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Since this is a school assignment I believe it should be grammatically correct.

Not really. The complete sentence should not have quote marks, and "coastal" alone is a little vague. The ladybug (is it Linda ?) had it right.

Good grief Marina, what are you writing? Is this a school assignment?

You can say either;
The envelope of air surrounding the earth is at least fifty miles thick.
or
There is an envelope of air surrounding the earth that is at least fifty miles thick.


I believe...