What is the effect of using imagery to describe the "all-embracing sky”?

It allows readers to remember and “feel” for a moment what an embrace feels like.

Explanation:

The use of imagery to describe the “all-embracing sky” is intentionally done in order to create a vivid picture of what is taken for granted as a common natural scene. It is not only an ordinary picture that is painted here. The personification of the sky as all-embracing gives it a nurturing and protective nature. To embrace may mean to protect or to comfort. The scope of this act of protection and comfort is limitless. The use of “all” may refer to being unconditional – not limited by space, race, color of skin, social and economic status. It goes beyond bounds and the effect is very reassuring. Moreover, it gives some kind of belongingness – that everyone and everything under the sky is united by that embrace.

This imagery has accomplished so much in two words the meaningful connection between nature and humans as well as the ever-protective and caring assurance of the sky above. There is a sense of assurance that everything is under control and life must go on.

"Research suggests that Mars has some features that would support life, but most conditions are too harsh for it." is definitely a main idea as for the second one your best bet is to talk about possible life on mars in the future so that 4th sentence is going to be your friend.