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Comets are bodies consisting of ice and dust that are thought to have formed further from the Sun than the
asteroids where temperatures were low enough for water and even carbon dioxide gas to condense into solid ice
grains. Although close to the Sun comets are heated and their ices turn into gas to form tails millions of
kilometres in length, most comets are still found far from the Sun and are at temperatures less than -225 degrees Celsius.
Because of their "cold storage" comets have been considered pristine samples that preserve the materials that
went to build our Solar System.
Recent research, reports Dr Alan Stern in today's edition of the journal Nature, however, suggests that even at the
enormously low temperatures of the outer solar system, comets can change with age. Comets are found in two main places in our
Solar System, in the Kuiper Belt, which is a disk shaped cloud of comets beyond the planet Neptune, and the Oort Cloud, which is
a spherical cloud of comets that extends one third of the way to the nearest star. Stern explains how comets in the Kuiper Belt and
Oort Cloud are likely to change in different ways.
In the Kuiper Belt, says Stern, comets are present in a much more confined region than in the Oort cloud and collisions are relatively frequent. The
highly fragile original comets are, therefore, likely to be compacted by repeated impacts. Collisions in the Kuiper Belt also are thought
to wear down comets, rather than cause them to build into larger objects. Although collisions are unlikely in the Oort cloud, Stern says that even Oort Cloud
comets may have experienced collisions early in their history since these icy bodies are thought to have originally formed between Jupiter and the Kuiper Belt and then
to have been thrown out into the Oort Cloud.
Kuiper Belt comets are warmer than those in the Oort cloud that have temperatures as low as -268 degrees Celsius. Most Kuiper Belt comets receive enough
heat from the Sun to boil away low-temperature ices such as carbon monoxide, which may survive at lower temperatures. Even in the Oort Cloud, however, things
can hot up occasionally suggests Stern. When large stars have passed close by our Solar System during the last 4.5 billion years, the lowest-temperature ices may
have been lost from the outer few tens of metres of these bodies.
Stern also points out that Oort Cloud comets, although protected from the Sun by there great distances, are exposed
to the harsh environment of interstellar space. Dust particles in molecular clouds in interstellar clouds encountered
by our Solar System may wear away up to 20 metres of the surface of Oort Cloud comets whilst the Kuiper Belt is protected
from the dust, most of which, is kept out of the Solar System by radiation from the Sun.
Stern's review in Nature highlights that although comets have changed since there formation they are still the
least altered bodies in our Solar System. Comet Space missions such as NASA's Deep Impact and Stardust, and ESA's Rosetta mission will, therefore, provide us
with valuable insights not only into how our Solar System formed, but also how it has interacted with our cosmic environment over the
last 4.5 billion years.
More info: Nature
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