Sentence:  Part of the time saved was spent on a preliminary estimate for a long-distance plan to free bound <WSD>oxygen</WSD> in the sands of Mars to make the planet more friendly to future human generations . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust