It wasn’t the Martians’ fault their planet died. If they existed – once – Martians were likely microbes, living in a world much like our own, warmed by an atmosphere and crisscrossed by waterways. But Mars began to lose that atmosphere, perhaps because its gravity wasn’t strong enough to hold onto it after an asteroid impact, or perhaps it was gradually blown away by solar winds. The cause is still mysterious, but the ending is clear: Mars’s liquid water dried up or froze into ice caps, leaving life without its most precious resource. Any Martians would have been victims of...