The file extension MARS marks so-called Acrobat MARS documents. It is the codename of a new format that represents an XML based PDF, also called pdfxml. The Mars format includes additional standards such as SVG, PNG, JPG, JPG2000, OpenType, Xpath and XML within a ZIP-compressed document container.
To create or read a MARS document you need the MARS plugin in addition to Acrobat or Acrobat Reader. Meanwhile, the Mars project was discontinued by Adobe and these plugins are no longer available on the manufacturer side.