Sentence:  Part of the time saved was spent on a preliminary estimate for a long-distance plan to free bound oxygen in the sands of <WSD>Mars</WSD> to make the planet more friendly to future human generations . 
Meaning of the word marked with <WSD> tags:  a small reddish planet that is the 4th from the sun and is periodically visible to the naked eye; minerals rich in iron cover its surface and are responsible for its characteristic color