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The mysteries of the Uranus system can be unlocked through interdisciplinary exploration
By KATHLEEN E. MANDT Authors Info & Affiliations
SCIENCE
16 Feb 2023
Vol 379, Issue 6633
pp. 640-642
DOI: 10.1126/science.ade8446
Uranus and Neptune are commonly called ice giants because they have a greater percentage of heavy elements compared to the gas giants, Jupiter and Saturn. The only spacecraft to visit Uranus and Neptune was Voyager 2 with brief flybys in 1986 and 1989—more than 30 years ago. These encounters presented a captivating view of two exotic planetary systems, leaving more questions than previously imagined. A recent survey conducted by the National Academies of Sciences, Engineering, and Medicine identified a Uranus Orbiter and Probe (UOP) as the planetary science community’s top priority for the next NASA large-scale mission. It will explore how Uranus formed; how much it migrated after formation; the planet’s interior structure, atmosphere, magnetosphere, and ring system; and whether any moons have or once had subsurface liquid water oceans. This mission will serve to inspire and educate multiple generations about solar system history and the mysteries at its farthest reaches.
Each decade, NASA tasks the National Academies to conduct a “decadal survey” of the planetary science community to define priorities for the next 10 years. NASA and Congress view these surveys as the US space science community’s formal statement of priority. They cost millions of dollars to execute and include independently evaluated mission concept studies. This large investment is critical to ensure that the survey recommends a technologically feasible program with realistic cost assumptions. It is a consensus report representing the voice of the community who conduct the survey and provide input through white papers. Without this process, the community would have limited influence with NASA in communicating top science priorities.
The planetary science decadal survey released in 2022, Origins, Worlds, and Life (OWL) (1), was one of the most unusual conducted to date because it took place during the COVID-19 pandemic. Although conditions created by the pandemic made the effort an extraordinary sacrifice for the many volunteers working on the survey, participants were determined to provide a true consensus. Over a period of 1 year, panelists held 176 meetings, carefully reviewed over 500 white papers, and sought further input through more than 300 presentations by external speakers given in open session. The survey was built on a framework of 12 priority science questions each outlined in individual chapters. Through these chapters, the “dearth of knowledge on the ice giants” was identified as a problem of highest priority for resolving in the coming decade.
To begin addressing this issue, a UOP was recommended as the next planetary flagship, or large-scale, mission because of the importance of full system science. This recommendation is the culmination of over 20 years of mission concept studies. UOP was first identified as a high priority in the 2003–2013 decadal survey New Frontiers in the Solar System (2). It was ranked as the third-highest priority flagship in the 2013–2023 decadal survey, Visions and Voyages (V&V) (3), following Mars Sample Return and the Europa Clipper, two missions now well advanced in their development. OWL reiterated the community consensus from V&V that UOP should be the next step. UOP will address science goals ranging from the origin and evolution of the Solar System to understanding processes only existing in this mysterious planetary system, and will provide results to help answer fundamental questions about one of the most common types of known exoplanets (planets that exist outside the Solar System).
Understanding how the giant planets formed and then migrated has broad implications for explaining the distribution of small bodies in our Solar System; the delivery through these small bodies of water and life-supporting elements to the inner Solar System; and, extending beyond the Solar System, the formation of exoplanets and their system architectures (1). Solar System formation and evolution are evaluated by connecting current measurements to conditions in the protosolar nebula (PSN)—the disk of gas, dust, and ice in which Solar System objects formed (4). Studying all four giant planets (Uranus, Neptune, Jupiter, and Saturn) is vital for reconstructing Solar System history (5, 6).
The giant planets formed by collecting gas and solids from the PSN, migrating to their current locations, and then collecting more solid materials through postformation impacts by icy and rocky planetesimals. Models predict different scenarios for formation and migration (7), but require measurements of isotope ratios and noble gas abundances in each planet’s atmosphere to verify which modeled scenario is most accurate (1, 4–6). An atmospheric probe is required to measure them.
The nitrogen isotope ratio, 14N/15N, is one of the more important isotopic measurements because it reveals the planet’s source of nitrogen. Jupiter’s 14N/15N, the only giant planet measurement currently available, is similar to the Sun’s. This suggests that Jupiter obtained its nitrogen either from PSN gas that was enriched in N2 through heating of PSN ices, or from building blocks formed in extremely cold conditions (6). A lower limit for Saturn’s tropospheric 14N/15N suggests a similar origin (8), but an atmospheric probe is needed to confirm a similarly high ratio in the well-mixed atmosphere. No measurement of 14N/15N is currently available for the ice giants.
The heavy noble gases provide another constraint for formation and evolution of the giant planets. Comparing noble gas abundances in giant planet atmospheres with abundances in analogs for solid materials in the PSN, comets and chondrites, can establish the relative contributions of rocky and icy material to the planet’s formation and reveal how these solid materials were distributed within the PSN. Noble gas abundances are only available for Jupiter, where they also imply either enriched PSN gas or solid building blocks formed in very cold conditions (5). Similar measurements are needed for Saturn and the ice giants. The UOP mission concept design includes a probe that will measure the noble gas abundances and 14N/15N in the atmosphere (see the figure).
UOP also includes an orbiter that is designed for full system science. The Uranus system is especially interesting because it could bear testimony to a massive collision that tilted Uranus on its side (9), resulting in a system with rings and moons at an obliquity of 98°. This orientation causes extreme atmospheric seasonal variation over its 84–Earth-year orbit, but observations of haze and clouds from Earth cannot provide enough information to explain atmospheric circulation and wind patterns. The UOP probe will measure wind and temperature at one location, and the orbiter will make global observations that will determine how this distinct atmosphere works.
The system’s extreme obliquity also limits visibility of the moons to one hemisphere during southern and northern summers. Voyager 2 could only image the moons’ southern hemispheres, but what was seen was unexpected. The five largest moons, predicted to be cold dead worlds, all showed evidence of recent resurfacing (10), suggesting that geologic activity might be ongoing. One or more of these moons could have potentially habitable liquid water oceans under an ice shell, making them “ocean worlds.” Ariel, the most extensively resurfaced moon, is a strong ocean worlds candidate along with the two largest moons, Titania and Oberon (10) (see the figure). UOP will image and measure the composition of the full surfaces of the moons to search for ongoing geologic activity, and measure whether magnetic fields vary in their interiors owing to the presence of liquid water.
The proposed Uranus Orbiter and Probe (UOP) will help to determine how Uranus formed and its migration since it formed—the planet’s interior structure, atmosphere, magnetosphere, and ring system—and whether any moons have habitable liquid water oceans.
Uranus has nine very dense narrow rings that would require resonances with a fleet of shepherding satellites to prevent them from rapidly spreading out and losing their sharp edges (11). Although Voyager 2 found resonances with small moons Cordelia, Ophelia, and Cressida, they are not sufficient to explain all of the ring features. Either a large number of moons too small for Voyager 2 detection are present, or resonances are created by the internal structure of Uranus (12). Furthermore, the composition of the surprisingly dark ring particles is not known and is clearly different from the surface composition of the moons (11). UOP will search for shepherding moons, monitor the motions of ring particles, and measure their composition.
The magnetosphere of Uranus is also very unusual. The magnetic field is not only tilted 60° from the rotation axis, it is also offset from the center of the planet (13). It is not clear how such a field can be produced, and this unusual orientation, combined with the extreme obliquity of the planet, causes seasonal and diurnal variations that might alter the planet’s atmosphere and moons. UOP will monitor the magnetosphere over the duration of the mission, recommended by OWL to be at least 5 years.
The interior structure of Uranus is a great mystery made more compelling by the discovery that the cores of Jupiter and Saturn are not solid with a clear boundary, but are instead diffuse and exchange material with the atmosphere. This could be the result either of planet-formation processes in which the core reaches a critical mass, vaporizes, and produces composition gradients; or from core erosion, in which heavy elements dissolve and erode into the atmosphere after formation (14). Determining whether the core of Uranus is solid or diffuse, and whether it is dominated by rock or ice, is important not only for understanding the ice giants, but also has implications for the origin and internal structure of similarly sized exoplanets. UOP will determine the interior structure of Uranus with atmospheric probe and orbiter gravity measurements.
The ideal next steps will build on past studies (1–3, 15), particularly the OWL study (1). OWL recommended launching by 2032 to allow the spacecraft to use Jupiter’s massive gravity to gain speed out to Uranus. This also guarantees arrival well before northern autumn equinox in 2050, ensuring full visibility of the moons. As an added benefit, the recommended trajectory slingshots the spacecraft over the ecliptic, providing a polar view of the Sun—a perspective of interest to the heliophysics community for making measurements to better understand solar wind differences between the poles and the equator. Trajectories after 2032 that do not use Jupiter’s gravity but still arrive before equinox are possible. However, they either deliver a smaller spacecraft into orbit carrying fewer instruments, or take longer to arrive. OWL had limited time to evaluate the millions of possible trajectories, so it will be necessary to use NASA-provided constraints on possible launch years to refine trajectory options and determine specific mass constraints. This allows development of a spacecraft design optimized for the mass constraints and selection of instruments. Evaluating other contributions is also of great importance. The European Space Agency (ESA) has done several studies of ice giants missions and represents a community prepared to make essential scientific and technical contributions.
As preparation begins for UOP, it is important to recognize that although Neptune is also an ice giant, its system is very different from that of Uranus and presents questions that cannot be answered by UOP (15). Neptune’s moon system is dominated by Triton, a Kuiper Belt object that reconfigured the entire system when it was captured. Additionally, the heat balance of Neptune and Uranus differs by an order of magnitude, potentially indicating substantial differences in internal structure (14). Because of its greater distance from the Sun, Neptune is more challenging to reach. Technology development and focused mission studies should begin in this decade to prepare to explore this equally interesting planetary system. The space science community has waited more than 30 years to explore the ice giants, and missions to them will benefit many generations to come.
K.E.M. served on the Pre-decadal Ice Giants Science Definition Team and the OWL Giant Planet Systems panel. Thanks to members of the science community, especially M. Hofstadter and A. Simon for insightful discussions.
References and Notes
1
National Academies of Sciences, Engineering, and Medicine, Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032 (National Academies Press, 2022).
GOOGLE SCHOLAR
2
National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy (National Academies Press, 2003).
GO TO REFERENCE
GOOGLE SCHOLAR
3
National Research Council, Vision and Voyages for Planetary Science in the Decade 2013–2022 (National Academies Press, 2011).
GOOGLE SCHOLAR
4
K. E. Mandt et al., Space Sci. Rev. 197, 297 (2015).
CROSSREF
PUBMED
ISI
GOOGLE SCHOLAR
5
K. Mandt et al., Space Sci. Rev. 216, 1 (2022).
GOOGLE SCHOLAR
6
O. Mousis et al., Exp. Astron.2021).
CROSSREF
GOOGLE SCHOLAR
7
D. Nesvorný, Annu. Rev. Astron. Astrophys. 56, 137 (2018).
GO TO REFERENCE
CROSSREF
ISI
GOOGLE SCHOLAR
8
L. Fletcher et al., Icarus 238, 170 (2014).
GO TO REFERENCE
CROSSREF
ISI
GOOGLE SCHOLAR
9
R. Rufu, R. Canup, Astrophys. J. 928, 123 (2022).
GO TO REFERENCE
CROSSREF
ISI
GOOGLE SCHOLAR
10
P. Schenk, J. Moore, Philos. Trans. R.Soc. A 378, 20200102 (2020).
CROSSREF
PUBMED
ISI
GOOGLE SCHOLAR
11
P. Nicholson et al., in Planetary Ring Systems: Properties, Structure, and Evolution, M. S. Tiscareno, C. D. Murray, Eds. (Cambridge Planetary Science, 2018), pp. 93–111.
GOOGLE SCHOLAR
12
J. A’Hearn, M. M. Hedman, C. R. Mankovich, H. Aramona, M. S. Marley, Planet. Sci. J. 3, 194 (2022).
GO TO REFERENCE
CROSSREF
GOOGLE SCHOLAR
13
C. Arridge, C. Paty, in Magnetospheres in the Solar System, R. Maggiolo et al., Eds. (American Geophysical Union, 2021), pp. 515–534.
GO TO REFERENCE
GOOGLE SCHOLAR
14
R. Helled, J. Fortney, Philos. Trans. R. Soc. A 378, 20190474 (2020).
CROSSREF
PUBMED
ISI
GOOGLE SCHOLAR
15
M. Hofstadter et al., Planet. Space Sci. 177, 104680 (2019).
CROSSREF
ISI
GOOGLE SCHOLAR
SHOW FEWER
See: https://www.science.org/doi/10.1126/science.ade8446
See: https://arxiv.org/pdf/2008.12125.pdf
Ice giants Uranus, Neptune physics.org
The realm of the two Ice Giants, lying between Uranus (19.2 AU) and Neptune (30.1 AU), remains largely unexplored. Like the Gas Giants Jupiter and Saturn (which have been well-characterised by the lengthy Galileo, Juno, and Cassini orbital missions over the past three decades), these giant worlds accreted from the reservoirs of rocks, ices, and gases present in the protosolar nebula at the epoch of planet formation and they feature dynamic atmospheres with banded structures and localised storms; convective interiors enriched in heavy elements (i.e., heavier than helium) compared to protosolar values; they exhibit powerful magnetic fields driven by hidden processes far below the clouds; and they are attended by delicate ring systems and ice-rich satellites. Uranus (14.5 Earth masses and 4.0 Earth radii) and Neptune (17.1 Earth masses and 3.8 Earth radii) represent an intermediate class of planetary object between the enormous Gas Giants and the smaller terrestrial rocky worlds.
Hartmann352
By KATHLEEN E. MANDT Authors Info & Affiliations
SCIENCE
16 Feb 2023
Vol 379, Issue 6633
pp. 640-642
DOI: 10.1126/science.ade8446
Uranus and Neptune are commonly called ice giants because they have a greater percentage of heavy elements compared to the gas giants, Jupiter and Saturn. The only spacecraft to visit Uranus and Neptune was Voyager 2 with brief flybys in 1986 and 1989—more than 30 years ago. These encounters presented a captivating view of two exotic planetary systems, leaving more questions than previously imagined. A recent survey conducted by the National Academies of Sciences, Engineering, and Medicine identified a Uranus Orbiter and Probe (UOP) as the planetary science community’s top priority for the next NASA large-scale mission. It will explore how Uranus formed; how much it migrated after formation; the planet’s interior structure, atmosphere, magnetosphere, and ring system; and whether any moons have or once had subsurface liquid water oceans. This mission will serve to inspire and educate multiple generations about solar system history and the mysteries at its farthest reaches.
Each decade, NASA tasks the National Academies to conduct a “decadal survey” of the planetary science community to define priorities for the next 10 years. NASA and Congress view these surveys as the US space science community’s formal statement of priority. They cost millions of dollars to execute and include independently evaluated mission concept studies. This large investment is critical to ensure that the survey recommends a technologically feasible program with realistic cost assumptions. It is a consensus report representing the voice of the community who conduct the survey and provide input through white papers. Without this process, the community would have limited influence with NASA in communicating top science priorities.
The planetary science decadal survey released in 2022, Origins, Worlds, and Life (OWL) (1), was one of the most unusual conducted to date because it took place during the COVID-19 pandemic. Although conditions created by the pandemic made the effort an extraordinary sacrifice for the many volunteers working on the survey, participants were determined to provide a true consensus. Over a period of 1 year, panelists held 176 meetings, carefully reviewed over 500 white papers, and sought further input through more than 300 presentations by external speakers given in open session. The survey was built on a framework of 12 priority science questions each outlined in individual chapters. Through these chapters, the “dearth of knowledge on the ice giants” was identified as a problem of highest priority for resolving in the coming decade.
To begin addressing this issue, a UOP was recommended as the next planetary flagship, or large-scale, mission because of the importance of full system science. This recommendation is the culmination of over 20 years of mission concept studies. UOP was first identified as a high priority in the 2003–2013 decadal survey New Frontiers in the Solar System (2). It was ranked as the third-highest priority flagship in the 2013–2023 decadal survey, Visions and Voyages (V&V) (3), following Mars Sample Return and the Europa Clipper, two missions now well advanced in their development. OWL reiterated the community consensus from V&V that UOP should be the next step. UOP will address science goals ranging from the origin and evolution of the Solar System to understanding processes only existing in this mysterious planetary system, and will provide results to help answer fundamental questions about one of the most common types of known exoplanets (planets that exist outside the Solar System).
Understanding how the giant planets formed and then migrated has broad implications for explaining the distribution of small bodies in our Solar System; the delivery through these small bodies of water and life-supporting elements to the inner Solar System; and, extending beyond the Solar System, the formation of exoplanets and their system architectures (1). Solar System formation and evolution are evaluated by connecting current measurements to conditions in the protosolar nebula (PSN)—the disk of gas, dust, and ice in which Solar System objects formed (4). Studying all four giant planets (Uranus, Neptune, Jupiter, and Saturn) is vital for reconstructing Solar System history (5, 6).
The giant planets formed by collecting gas and solids from the PSN, migrating to their current locations, and then collecting more solid materials through postformation impacts by icy and rocky planetesimals. Models predict different scenarios for formation and migration (7), but require measurements of isotope ratios and noble gas abundances in each planet’s atmosphere to verify which modeled scenario is most accurate (1, 4–6). An atmospheric probe is required to measure them.
The nitrogen isotope ratio, 14N/15N, is one of the more important isotopic measurements because it reveals the planet’s source of nitrogen. Jupiter’s 14N/15N, the only giant planet measurement currently available, is similar to the Sun’s. This suggests that Jupiter obtained its nitrogen either from PSN gas that was enriched in N2 through heating of PSN ices, or from building blocks formed in extremely cold conditions (6). A lower limit for Saturn’s tropospheric 14N/15N suggests a similar origin (8), but an atmospheric probe is needed to confirm a similarly high ratio in the well-mixed atmosphere. No measurement of 14N/15N is currently available for the ice giants.
The heavy noble gases provide another constraint for formation and evolution of the giant planets. Comparing noble gas abundances in giant planet atmospheres with abundances in analogs for solid materials in the PSN, comets and chondrites, can establish the relative contributions of rocky and icy material to the planet’s formation and reveal how these solid materials were distributed within the PSN. Noble gas abundances are only available for Jupiter, where they also imply either enriched PSN gas or solid building blocks formed in very cold conditions (5). Similar measurements are needed for Saturn and the ice giants. The UOP mission concept design includes a probe that will measure the noble gas abundances and 14N/15N in the atmosphere (see the figure).
UOP also includes an orbiter that is designed for full system science. The Uranus system is especially interesting because it could bear testimony to a massive collision that tilted Uranus on its side (9), resulting in a system with rings and moons at an obliquity of 98°. This orientation causes extreme atmospheric seasonal variation over its 84–Earth-year orbit, but observations of haze and clouds from Earth cannot provide enough information to explain atmospheric circulation and wind patterns. The UOP probe will measure wind and temperature at one location, and the orbiter will make global observations that will determine how this distinct atmosphere works.
The system’s extreme obliquity also limits visibility of the moons to one hemisphere during southern and northern summers. Voyager 2 could only image the moons’ southern hemispheres, but what was seen was unexpected. The five largest moons, predicted to be cold dead worlds, all showed evidence of recent resurfacing (10), suggesting that geologic activity might be ongoing. One or more of these moons could have potentially habitable liquid water oceans under an ice shell, making them “ocean worlds.” Ariel, the most extensively resurfaced moon, is a strong ocean worlds candidate along with the two largest moons, Titania and Oberon (10) (see the figure). UOP will image and measure the composition of the full surfaces of the moons to search for ongoing geologic activity, and measure whether magnetic fields vary in their interiors owing to the presence of liquid water.
The proposed Uranus Orbiter and Probe (UOP) will help to determine how Uranus formed and its migration since it formed—the planet’s interior structure, atmosphere, magnetosphere, and ring system—and whether any moons have habitable liquid water oceans.
Uranus has nine very dense narrow rings that would require resonances with a fleet of shepherding satellites to prevent them from rapidly spreading out and losing their sharp edges (11). Although Voyager 2 found resonances with small moons Cordelia, Ophelia, and Cressida, they are not sufficient to explain all of the ring features. Either a large number of moons too small for Voyager 2 detection are present, or resonances are created by the internal structure of Uranus (12). Furthermore, the composition of the surprisingly dark ring particles is not known and is clearly different from the surface composition of the moons (11). UOP will search for shepherding moons, monitor the motions of ring particles, and measure their composition.
The magnetosphere of Uranus is also very unusual. The magnetic field is not only tilted 60° from the rotation axis, it is also offset from the center of the planet (13). It is not clear how such a field can be produced, and this unusual orientation, combined with the extreme obliquity of the planet, causes seasonal and diurnal variations that might alter the planet’s atmosphere and moons. UOP will monitor the magnetosphere over the duration of the mission, recommended by OWL to be at least 5 years.
The interior structure of Uranus is a great mystery made more compelling by the discovery that the cores of Jupiter and Saturn are not solid with a clear boundary, but are instead diffuse and exchange material with the atmosphere. This could be the result either of planet-formation processes in which the core reaches a critical mass, vaporizes, and produces composition gradients; or from core erosion, in which heavy elements dissolve and erode into the atmosphere after formation (14). Determining whether the core of Uranus is solid or diffuse, and whether it is dominated by rock or ice, is important not only for understanding the ice giants, but also has implications for the origin and internal structure of similarly sized exoplanets. UOP will determine the interior structure of Uranus with atmospheric probe and orbiter gravity measurements.
The ideal next steps will build on past studies (1–3, 15), particularly the OWL study (1). OWL recommended launching by 2032 to allow the spacecraft to use Jupiter’s massive gravity to gain speed out to Uranus. This also guarantees arrival well before northern autumn equinox in 2050, ensuring full visibility of the moons. As an added benefit, the recommended trajectory slingshots the spacecraft over the ecliptic, providing a polar view of the Sun—a perspective of interest to the heliophysics community for making measurements to better understand solar wind differences between the poles and the equator. Trajectories after 2032 that do not use Jupiter’s gravity but still arrive before equinox are possible. However, they either deliver a smaller spacecraft into orbit carrying fewer instruments, or take longer to arrive. OWL had limited time to evaluate the millions of possible trajectories, so it will be necessary to use NASA-provided constraints on possible launch years to refine trajectory options and determine specific mass constraints. This allows development of a spacecraft design optimized for the mass constraints and selection of instruments. Evaluating other contributions is also of great importance. The European Space Agency (ESA) has done several studies of ice giants missions and represents a community prepared to make essential scientific and technical contributions.
As preparation begins for UOP, it is important to recognize that although Neptune is also an ice giant, its system is very different from that of Uranus and presents questions that cannot be answered by UOP (15). Neptune’s moon system is dominated by Triton, a Kuiper Belt object that reconfigured the entire system when it was captured. Additionally, the heat balance of Neptune and Uranus differs by an order of magnitude, potentially indicating substantial differences in internal structure (14). Because of its greater distance from the Sun, Neptune is more challenging to reach. Technology development and focused mission studies should begin in this decade to prepare to explore this equally interesting planetary system. The space science community has waited more than 30 years to explore the ice giants, and missions to them will benefit many generations to come.
K.E.M. served on the Pre-decadal Ice Giants Science Definition Team and the OWL Giant Planet Systems panel. Thanks to members of the science community, especially M. Hofstadter and A. Simon for insightful discussions.
References and Notes
1
National Academies of Sciences, Engineering, and Medicine, Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032 (National Academies Press, 2022).
GOOGLE SCHOLAR
2
National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy (National Academies Press, 2003).
GO TO REFERENCE
GOOGLE SCHOLAR
3
National Research Council, Vision and Voyages for Planetary Science in the Decade 2013–2022 (National Academies Press, 2011).
GOOGLE SCHOLAR
4
K. E. Mandt et al., Space Sci. Rev. 197, 297 (2015).
CROSSREF
PUBMED
ISI
GOOGLE SCHOLAR
5
K. Mandt et al., Space Sci. Rev. 216, 1 (2022).
GOOGLE SCHOLAR
6
O. Mousis et al., Exp. Astron.2021).
CROSSREF
GOOGLE SCHOLAR
7
D. Nesvorný, Annu. Rev. Astron. Astrophys. 56, 137 (2018).
GO TO REFERENCE
CROSSREF
ISI
GOOGLE SCHOLAR
8
L. Fletcher et al., Icarus 238, 170 (2014).
GO TO REFERENCE
CROSSREF
ISI
GOOGLE SCHOLAR
9
R. Rufu, R. Canup, Astrophys. J. 928, 123 (2022).
GO TO REFERENCE
CROSSREF
ISI
GOOGLE SCHOLAR
10
P. Schenk, J. Moore, Philos. Trans. R.Soc. A 378, 20200102 (2020).
CROSSREF
PUBMED
ISI
GOOGLE SCHOLAR
11
P. Nicholson et al., in Planetary Ring Systems: Properties, Structure, and Evolution, M. S. Tiscareno, C. D. Murray, Eds. (Cambridge Planetary Science, 2018), pp. 93–111.
GOOGLE SCHOLAR
12
J. A’Hearn, M. M. Hedman, C. R. Mankovich, H. Aramona, M. S. Marley, Planet. Sci. J. 3, 194 (2022).
GO TO REFERENCE
CROSSREF
GOOGLE SCHOLAR
13
C. Arridge, C. Paty, in Magnetospheres in the Solar System, R. Maggiolo et al., Eds. (American Geophysical Union, 2021), pp. 515–534.
GO TO REFERENCE
GOOGLE SCHOLAR
14
R. Helled, J. Fortney, Philos. Trans. R. Soc. A 378, 20190474 (2020).
CROSSREF
PUBMED
ISI
GOOGLE SCHOLAR
15
M. Hofstadter et al., Planet. Space Sci. 177, 104680 (2019).
CROSSREF
ISI
GOOGLE SCHOLAR
SHOW FEWER
See: https://www.science.org/doi/10.1126/science.ade8446
See: https://arxiv.org/pdf/2008.12125.pdf
Ice giants Uranus, Neptune physics.org
The realm of the two Ice Giants, lying between Uranus (19.2 AU) and Neptune (30.1 AU), remains largely unexplored. Like the Gas Giants Jupiter and Saturn (which have been well-characterised by the lengthy Galileo, Juno, and Cassini orbital missions over the past three decades), these giant worlds accreted from the reservoirs of rocks, ices, and gases present in the protosolar nebula at the epoch of planet formation and they feature dynamic atmospheres with banded structures and localised storms; convective interiors enriched in heavy elements (i.e., heavier than helium) compared to protosolar values; they exhibit powerful magnetic fields driven by hidden processes far below the clouds; and they are attended by delicate ring systems and ice-rich satellites. Uranus (14.5 Earth masses and 4.0 Earth radii) and Neptune (17.1 Earth masses and 3.8 Earth radii) represent an intermediate class of planetary object between the enormous Gas Giants and the smaller terrestrial rocky worlds.
Hartmann352
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