An illustration of water in our Solar
System through time from before the Sun’s birth through the creation of
the planets. The image is credited to Bill Saxton, NSF/AUI/NRAO. A
larger version is available here. Another image is available here.
Washington, D.C.—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.
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This research was supported by the NSF, the Rackham Predoctoral Fellowship, NASA Astrobiology, NASA Cosmochemistry and NASA.
