We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord ...occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius R sub(equiv) = 555+ or -2.5 km and geometric visual albedo p sub(v) = 0.109+ or -0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of member of = 0.087 super(+0.0268) sub(-0.0175), an equatorial radius of 569 super(+24) sub(-17)km, and a density of 1.99 + or - 0.46 g cm super(-3). The orientation of our preferred solution in the plane of the sky implies that Quaoar's satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.
Aims. Using data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal ...inertia (TI) and roughness in several regions on the largest lobe of the nucleus. Methods. We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014. Results. The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be <80 JK−1 m−2 s−0.5 for the Seth, Ash, and Aten regions. The submillimetre channel implies similar results but also suggests that higher values could be possible. A low TI is consistent with other MIRO measurements and in situ data from the MUPUS instrument at the final landing site of Philae. The VIRTIS results give a best-fitting value of 80 JK−1 m−2 s−0.5 and values in the range 40–160 JK−1 m−2 s−0.5 in the same areas. These observations also allow the subpixel scale surface roughness to be estimated and compared to images from the OSIRIS camera. The VIRTIS data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre–millimetre) scales, and visually rough terrain can be smooth at small scales.
Context. The recently announced Oort-cloud comet C/2014 UN271 (Bernardinelli-Bernstein) is remarkable in at least three respects: (i) it was discovered inbound as far as ∼29 au from the Sun (with ...prediscovery images up to ∼34 au); (ii) it already showed cometary activity at almost 24 au; and (iii) its nuclear magnitude (Hr ∼ 8.0) indicates an exceptionally large object. Detection of gases is expected in the upcoming years as the comet heads toward a perihelion of ∼11 au in 2031.Aims. The goal is to determine the object’s diameter and albedo from thermal measurements.Methods. We used ALMA in extended configuration (resolution ∼0.064″) to measure the 1287 μm (233 GHz) continuum flux of the comet. Observations were performed on August 8, 2021, at a 20.0 au distance from the Sun. The high spatial resolution was chosen in order to filter out any dust contribution. We also used a recently published Afρ value to estimate the dust production rate and the expected dust thermal signal for various assumptions on particle size distribution.Results. We detected the thermal emission of the object at ∼10σ, with a flux of 0.128 ± 0.012 mJy. Based on observational constraints and our theoretical estimates of the dust contribution, the entirety of the measured flux can be attributed to the nucleus. From NEATM modeling combined with the Hr magnitude, we determine a surface-equivalent diameter of 137 ± 17 km and a red geometric albedo of 5.3 ± 1.2%. This confirms that C/2014 UN271 is by far the largest Oort-cloud object ever found (almost twice as large as comet C/1995 O1 Hale-Bopp) and, except for the Centaur 95P/Chiron, which shows outburst-like activity, the largest known comet in the Solar System. On the other hand, the C/2014 UN271 albedo is typical of comets, adding credence to a “universal” comet nucleus albedo.Conclusions. With its distant perihelion and uniquely large size, C/2014 UN271 (Bernardinelli-Bernstein) is the prominent archetype of distant comets whose activity is driven by hypervolatiles. Monitoring of dust and gas emission as the comet approaches and passes perihelion will permit its activity time pattern to be studied and compared to the distant (outbound) activity of Hale-Bopp. Post-perihelion thermal measurements will permit the study of possible albedo changes, such as a surface brightening compared to pre-perihelion, as was observed for Hale-Bopp.
Aims. Using spectroscopic and continuum data measured by the MIRO instrument on board Rosetta of comet 67P/Churyumov-Gerasimenko, it is possible to derive and track the change in the water production ...rate, to learn how the outgassing evolves with heliocentric distance. The MIRO data are well suited to investigate the evolution of 67P, in unprecedented spatial and temporal detail. Methods. To obtain estimates of the local effective Haser production rates we developed an efficient and reliable retrieval approach with precalculated lookup tables. We employed line area ratios (H216O/H218O) from pure nadir observations as the key variable, along with the Doppler shift velocity, and continuum temperature. This method was applied to the MIRO data from August 2014 until April 2016. Perihelion occurred on August 13, 2015 when the comet was 1.24 AU from the Sun. Results. During the perihelion approach, the water production rates increased by an order of magnitude, and from the observations, the derived maximum for a single observation on August 29, 2015 is (1.42 ± 0.51) ×1028. Modeling the data indicates that there is an offset in the peak outgassing, occurring 34 ± 10 days after perihelion. During the pre-perihelion phase, the production rate changes with heliocentric distance as rh−3.8±0.2; during post-perihelion, the dependence is rh−4.3±0.2. The comet is calculated to have lost 0.12 ± 0.06 % of its mass during the perihelion passage, considering only water ice sublimation. Additionally, this method provides well sampled data to determine the spatial distribution of outgassing versus heliocentric distance. The time evolution is definitely not uniform across the surface. Pre- and post-perihelion, the surface temperature on the southern hemisphere changes rapidly, as does the sublimation rate with an exponent of ~−6. There is a strong latitudinal dependence on the rh exponent with significant variation between northern and southern hemispheres, and so the average over the comet surface may only be of limited importance. We present more detailed regional variation in the outgassing, demonstrating that the highest derived production rates originate from the Wosret, Neith and Bes regions during perihelion.
Context . On October 31, 2009, the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory observed far-infrared spectra of Jupiter in the wavelength range between ...50 and 220 µm as part of the program “Water and Related Chemistry in the Solar System”. The spectra have an effective spectral resolution between 900 and 3500, depending on the wavelength and grating order. Aims . We investigate the disk-averaged chemical composition of Jupiter’s atmosphere as a function of height using these observations. Methods . We used the Planetary Spectrum Generator and the least-squares fitting technique to infer the abundances of trace constituents. Results . The PACS data include numerous spectral lines attributable to ammonia (NH 3 ), methane (CH 4 ), phosphine (PH 3 ), water (H 2 O), and deuterated hydrogen (HD) in the Jovian atmosphere and probe the chemical composition from p ~ 275 mbar to p ~ 900 mbar. From the observations, we infer an ammonia abundance profile that decreases from a mole fraction of (1.7 ± 0.8) × 10 −4 at p ~ 900 mbar to (1.7 ± 0.9) × 10 −8 at p ~ 275 mbar, following a fractional scale height of about 0.114. For phosphine, we find a mole fraction of (7.2 ± 1.2) × 10 −7 at pressures higher than (550 ± 100) mbar and a decrease of its abundance at lower pressures following a fractional scale height of (0.09 ± 0.02). Our analysis delivers a methane mole fraction of (1.49 ± 0.09) × 10 −3 . Analyzing the HD R (0) line at 112.1 µm yields a new measurement of Jupiter’s D/H ratio, D/H = (1.5 ± 0.6) × 10 −5 . Finally, the PACS data allow us to put the most stringent 3 σ upper limits yet on the mole fractions of hydrogen halides in the Jovian troposphere. These new upper limits are <1.1 × 10 −11 for hydrogen fluoride (HF), <6.0 × 10 −11 for hydrogen chloride (HCl), <2.3 × 10 −10 for hydrogen bromide (HBr) and <1.2 × 10 −9 for hydrogen iodide (HI) and support the proposed condensation of hydrogen halides into ammonium halide salts in the Jovian troposphere.
Context. The Kuiper belt is formed of planetesimals which failed to grow to planets and its dynamical structure has been affected by Neptune. The classical Kuiper belt contains objects both from a ...low-inclination, presumably primordial, distribution and from a high-inclination dynamically excited population. Aims. Based on a sample of classical trans-Neptunian objects (TNOs) with observations at thermal wavelengths we determine radiometric sizes, geometric albedos and thermal beaming factors for each object as well as study sample properties of dynamically hot and cold classical Methods. Observations near the thermal peak of TNOs using infrared space telescopes are combined with optical magnitudes using the radiometric technique with near-Earth asteroid thermal model (NEATM). We have determined three-band flux densities from Herschel/PACS observations at 70.0, 100.0 and 160.0 mu m and Spitzer/MIPS at 23.68 and 71.42 mu m when available. We use reexamined absolute visual magnitudes from the literature and ground based programs in support of Herschel observations. Results. We have analysed 18 classical TNOs with previously unpublished data and re-analysed previously published targets with updated data reduction to determine their sizes and geometric albedos as well as beaming factors when data quality allows. We have combined these samples with classical TNOs with radiometric results in the literature for the analysis of sample properties of a total of 44 objects. We find a median geometric albedo for cold classical TNOs of 0.14 sub(-0.07) super(+0.09) and for dynamically hot classical TNOs, excluding the Haumea family and dwarf planets, 0.085 sub(-0.045) super(+0.084) . We have determined the bulk densities of Borasisi-Pabu (2.1 sub(-1.2) super(+2.6) g cm super(-3)), Varda-Ilmare (1.25 sub(-0.43) super(+0.40) g cm super(-3)) and 2001 QC sub(298) (1.14 sub(-0.30) super(+0.34) g cm super(-3)) as well as updated previous density estimates of four targets. We have determined the slope parameter of the debiased cumulative size distribution of dynamically hot classical TNOs as q = 2.3 + or - 0.1 in the diameter range 100 < D < 500 km. For dynamically cold classical TNOs we determine q = 5.1 + or - 1.1 in the diameter range 160 < D < 280 km as the cold classical TNOs have a smaller maximum size.
We have developed a one‐dimensional, diurnally averaged, photochemical model for Jupiter's stratosphere that couples photodissociation, chemical kinetics, vertical diffusion, and radiative transport. ...The predictions regarding the abundances and vertical profiles of hydrocarbon compounds are compared with observations from the Infrared Space Observatory (ISO) to better constrain the atmospheric composition, to better define the eddy diffusion coefficient profile, and to better understand the chemical reaction schemes that produce and destroy the observed constituents. From model‐data comparisons we determine that the C2H6 mole fraction on Jupiter is (4.0 ± 1.0) × 10−6 at 3.5 mbar and (2.7 ± 0.7) × 10−6 at 7 mbar, and the C2H2 mole fraction is (1.4 ± 0.8) × 10−6 at 0.25 mbar and (1.5 ± 0.4) × 10−7 at 2 mbar. The column densities of CH3C2H and C6H6 are (1.5 ± 0.4) × 1015 cm−2 and (8.0 ± 2) × 1014 cm−2, respectively, above 30 mbar. Using identical reaction lists, we also have developed photochemical models for Saturn, Uranus, and Neptune. Although the models provide good first‐order predictions of hydrocarbon abundances on the giant planets, our current chemical reaction schemes do not reproduce the relative abundances of C2Hx hydrocarbons. Unsaturated hydrocarbons like C2H4 and C2H2 appear to be converted to saturated hydrocarbons like C2H6 more effectively on Jupiter than on the other giant planets, more effectively than is predicted by the models. Further progress in our understanding of photochemistry at low temperatures and low pressures in hydrogen‐dominated atmospheres hinges on the acquisition of high‐quality kinetics data.
Context.
Past observations of Saturn with ground-based and space telescopes have enabled the monitoring of tropospheric wind speeds using cloud-tracking techniques. The most remarkable feature is a ...broad and fast prograde jet at the equator that reaches speeds of ~400 m s
−1
. Saturn’s stratospheric dynamics are less well-known. At low latitudes, they are characterized by the thermal signature of an equatorial oscillation; the observed thermal structure implies that there is a strong oscillating vertical shear of the zonal winds throughout the stratosphere. However, wind speeds in this region cannot be measured by cloud-tracking techniques and remain unknown.
Aims.
The objective of this study is to measure directly and for the first time the zonal winds in Saturn’s stratosphere using the ALMA interferometer.
Methods.
We observed the spectral lines of CO at 345.796 GHz and HCN at 354.505 GHz with the high spatial (~0.6″ × 0.5″) and spectral resolutions enabled by ALMA, and measured the Doppler shift induced by the winds on the lines at the planet limb where the emission is the strongest. After subtracting the beam-convolved planet rotation, we derived the zonal wind speeds as a function of latitude.
Results.
We measured the zonal winds from ~20°S to the northern polar latitudes. Latitudes between 20°S and 45°S were obscured by the rings and were inaccessible southward of 45°S. The zonal wind profiles obtained on the eastern and western limbs are consistent within the error bars and probe from the 0.01 to the 20 mbar level. We most noticeably detect a broad super-rotating prograde jet that spreads from 20°S to 25°N with an average speed of 290 ± 30 m s
−1
. This jet is asymmetrical with respect to the equator, a possible seasonal effect. We tentatively detect the signature of the Saturn semi-annual oscillation (SSAO) at the equator, in the form of a ~−50 ± 30 m s
−1
peak at the equator which lies on top of the super-rotating jet. We also detect a broad retrograde wind (−45 ± 20 m s
−1
) of about 50 m s
−1
in the mid-northern latitudes. Finally, in the northern polar latitudes, we observe a possible auroral effect in the form of a ~200 m s
−1
jet localized on the average position of the northern main auroral oval and in couter-rotation, like the Jovian auroral jets.
Conclusions.
Repeated observations are now required to monitor the temporal evolution of the winds and quantify the variability of the SSAO jet, to test the seasonality of the asymmetry observed in the broad super-rotating jet, and to verify the presence of auroral jets in the southern polar region of Saturn.
Context. Carbon monoxide (CO) has been detected in all giant planets and its origin is both internal and external in Jupiter and Neptune. Despite its first detection in Uranus a decade ago, the ...magnitude of its internal and external sources remains unconstrained. Aims. We targeted CO lines in Uranus in the submillimeter range to constrain its origin. Methods. We recorded the disk-averaged spectrum of Uranus with very high spectral resolution at the frequencies of CO rotational lines in the submillimeter range in 2011−2012. We used empirical and diffusion models of the atmosphere of Uranus to constrain the origin of CO. We also used a thermochemical model of its troposphere to derive an upper limit on the oxygen-to-hydrogen (O/H) ratio in the deep atmosphere of Uranus. Results. We have detected the CO(8−7) rotational line for the first time with Herschel-HIFI. Both empirical and diffusion models results show that CO has an external origin. An empirical profile in which CO is constant above the 100 mbar level with a mole fraction of 7.1−9.0 × 10-9, depending on the adopted stratospheric thermal structure, reproduces the data. Sporadic and steady source models cannot be differentiated with our data. Taking the internal source model upper limit of a mole fraction of 2.1 × 10-9 we find, based on our thermochemical computations, that the deep O/H ratio of Uranus is less than 500 times solar. Conclusions. Our work shows that the average mole fraction of CO decreases from the stratosphere to the troposphere and thus strongly advocates for an external source of CO in Uranus. Photochemical modeling of oxygen species in the atmosphere of Uranus and more sensitive observations are needed to reveal the nature of the external source.
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