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

carbon dioxide atmosphere, laden with sulfuric acid clouds, circle the planet every 4 days. Venus Express
discovered that lightning, aurora, and nightglows light up the planet’s sky. 70 Evidence of active
volcanism is also suggested, supporting the idea that ongoing volcanic emission of sulfur dioxide feeds
the thick sulfuric acid clouds. What mechanisms triggered Venus’s runaway greenhouse and on what
timescale remain open questions. 71 Addressing them can help us better understand the principles of
greenhouse atmospheres in general, placing Earth’s in a broader context. For Venus, addressing these
questions requires measurements of atmospheric chemistry, notably of the isotopic and noble gas
chemistry of the lower atmosphere. Establishing the initial climate conditions and modern states of
Venus and Mars can help us to understand how their environments diverged so dramatically from Earth’s.
Mars has perhaps the most Earth-like modern planetary atmosphere, and its earliest climate may
have been very Earth-like. Studying it therefore provides opportunities to validate terrestrial climate and
global circulation models under very different atmospheric conditions. Mars’s polar layered deposits
suggest climate change in the last 10 million years, and dynamical models predict large recent excursions
in axial tilt and orbital eccentricity. 72 These considerations point to recent climatic change, analogous to
ice ages on Earth, detailed records of which are likely preserved in the polar layered deposits. During
Mars’s postulated early warm wet climate solar luminosity is thought to have been ~25 percent lower than
today. This fact has made it difficult for atmosphere modelers to understanding how Mars’s greenhouse
could have sustained such warm conditions, but the geologic evidence for Noachian rivers and lakes is
compelling. The continued investigation of Mars’s climate through time and the study of its modern
atmospheric processes from orbit, from the surface, and ultimately from analysis of returned samples
conditions remain high priority scientific objectives.
Flow within giant planet atmospheres is largely organized in east-west jet streams. Whereas
Jupiter and Saturn exhibit alternating east-west jets, Uranus and Neptune show broad belts of retrograde
winds at the equator shifting to prograde with increasing latitude. Vortices, cyclonic and anticyclonic, at
many scales spin between the jets and resemble weather features seen on Earth ranging from tornados to
hurricanes. North-south circulation continually overturns belt-zone system in Hadley-like convection
cells and by wave forcing. 73,74 Major questions remain as to how these motions, visible in the layered
cloud decks, couple to the interior structure and deep circulation. The Juno and Cassini Solstice missions
may detect gravitational signatures of deep internal flow in Jupiter and Saturn. The most serious gap in
our understanding of planetary atmospheres remains for the ice giants, Neptune and Uranus.
The giant planets also provide the only examples of processes common to Earth in which strong
internal magnetic fields interact with the solar wind. This includes the fluorescing spectacle of Earth’s
northern lights and similar auroral displays seen near the magnetic poles of Jupiter and Saturn. At Jupiter
and Saturn the main sources feeding the magnetospheric plasma appear to be Io, Enceladus, and Saturn’s
rings, while most of the magnetospheric plasma at Earth is trapped solar wind. These interactions pose
major consequences for humans; understanding and predicting them is important. The solar wind induces
magnetospheric storms that disrupt power and communication systems worldwide. The giant planets
provide a wide spectrum of observable magnetospheric processes that can contribute directly to an
understanding of the physics at work in Earth’s space environment.
One of the most startling revelations of the last decade is how much the processes ongoing in
Titan’s thick global atmosphere and on its surface resemble those of Earth. Both worlds have nitrogendominated
atmospheres with about the same surface pressure. 75 However in Titan’s ultra cold
meteorology, methane migrates through a global system of clouds, rain, rivers, lakes, seas, and aquifers:
the analogy to Earth’s hydrological cycle is obvious. The mechanics and chemistry of this atmosphere
are complex but pale in comparison to the complexity of Earth’s. In our quest to understand greenhouse
mechanisms, Titan’s atmosphere manifests both greenhouse warming and anti-greenhouse cooling,
puzzling diametric cases in which thermal radiation is sometimes trapped and sometimes radiated to
space. The Cassini-Huygens spacecraft arrived at Saturn near northern winter/summer solstice and the
mission will be extended through northern summer solstice, allowing unprecedented view of Titan’s
seasonal behavior. Continued exploration of this fascinating Earth-like atmosphere, both from orbit and
in situ, remains one of the most important objectives for planetary science.
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