The Hellas Find. Fox Meridian, #9

For a long time, humans have observed Mars through telescopes and seen evidence of life, even of complex civilisations. When first machines and then humans actually got to the Red Planet, they discovered a dead rock with a thin, oxygen-free atmosphere, but the hope of discovering some form of life there just would not die. Now a geology team working in the Hellas Planitia region has discovered evidence that there used to be a great civilisation on the dead world.
Or maybe it's not so dead, given that the team who found the antediluvian city have gone missing, leaving behind videos which suggest they were killed by Martians. With the American government worried about public reaction and the possibility of living Martians, Fox Meridian, the world's first and only human infomorph, finds herself being transmitted to Mars to head up the investigation of what she is sure is a hoax.
It must be a hoax, right? There are no Martians. Or are there?