23 DECEMBER 2016 • VOL 354 ISSUE 6319 1537 SCIENCE sciencemag.org
days are numbered as heating from the Sun
erodes the comet in a generally gradual but
occasionally dramatic fashion. On pages
1563 and 1566 of this issue, Filacchione et
al. (1) and Fornasier et al. (2), respectively,
report on the long-term and close-up sur-
veillance of 67P by Rosetta, revealing for the
first time both the day-to-day and seasonal
evolution of a comet nucleus (1, 2).
Comets contain fingerprints of the early
solar system, but recognizing those signatures is difficult. The effects of evolutionary
changes from radiation processing during
billions of years in cold storage followed by
recent extreme heating from many close solar passages must be recognized and disentangled from natal characteristics in order to
read the primitive record of the early solar
system. Until recently, the nature and properties of comet nuclei were based largely on
inferences with little direct observational
information. The typically several kilome-ter-sized nucleus of a comet is only directly
observable from Earth when it is inactive
and far away and thus faint and difficult to
characterize. At closer distances, the Sun activates the release of large quantities of gas
and dust forming a coma (or atmosphere)
that shrouds the nucleus, allowing only the
inference of nucleus characteristics from the
properties of coma gas and dust.
The situation improved once spacecraft
designed to study a comet’s nucleus got up
close and personal, piercing the coma to
reveal unprecedented detail about nucleus
surface morphology and the near-nucleus
coma environment (3–7). But, as revolutionary as these missions were, they occurred
during fleeting high-speed encounters,
leaving important gaps about the life cycle
of comets that snapshots in time cannot
reveal. We know from Earth-based observations that comet evolution is not always
steady but is punctuated by bursts of activity
that cause sudden, often dramatic, brightening (8), occasionally shattering a comet to
pieces (9), and sometimes even resulting
in catastrophic disintegration (10). Rosetta
was transformational because it was able to
study the evolution of the nucleus up close
over a 2-year period beginning with a phase
of near inactivity, following it through its
closest approach to the Sun when activity is
highest, and staying with it as it retreated
from the Sun and once more headed to dormancy (see the figure).
Rosetta was particularly well equipped to
characterize comet surface morphology and
track changes with time. Using a combina-
tion of spectroscopic and imaging systems
spanning visible to infrared wavelengths, it
could reveal the composition of ices on the
nucleus through detection of diagnostic
spectral signatures (1) and the extent of ice
and dust coverage on the nucleus through
changes in surface color (2). A surprising
result was the detection of volatile carbon
dioxide ice (dry ice) in a surface region just
coming out of the shadow of a 4-year win-
ter while there was an absence in the same
region of water ice, which is by far the domi-
nant ice in comets (1). However, sheets of
mixed water ice and dust existed over large
portions of the nucleus just below a dusty
surface (2). This compositional structure in
the upper layers of the nucleus evolved as
the comet approached the Sun, stripping
off the dust layers to temporarily reveal an
ice-rich surface before sublimation removed
the ice, leaving again a dusty surface as 67P
receded from the Sun (2).
The temperature of the upper layers of
the nucleus, as controlled by both the daily
and seasonal exposure to sunlight, largely
dictates the transportation and sequestra-
tion of ices in 67P. Daily variations were
also observed as regions rotating in and out
of shadow caused a cycle of frost condensa-
tion and sublimation on the surface in an
ongoing interaction between the nucleus
and coma (2). Over longer time scales, re-
gions in shadow cool the surface to very
low temperatures, whereas thermal inertia
keeps interior layers at higher temperature,
allowing the more volatile carbon dioxide
to reach the colder surface from an interior
reservoir before condensing, whereas water
condenses deeper in the nucleus before it
can reach the surface. These processes of
surface erosion and volatile transport drive
the evolution of 67P.
Rosetta has provided fundamental insights into the importance of evolutionary
processes in comets. We now have direct
evidence about how the composition and
structure of the near-surface layers are
driven by both diurnal and seasonal evolution. That volatile ices such as carbon dioxide still exist in comets today supports the
idea that the interior of the nucleus still
retains much of its natal character. Rosetta
represents a major step forward in comet
science, but many questions remain. Comets are compositionally diverse, and the few
comets visited by spacecraft have remarkable variation in surface morphology, so it
is unclear that the processes that drive evolution on 67P are universal. What is clear
is that results from Rosetta will keep comet
scientists hard at work trying to understand
this diverse and evolving family of objects
containing the most primitive material
from the birth of our solar system—at least
until the next comet mission provides new
glimpses that both amaze us and dramatically alter our perceptions. j
1. G.Filacchione et al., Science 354, 1563(2016).
2. S. Fornasier et al., Science 354, 1566 (2016).
3. H. U. Keller, R. Kramm, N. Thomas, Nature 331, 227 (1988).
4. L. A. Soderblom et al., Science 296, 1087 (2002).
5. D. E. Brownlee et al., Science 304, 1764 (2004).
6. M. F. A’Hearn etal.,Science 310, 258 (2005).
7. M. F. A’Hearn etal.,Science 332, 1396 (2011).
8. M. Montaltoetal., Astron.Astrophys.479, L45 (2008).
9. H.A. Weaver et al., Science 267,1282(1995).
10. H. A. Weaver et al., Science 292, 1329 (2001).
Grayscale photograph of comet 67P/Churyumov-
Gerasimenko taken by the Rosetta spacecraft.
To rendezvous and track
The Rosetta mission followed comet 67P for over a 2-year period covering a large portion of its orbit
from near inactivity, through high levels of activity near closest approach to the Sun, and back out
again. The orbit of comet 67P is shown in red in relation to the orbits of the fve innermost planets.
Mission milestones and extent are illustrated.
approach to the Sun
13 August 2015
End of mission after controlled
impact onto surface
30 September 2016
6 August 2014