its surface. Gonggong is large enough to be able to retain trace amounts of
volatile methane on its surface, even when at its closest distance to the Sun (33.2
AU), where temperatures are higher than that of Quaoar. In particular, the large size of Gonggong means that it is likely to retain trace amounts of other volatiles, including
ammonia,
carbon monoxide, and possibly
nitrogen, which almost all trans-Neptunian objects lose over the course of their existence. Like Quaoar, Gonggong is expected to be near the mass limit at which it is able to retain those
volatile materials on its surface. In 2022, low resolution near-infrared (0.7–5 μm)
spectroscopic observations by the
James Webb Space Telescope (JWST) revealed the presence of significant amounts of
ethane ice (C2H6) on the surface of Gonggong, though there appears to be less ethane on Gonggong than on
Sedna. The JWST spectra also contain evidence of presence of small amounts of
carbon dioxide (CO2) complexed with either dark surface material or some ices as well as complex organics. On the other hand no evidence of presence of
methane (CH4) and
methanol (CH3OH) was found at variance with the earlier observations.
Possible atmosphere The presence of tholins on the surface of Gonggong implies the possible existence of methane ice on the surface and a tenuous methane atmosphere analogous to Quaoar. Gonggong occasionally comes closer to the Sun than Quaoar, where it becomes warm enough for the methane ice on the surface to sublimate, possibly creating an atmosphere. Gonggong's larger mass makes the retention of methane just possible. During aphelion, methane along with other
volatiles would condense on Gonggong's surface, allowing for long-term irradiation that would otherwise result in a decrease in surface albedo. The lower surface albedo would contribute to the loss of highly volatile materials such as nitrogen, as a lower albedo corresponds to more light being
absorbed by the surface rather than being reflected, thus resulting in greater surface heating. Hence, the nitrogen content of Gonggong's atmosphere is expected to be depleted to trace amounts while methane is likely retained. Gonggong is thought to have had cryovolcanic activity along with a more substantial atmosphere shortly after its
formation. Such cryovolcanic activity is expected to have been brief, and the resulting atmosphere gradually
escaped over time. Volatile gases, such as nitrogen and carbon monoxide, were lost, while less volatile gases such as methane are likely to remain in its present tenuous atmosphere.
Size As of 2019, Gonggong is estimated to have a diameter of , derived from
radiometric measurements, its calculated mass, and assuming a density similar to other similar bodies. This would make Gonggong the fifth-largest trans-Neptunian object, after
Pluto,
Eris,
Haumea and
Makemake. Gonggong is approximately the size of Pluto's moon
Charon, although Gonggong's current size estimate has an uncertainty of . Gonggong has been classified as a dwarf planet by several astronomers. Brown states that Gonggong "must be a dwarf planet even if predominantly rocky", based on the 2013 radiometric measurement of .
Scott Sheppard and colleagues think that it is likely to be a dwarf planet, based on its minimum possible diameter— under the assumption of a completely reflective surface with an albedo of 1{{refn|The resulting minimum diameter of 580 km is derived from the equation E=\frac{1329}{\sqrt{p}} 10^{-0.2H}, where H is the absolute magnitude of Gonggong, and p is the albedo of Gonggong, which in this case is assumed to be 1.|name=magnitude|group=lower-alpha}}—and what was at the time the expected lower size limit of around for
hydrostatic equilibrium in cold icy-rocky bodies. However,
Iapetus is not in equilibrium despite being in diameter, so this remains just a possibility. In 2010, astronomer
Gonzalo Tancredi initially estimated Gonggong to have a very large diameter of , though its dwarf planet status was unclear as there was no lightcurve data or other information to ascertain its size. Gonggong is too distant to be resolved directly; Brown placed a rough estimate of its diameter ranging from , based on an albedo of 0.18 which was the best fit in his model. A survey led by a team of astronomers using the
European Space Agency's
Herschel Space Observatory in 2012 determined its diameter to be (), based on the thermal properties of Gonggong observed in the
far infrared range. This measurement is consistent with Brown's estimate. Later observations in 2013 using combined thermal emission data from
Herschel and the
Spitzer Space Telescope suggested a smaller size of (), though this estimate had a larger range of uncertainty. In 2016, combined observations from the
Kepler spacecraft and archival thermal emission data from
Herschel suggested that Gonggong was much larger than previously thought, giving a size estimate of () based on an assumed equator-on view and a lower estimated albedo of 0.089. This would have made Gonggong the third-largest trans-Neptunian object after Eris and Pluto, larger than Makemake (). These observations of Gonggong were part of the
Kepler spacecraft's
K2 mission which includes studying
small Solar System bodies. Subsequent measurements in 2018 revised the size of Gonggong to (), based on the mass and density of Gonggong derived from the orbit of its satellite and the discovery that the viewing direction was almost
pole-on. With this size estimate, Gonggong is again thought to be the fifth-largest trans-Neptunian object.
Mass, density and rotation Based on the orbit of its satellite, the mass of Gonggong has been calculated to be , with a density of . Given the mass, the 2016 size estimate of would have implied an unexpectedly low (and likely erroneous) density of . Gonggong is the fifth most massive trans-Neptunian object, after Eris, Pluto, Haumea, and Makemake. It is slightly more massive and denser than Charon, which has a mass of and a density of . Due to its large size, mass, and density, Gonggong is expected to be in hydrostatic equilibrium, taking the shape of a
MacLaurin spheroid that is slightly
flattened due to its rotation. The rotation period of Gonggong was first measured in March 2016, through observations of variations in its brightness with the
Kepler space telescope. Gonggong's light curve
amplitude as observed by
Kepler is small, only varying in brightness by about 0.09
magnitudes. The small
light curve amplitude of Gonggong indicates that it is being viewed at a pole-on configuration, further evidenced by the observed inclined orbit of its satellite. The
Kepler observations provided ambiguous values of and hours for the rotation period. Based on a best-fit model for its rotation pole orientation, the value of hours is thought to be the more plausible one. Gonggong rotates slowly compared to other trans-Neptunian objects, which usually have periods between 6 and 12 hours. Due to its slow rotation, it is expected to have a low
oblateness of 0.03 or 0.007, for rotation periods of 22.4 or 44.81 hours, respectively. == Satellite ==