The Herschel Space Observatory has focused on Mars, the four giant
planets, and the two homes of comets to uncover new information
about them and about the nebula from which our solar system formed.
When Jupiter, Saturn, Uranus, and Neptune condensed out of the
solar system's parent nebula some 4.6 billion years ago, there
were millions of bits and pieces left over - the comets.
A region of comets lying beyond Neptune's orbit has remained fairly
undisturbed since then. It is known as the Kuiper Belt, and it's the
source of short-period comets like Halley.
But theory holds that the comets scattered among the giant planets
were relocated - drawn to and then flung away by the planets'
gravitational pull. (NASA has made good use of this "slingshot
effect" to give spacecraft a boost on their way to the outer planets.)
Some of the comets are thought to have headed sunward and collided
with the inner planets. In fact, one school of thought has it that
these comets provided the water for Earth's oceans, and may also
have contributed the complex organic molecules that led to life.
But most of the comets were thrown much farther. Massive Jupiter
hurled most of the comets in its vicinity clear out of the solar
system. The smaller giants propelled their comets with less force,
and fewer gained the velocity needed to leave the solar system. The
comets that didn't quite escape wound up forming the Oort Cloud, a
vast storehouse of comets at the outskirts of the solar system,
trillions of miles from the sun (compared to Pluto's average distance
of less than 6 billion miles).
Since they have undergone little change since the beginning of the
solar system, comets are thought to contain fairly pristine samples
of the materials that formed the primordial nebula. So studying them
is a good way to glimpse the original cloud of gas and dust from which
the Sun and planets formed.
Herschel has made detailed observations of comets to help scientists
reconstruct the early development of the solar system, and also determine
whether comets were the source of water and pre-life chemicals on primitive
Earth. (And if on Earth, then also perhaps on Mars and moons of the giant
Herschel has inventoried comets' chemical composition, and study their
physical and chemical processes. Comparing Kuiper Belt comets with those
in the Oort Cloud has enabled scientists to infer conditions in the
different parts of the nebula where they formed.
The deuterium/hydrogen ratio
One measurement of particular interest is the ratio of deuterium to
hydrogen (D/H ratio) in cometary water.
Hydrogen is the simplest atom, with one electron in orbit around a nucleus
of one proton. Deuterium is an isotope of hydrogen, in which the nucleus
also has one neutron. Chemical compounds that contain hydrogen also come
in versions made with deuterium.
Water that contains deuterium is known as "heavy water." Earth's oceans
contain a characteristic percentage of heavy water. So do comets, which
are mostly water, and some of the other planets and moons.
Knowing the D/H ratio of water and other substances is useful to scientists
for two basic reasons.
First, it acts as a fingerprint. By comparing the D/H ratios in Earth's water
with that in cometary water, for example, scientists can determine whether
our planet's water could have come from comets. The same determination can
be made for other planets and moons in the solar system.
Second, the D/H ratio in comets provides a clue to conditions at the
beginning of the Universe!
Scientists think the D/H ratio of pristine comets represents that of our
original nebula, which in turn is typical of the rest of the Universe.
And all of the hydrogen and deuterium in the Universe is thought to have
formed during the first three minutes after the Big Bang (the nuclei, that
is - stable atoms didn't develop for another 300,000 years or so), so the
D/H ratio of the Universe was fixed in place at that time.
Knowing that ultimate D/H ratio would help scientists determine the
conditions that could have generated hydrogen and deuterium in that
particular ratio, and therefore help them deduce the nature of the
Universe in its earliest stages.
Scientists are also interested in comparing the D/H ratios in the
atmospheres of the four giant planets, and Herschel will measure them
Montage of planets photographed by the Voyager spacecraft. From
upper right: Jupiter, Saturn, Uranus, and Neptune.
One important reason is to test a prevailing theory of planet formation.
Scientists think that as our solar system developed, Jupiter and Saturn
formed mostly from gas, while Uranus and Neptune - out in the more
tenuous suburbs of the primordial nebula – formed from a much higher
percentage of ice-covered dust grains.
If that is correct, then Uranus and Neptune should have a higher D/H
ratio than Jupiter and Saturn, since icy dust mantles tend to have a
higher ratio than the surrounding gas.
Comparing the ratios in the two pairs of planets will reveal information
about the composition of that icy dust. And comparing them with the
ratios in comets will further enhance our understanding of how all
those bodies formed, and the structure of the primordial nebula.
The Infrared Space Observatory (ISO) surprised scientists when it detected
water in the high atmospheres of the four giants. Herschel will help to
determine whether that water came from their rings and moons or from the
icy crusts of interplanetary dust.
Tracking water and other molecules at various altitudes (the "vertical
profiles") provides information on how those atmospheres work - how
convection moves gases up and down, and how winds blow them around.
Herschel has studied the vertical mixing profiles of many molecules in
the atmospheres of the giant planets, providing new insights about the
chemistry and dynamics of the atmospheric layers. Ammonia and phosphine,
which are affected by condensation, photochemical processes, and vertical
transport, are expected to be among the most valuable guides to information
about atmospheric circulation in the giant planets.
Herschel has mapped the spectral continuum of Jupiter and Saturn in the
far-infrared for the first time to reveal properties of their clouds,
especially particle size and density.
Looking further out to Uranus and Neptune, Herschel has measured the
methane in their stratospheres.
Herschel has also searched all four gas giants for chemicals never before
seen in planetary atmospheres.
Understanding the atmospheric chemistry of Mars is important both for an
understanding of Mars' history - including the possibility that it was
more like Earth in its earlier days - and as a tool for comparing how
atmospheres differ on different planets. Such studies may provide insights
into the workings and possible future of our own atmosphere here on Earth.
Herschel has explored the Martian atmosphere in the 200-670 micron range
for the first time, enabling scientists to determine the vertical profiles
of water vapor and oxygen molecules. Monitoring water at various times
during the Martian year has revealed seasonal changes.
Herschel has also measured deuterium and carbon monoxide, and may detect other
compounds, such as hydrogen peroxide, that are predicted by models of
Finally, Herschel has obtained information about the composition and
emissivity of minerals covering Mars' surface.