Xenon
, atomic number 54, is a highly
inert noble gas that can be purified from liquid
air and is found in the Earth’s atmosphere in trace
amounts. It is commercially used in the production
of lamps and laser equipments because of its high
luminous intensity. Although typically transparent
and colourless, xenon will emanate a blue or
green colour within a discharge tube. It was first
discovered by William Ramsay and Mor ris W.
Travers in 1897 and its name stems from the Greek
word for “unknown”.
Despite the wide use of xenon in lamp and
laser-related industries, the element itself is a
scarce resource in comparison to other noble
gases. German scientists have found that the
Earth’s atmosphere has a far lower content of
xenon than the lighter noble gases such as argon
and k r ypton. In addition, the abundance of
xenon in the atmosphere is also lower than that in
meteorites, which is similar in composition to the
formation of rocks on Earth
[1]
. Several hypotheses
exist that attempt to explain the phenomenon of
the ‘missing’ noble gas.
It is believed that xenon had not actually
‘escaped’ from the Earth, but is effectively ‘hidden’
somewhere other than the atmosphere, namely
glaciers, minerals and the Earth’s core. Scientists
suspected that the ‘hiding spot’ is in magnesium
silicate perovskite (MgSO
3
), a major component of
the lower mantle forming the rock layer between
the crust and the core of the Earth. They theorised
that these rocks may have squi r reled xenon
from the atmosphere and trapped it within the
composition of the rocks. Thus, an experiment was
performed in an attempt to replicate this scenario.
Xenon and argon were dissolved in perovskite
under conditions of high temperature and pressure
to mimic the actual conditions of the Earth’s lower
mantle. The results proved disappointing, as argon
was successfully dissolved, but xenon was not,
indicating that this theory is likely to be untrue.
The results for this experiment gave rise to a
different idea and the search for xenon was on.
An examination of the Earth’s history revealed that
the Earth was struck by meteorites 4 billion years
ago, losing much of its primordial atmosphere.
Scientists suggested that during the formation of
the new atmosphere, noble gases such as argon,
which temporarily ‘hid’ in perovskite and were
released back into the atmosphere. Meanwhile,
poorly-dissolved xenon escaped into space. While
difficult to prove, the relative ratio of xenon, argon
and kr ypton in atmosphere content and their
individual solubility in rocks correspond, supporting
this school of thought
[2]
. In addition, the fact that
the lighter isotopes of xenon is more difficult to
find in the atmosphere than the heavier one may
also help explain the hypothesis since the lighter
isotopes are more likely to escape during the loss
of primordial atmosphere than the heavier ones.
Concrete evidence for either of these theories
is non-existent and while the latter appears to
be a more plausible explanation of Earth’s low
xenon levels, there are still many uncertainties. The
theory does not address noble gases resulting from
radioactive decay from heavier elements such as
uranium and plutonium, nor does it consider the
logical paradox in the application of the same
theory on Mars. More research must be done in
order to solve this mind-boggling puzzle.
氙到底在哪裏？
By Yi Qi
戚益
This article may be useful for classes learning about “Planet Earth” based on the DSE Chemistry syllabus.
根據 DSE 化學科目，這篇文章有助學習關於 Planet Earth 的課程
