Paradox Interactive is letting players keep Surviving Mars for free on Steam in a very time-limited promotion, and that offer expires today (September 8).Unlike time-limited free trial promotions, anyone who adds Surviving Mars (usually priced at £26.99) to their Steam account during this offer window will be able to keep the game in their library forever.The offer is only for 24 hours, and is set to end on September 8 at 10am PDT / 1pm EDT / 6pm BST.Attention, Survivors!