Sky and Telescope - May 2017 - 20

SPOUTING OFF A key
objective for Rosetta
was capturing the
comet when it was
most active. The
misshapen nucleus,
here silhouetted by
dust in its own coma,
releases one very
strong jet and several
weaker ones on August 12, 2015 - one
day before the comet
reached perihelion.

the ratio of deuterium ("heavy hydrogen") to normal hydrogen in water molecules. This D:H ratio varies based on where
it's measured in the solar system. Meteorites and asteroids
have D:H values similar to Earth's. Although we think of
these objects as very dry, some types of carbonaceous chondrite
meteorites can have a water fraction as high as 20%. But
whether these could have delivered enough water to Earth is
still debated.
Meanwhile, the most distant, longest-period comets -
those coming from the Oort Cloud - all have D:H ratios that
are too high, compared with our planet. However, the value
measured in a short-period comet, 103P/Hartley 2, has a very
Earth-like value. This object is a Jupiter-family comet (JFC), an
object with a relatively short-period orbit that is dynamically
controlled by Jupiter. So we were eager to know if 67P, which
is also a JFC, likewise has an Earth-like D:H ratio.
The first and strongest result regarding this question
came from Rosetta's ROSINA mass spectrometer, which
found a D:H ratio 3½ times higher than Earth's - a value
even higher than those of Oort Cloud comets! This confirms
what many planetary scientists had already come to suspect:
Perhaps comets of any type are not the primary source of

Earth's water. Instead, maybe most of it really did come from
asteroids - or was here from the outset in the materials from
which Earth formed.
Still, comets have and will continue to hit Earth and
deliver some water. So every glass of water you drink has
comet water in it, just not as much as once thought.
An exciting implication of the different D:H values in
comets Churyumov-Gerasimenko and Hartley 2 is that JFCs
might have originated from widely different regions of the
solar system. Moreover, apparently the water ice in 67P is
original to the comet - it was never "reprocessed" and mixed
with water from other sources in the primordial solar nebula.
So do we have any clue where Comet 67P/Churyumov-Gerasimenko formed? One hint comes from the ROSINA and Alice
instruments, which detected molecular oxygen (O2). This is
surprising because O2 is very reactive - it's hard to understand
how it could have been retained in a 4.5-billion-year-old comet.
One idea that's become a lively topic of discussion among
Rosetta's scientists is that the O2 became trapped in "cages"
of ice (called clathrates). This would keep the oxygen from
escaping or reacting until the ice sublimates whenever the
comet comes close to the Sun.
ROSINA also discovered molecular nitrogen (N2). This
finding, the detection of O2, and the high D:H ratio all indicate that 67P formed at very cold temperatures (25 to 30 K).
This would place its origin in the outer fringe of our planetary system, likely in the Kuiper Belt. That's also dynamically
consistent with current models of solar system formation.
So let's recap: Measurements from Rosetta and dynamical
models imply that 67P formed in the Kuiper Belt, and that its
two lobes were separate bodies that gently collided and stuck
together. But how did those 2-km-wide bodies come to exist
so far from the Sun in the first place?
One clue comes from the "goose bumps" observed on the
nucleus - perhaps an appropriate term considering how cold
it is out there. On the walls of pits and cliffs we identified textures that look like they are built of many small boulders just

Rosetta, circa 1992

PLAN A ESA's original concept for Rosetta.
Note the sample-return capsule (brown).

20

M A Y 2 0 1 7 * SK Y & TELESCOPE

º This mission didn't end up the way the
European Space Agency conceived it in the
early 1990s. The original plan called for a
spacecraft able to return a sample of comet
nucleus. Then it became a mission with two
landers (NASA's Champollion and ESA's
RoLand) before morphing to the orbiter-lander
configuration finally built by ESA.
Even the original target, a comet named
46P/Wirtanen, had to be changed. The failure
of an Ariane V rocket just one month prior
to Rosetta's planned January 2003 launch
caused a year-long postponement. That delay
put a rendezvous with Comet Wirtanen out of

reach. So the Rosetta team scrambled to find
another target that was scientifically interesting, similarly sized (Philae had been designed
specifically for the mass of Comet Wirtanen),
and in an orbit that could be reached.
The decision to go to 67P/ChuryumovGerasimenko was a compromise. This body,
like Wirtanen, is also a Jupiter-family comet
- but one that's at least three times larger
and up to 30 times more massive. So the
lander did have to be modified a bit. And
while 67P could be reached with decent
fuel reserves, it also took the longest time to
reach- which made it a more risky choice.
Comet 67P was amazing, so imagine what
that "other" mission would have been like.

CO ME T: ESA / ROSE T TA / MPS FOR OSIRIS TE A M; ROSE T TA CONCEPT: ESA

Rosetta Results



Sky and Telescope - May 2017

Table of Contents for the Digital Edition of Sky and Telescope - May 2017

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