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Views of Uranus (top) and Neptune (bottom) from spacecraft observations. Hubble Space Telescope observations (right) reveal changes in Uranus’s polar hazes, as well as new dark vortices on Neptune, since the Voyager 2 flybys (left). These features hint at the atmospheric structure that only an in situ probe can confirm
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For the Ice Giants, atmospheric entry probes provide critical measurements not attainable via remote observations. Including the 2013–2022 NASA Planetary Decadal Survey, there have been at least five comprehensive atmospheric probe engineering design studies performed in recent years by NASA and ESA. International science definition teams have asse...
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... Almost half a century after the launch of the Voyager 2 space probe, the prospect of a new visit to the ice giant Uranus is finally crystallizing again: A flagship mission to the planet Uranus (hereafter referred to as "UOP", standing for Uranus Orbiter and Probe) has been declared a priority, according to the announcement by NASA's Planetary Science and Astrobiology Decadal Survey 2023-2032. 1 The announcement is timely; numerous publications have underlined the rich potential of such a mission in terms of planetary science over the past few years [42,43,49,51,54,58,59,63,93]. Yet, any mission to the outer Solar system must undergo a long cruise in interplanetary space before reaching its desti-nation. ...
With the recent announcement by NASA's Planetary Science and Astrobiology Decadal Survey 2023-2032, a priority flagship mission to the planet Uranus is anticipated. Here, we explore the prospects of using the mission's radio Doppler tracking equipment to detect gravitational waves (GWs) and other analogous signals related to dark matter (DM) over the duration of its interplanetary cruise. By employing a methodology to stack tracking data in combination with Monte-Carlo Markov-Chain parameter recovery tests, we show that the mission will be sensitive to GWs over the wide frequency range of $3\times 10^{-9}$ Hz to $10^{-1}$ Hz, provided that tracking data is taken consistently over a large fraction of the cruise duration. Thus, the mission has the potential to fill the gap between pulsar timing and space-based-interferometry GW observatories. Within this assumption, we forecast the detection of $\mathcal{\mathcal{O}}(1 - 100)$ individual massive black hole binaries using two independent population models. Additionally, we determine the mission's sensitivity to both astrophysical and primordial stochastic gravitational wave backgrounds, as well as its capacity to test, or even confirm via detection, ultralight DM models. In all these cases, the tracking of the spacecraft over its interplanetary cruise would enable coverage of unexplored regions of parameter space, where signals from new phenomena in our Universe may be lurking.
... CH 4 clouds condense much higher, around 1.3 bar (Irwin et al. 2022), and can be easily sampled by an entry probe assuming the probe passes through these ephemeral and localized clouds. The purported globally distributed H 2 S cloud deck around 3-6-bar will also be accessible to the entry probe; most probe designs target at least the 10 bar pressure level (Mousis et al. , 2018Simon et al. 2020;Orton et al. 2021a). Thermochemical equilibrium models suggest that a deeper NH 4 SH cloud layer exists around 30-40 bar (Weidenschilling and Lewis 1973; Atreya and Wong 2005), but this may be too deep for the probe to measure within the constraints of battery power, telecommunication restrictions, or increasing radio wavelength opacity at and below that pressure level due to the presence of NH 3 . ...
The Ice Giants represent a unique and relatively poorly characterized class of planets that have been largely unexplored since the brief Voyager 2 flyby in the late 1980s. Uranus is particularly enigmatic, due to its extreme axial tilt, offset magnetic field, apparent low heat budget, mysteriously cool stratosphere and warm thermosphere, as well as a lack of well-defined, long-lived storm systems and distinct atmospheric features. All these characteristics make Uranus a scientifically intriguing target, particularly for missions able to complete in situ measurements. The 2023-2032 Decadal Strategy for Planetary Science and Astrobiology prioritized a flagship orbiter and probe to explore Uranus with the intent to “...transform our knowledge of Ice Giants in general and the Uranian system in particular” (National Academies of Sciences, Engineering, and Medicine in Origins, worlds, and life: a decadal strategy for planetary science and astrobiology 2023-2032, The National Academies Press, Washington, 2022). In support of this recommendation, we present community-supported science questions, key measurements, and a suggested instrument suite that focuses on the exploration and characterization of the Uranian atmosphere by an in situ probe. The scope of these science questions encompasses the origin, evolution, and current processes that shape the Uranian atmosphere, and in turn the Uranian system overall. Addressing these questions will inform vital new insights about Uranus, Ice Giants and Gas Giants in general, the large population of Neptune-sized exoplanets, and the Solar System as a whole.
... The peak deceleration is in the range of 4-10 g, which is well within the range of decelerations typical entry vehicles are subjected to [20,21]. The peak heat rate is in the range of 1400-1800 W/cm 2 , which is well within the tested limits of the HEEET thermal protection system [22], and much less than that encountered during the steep entry of an entry probe [23]. The integrated heat load is about 200-300 kJ/cm 2 , which is quite high but still reasonable for HEEET. ...
At the far reaches of the outer Solar System, the ice giants remain the last class of planets yet to be studied using orbiters. The 2023-2032 Planetary Science Decadal Survey has underscored the importance of the ice giants in understanding the origin, formation, and evolution of our Solar System. The enormous heliocentric distance of Uranus presents considerable mission design challenges, the most important being able to reach Uranus within a reasonable time. The present study presents two examples of aerocapture enabled short flight time, fast trajectories for Uranus orbiter missions, and highlights the enormous benefits provided by aerocapture. The first is an EEJU trajectory with a launch opportunity in July 2031 with a flight time of 8 years. The second is an EJU trajectory with a launch opportunity in June 2034 with a flight time of only 5 years. Using the Falcon Heavy Expendable, the available launch capability is 4950 kg and 1400 kg respectively for the two trajectories. Both trajectories have a high arrival speed of 20 km/s, which provides sufficient corridor width for aerocapture. Compared to propulsive insertion architectures which take 13 to 15 years, the fast trajectories offer significant reduction in the flight time.
... Uranus and Neptune are still underexplored compared to the gas giants, and the scientific yield of prospective in-situ missions to these planets are being rigorously examined by the planetary science community (see e.g., Hofstadter et al. 2019;Beddingfield et al. 2020;Dahl et al. 2020;Fletcher et al. 2020aFletcher et al. , 2020bHelled & Fortney 2020b;Moore et al. 2020;Zwick et al. 2022). Furthermore, various mission concepts have already been suggested (e.g., Cartwright et al. 2020;Cohen et al. 2020;Jarmak et al. 2020;Simon et al. 2020;Rymer et al. 2021). Currently we do not have measurements of the higher order even harmonics (J 6,8,10... ) and odd harmonics (J 2i+1 ) for Uranus and Neptune. ...
Uranus and Neptune exhibit fast surface zonal winds that can reach up to a few hundred meters per second. Previous studies on zonal gravitational harmonics and ohmic dissipation constraints suggest that the wind speeds diminish rapidly in relatively shallow depths within the planets. Through a case-by-case comparison between the missing dynamical gravitational harmonic J 4 ′ from structure models, and with that expected from fluid perturbations, we put constraints on zonal wind decay in Uranus and Neptune. To this end, we generate polytropic empirical structure models of Uranus and Neptune using fourth-order theory of figures that leave hydrostatic J 4 as an open parameter. Allotting the missing dynamical contribution to density perturbations caused by zonal winds (and their dynamic self-gravity), we find that the maximum scale height of zonal winds are ∼2%–3% of the planetary radii for both planets. Allowing the models to have J 2 solutions in the ±5 × 10 ⁻⁶ range around the observed value has similar implications. The effect of self-gravity on J 4 ′ is roughly a factor of ten lower than that of zonal winds, as expected. The decay scale heights are virtually insensitive to the proposed modifications to the bulk rotation periods of Uranus and Neptune in the literature. Additionally, we find that the dynamical density perturbations due to zonal winds have a measurable impact on the shape of the planet, and could potentially be used to infer wind decay and bulk rotation period via future observations.
... Furthermore, various mission concepts have already been suggested (e.g. Jarmak et al. 2020;Cohen et al. 2020;Cartwright et al. 2020;Simon et al. 2020;Rymer et al. 2021). Currently we do not have measurements of the higher order even harmonics (J 6,8,10... ) and odd harmonics (J 2i+1 ) for Uranus and Neptune. ...
Uranus and Neptune exhibit fast surface zonal winds that can reach up to few hundred meters per second. Previous studies on zonal gravitational harmonics and Ohmic dissipation constraints suggest that the wind speeds diminish rapidly in relatively shallow depths within the planets. Through a case-by-case comparison between the missing dynamical gravitational harmonic $J^\prime_4$ from structure models, and with that expected from fluid perturbations, we put constraints on zonal wind decay in Uranus and Neptune. To this end, we generate polytropic empirical structure models of Uranus and Neptune using $4^{\rm th}$-order Theory of Figures (ToF) that leave hydrostatic $J_4$ as an open parameter. Allotting the missing dynamical contribution to density perturbations caused by zonal winds (and their dynamic self-gravity), we find that the maximum scale height of zonal winds are $\sim 2-3\%$ of the planetary radii for both planets. Allowing the models to have $J_2$ solutions in the $\pm 5 \times 10^{-6}$ range around the observed value has similar implications. The effect of self-gravity on $J^\prime_4$ is roughly a factor of ten lower than that of zonal winds, as expected. The decay scale heights are virtually insensitive to the proposed modifications to the bulk rotation periods of Uranus and Neptune in the literature. Additionally, we find that the dynamical density perturbations due to zonal winds have a measurable impact on the shape of the planet, and could potentially be used to infer wind decay and bulk rotation period via future observations.
... Uranus and Neptune are so cold that many of the dominant carbon-, nitrogen-, and oxygen-bearing molecules have condensed out of the observable atmosphere, leaving only methane accessible by remote observation (Helled et al. 2020). There are calls for a space mission to explore one of the ice giants in situ and measure its atmospheric abundances directly with a probe; however, such a mission is over a decade away (Simon et al. 2020). ...
We present a transmission spectrum for the Neptune-sized exoplanet HD 106315c from optical to infrared wavelengths based on transit observations from the Hubble Space Telescope/Wide Field Camera 3, K2, and Spitzer. The spectrum shows tentative evidence for a water absorption feature in the 1.1–1.7 μ m wavelength range with a small amplitude of 30 ppm (corresponding to just 0.8 ± 0.04 atmospheric scale heights). Based on an atmospheric retrieval analysis, the presence of water vapor is tentatively favored with a Bayes factor of 1.7–2.6 (depending on prior assumptions). The spectrum is most consistent with either an enhanced metallicity or high-altitude condensates, or both. Cloud-free solar composition atmospheres are ruled out at >5 σ confidence. We compare the spectrum to grids of cloudy and hazy forward models and find that the spectrum is fit well by models with moderate cloud lofting or haze formation efficiency over a wide range of metallicities (1–100× solar). We combine the constraints on the envelope composition with an interior structure model and estimate that the core mass fraction is ≳0.3. With a bulk composition reminiscent of that of Neptune and an orbital distance of 0.15 au, HD 106315c hints that planets may form out of broadly similar material and arrive at vastly different orbits later in their evolution.
... Modern explorations of possible mission scenarios to the Ice Giants Uranus and Neptune [1,2,3,4] have motivated a revision of what we know about these planets and their planetary systems. Recent reviews explore their atmospheric dynamics [5], mean circulation patterns [6], and vertical structure [5,7]. ...
The Ice Giants Uranus and Neptune have hydrogen-based atmospheres with several constituents that condense in their cold upper atmospheres. A small number of bright cloud systems observed in both planets are good candidates for moist convective storms, but their observed properties (size, temporal scales and cycles of activity) differ from moist convective storms in the Gas Giants. These clouds and storms are possibly due to methane condensation and observations also suggest deeper clouds of hydrogen sulfide (H$_2$S) at depths of a few bars. Even deeper, thermochemical models predict clouds of ammonia hydrosulfide (NH$_4$SH) and water at pressures of tens to hundreds of bars, forming extended deep weather layers. Because of hydrogen's small molecular weight and the high abundance of volatiles, their condensation imposes a strongly stabilizing vertical gradient of molecular weight larger than the equivalent one in Jupiter and Saturn. The resulting inhibition of vertical motions should lead to a moist convective regime that differs significantly from the one occurring on nitrogen-based atmospheres like those of Earth or Titan. As a consequence, the thermal structure of the deep atmospheres of Uranus and Neptune is not well understood. Similar processes might occur at the deep water cloud of Jupiter in Saturn, but the Ice Giants offer the possibility to study these physical aspects in the upper methane cloud layer. A combination of orbital and in situ data will be required to understand convection and its role in atmospheric dynamics in the Ice Giants, and by extension, in hydrogen atmospheres including Jupiter, Saturn and giant exoplanets.
... Hofstadter et al. 2019;Beddingfield et al. 2020;Dahl et al. 2020;Fletcher et al. 2020a,b;Helled & Fortney 2020;Moore et al. 2020;Soyuer et al. 2021), and various mission concepts have already being discussed (e.g. Cartwright et al. 2020;Cohen et al. 2020;Jarmak et al. 2020;Simon et al. 2020). The underexploration of Uranus and Neptune is unfortunate, as these planets exhibit highly multipolar and non-axisymmetric magnetic fields (Connerney, Acuna & Ness 1987Holme & Bloxham 1996), are compositionally more diverse than the gas giants (Helled et al. 2011;Nettelmann et al. 2013), and have a significant contrast in their thermal flux. ...
The low luminosity of Uranus is still a puzzling phenomenon and has key implications for the thermal and compositional gradients within the planet. Recent studies have shown that planetary volatiles become ionically conducting under conditions that are present in the ice giants. Rapidly growing electrical conductivity with increasing depth would couple zonal flows to the background magnetic field in the planets, inducing poloidal and toroidal field perturbations ${B}^{\omega } = {B}^{\omega }_P + {B}^{\omega }_T$ via the ω–effect. Toroidal perturbations ${B}^{\omega }_T$ are expected to diffuse downwards and produce poloidal fields ${B}^{\alpha }_P$ through turbulent convection via the α–effect, comparable in strength to those of the ω–effect; ${B}^{\omega }_P$. To estimate the strength of poloidal field perturbations for various Uranus models in the literature, we generate wind decay profiles based on Ohmic dissipation constraints assuming an ionically conducting H–He–H2O interior. Due to the higher metallicities in outer regions of hot Uranus models, zonal winds need to decay to ∼0.1% of their surface values in the outer 1% of Uranus to admit decay solutions in the Ohmic framework. Our estimates suggest that colder Uranus models could potentially have poloidal field perturbations that reach up to $\mathcal {O}(0.1)$ of the background magnetic field in the most extreme case. The possible existence of poloidal field perturbations spatially correlated with Uranus’ zonal flows could be used to constrain Uranus’ interior structure, and presents a further case for the in situ exploration of Uranus.
... Hofstadter et al. 2019;Helled & Fortney 2020;Dahl et al. 2020;Fletcher et al. 2020a,b;Beddingfield et al. 2020;Moore et al. 2020;Soyuer et al. 2021), and various mission concepts have already being discussed (e.g. Jarmak et al. 2020;Cohen et al. 2020;Cartwright et al. 2020;Simon et al. 2020). The under-exploration of Uranus and Neptune is unfortunate, as these planets exhibit highly multipolar and nonaxisymmetric magnetic fields (Connerney et al. 1987(Connerney et al. , 1991Holme & Bloxham 1996), are compositionally more diverse than the gas giants (Helled et al. 2011;Nettelmann et al. 2013), and have a significant contrast in their thermal flux. ...
The low luminosity of Uranus is still a puzzling phenomenon and has key implications for the thermal and compositional gradients within the planet. Recent studies have shown that planetary volatiles become ionically conducting under conditions that are present in the ice giants. Rapidly growing electrical conductivity with increasing depth would couple zonal flows to the background magnetic field in the planets, inducing poloidal and toroidal field perturbations $\mathbf{B}^{\omega} = \mathbf{B}^{\omega}_P + \mathbf{B}^{\omega}_T$ via the $\omega$-effect. Toroidal perturbations $\mathbf{B}^{\omega}_T$ are expected to diffuse downwards and produce poloidal fields $\mathbf{B}^{\alpha}_P$ through turbulent convection via the $\alpha$-effect, comparable in strength to those of the $\omega$-effect; $\mathbf{B}^{\omega}_P$. To estimate the strength of poloidal field perturbations for various Uranus models in the literature, we generate wind decay profiles based on Ohmic dissipation constraints assuming an ionically conducting H-He-H$_2$O interior. Due to the higher metallicities in outer regions of hot Uranus models, zonal winds need to decay to $\sim$0.1% of their surface values in the outer 1% of Uranus to admit decay solutions in the Ohmic framework. Our estimates suggest that colder Uranus models could potentially have poloidal field perturbations that reach up to $\mathcal{O}(0.1)$ of the background magnetic field in the most extreme case. The possible existence of poloidal field perturbations spatially correlated with Uranus' zonal flows could be used to constrain Uranus' interior structure, and presents a further case for the $\textit{in situ}$ exploration of Uranus.
... This enthusiasm for Ice Giants from both sides of the Atlantic led to a joint study , which looked closely at a number of potential mission architectures [10]. Since then, ESA has considered a palette of potential contributions to a US-led mission 3 (2018-19), including secondary orbiters and atmospheric probes, as reviewed by Simon et al. [11] and advocated by Mousis et al. ([12], this issue). At the time of our submission to Voyage 2050, it had been hoped that such a contribution, at the ∼e0.5bn level, might be included in a proposed increase in ESA's science budget in 2019. ...
Of all the myriad environments in our Solar System, the least explored are the distant Ice Giants Uranus and Neptune, and their diverse satellite and ring systems. These ‘intermediate-sized’ worlds are the last remaining class of Solar System planet to be characterised by a dedicated robotic mission, and may shape the paradigm for the most common outcome of planetary formation throughout our galaxy. In response to the 2019 European Space Agency call for scientific themes in the 2030s and 2040s (known as Voyage 2050 ), we advocated that an international partnership mission to explore an Ice Giant should be a cornerstone of ESA’s science planning in the coming decade, targeting launch opportunities in the early 2030s. This article summarises the inter-disciplinary science opportunities presented in that White Paper [1], and briefly describes developments since 2019.
