I think you mean earth's protective shield. The ozone gas forms
it.
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The earth's protective shield is formed by one of the molecules
of the oxygen. It is ozone.
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Aegis
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Ozone acted as a miracle shield. It is a protective layer.
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The gas which help in forming the earth's protective shield is
ozone gas. It is present in the atmosphere as the ozone layer in
the stratospheric regionof atmosphere.