There are several ways to render a depth pass with Air.  Here are two that work with an existing set of rib files (so you do not need to set up a separate render pass in Massive or export an additional set of files).

Method 1:  Save the standard depth buffer z as the output image.  First, create a small text file with the following commands:

Display "depth#4f.tif" "file" "z"
Option "render" "commands" "-Display"
Option "render" "zrange" [1000 10000]

The zrange option sets the minimum and maximum depth values which correspond to 0 and 1 respectively in the exported z value.  If you wish to save raw depth values, omit the line that sets the zrange option.

Then render from a command shell including the extra file prior to main scene name:

air zdepth.rib scene0001.rib

For a sequence use:

air -frames 1 100 zdepth.rib scene#4f.rib

Method 2:  Render a depth gradient using Air’s depthpass surface shader.  As in method 1, first create a small text file with a few RIB commands:

Display "depth#4f.tif" "file" "r"
Option "render" "commands" "-Display"

Surface "depthpass"
  "mindistance" 1000
  "maxdistance" 10000

Option "render" "commands" "-Surface"

Then include the extra file on the command line:

air depthpass.rib scene0001.rib