No. Sorry. NASA's Mars announcement, scheduled for 28 September, 15:30, almost certainly has nothing to do with Martians.
However, the scientists set to appear at the briefing do provide a hefty hint at what NASA is set to announce: water.
Participants include Jim Green, director of planetary science; Lujendra Ojha of the Georgia Institute of Technology; Michael Meyer, lead scientist for the Mars Exploration Program; Mary Beth Wilhelm from NASA’s Ames Research Center; and Alfred McEwen, principal investigator for the High Resolution Imaging Science Experiment.
Back in 2011, Ojha and McEwen published a report on "warm-season flows" - dark patterns in the sands of Mars that have been observed to grow from early spring of one Mars year to mid-summer of the following year (seen above). The report proposed that the tendrils might indicate the flow of salt water - though further evidence was required.
"We still don't have a smoking gun for existence of water in RSL, although we're not sure how this process would take place without water," said Ojha.
With both Ojha and McEwen present at Monday's conference, along with figures such as Green and Meyer (they're always involved in big Mars findings), we'd bet all our Moon rocks that NASA has some firm knowledge of the cause of these strange seasonal sand flows.
That, or its aliens. Tune in to the NASA TV stream on Monday to find out.