A study of the potential future mission to ‘ice giant’ planets Uranus and Neptune and their moons has been released — the first in a series of mission studies NASA scientists will conduct in support of the next Planetary Science Decadal Survey.
Uranus and Neptune are known as ‘ice giants.’ In spite of this name, relatively little solid ice is thought to be in them today, but it is believed there is a massive liquid ocean beneath their clouds. This makes them fundamentally different from Solar System’s terrestrial planets and gas giants.
To date, Uranus and Neptune have been visited briefly by one spacecraft, NASA’s Voyager 2: it rapidly flew by Uranus in 1986 and Neptune in 1989, as part of its grand tour of discovery that previously took it by Jupiter and Saturn.
“Exploration of at least one ‘ice giant’ system is critical to advance our understanding of the Solar System, exoplanetary systems, and to advance our understanding of planetary formation and evolution,” the study authors said.
“Three key points highlight the importance of sending a mission to Uranus and Neptune.”
“First, they represent a class of planet that is not well understood, and which is fundamentally different from the gas giants (Jupiter and Saturn) and the terrestrial planets. Ice giants are, by mass, about 65% water and other so-called ‘ices,’ such as methane and ammonia. In spite of the ‘ice’ name, these species are thought to exist primarily in a massive, super-critical liquid water ocean. No current model for their interior structure is consistent with all observations.”
“A second key factor in their importance is that ice giants are extremely common in our Galaxy. They are much more abundant than gas giants such as Jupiter, and the majority of planets discovered so far appear to be ice giants. Exploration of our local ice giants would allow us to better characterize exoplanets.”
“The final point to emphasize about ‘ice giants’ is that they challenge our understanding of planetary formation, evolution and physics.”
The study team, led by NASA’s Jet Propulsion Laboratory (JPL) and Science Definition Team, identified 12 priority science objectives for ice giant exploration.
“The two most important objectives relate to the formation, structure, and evolution of ice giants:
(i) constrain the structure and characteristics of the planet’s interior, including layering, locations of convective and stable regions, and internal dynamics;
(ii) determine the planet’s bulk composition, including abundances and isotopes of heavy elements, He and heavier noble gases.
The remaining ten objectives, which are of equal importance, touch upon all aspects of the ice giant system:
(iii) improve knowledge of the planetary dynamo;
(iv) determine the planet’s atmospheric heat balance;
(v) measure the planet’s tropospheric 3D flow (zonal, meridional, vertical) including winds, waves, storms and their lifecycles, and deep convective activity;
(vi) characterize the structures and temporal changes in the rings;
(vii) obtain a complete inventory of small moons, including embedded source bodies in dusty rings and moons that could sculpt and shepherd dense rings;
(viii) determine the surface composition of rings and moons, including organics; search for variations among moons, past and current modification, and evidence of long-term mass exchange / volatile transport;
(ix) map the shape and surface geology of major and minor satellites;
(x) determine the density, mass distribution, and internal structure of major satellites and, where possible, small inner satellites and irregular satellites;
(xi) determine the composition, density, structure, source, spatial and temporal variability, and dynamics of Triton’s atmosphere;
(xii) investigate solar wind-magnetosphere-ionosphere interactions and constrain plasma transport in the magnetosphere.”
A variety of potential mission concepts are discussed in the study, including flight elements, orbiters, instruments, and probes that would dive into an ice giant’s atmosphere to study its composition.
A narrow-angle camera would send data back to Earth about the planets and their moons.
“Uranus boasts the most densely packed system of satellites in the Solar System, including 13 low-mass inner moons, five large moons, and dozens of irregular satellites thought to represent captured Centaurs, trans-neptunian objects, and comets,” the scientists said.
“Neptune has 14 known satellites, at least eight of which have diameters greater than 25 miles.”
“This study argues the importance of exploring at least one of these planets and its entire environment, which includes surprisingly dynamic icy moons, rings, and bizarre magnetic fields,” said JPL scientist Dr. Mark Hofstadter, one of the two co-chairs of the team.
“We do not know how these planets formed and why they and their moons look the way they do. There are fundamental clues as to how our Solar System formed and evolved that can only be found by a detailed study of one, or preferably both, of these planets,” said co-chair Dr. Amy Simon, of NASA’s Goddard Space Flight Center.
The results of this and future studies will be used as the Decadal Survey deliberates on NASA’s planetary science priorities from 2022-2032.
Mark Hofstadter et al. 2017. Ice Giants: Pre-Decadal Survey Mission Study Report. NASA, JPL D-100520