Sky and Telescope - May 2017 - 21

10-5

PLOT: K ATHRIN A LT WEGG E T A L. / SCIENCE; IM AG ES: ESA / ROSE T TA / MPS FOR OSIRIS TE A M (3)

C/1996 B2 (Hyakutake)

67P/C-G

45P/H-M-P
103P/Hartley 2

1P/Halley

8P/Tuttle
C/2009 P1 (Garradd)

C/2001 Q4 (NEAT)
C/2002 T7 (LINEAR)

153P/Ikeya-Zhang

Outer planets

C/1995 O1 (Hale-Bopp)

Enceladus
Saturn

Asteroids
Jupiter

10-4

Uranus
Neptune

Earth

Deuterium-to-hydrogen ratio (D:H)

10-3

Jupiter-family
comets

Protosolar nebula

Oort Cloud comets

p WATER'S ORIGINS The proportions of hydrogen and its heavier
isotope deuterium vary widely across the solar system. The ratio is
unexpectedly high in Rosetta's comet, 67P/Churyumov-Gerasimenko,
implying that most of Earth's water must have come from water-rich
asteroids or was already here when our planet formed.

a few meters wide. Could these be our first peek at the basic
building blocks that created comets and eventually planets?

Results From Philae
In addition to the CONSERT measurements of the interior of
the nucleus, Philae provided valuable measurements about
the properties of the comet's surface. Even the unplanned
"bounces" helped us gauge the hardness of the surface. Thanks
to MUPUS, which tried to hammer an instrumented penetrator
into the nucleus, we know that there's a surface layer of dust
covering a very hard, compacted dust-ice crust. The lander also
confirmed that the nucleus has no intrinsic magnetic field.
COSAC, which paired a gas chromatograph and mass spectrometer, identified 16 organic compounds, four of which -
including acetone and acetamide - had never been detected
in a comet. Gases measured by Ptolemy, another mass
spectrometer, contained not only the expected water, carbon

monoxide, and carbon dioxide, but also organic compounds
including formaldehyde. Many of these compounds are
important in creating amino acids, sugars, and nucleobases.
In addition, ROSINA detected the amino acid glycine. Even
if comets didn't supply the majority of water on Earth, they
could have provided essential raw materials for life.
We'll never hear from Rosetta again, but this is not the
last that we'll hear from its host. The comet will have a
fairly close encounter with Jupiter, only about 56 million
kilometers, at its next aphelion in November 2018. Jupiter's
gravitational tweak will decrease 67P's perihelion. So when
the comet next comes its closest to the Sun, on November 2,
2021, chances are good that we'll see more vigorous activity. That coming apparition also offers quite good observing
geometry from Earth, with the comet just 0.42 astronomical
unit (63 million km) away and well positioned high in the
northern night sky (declination +26°).
But it will be a long time, if ever, that we'll see Comet 67P/
Churyumov-Gerasimenko as well as we have these past three
years. And as I watched Rosetta take its final plunge to the
surface, streaming to us in real time images of new vistas
seen in more detail than ever before, I couldn't help but think
of this quote from the movie Blade Runner:
"I've seen things you people wouldn't believe. Attack ships on
fire off the shoulder of Orion. I watched C-beams glitter in
the dark near the Tannhauser Gate. All those moments will
be lost in time, like tears in rain. Time to die."
Rest in peace, Rosetta and Philae. I'm sure those moments
will not be lost as we continue to use this mission's vast
stockpile of data to further our knowledge about comets, the
origin of our solar system, and perhaps ourselves.
¢ JOEL PARKER (Southwest Research Institute) is on the science teams of Rosetta and several other space missions. He
produces and hosts the radio science show "How on Earth."

ICY FIREWORKS On July 29, 2015, just
two weeks before reaching perihelion,
the comet unleashed an intense but
short outburst of gas and dust. These
three images span just 36 minutes.

s k y a n d t e l e s c o p e .c o m

* MAY 2017

21


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Sky and Telescope - May 2017

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