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Global views of Uranus and Neptune. Upper row Uranus images in : (a) visible wavelengths from Voyager 2 ; (b) Near IR with extreme processing of cloud features from Fry et al. (2012) ; (c) Near IR of bright features from de Pater et al. (2014). Bottom row Neptune images in : (d) visible wavelengths from Voyager 2 ; (e) Visible wavelengths from HST (image credits : NASA, ESA, and M.H. Wong and J. Tollefson from UC Berkeley) ; (f) near IR (observations courtesy of I. de Pater).
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The ice giants Uranus and Neptune are the least understood class of planets in our solar system but the most frequently observed type of exoplanets. Presumed to have a small rocky core, a deep interior comprising ∼70% heavy elements surrounded by a more dilute outer envelope of H2 and He, Uranus and Neptune are fundamentally different from the bett...
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... results from an ice giant atmospheric probe would have to be interpreted in light of the different meteorological fea- tures that have been observed in Uranus and Neptune. Figure 4 shows the visual aspect of both planets at a variety of wa- velengths from the visible to the near infrared. Both planets show a recursive but random atmospheric activity at cloud le- vel that can be observed in the methane absorption bands as bright spots ( Sromovsky et al., 1995). ...
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The exploration of carbon-to-oxygen ratios has yielded intriguing insights into the composition of close-in giant exoplanets, giving rise to a distinct classification: carbon-rich planets, characterized by a carbon–to–oxygen ratio ≥ 1 in their atmospheres, as opposed to giant planets exhibiting carbon–to–oxygen ratios close to the protosolar value....
Citations
... Deriving the deep oxygen in the ice giants thus requires the use of indirect measurements and thermochemical modeling. From the detection of CO in the atmosphere of Neptune (Marten et al. 1993) and using estimates of the deep atmospheres vertical mixing, Lodders and Fegley (1994) found that Neptune could have a rather extreme oxygen enrichment factor of 400 times solar, difficult to explain with formation models (e.g., Mousis et al. (2018)). The lack of direct detection of CO in the atmosphere of Uranus resulted in the derivation of an O/H enrichment upper limit of 260 times solar (Lodders and Fegley 1994). ...
The exploration of carbon-to-oxygen ratios has yielded intriguing insights into the composition of close-in giant exoplanets, giving rise to a distinct classification: carbon-rich planets, characterized by a carbon–to–oxygen ratio ≥ 1 in their atmospheres, as opposed to giant planets exhibiting carbon–to–oxygen ratios close to the protosolar value. In contrast, despite numerous space missions dispatched to the outer solar system and the proximity of Jupiter, Saturn, Uranus, and Neptune, our understanding of the carbon-to-oxygen ratio in these giants remains notably deficient. Determining this ratio is crucial as it serves as a marker linking a planet’s volatile composition directly to its formation region within the disk. This article provides an overview of the current understanding of the carbon-to-oxygen ratio in the four gas giants of our solar system and explores why there is yet no definitive dismissal of the possibility that Jupiter, Saturn, Uranus, or Neptune could be considered carbon-rich planets. Additionally, we delve into the three primary formation scenarios proposed in existing literature to account for a bulk carbon-to-oxygen ratio ≥ 1 in a giant planet. A significant challenge lies in accurately inferring the bulk carbon-to-oxygen ratio of our solar system’s gas giants. Retrieval methods involve integrating in situ measurements from entry probes equipped with mass spectrometers and remote sensing observations conducted at microwave wavelengths by orbiters. However, these methods fall short of fully discerning the deep carbon-to-oxygen abundance in the gas giants due to their limited probing depth, typically within the 10–100 bar range. To complement these direct measurements, indirect determinations rely on understanding the vertical distribution of atmospheric carbon monoxide in conjunction with thermochemical models. These models aid in evaluating the deep oxygen abundance in the gas giants, providing valuable insights into their overall composition.
... Deriving the deep oxygen in the ice giants thus requires the use of indirect measurements and thermochemical modeling. From the detection of CO in the atmosphere of Neptune (Marten et al, 1993) and using estimates of the deep atmospheres vertical mixing, Lodders and Fegley (1994) found that Neptune could have a rather extreme oxygen enrichment factor of 400 times solar, difficult to explain with formation models (e.g., Mousis et al (2018)). The lack of direct detection of CO in the atmosphere of Uranus resulted in the derivation of an O/H enrichment upper limit of 260 times solar (Lodders and Fegley, 1994). ...
The exploration of carbon-to-oxygen ratios has yielded intriguing insights into the composition of close-in giant exoplanets, giving rise to a distinct classification: carbon-rich planets, characterized by a carbon-to-oxygen ratio $\ge$ 1 in their atmospheres, as opposed to giant planets exhibiting carbon-to-oxygen ratios close to the protosolar value. In contrast, despite numerous space missions dispatched to the outer solar system and the proximity of Jupiter, Saturn, Uranus, and Neptune, our understanding of the carbon-to-oxygen ratio in these giants remains notably deficient. Determining this ratio is crucial as it serves as a marker linking a planet's volatile composition directly to its formation region within the disk. This article provides an overview of the current understanding of the carbon-to-oxygen ratio in the four gas giants of our solar system and explores why there is yet no definitive dismissal of the possibility that Jupiter, Saturn, Uranus, or Neptune could be considered carbon-rich planets. Additionally, we delve into the three primary formation scenarios proposed in existing literature to account for a bulk carbon-to-oxygen ratio $\ge$ 1 in a giant planet. A significant challenge lies in accurately inferring the bulk carbon-to-oxygen ratio of our solar system's gas giants. Retrieval methods involve integrating in situ measurements from entry probes equipped with mass spectrometers and remote sensing observations conducted at microwave wavelengths by orbiters. However, these methods fall short of fully discerning the deep carbon-to-oxygen abundance in the gas giants due to their limited probing depth, typically within the 10-100 bar range.
... Such planets form with a silicate-to-ice ratio of 1:1 (e.g., Lodders 2003;Lopez 2017;Zeng et al. 2019;Mousis et al. 2020;Aguichine et al. 2021) beyond the snowline. These are analogous to the water-rich minor planets and moons (e.g., Enceladus, Pluto, etc.) of the solar system and the water-rich cores of Uranus and Neptune (Mousis et al. 2018). However, the existence of such planets around other stars at short orbital distances remains elusive to date. ...
Planet formation models suggest that the small exoplanets that migrate from beyond the snowline of the protoplanetary disk likely contain water-ice-rich cores (∼50% by mass), also known as water worlds. While the observed radius valley of the Kepler planets is well explained by the atmospheric dichotomy of the rocky planets, precise measurements of the mass and radius of the transiting planets hint at the existence of these water worlds. However, observations cannot confirm the core compositions of those planets, owing to the degeneracy between the density of a bare water-ice-rich planet and the bulk density of a rocky planet with a thin atmosphere. We combine different formation models from the Genesis library with atmospheric escape models, such as photoevaporation and impact stripping, to simulate planetary systems consistent with the observed radius valley. We then explore the possibility of water worlds being present in the currently observed sample by comparing them with simulated planets in the mass–radius–orbital period space. We find that the migration models suggest ≳10% and ≳20% of the bare planets, i.e., planets without primordial H/He atmospheres, to be water-ice-rich around G- and M-type host stars, respectively, consistent with the mass–radius distributions of the observed planets. However, most of the water worlds are predicted to be outside a period of 10 days. A unique identification of water worlds through radial velocity and transmission spectroscopy is likely to be more successful when targeting such planets with longer orbital periods.
... However, these probes could also be capable of studying the hazes composed of small photochemically produced particles (hydrocarbon ices) and the abundances of species that can condense in the atmosphere and form clouds, such as NH 3 , CH 4 , H 2 S, and H 2 O. These aerosol particles are crucial in the radiative heat budget of the planet, and measuring their optical properties and spatial distribution can provide useful information about the chemistry and dynamics of the Uranus stratosphere and troposphere, constraining planetary formation models (e.g., Atreya et al. 2020;Mousis et al. 2018). Orbital observations could give partial information about these atmospheric structured layers but to give a "ground truth" of the vertical distribution and composition of the aerosols we need the use of a descent probe with dedicated instrumentation and in-situ observations compatible with the resources of the probe's primary mission. ...
The aerosols (clouds and hazes) on Uranus are one of the main elements for understanding the thermal structure and dynamics of its atmosphere. Aerosol particles absorb and scatter the solar radiation, directly affecting the energy balance that drives the atmospheric dynamics of the planet. In this sense, aerosol information such as the vertical distribution or optical properties is essential for characterizing the interactions between sunlight and aerosol particles at each altitude in the atmosphere and for understanding the energy balance of the
planet’s atmosphere. Moreover, the distribution of aerosols in the atmosphere provides key information on the global circulation of the planet (e.g., regions of upwelling or subsidence).
To address this challenge, we propose the Uranus Multi-experiment Radiometer (UMR), a lightweight instrument designed to characterize the aerosols in Uranus’ atmosphere as part of the upcoming Uranus Flagship mission’s descending probe payload. The scientific goals of UMR are: (1) to study the variation of the solar radiation in the ultra-violet (UV) with altitude and characterize the energy deposition in the atmosphere; (2) to study the vertical distribution of the hazes and clouds and characterize their scattering and optical properties; (3) to investigate the heating rates of the atmosphere by directly measuring the upward and downward fluxes; and (4) to study the cloud vertical distribution and composition at pressures where sunlight is practically negligible (p > 4-5 bars).
The instrument includes a set of photodetectors, field-of-view masks, a light infrared lamp, and interference filters. It draws on the heritage of previous instruments developed at the Instituto Nacional de Técnica Aeroespacial (INTA) that participated in the exploration of Mars, where similar technology has demonstrated its endurance in extreme environments while utilizing limited resources regarding power consumption, mass and volume footprints, and data budget. The radiometer’s design and characteristics make it a valuable complementary payload for studying Uranus’ atmosphere with a high scientific return.
... abundant and occurs at low optical depth (Mousis et al. 2018). Given our limited knowledge of the ice giants and their large total mass contribution to shape our solar system, it is essential that we move forward on constraining their interior structure and bulk composition. ...
In this paper, the improved design of an Ice Giants Net Flux Radiometer (IG-NFR), for inclusion as a payload on a future Uranus probe mission, is given. IG-NFR will measure the net radiation flux, in seven spectral bands, each with a 10° Field-Of-View (FOV) and in five viewing angles as a function of altitude. Net flux measurements within spectral filter bands, ranging from solar to far-infrared, will help derive radiative heating and cooling profiles, and will significantly contribute to our understanding of the planet’s atmospheric heat balance and structure, tropospheric 3-D flow, and compositions and opacities of the cloud layers. The IG-NFR uses an array of non-imaging Winston cones integrated to a matched thermopile detector Focal Plane Assembly (FPA), with individual bandpass filters and windows, housed in a vacuum micro-vessel. The FPA thermopile detector signals are read out in parallel mode, amplified and processed by a multi-channel digitizer application specific integrated circuit (MCD ASIC) under field programmable gate array (FPGA) control. The vacuum micro-vessel rotates providing chopping between FOV’s of upward and downward radiation fluxes. This unique design allows for small net flux measurements in the presence of large ambient fluxes and rapidly changing temperatures during the probe descent to ≥10 bar pressure.
... The Ice Giants represent a class of planets in our solar systems that are the least understood but the most frequently observed type of exoplanet [1]. Our knowledge of atmospheric processes occurring on these planets and thus their role in the evolution of the solar system is primarily derived from Earth based telescopes which are associated with a high level of uncertainty. ...
... However, observations show that giant planets present a range of supersolar metallicities. In Jupiter's atmosphere, the abundances of volatile elements were found to be ∼1.5-6.1 times higher than their protosolar values (Atreya et al. 2003;Mousis et al. 2018;Li et al. 2020), with a few exceptions attributed to interior processes. In Uranus and Neptune, volatile abundances can reach up to about 100 times their protosolar values (Lindal et al. 1987(Lindal et al. , 1990Baines et al. 1995;Karkoschka & Tomasko 2009;Sromovsky et al. 2014). ...
... One-σ error bar measurements made at Jupiter by the Galileo probe and the Juno spacecraft indicate C, N, O, S, P, Ar, Kr, and Xe abundances that are ∼1.5 to 6 times higher than the protosolar values (Atreya et al. 2003;Wong et al. 2004;Mousis et al. 2018;Li et al. 2020). To explain those features, it has been proposed that Jupiter's atmosphere could reflect the composition of icy planetesimals either made of amorphous ice (Owen et al. 1999) or from pure condensates and/or clathrates (Gautier et al. 2001;Gautier & Hersant 2005;Mousis et al. 2018Mousis et al. , 2021b. ...
... One-σ error bar measurements made at Jupiter by the Galileo probe and the Juno spacecraft indicate C, N, O, S, P, Ar, Kr, and Xe abundances that are ∼1.5 to 6 times higher than the protosolar values (Atreya et al. 2003;Wong et al. 2004;Mousis et al. 2018;Li et al. 2020). To explain those features, it has been proposed that Jupiter's atmosphere could reflect the composition of icy planetesimals either made of amorphous ice (Owen et al. 1999) or from pure condensates and/or clathrates (Gautier et al. 2001;Gautier & Hersant 2005;Mousis et al. 2018Mousis et al. , 2021b. Alternatively, it has been proposed that this supersolar metallicity could result from the accretion of already pre-enriched PSN gas (Mousis et al. 2019;Aguichine et al. 2022). ...
The supersolar abundances of volatiles observed in giant planets suggest that a compositional gradient was present at the time of their formation in the protosolar nebula. To explain this gradient, several studies have investigated the radial transport of trace species and the effect of icelines on the abundance profiles of solids and vapors formed in the disk. However, these models only consider the presence of solids in the forms of pure condensates or amorphous ice during the evolution of the protosolar nebula. They usually neglect the possible crystallization and destabilization of clathrates, along with the resulting interplay between the abundance of water and those of these crystalline forms. This study is aimed at pushing this kind of investigation further by considering all possible solid phases together in the protosolar nebula: pure condensates, amorphous ice, and clathrates. To this end, we used a one-dimensional (1D) protoplanetary disk model coupled with modules describing the evolution of trace species in the vapor phase, as well as the dynamics of dust and pebbles. Eleven key species are considered here, including H$_2$O, CO, CO$_2$, CH$_4$, H$_2$S, N$_2$, NH$_3$, Ar, Kr, Xe, and PH$_3$. Two sets of initial conditions are explored for the protosolar nebula. In a first scenario, the disk is initially filled with icy grains in the forms of pure condensates. In this case, we show that clathrates can crystallize and form enrichment peaks up to about ten times the initial abundances at their crystallization lines. In a second scenario, the volatiles were delivered to the protosolar nebula in the forms of amorphous grains. In this case, the presence of clathrates is not possible because there is no available crystalline water ice in their formation region. Enrichment peaks of pure condensates also form beyond the snowline up to about seven times the initial abundances.
... The scientific potential for such missions is manyfold (Arridge et al., 2014;Mousis et al., 2018). This includes, among others, knowledge on their ring systems and all the major satellites which may possibly be ocean worlds; study of the planets magnetospheres; and finally insight on the dynamics and composition of their different atmospheric layers. ...
An aerothermodynamic analysis of representative aerocapture (29 km/s) and entry (18 km/s) flows in Neptune is discussed. Two 60° and 45° sphere-cone shapes are considered, and chemically-reactive, nonequilibrium flowfield solutions are computed for the forebody region, yielding surface heat fluxes from convective heating. A radiative transfer calculation using a line-by-line approach coupled with a ray-tracing routine, yields surface heat fluxes from radiative heating.
If the trace amounts of CH4 in Neptune’s atmosphere are accounted for, they dramatically enchance radiative heating, as a result of strong radiative emission from carbonaceous species in the shock layer. For the 18 km/s entry point, radiative heat fluxes accordingly increase by several orders of magnitude, from 0 % to about 50 % of the total heat fluxes.
The post-shock flow features further differ significantly depending on the capsule shape. The sonic line is near the sphere-cone transition zone for the 45° shape and starts detaching from the boundary layer for angles above 55°. The post-shock flow becomes entirely subsonic up to the spacecraft shoulder at 60°. More streamlined shapes will accordingly be more aerodynamically stable.
... However, observations show that giant planets present a range of supersolar metallicities. In Jupiter's atmosphere, the abundances of volatile elements were found to be ∼1.5-6.1 times higher than their protosolar values (Atreya et al. 2003;Mousis et al. 2018;Li et al. 2020), with a few exceptions attributed to interior processes. In Uranus and Neptune, volatile abundances can reach up to about 100 times their protosolar values (Lindal et al. 1987(Lindal et al. , 1990Baines et al. 1995;Karkoschka & Tomasko 2009;Sromovsky et al. 2014). ...
... One-σ error bar measurements made at Jupiter by the Galileo probe and the Juno spacecraft indicate C, N, O, S, P, Ar, Kr, and Xe abundances that are ∼1.5 to 6 times higher than the protosolar values (Atreya et al. 2003;Wong et al. 2004;Mousis et al. 2018;Li et al. 2020). To explain those features, it has been proposed that Jupiter's atmosphere could reflect the composition of icy planetesimals either made of amorphous ice (Owen et al. 1999) or from pure condensates and/or clathrates (Gautier et al. 2001;Gautier & Hersant 2005;Mousis et al. 2018Mousis et al. , 2021b. ...
... One-σ error bar measurements made at Jupiter by the Galileo probe and the Juno spacecraft indicate C, N, O, S, P, Ar, Kr, and Xe abundances that are ∼1.5 to 6 times higher than the protosolar values (Atreya et al. 2003;Wong et al. 2004;Mousis et al. 2018;Li et al. 2020). To explain those features, it has been proposed that Jupiter's atmosphere could reflect the composition of icy planetesimals either made of amorphous ice (Owen et al. 1999) or from pure condensates and/or clathrates (Gautier et al. 2001;Gautier & Hersant 2005;Mousis et al. 2018Mousis et al. , 2021b. Alternatively, it has been proposed that this supersolar metallicity could result from the accretion of already pre-enriched PSN gas (Mousis et al. 2019;Aguichine et al. 2022). ...
To be published in Astronomy & Astrophysics 19 pages, 11 figures
... The ice giants Uranus and Neptune represent a unique class of planets in the Solar System that have not yet been studied by orbiter spacecraft but hold important clues about the formation and evolution of our Solar System. In contrast to the terrestrial inner planets which are mainly composed of rock and 5 the gas giant planets made of hydrogen and helium, the ice giants are mainly composed of planetary ices such as water, ammonia, and methane making them fundamentally different from the inner planets and the gas giants [1,2,3]. The ...
The conceptual design for a Flagship-class Uranus Orbiter and Probe (UOP) mission using aerocapture is presented. Uranus is historically the least studied destination for aerocapture, primarily attributed to the lack of an engineering-level atmosphere model until UranusGRAM was released by NASA in 2021. The present study is the first detailed end-to-end study of a Uranus aerocapture mission concept taking into account constraints arising from launch vehicle performance, interplanetary trajectory, aerocapture vehicle design, thermal protection system, and probe delivery. The mission concept uses a Falcon Heavy Expendable launcher and a high-energy, fast-arrival V∞ Earth–Earth–Jupiter–Uranus (EEJU) gravity assist trajectory to deliver a 1400 kg orbiter and a 300 kg entry probe to Uranus. The aerocapture vehicle is a derivative of the Mars Science Laboratory entry system with extensive flight heritage, and uses the state-of-the-art HEEET thermal protection system. Compared to the current baseline UOP mission using conventional propulsive orbit insertion with a 13-year flight time and 5-year orbital mission at Uranus, the proposed 8-year aerocapture mission concept enables a 10-year orbital mission at Uranus within the budgetary and schedule constraints of a Flagship-class mission.
