Abstract
Mars Sample Return (MSR) has been a high priority for the planetary science community for more than four decades (for a recent review, see Beaty et al., 2019). Analyzing martian samples in terrestrial laboratories would advance our understanding of Mars in multiple ways that are impossible using in situ missions alone.
The overall MSR concept includes three distinct phases: Selecting and collecting scientifically suitable samples on Mars, currently being carried out by the Mars 2020 mission with the Perseverance rover; Retrieving the samples on Mars and transporting them to Earth; Receiving the samples on Earth, making them available for analysis by the science community for decades to come.
With the recent successful collection of the first samples by the Perseverance rover and the ongoing progress by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) on the development of the missions that could retrieve and transport the samples to Earth, MSR continues to move closer to becoming a reality. As Perseverance and the MSR retrieval and return missions progress, it becomes increasingly imperative to develop detailed plans for the receipt and analysis of the samples on Earth to ensure that the full science potential of MSR can be realized.
Although the topic of post-receipt sample containment, curation, distribution, and analysis has been discussed for about two decades, there has been a need to review, update, and refine these plans. Beginning in 2017, NASA and ESA have engaged in several joint planning activities related to the science of MSR. This has resulted in an updated analysis of the potential science objectives of MSR by the International MSR Objectives and Samples Team (iMOST) (Beaty et al., 2019). Subsequent planning by the first MSR Science Planning Group (MSPG) in 2019 resulted in a set of committee and workshop reports related to MSR science management, scientific analyses that would need to be carried out inside biocontainment at a Sample Receiving Facility (SRF), and contamination control considerations for the MSR samples (MSPG 2019a,b,c).
Most recently, the MSR Science Planning Group 2 (MSPG2) was established to build on the foundation provided by iMOST and MSPG. MSPG2, an international planning team selected through an open competition, formally began its work in June, 2020, under terms of reference provided by NASA and ESA. The committee had 31 members and was led by Dr. Gerhard Kminek (ESA) and Dr. Michael Meyer (NASA). Final results were reported in June, 2021, and consisted of six topical reports, described below, along with an overall integrated summary. The MSPG2 products encompass previous deliberations and represent an up-to-date status of MSR science planning.
During approximately the same time period that MSPG2 was active, three other working groups were also preparing reports related to MSR:
Sample Safety Assessment Protocol-Working Group (SSAP-WG): SSAP-WG was established by the Committee on Space Research (COSPAR) President in response to a recommendation of the International Mars Architecture for the Return of Samples (iMars Phase II) Working Group (Haltigin et al., 2017). The mandate of the SSAP-WG was to develop a protocol to assess whether there are indications of martian life in any martian material, or spacecraft hardware exposed to martian material, and whether this would constitute a hazard to Earth while maintaining the scientific integrity of the overall material from Mars to the maximum extent possible. The working group started their work in September 2018 and completed the Sample Safety Assessment Framework (SSAF) report in the fourth quarter of 2021.
Atmospheric Sample Tiger Team (ATT): ATT was assembled in December, 2020, to provide input to formulation of a requirement for a sample of the martian atmosphere to be collected and transported to Earth as part of MSR. Output was reported in the form of an internal report (January, 2021) that was further expanded to include other implementation possibilities and finalize the recommendations for a martian atmospheric sample.
Orbiting Sample Tiger Team (OSTT): OSTT was assembled in February, 2021, to inform the planetary protection categorization for the Orbiting Sample container (OS) that is designed to hold the samples during their journey from Mars to Earth.
The purpose of this special issue of Astrobiology is to collate and publish these scientific and planning reports for MSR in a single location and make them available to the community of scientists interested in MSR. The components of this special issue are summarized below.
The final report is an integrated summary of the MSPG2 activities and reports, including a notional timeline of key decision points that reflect the timelines of the flight elements (Mars 2020, Earth Return Orbiter (ERO), Sample Retrieval Lander (SRL)), the SRF, the National Environmental Policy Act (NEPA) process, and MSR science management.
A fundamental premise of the MSR Campaign is that the scientific benefit and discoveries are meant to be shared between the MSR Partners and the world's scientific community. Because there are so many scientific elements that must be executed to achieve the full science potential of the MSR Campaign, significant coordination is required. It is thus critical to ensure that the appropriate planning, resources, management, and oversight are available. This report outlines the rationale for an overarching MSR Science Program and proposes implementations for achieving the necessary functionalities of such a program.
All material collected from Mars (including gases, dust, rock, regolith) would need to be carefully handled, stored, and analyzed following Earth return to minimize the alteration or degradation that could occur on Earth and to optimize the scientific measurements that can be done on the samples now and into the future. This includes performing a set of initial sample characterization activities in order to generate a sample catalog so that appropriate materials can be allocated for specific science investigations. This report defines a set of curation-related activities that would need to take place inside an SRF, including the expected analytical capabilities that would be necessary to execute these activities successfully.
It is certain that the process of breaking the sample tube seals, extracting the headspace gas, and exposing the solid samples to non-Mars atmosphere would perturb local equilibrium conditions between gas and solid sample material and set in motion irreversible exchange processes that proceed as a function of time. There is a substantial amount of uncertainty related to the timeline for release of samples from the SRF. Determining whether samples are safe for release from biocontainment, which may involve detailed analysis and/or sterilization, is expected to take several months per sample and longer for the full collection. The purpose of this report is to understand the sample properties that are vulnerable to time-dependent alteration as well as their characteristic time scales. It will be necessary to plan for measuring these properties before the scientific information is irretrievably lost.
Potential biosignatures and associated indicators of ancient environmental conditions in the returned Mars samples would be characterized by laboratory measurements of many different properties using multiple instruments. Subsamples determined to be free of biohazards before or after sterilization could be released to state-of-the-art laboratories outside biocontainment. Some of the scientific measurements can be made on sterilized subsamples, while others cannot. Sterilization changes some sample properties such that the measurement no longer represents the sample's intrinsic paleoenvironmental and biosignature properties as they were before the sample arrived on Earth. This report assesses how the diverse consequences of sterilization affect the scientific usefulness of different subsamples for various measurements that support the science goals of the MSR Campaign, and the capabilities that would be necessary in the SRF to successfully accomplish high-quality sample-science investigations that are time-critical and incompatible with sterilization.
There are several high-priority science questions that can be answered with a sample of martian atmosphere, including how the martian atmosphere has evolved over time. In addition, it is essential to establish a baseline for solid-gas interactions that affect the rock samples. Given the design of the Mars 2020 sampling system, all of the sample tubes that contain rock cores will inevitably have some quantity of martian atmospheric gas in the headspace over the solid material. There would be additional value in taking a dedicated atmospheric sample that does not also have rock material in the volume, since there will be interactions between the rock samples and the headspace gas that alters the composition of the core samples and the gas. This report addresses the science impact of various possible implementations for collection of a dedicated sample of the martian atmosphere.
Dust that is lifted into the martian atmosphere is of high interest to martian atmospheric scientists, as well as geologists, astrobiologists, and planners for future human missions. MSR, as it is presently envisioned, will have a number of surfaces that will be exposed to the martian environment and will serve as vehicles for the serendipitous collection and transport of dust that falls out of the atmosphere by means of airfall sedimentation. Foremost among these is the exterior of the rock sample tubes, which may lie on the martian surface for multiple years before retrieval. This report contains an analysis of the potential quantities of this dust and possible scientific use once received on Earth.
The SSAP-WG's report, titled the Sample Safety Assessment Framework (SSAF), has four elements, each of which is necessary but, on its own, not sufficient to qualify for a safety assessment. The four elements are: (1) Application of the Bayesian statistics; (2) Informed sub-sampling strategy; (3) Use of a test sequence to search for evidence of life in the samples; (4) Decision criteria. The SSAF has been established with sufficient detail to allow proper planning for the Sample Receiving Facility (SRF) and preparing for the scientific analysis of the samples. Once any open gaps with respect to the test sequence are closed, the SSAF can be transformed into a Sample Safety Assessment Protocol (SSAP) with the identified quality control measures, tested on analogue material and applied to the samples from Mars.
The most important single element of the ground portion of MSR is the Sample Receiving Facility (SRF). The SRF would need to be designed and equipped to enable each of the following: the receipt of the returned spacecraft; extracting and opening of the sealed sample container; extracting the samples from the sample tubes; and a set of evaluations and analyses of the samples—all under strict protocols of biocontainment, cleanliness, and contamination control. One key open question for planning the SRF relates to the minimum size and cost needed to achieve its performance requirements. This, in turn, naturally leads to the question—what are those requirements? This report presents an integration of SRF functionalities and potential requirements implied by the previous reports in this issue.
To accomplish the delivery of the MSR samples to Earth, the “Orbiting Sample” container (OS) would be sent to Mars to accommodate the collected samples. Once the OS arrives at Earth, the samples would be removed to be examined in the SRF. The MSR OS “Tiger Team” (OSTT) was convened to determine the appropriate cleanliness level options of the interior of the OS. The team's remit was primarily focused on deciding on the trade-offs between Planetary Protection cleanliness levels IVa and IVb (COSPAR, 2021). These cleanliness levels are determined by COSPAR planetary protection regulations, where IVa requires an average of <300 bacterial spores/m2 and <3 x 105 bacterial spores on the landed system, and IVb mandates the more stringent requirement of <30 bacterial spores on the landed system. This report documents the consensus opinion submitted by the OSTT that recommended the interior of the OS be cleaned to a IVa requirement with any feasible added effort toward IVb, and it provides the rationale for that recommendation.
Footnotes
Acknowledgments
We would like to acknowledge the dedication and efforts of all scientists that worked on the various reports represented in this special issue during a global pandemic. We also would like to acknowledge the support of the Astrobiology Editor-in-Chief, Sherry L. Cady, for making this special issue a reality. The decision to implement Mars Sample Return will not be finalized until NASA's completion of the National Environmental Policy Act (NEPA) process. This document is being made available for planning and information purposes only. A portion of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).