Sky & Telescope - August 2022 - 9

NASA / JPL-CALTECH
SPACE & SOCIETY
Planning the Next Decade
of Planetary Science
THE NATIONAL ACADEMIES of Science,
Engineering and Medicine have
released the decadal survey for the
planetary science and astrobiology community,
providing marching orders for
solar system exploration through 2032.
The decadal survey prioritized the
development of a new flagship mission:
the Uranus Orbiter and Probe. Costing
on the order of $1 billion, this mission
would send a Cassini-style spacecraft
to the ice giant and its moons and will
include a probe to enter the atmosphere.
(Uranus won out over Neptune due to
launch and trajectory logistics.)
The second-highest priority flagship
identified in the report is a combination
orbiter-and-lander that could head to
Saturn's moon Enceladus. The Enceladus
Orbilander would sample plumes
emanating from geysers on the icy
moon's surface.
The report also recommended the
continued development and support of
projects from previous decadals, such
as the Perseverance rover and sampleSOLAR
SYSTEM
Where Did the Ice Giants Form?
CONVENTIONAL PLANET-FORMATION
scenarios have Neptune and Uranus
forming close to the Sun before migrating
outward. But a new study to appear
in the Astrophysical Journal shows that
the ice giants could have formed right
where they are now.
In the classic picture of planet formation,
the dust grains in a protoplanetary
disk clump together into larger and
larger bodies. Once these bodies reach
the mass of planetary cores, they gather
gas from the disk.
But according to this classic scenario,
planetesimal growth would have
been slower farther away from the Sun.
Uranus and Neptune, at their current
distances, wouldn't have had time to
amass their mantles before the gaseous
disk had dissipated.
However, this picture neglects the
role of pebbles, centimeter-size particles
whose presence speeds up accretion
onto planetary cores, such as those in
Uranus and Neptune, argue Claudio
Valletta and Ravit Helled (University of
Zürich, Switzerland).
" The formation timescale problem,
which was there for a very long time,
from my perspective no longer exists, "
Helled says.
Valletta and Helled ran 24 versions
of a simulation that followed the accretion
of pebbles and gas onto a single
planetary core, considering the growth
of Uranus and Neptune individually.
They successfully reproduced the ice
giants' total mass as well as the heft
of their gaseous envelopes. However,
several of the simulation runs formed
(220 pounds) of regolith, which Artemis
astronauts would return to Earth.
The decadal also endorsed planetary
defense and supports the NEO Surveyor
mission, which would fly a mid-infrared
camera to seek objects more than
140 meters in size.
The report did not recommend
The Voyager 2 flyby of Uranus in 1986 provided
us with our closest view of the enigmatic
ice giant.
caching mission as well as the underconstruction
Europa Clipper. As part
of continued Mars exploration, the
committees also prioritized a probe (the
Mars Life Explorer) to seek extant life
on the Red Planet.
For lunar exploration, the survey
recommended the development of
Endurance A, a large sample-return
rover that would visit the lunar south
pole. It would collect 100 kilograms
any specific missions for NASA's New
Frontiers program but rather set themes
for consideration. To support smallerbudget
exploration under the Discovery
and Small Innovative Missions for Planetary
Exploration programs, the survey
suggests a cost-cap increase for both
mission classes.
In alignment with the astronomy
and astrophysics decadal survey (S&T:
Mar. 2022, p. 8), the planetary-science
community highlighted the urgent need
to promote diversity and equity in the
field. Both reports also called for measures
to monitor and mitigate the rising
impact of satellite constellations.
" I think [the decadal] is a very
compelling vision for space exploration
for the next decade and beyond, " says
planetary scientist Jonathan Fortney
(University of California, Santa Cruz).
¢DAVID DICKINSON
a planet missing heavy elements: up to
3 Earth masses' worth. Giant impacts
might have delivered this missing mass,
the team suggests.
André Izidoro (Rice University), who
was not involved in the study, cautions
that migration is hard to avoid.
" As planets grow, especially when they
become larger than one Earth or Mars
mass, they start to move around, " he
says. Simulations have in fact demonstrated
that Uranus and Neptune could
have formed from a series of collisions
among migrating protoplanets.
A future mission to Uranus (see
above) might shed light on the planet's
migration history or lack thereof. But
regardless of what any such mission
finds, understanding the ice giants'
origin will likely bring together multiple
processes. As both Helled and Izidoro
repeatedly emphasized: It's complicated.
¢JURE JAPELJ
sk yand tele scope .o r g * AUGUST 2022
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Sky & Telescope - August 2022

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