After having worked as Senior Project Hydrogeologist at Envirogen, a Princeton-based research and consulting firm, for which he investigated subsurface hydrocarbon spills, he became in 1997 adjunct professor at the University of Wisconsin-La Crosse.

In 1998 he joined the University of Texas at El Paso as assistant professor, investigating microbe and chemical transport in groundwater, and microbial interaction in a planetary environment.

Since 2013 he is a professor at Technische Universität Berlin (Germany) and led as Principal Investigator the European Union – funded ERC Advanced Grant project on the “Habitability of Martian Environments” from 2013 to 2019.

[25] With Ian Crawford he proposes that microbial life may have existed temporarily on Earth´s Moon, at a time of major volcanic outgassing about 3.5 billion years ago.

[30] Also, he proposes in a paper with Ian Crawford that the solution to the Fermi Paradox may either be the Zoo Hypothesis (the aliens do not interfere and consider us as a nature preserve or as a developing civilization, not to be meddled with akin to the Prime Directive in Star Trek) or that – as a technologically advanced species – we are (nearly) alone in the Universe.

It has been the subject of TV programs on the BBC, the National Geographic and the Discovery Channel, and of numerous articles in magazines such as New Scientist, The Guardian and Der Spiegel.