Dr. rer. nat. Petra Rettberg is an outstanding expert in Planetary Protection who works as a Team Leader of the Astrobiology-Group in the Institute of Aerospace Medicine of the
German Aerospace Center (DLR). Dr. Rettberg is a co-I of HABIT and is furthermore and expert on planetary protection that assists the
European Space Agency (ESA) monitoring the bioburden levels of instruments, and spacecraft that are sent to Mars and participating in the training activities that are given to engineers and scientists
involved in space missions. She develops also her own research line about the micro-organisms that may survive the extreme conditions that are relevant for other potentially habitable environments
such as Mars and Europa, to understand the limits of life and habitability. She recently cooperated with the GAS Group in the planetary protection procedures for
As an expert in this field, can you explain to us what is exactly Planetary Protection and why it is important for space exploration?
UN Outer Space Treaty guarantees the freedom to explore outer space. However, this is linked to the responsibility to avoid the harmful contamination of celestial bodies.
Planetary protection has two different goals: We have to make sure that scientific investigations related to the origin, evolution, and distribution of life are not compromised
(“forward planetary protection”). Space missions to extraterrestrial habitable environments such as Mars or the icy moons in the outer solar system offer a unique opportunity to learn more about
the origin of life in a way that is no longer possible on Earth. However, the uncontrolled introduction of terrestrial microorganisms and biochemical compounds and their rediscovery by the actual
mission or by future missions would lead to false-positive results. These results could be misinterpreted as the detection of signs of extraterrestrial life with severe consequences for the direction of
subsequent research activities and unnecessary planetary protection restrictions in the future. In addition, we have to protect the Earth from the potential hazards posed by extraterrestrial
matter carried by spacecraft returning from an interplanetary mission (“backward planetary protection”). This is simple prudence to protect the Earth, even if the probability of hazardous effects is very low.
This approach is in line with the precautionary principle of environmental protection we have on Earth.
What are the main tasks that you develop in Planetary Protection as Team Leader of the Astrobiology-Group in the Institute of Aerospace Medicine?
Contamination by terrestrial microorganisms can only occur if these organisms can survive and replicate on other planets or moons. In my astrobiology group,
we are investigating microorganisms from different environments on Earth which can be described as extreme from a human point of view. We want to find out if different groups of Earth organisms could life on
Mars or in the subsurface oceans of Enceladus and Europa. To do this we are exposing these microorganisms to Mars or Enceladus/Europa-like conditions in the lab and analyze their physiological capability and
the molecular basis of the observed resistances. Examples of our test parameters are ionizing and non-ionising radiation, low pressure, different atmospheres, high salt concentrations, oxidizing compounds,
high and low temperature. We are also contributing to the development of bioburden reduction methods, e.g. the application of cold atmospheric plasma for sterilization of space hardware or the assessment of the
efficiency of antimicrobial surfaces. Besides, we are performing planetary protection measurements for space instruments such as HABIT as well as planetary protection verification assays on behalf
of ESA, just now for the final stages of the assembly, integration, and testing of ExoMars2020.
What aspects would you like to highlight about Planetary Protection in the ExoMars mission?
When the actual work on the HABIT instrument began back in 2016, I visited Omnysys and gave a planetary
protection course to engineers, managers, and students involved in the project. This course consisted of lectures about planetary protection in general and the implementation of planetary protection in the
ExoMars mission in particular. Also, I demonstrated the planetary protection sampling with swabs, wipes and air samplers and explained the
biological assays for sample analysis. After detailed discussions with the whole HABIT team, we concluded that the easiest way to process the samples from the instrument itself
and the BOTTLE salts would be to send them to my lab in Cologne, Germany, for analysis. Later, the knowledge about planetary protection in the
HABIT team was refreshed by a visit of some team members in Cologne, where they also could do some hands-on training in our microbiological labs in addition
to hearing lectures about the planetary protection background, the requirements and the implementation. The preparation of the HABIT BOTTLE
salt samples and their bioburden measurement turned out to be challenging due to their pronounced deliquescence. This property, however, was the reason why these salts have been chosen, namely
to absorb water from the Martian atmosphere. Ideas about how to measure the bioburden of the HABIT BOTTLE salts and how to reduce it was discussed with
the HABIT scientists on different occasions. We analyzed the bioburden in these deliquescent salts, suggested sterile filtration of dissolved salts followed by drying.
We tested the microbial contamination in different treated samples. Finally, the results were satisfying concerning sample preparation as well as planetary protection.
Besides the already mentioned astrobiological research activities to understand the limits of life and to define habitability, I am working on ECSS
(European Cooperation for Space Standardization) standards for planetary protection. I am a member of ESA’s planetary protection working group.
This group gives scientific advice to ESA for the formulation and updating of the planetary protection policy and requirements. Besides,
I am DLR’s representative in the COSPAR planetary protection panel.
The next decade is going to be especially active in planetary space missions. In your opinion what will be the “big questions” that researchers should face related to Planetary Protection? What are your ideas about sample return missions?
Can we detect biosignatures as signs of past or present life on Mars or the icy moons? How can we avoid the unwanted contamination of habitable
environments with adequate measures? To answer these questions the analysis of returned samples is very necessary. Only on Earth, we can use the best available technologies with a variety of
sophisticated large instruments that can not be flown in a space mission.