Change 9 to nine. (Even the Associated Press Stylebook says to spell this out rather than using the numeral.)
Add a comma after recon team.
Change 9th to ninth.
Bizarre word choice alert: Why say biosphere instead of surface? (Mars doesn’t have much of a biosphere anyway, and if this part refers to Earth instead of Mars, the sentence needs to say so.) I strongly recommend changing it… and not feeding pepper to the pet thesaurus anymore, because it’s sneezing all over the writing. (Weird, inappropriate, or unnecessarily “fancy” word choices = thesaurus snot, meaning the manuscript looks as if a thesaurus sneezed on it.)
I also recommend getting rid of the ellipsis at the end, because that sentence doesn’t actually trail off, incomplete, and only in book blurbs and such should an ellipsis be used just to
create fake a sense of dramatic tension or whatever.
The first group sent to colonize Mars goes silent after nine days. As part of the recon team, you are sent to find out what’s happened to them.
On the ninth day, a shadow falls over the surface of the planet.