Space station experiment suggests Mars rovers will need to dig deep to find life

It seems to me that the depth of any currently surviving biomarkers of past life on Mars may be much more than a few millimeters, or even six feet deep in Martian "soil" because that soil has had maybe a few billion years to get moved around by atmospheric winds and maybe marsquakes and volcanic events, not to mention meteorite impacts.

I would think that the best chance to find ancient biomarkers, and even current life, would be in some wet environments located well underground, even if they freeze and thaw annually.