Vision and Voyages for Planetary Science in the - Solar System ...

How Have the Myriad of Chemical and Physical Processes that Shaped the Solar System
Operated, Interacted, and Evolved Over Time?
In searching for answers to the overarching questions, we first seek a deep understanding of the
chemical and physical processes that have shaped planetary interiors, surfaces, atmospheres, rings, and
magnetospheres through time. Since 1977 when the Voyager spacecraft left Earth, our perspectives
regarding the complexity and diversity of the solar system have undergone immense revision and
expansion. Ranger, Surveyor, and Apollo data showed that the Moon’s geologic evolution ended long
ago. Mariner 10’s visit to Mercury showed a similar picture—an ancient, impact-riddled, and
geologically dead world. Mariners 4, 6, 7, and 9 flew by and orbited Mars, and again revealed a mostly
cratered volcanic world, but one also with jumbled chaotic terrains, gargantuan canyons, exotic polar
deposits, and outflow channels and drainage networks of a watery but ancient origin. Even with Mars’s
profusion of geologic processes, it too appeared to be inactive, a frigid desert world.
As had been predicted, Voyager found that Jupiter’s moon Io is the most intensely volcanic object
in the solar system; volcanic plumes fountain up to 300 kilometers and not a single impact crater has been
found anywhere on its young volcanic plains. Galileo confirmed that the surface of Io continues to evolve
rapidly, discovering molten lakes of silicates and sulfur-rich lavas and active fire fountains. New
Horizons provided elegant movies of an eruption in progress. Caught in a celestial dance that also
involves Jupiter, Europa, and Ganymede, Io is intensely heated by tides and remains one of the best
places in the solar system to study active volcanism and tidal heating.
Turning to the other Galilean satellites, the Galileo mission found all three to have internal
oceans. 76 Of the three, the ceiling of Europa’s ocean chamber is thought to be at the shallowest depth
because, although less so than Io, it is also tidally heated. In addition because its large rocky interior,
upon which the ocean rests, is subjected to both tidal and radiogenic heating, it is reasonable to expect
seafloor volcanism and hydrothermal activity that could provide nutrients and energy to support
metabolism. 77 These factors combine to make Europa, along with Mars, the highest priority destinations
in the solar system as potential planetary habitats.
For the jovian system we have learned to expect the unexpected. Galileo showed Ganymede, the
only satellite in the outer solar system known to have an internal magnetic field and a magnetosphere.
Galileo’s probe into the jovian atmosphere revealed the noble gas abundances were very unlike the
Sun’s—processes like helium rain falling into Jupiter’s core have been invoked as possible explanations,
and more recent observations have shown a dynamic, ever-changing atmosphere, riddled by impacts. 78
Saturn’s excessive thermal energy might also signal helium rain; direct measurement of its noble
gas abundance and isotopic chemistry is required to get an answer. Cassini confirmed exquisite features
in Saturn’s atmosphere: the Voyager-detected hexagonal circumpolar jet rotating around Saturn’s north
pole and a hot, hurricane-like, vortex with newly discovered well-defined eye wall circles in the south.
That a tiny icy moon could be warm enough to maintain liquid water in its interior driving jets out if its
broken crust, make Enceladus a key destination. 79,80 Mimicking Earth, Titan has seas of organic sand
dunes, hydrocarbon lakes, dendritic river systems, putative icy volcanism, mountain chains, and global
fault systems—Titan ranks near the top as a target of future exploration. 81
At Neptune Voyager found nitrogen geysers fountaining up into the stratosphere from Triton’s
ultra-cold, 37 K surface—perhaps driven by a greenhouse effect as subliming gas rushes to vents under
clear nitrogen ice. Originally we thought only Saturn among the giant planets possesses a ring system.
As it turns out so do Jupiter, Uranus, and Neptune, and these systems all differ dramatically. Neptune
possesses orbiting arcs forming partial rings; dense dark rings interspersed with broad sheets of nearly
invisible dust encircle Uranus; Jupiter’s gossamer ring orbits as a dusty wreath. 82,83 We are only
beginning to uncover the nature and ages of the ring materials.
We have had tantalizing glimpse of conditions and configurations in the ice giants themselves:
oddly tilted and offset magnetic fields, unexplained sources of heat, supersonic atmospheric motions. Of
the major objects of the solar system, we understand these ice-giant worlds least. Because recent
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