“And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. And God called the firmament Heaven. And the evening and the morning were the second day.” (Genesis 1:6-8).
At this point I’m assuming the earth had coalesced into a globe of its present size and shape. As Day 2 begins, the earth was surrounded by a cocoon of water of some sort. This is not unlike a babe being surrounded by a placenta of waters. Of course, the waters surrounding the earth may have been far more extensive than a cocoon. We have no way of knowing, but I like the symbology of the earth being wrapped in a womb of water. As to there being water in outer space, you might want to check out the article,
“Water in Space More Abundant than Expected”.
But what is a firmament? The Hebrew word suggests “an expanse” or “visible arch of the sky.” The Hebrew word for heaven suggests “the sky,” “perhaps alluding to the visible arch in which the clouds move.” It also refers to “the higher ether where the celestial bodies revolve”—basically, the sun and everything you can see in the night sky. Based on the Hebrew, it would appear the firmament is none other than the earth’s atmosphere. (See the NASA photo below.)
Earth’s atomosphere (a NASA photo)