Water was crucial to the rise of life on Earth and is also important to
evaluating the possibility of life on other planets. Identifying the original
source of Earth's water is key to understanding how life-fostering
environments come into being and how likely they are to be found elsewhere.
New work from a team including Carnegie's Conel Alexander found that much of
our Solar System's water likely originated as ices that formed in interstellar
space. Their work is published in Science.
Water is found throughout our Solar System. Not just on Earth, but on icy
comets and moons, and in the shadowed basins of Mercury. Water has been
found included in mineral samples from meteorites, the Moon, and Mars.
Comets and asteroids in particular, being primitive objects, provide a
natural "time capsule" of the conditions during the early days of our Solar
System. Their ices can tell scientists about the ice that encircled the Sun
after its birth, the origin of which was an unanswered question until now.
In its youth, the Sun was surrounded by a protoplanetary disk, the so-called
solar nebula, from which the planets were born. But it was unclear to
researchers whether the ice in this disk originated from the Sun's own
parental interstellar molecular cloud, from which it was created, or whether
this interstellar water had been destroyed and was re-formed by the chemical
reactions taking place in the solar nebula.
"Why this is important? If water in the early Solar System was primarily
inherited as ice from interstellar space, then it is likely that similar
ices, along with the prebiotic organic matter that they contain, are
abundant in most or all protoplanetary disks around forming stars,"
Alexander explained. "But if the early Solar System's water was largely the
result of local chemical processing during the Sun's birth, then it is
possible that the abundance of water varies considerably in forming
planetary systems, which would obviously have implications for the potential
for the emergence of life elsewhere."
In studying the history of our Solar System's ices, the team -- led by L.
Ilsedore Cleeves from the University of Michigan -- focused on hydrogen and
its heavier isotope deuterium. Isotopes are atoms of the same element that
have the same number of protons but a different number of neutrons. The
difference in masses between isotopes results in subtle differences in their
behavior during chemical reactions. As a result, the ratio of hydrogen to
deuterium in water molecules can tell scientists about the conditions under
which the molecules formed.
For example, interstellar water-ice has a high ratio of deuterium to
hydrogen because of the very low temperatures at which it forms. Until now,
it was unknown how much of this deuterium enrichment was removed by chemical
processing during the Sun's birth, or how much deuterium-rich water-ice the
newborn Solar System was capable of producing on its own.
So the team created models that simulated a protoplanetary disk in which all
the deuterium from space ice has already been eliminated by chemical
processing, and the system has to start over "from scratch" at producing ice
with deuterium in it during a million-year period. They did this in order to
see if the system can reach the ratios of deuterium to hydrogen that are
found in meteorite samples, Earth's ocean water, and "time capsule" comets.
They found that it could not do so, which told them that at least some of
the water in our own Solar System has an origin in interstellar space and
pre-dates the birth of the Sun.
"Our findings show that a significant fraction of our Solar System's water,
the most-fundamental ingredient to fostering life, is older than the Sun,
which indicates that abundant, organic-rich interstellar ices should
probably be found in all young planetary systems," Alexander said.
Reference:
L. Ilsedore Cleeves, Edwin A. Bergin, Conel M. O’D. Alexander, Fujun Du, Dawn
Graninger, Karin I. Öberg, and Tim J. Harries. The ancient heritage of water
ice in the solar system. Science, 26 September 2014: 1590-1593
DOI: 10.1126/science.1258055