4 billion versus 3.5 billion years
Parts of the article say cold for 4 billion years, other parts say 3.5 billion. That's a big difference for astrobiology - Earth probably had life 3.5 billion years ago, and may well have not had life 4 billion years ago. If life started on Mars before it got cold, then there's a chance it could have migrated underground/suvived at localized hot springs.
Maybe someone with a Science subscription can tell us which figure is correct.