Conference Report: Biosignature Preservation and Detection in Mars Analog Environments

The Conference on Biosignature Preservation and Detection in Mars Analog Environments brought together 90 Mars scientists on May 16–18, 2016, at the Hyatt Regency Lake Tahoe in Incline Village, Nevada. Most of the attendees at the conference came from academia (including an encouraging number of graduate students and postdocs), from several of the NASA centers, and from research institutes. About 10% of the participants were international. The conference was sponsored by LPI, USRA, and NASA.

The purpose of the conference was to discuss the attributes and preservation potential of a range of microbial biosignatures in Mars analog habitable environments on Earth. There has been substantial recent field and laboratory work on these features, including in rocks of widely varying ages. Understanding the properties of these biosignatures on Earth is a key input into discussion of strategies for detection of potential biosignatures on Mars. The conference had 84 submitted abstracts ¹ and several additional invited talks. Most of the abstracts relate to the different types of biosignatures observed in specific terrestrial environments. The conference program was therefore primarily organized around environmental commonalities. Even though a fundamental comparison of Earth versus Mars from a paleobiology perspective was included in the call for abstracts, the response in this area was minor, though impactful. The workshop emphasized discussion time, which was used in four ways: immediately following each talk, a block of time at the end of each session, a conference-level block at the end, and unstructured discussions in association with the poster session.

The five analog environments most prominently discussed in the workshop include (1) hydrothermal spring systems (wherever they intersected the surface), (2) subaqueous environments (including deltas, lakes, playas, and shallow oceanic environments), (3) subaerial environments (all environments where water comes from precipitation, snow melt, or ambient-temperature groundwater), (4) subsurface environments (all those below the active regolith, except those impacted by hydrothermal circulation), and (5) iron-rich systems (where circulating groundwater or hydrothermal systems mobilize iron). A writing committee was established at the beginning of the conference to collect ideas and concepts from the assembled group and prepare a synthesis review (see Hays et al., 2017). Finally, at the conclusion of the workshop, a field trip was led by Dr. Wendy Calvin and Dr. Jack Farmer to the nearby Steamboat Springs geothermal area, an important example of one of the major Mars analog environments discussed.

For biomarkers in these environments, discussion focused on four key questions:

(1) How does the combination of habitability potential and preservation potential combine to create biosignatures in the geological record?

(2) How do we use biosignatures to interpret the presence or absence of life in ancient analog environments?

(3) How might we translate what we learn about preservation of biosignatures in Mars analogs to the different physical conditions and environments on Mars?

(4) How could or should this knowledge influence the strategies and priorities for the astrobiological exploration of Mars?

Based on the content of the abstracts, the presentations that were given, and the various discussion sessions, the following major messages rose to prominence:

• Most of the submitted papers addressed one aspect or another of the biosignatures of microbial life in past subaqueous or hydrothermal environments on Earth. These two paleo-environments are relatively common in Earth's geological record and have been well studied.

• Of the papers submitted to the conference, there was almost no content relating to biosignatures of ancient life associated with the deep biosphere on Earth, although this topic was addressed during the discussions. Even though the habitability of a modern deep biosphere on Earth is clear, research into the biosignature preservation potential of these rocks is in its infancy. Accordingly, the potential for the martian deep subsurface environment to preserve any biosignatures is much less certain than that of other environments under consideration.

• A number of participants emphasized the importance of silica for the preservation of biosignatures. Silica is known to be stable relative to a variety of postdepositional geological processes, including over long geological timescales, and it can be very effective at protecting the objects that it encases.

• Key features of the five environments discussed showed that, in most cases, there is a fundamental mismatch in scale between what can be observed from orbit (e.g., on Mars) and what we have traditionally looked for on the ground (in Mars analogues). On Earth, biosignature investigations typically involve hand lens–scale observation by a human in the field followed by laboratory work, and it will be challenging to translate these lessons to key Mars efforts such as site selection. In addition, this spatial heterogeneity of biomarkers distributed in the environment will also be a factor in the identification and selection of samples to be collected.

• Factors involving timescales are key to considering biosignature preservation and highlighted fundamental differences between the Mars analogs and Mars itself. These factors include how old a particular deposit is, as specific time periods on Mars are more likely to have been habitable than others, whereas Earth has been continuously habitable for at least the past 4 billion years. For similar reasons, the duration of time for which a potential environment was habitable could also affect biosignature preservation. Additionally, how long a preserved deposit has been exposed to the radiation environment on the surface of Mars could have a significant effect on the detectability of organic compound biomarkers.

• Submitted papers that specifically focus on the application of habitability and biosignature preservation lessons from Earth to the different environmental conditions on Mars were not sufficient to generate systematic analysis or consensus positions. Additionally, the habitability and preservation potential of deep subsurface environments were similarly underrepresented. These subjects merit further research and consideration and would be good topics for future conferences.

In the review paper to follow (Hays et al., 2017), 10 participants used the conference discussions to catalyze a postconference review of the issues discussed. This includes a major literature review of prior work on the major biosignature analog environments discussed above, some interpretations as to how these analog studies may be useful in guiding strategic planning for the astrobiological exploration of Mars, and participants' thoughts on urgent needs for research and future research directions.