68 ASTRONOMY • DECEMBER 2019
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
THE QUANTUM GRAVITY SECTION
IN THE APRIL ISSUE NOTED THAT
WATER WITHIN SATURN’S RINGS AND
MOONS IS SIMILAR TO EARTH’S, EXCEPT
FOR PHOEBE. HOW IS PHOEBE’S WATER
DIFFERENT?
Jack Stelnicki
San FranciscoAI
The origin of water throughout the solar
system is an area of continuous study. One
fingerprint for water’s origin is the isotopic signature:
the deuterium-to-hydrogen ratio (D/H). Deuterium is a
hydrogen atom with one proton and one neutron in the
nucleus, while normal hydrogen has only the proton.
The D/H value ref lects a combination of the source of
the water, the location of a body’s formation, and the
subsequent evolution of the body since its formation.
Models of solar system formation say that D/H values
should increase from the inner to outer solar system due
to the higher temperatures close to the Sun.
The D/H of terrestrial ocean water is about 0.00016,
and the bulk Earth value is 0.000146. The least altered
carbonaceous meteorites, which represent the solar
system’s earliest composition and formed relatively farHow is Phoebe
different?
from the Sun, are about 0.000145,
the same as the bulk Earth within
measurement error. Some comets
have D/H values around 0.0003
to 0.0005, all consistent with
increasing D/H values with
increasing distance from the Sun.
Some models for solar system
formation predict a D/H in the
Saturn system around 0.0004 to
0.0005 and others in the 0.003 to
0.006 range, all much higher
than terrestrial values. But
analysis of Cassini data from the
Visual and Infrared Mapping
Spectrometer, using a newly
developed method for deriving
isotopic ratios in solid surfaces,
measured the D/H in Saturn’s
rings at 0.00016, very close to the
terrestrial value within measure-
ment error. Saturn’s satellites
Enceladus, Rhea, Hyperion, and Iapetus also have D/H
values close to terrestrial. Phoebe, on the other hand,
has a D/H of 0.0013, many times higher than the other
satellites, rings, and terrestrial water. All this points to
several important conclusions: First, that the current
models of the formation of the solar system need revi-
sion. Second, that the water on Earth and in the Saturn
system appears to have a common origin. And third,
Phoebe had to have originated in the cold outer reaches
of the solar system.Roger Clark
Senior Scientist, Planetary Science Institute, Lakewood, ColoradoSaturn’s moon Phoebe
stretches about
132 miles (213 km)
across. Unlike the rest
of the Saturn system,
including the planet’s
rings and other
moons, Phoebe
appears to have water
that formed farther out
in the solar system.
NASA/ESA/JPL/SSI
Ridges such as this one, located in Mare Frigoris, indicate to
planetary scientists that the Moon’s crust is still changing. Some
changes, such as movement along fault lines, could cause
moonquakes. NASA