Thermal metamorphism of CM chondrites: A dehydroxylation-based peak-temperature thermometer and implications for sample return from asteroids Ryugu and Bennu

Planning Implications Related to Sterilization-Sensitive Science Investigations Associated with Mars Sample Return (MSR)

The NASA/ESA Mars Sample Return (MSR) Campaign seeks to establish whether life on Mars existed where and when environmental conditions allowed. Laboratory measurements on the returned samples are useful if what is measured is evidence of phenomena on Mars rather than of the effects of sterilization conditions. This report establishes that there are categories of measurements that can be fruitful despite sample sterilization and other categories that cannot. Sterilization kills living microorganisms and inactivates complex biological structures by breaking chemical bonds. Sterilization has similar effects on chemical bonds in non-biological compounds, including abiotic or pre-biotic reduced carbon compounds, hydrous minerals, and hydrous amorphous solids. We considered the sterilization effects of applying dry heat under two specific temperature-time regimes and the effects of γ-irradiation. Many measurements of volatile-rich materials are sterilization sensitive-they will be compromised by either dehydration or radiolysis upon sterilization. Dry-heat sterilization and γ-irradiation differ somewhat in their effects but affect the same chemical elements. Sterilization-sensitive measurements include the abundances and oxidation-reduction (redox) states of redox-sensitive elements, and isotope abundances and ratios of most of them. All organic molecules, and most minerals and naturally occurring amorphous materials that formed under habitable conditions, contain at least one redox-sensitive element. Thus, sterilization-sensitive evidence about ancient life on Mars and its relationship to its ancient environment will be severely compromised if the samples collected by Mars 2020 rover Perseverance cannot be analyzed in an unsterilized condition. To ensure that sterilization-sensitive measurements can be made even on samples deemed unsafe for unsterilized release from containment, contingency instruments in addition to those required for curation, time-sensitive science, and the Sample Safety Assessment Protocol would need to be added to the Sample Receiving Facility (SRF). Targeted investigations using analogs of MSR Campaign-relevant returned-sample types should be undertaken to fill knowledge gaps about sterilization effects on important scientific measurements, especially if the sterilization regimens eventually chosen are different from those considered in this report. Executive Summary A high priority of the planned NASA/ESA Mars Sample Return Campaign is to establish whether life on Mars exists or existed where and when allowed by paleoenvironmental conditions. To answer these questions from analyses of the returned samples would require measurement of many different properties and characteristics by multiple and diverse instruments. Planetary Protection requirements may determine that unsterilized subsamples cannot be safely released to non-Biosafety Level-4 (BSL-4) terrestrial laboratories. Consequently, it is necessary to determine what, if any, are the negative effects that sterilization might have on sample integrity, specifically the fidelity of the subsample properties that are to be measured. Sample properties that do not survive sterilization intact should be measured on unsterilized subsamples, and the Sample Receiving Facility (SRF) should support such measurements. This report considers the effects that sterilization of subsamples might have on the science goals of the MSR Campaign. It assesses how the consequences of sterilization affect the scientific usefulness of the subsamples and hence our ability to conduct high-quality science investigations. We consider the sterilization effects of (a) the application of dry heat under two temperature-time regimes (180°C for 3 hours; 250°C for 30 min) and (b) γ-irradiation (1 MGy), as provided to us by the NASA and ESA Planetary Protection Officers (PPOs). Measurements of many properties of volatile-rich materials are sterilization sensitive-they would be compromised by application of either sterilization mode to the subsample. Such materials include organic molecules, hydrous minerals (crystalline solids), and hydrous amorphous (non-crystalline) solids. Either proposed sterilization method would modify the abundances, isotopes, or oxidation-reduction (redox) states of the six most abundant chemical elements in biological molecules (i.e., carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur, CHNOPS), and of other key redox-sensitive elements that include iron (Fe), other first-row transition elements (FRTE), and cerium (Ce). As a result of these modifications, such evidence of Mars' life, paleoenvironmental history, potential habitability, and potential biosignatures would be corrupted or destroyed. Modifications of the abundances of some noble gases in samples heated during sterilization would also reset scientifically important radioisotope geochronometers and atmospheric-evolution measurements. Sterilization is designed to render terminally inactive (kill) all living microorganisms and inactivate complex biological structures (including bacterial spores, viruses, and prions). Sterilization processes do so by breaking certain pre-sterilization chemical bonds (including strong C-C, C-O, C-N, and C-H bonds of predominantly covalent character, as well as weaker hydrogen and van der Waals bonds) and forming different bonds and compounds, disabling the biological function of the pre-sterilization chemical compound. The group finds the following: No sterilization process could destroy the viability of cells whilst still retaining molecular structures completely intact. This applies not only to the organic molecules of living organisms, but also to most organic molecular biosignatures of former life (molecular fossils). As a matter of biological principle, any sterilization process would result in the loss of biological and paleobiological information, because this is the mechanism by which sterilization is achieved. Thus, almost all life science investigations would be compromised by sterilizing the subsample by either mode. Sterilization by dry heat at the proposed temperatures would lead to changes in many of the minerals and amorphous solids that are most significant for the study of paleoenvironments, habitability, potential biosignatures, and the geologic context of life-science observations. Gamma-(γ-)irradiation at even sub-MGy doses induces radiolysis of water. The radiolysis products (e.g., free radicals) react with redox-sensitive chemical species of interest for the study of paleoenvironments, habitability, and potential biosignatures, thereby adversely affecting measurements of those species. Heat sterilization and radiation also have a negative effect on CHNOPS and redox-sensitive elements. MSPG2 was unable to identify with confidence any measurement of abundances or oxidation-reduction states of CHNOPS elements, other redox-sensitive elements (e.g., Fe and other FRTE; Ce), or their isotopes that would be affected by only one, but not both, of the considered sterilization methods. Measurements of many attributes of volatile-rich subsamples are sterilization sensitive to both heat and γ-irradiation. Such a measurement is not useful to Mars science if what remains in the subsample is evidence of sterilization conditions and effects instead of evidence of conditions on Mars. Most measurements relating to the detection of evidence for extant or extinct life are sterilization sensitive. Many measurements other than those for life-science seek to retrieve Mars' paleoenvironmental information from the abundances or oxidation-reduction states of CHNOPS elements, other redox-sensitive elements, or their isotopes (and some noble gases) in returned samples. Such measurements inform scientific interpretations of (paleo)atmosphere composition and evolution, (paleo)surface water origin and chemical evolution, potential (paleo)habitability, (paleo)groundwater-porewater solute chemistry, origin and evolution, potential biosignature preservation, metabolic element or isotope fractionation, and the geologic, geochronological, and geomorphic context of life-sciences observations. Most such measurements are also sterilization sensitive. The sterilization-sensitive attributes cannot be meaningfully measured in any such subsample that has been sterilized by heat or γ-irradiation. Unless such subsamples are deemed biohazard-safe for release to external laboratories in unsterilized form, all such measurements must be made on unsterilized samples in biocontainment. An SRF should have the capability to carry out scientific investigations that are sterilization-sensitive to both PPO-provided sterilization methods (Figure SE1). The following findings have been recognized in the Report. Full explanations of the background, scope, and justification precede the presentation of each Finding in the Section identified for that Finding. One or more Findings follow our assessment of previous work on the effects of each provided sterilization method on each of three broad categories of measurement types-biosignatures of extant or ancient life, geological evidence of paleoenvironmental conditions, and gases. Findings are designated Major if they explicitly refer to both PPO-provided sterilization methods or have specific implications for the functionalities that need to be supported within an SRF. FINDING SS-1: More than half of the measurements described by iMOST for investigation into the presence of (mostly molecular) biosignatures (iMOST Objectives 2.1, 2.2 and 2.3) in returned martian samples are sterilization-sensitive and therefore cannot be performed with acceptable analytical precision or sensitivity on subsamples sterilized either by heat or by γ-irradiation at the sterilization parameters supplied to MSPG2. That proportion rises to 86% of the measurements specific to the investigation of extant or recent life (iMOST Objective 2.3) (see Section 2.5). This Finding supersedes Finding #4 of the MSPG Science in Containment report (MSPG, 2019). FINDING SS-2: Almost three quarters (115 out of 160; 72%) of the measurements described by iMOST for science investigations not associated with Objective 2 but associated with Objectives concerning geological phenomena that include past interactions with the hydrosphere (Objectives 1 and 3) and the atmosphere (Objective 4) are sterilization-tolerant and therefore can (generally) be performed with acceptable analytical precision or sensitivity on subsamples sterilized either by heat or by γ-irradiation at the sterilization parameters supplied to MSPG2 (see Section 2.5). This Finding supports Finding #6 of the MSPG Science in Containment report (MSPG, 2019). MSPG2 endorses the previously proposed strategy of conducting as many measurements as possible outside the SRF where the option exists. FINDING SS-3: Suggested strategies for investigating the potential for extant life in returned martian samples lie in understanding biosignatures and, more importantly, the presence of nucleic acid structures (DNA/RNA) and possible agnostic functionally similar information-bearing polymers. A crucial observation is that exposure of microorganisms to temperatures associated with sterilization above those typical of a habitable surface or subsurface environment results in a loss of biological information. If extant life is a target for subsample analysis, sterilization of material via dry heat would likely compromise any such analysis (see Section 3.2). FINDING SS-4: Suggested strategies for investigating the potential for extant life in returned martian samples lie in understanding biosignatures, including the presence of nucleic acid structures (DNA/RNA) and possible agnostic functionally similar information-bearing polymers. A crucial observation is that exposure of microorganisms to γ-radiation results in a loss of biological information through molecular damage and/or destruction. If extant life is a target for subsample analysis, sterilization of material via γ-radiation would likely compromise any such analysis (see Section 3.3). FINDING SS-5: Suggested strategies for investigating biomolecules in returned martian samples lie in detection of a variety of complex molecules, including peptides, proteins, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), as well as compounds associated with cell membranes such as lipids, sterols, and fatty acids and their geologically stable reaction products (hopanes, steranes, etc.) and possible agnostic functionally similar information-bearing polymers. Exposure to temperatures above MSR Campaign-Level Requirements for sample temperature, up to and including sterilization temperatures, results in a loss of biological information. If the presence of biosignatures is a target for subsample analysis, sterilization of material via dry heat would likely compromise any such analysis (see Section 4.2). FINDING SS-6: Suggested strategies for investigating biomolecules in returned martian samples lie in detection of a variety of complex molecules, including peptides, proteins, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), and compounds associated with cell membranes such as lipids, sterols and fatty acids and their geologically stable reaction products (hopanes, steranes, etc.) and possible agnostic functionally similar information-bearing polymers. Exposure to radiation results in a loss of biological information. If the presence of biosignatures is a target for subsample analysis, sterilization of material via γ-irradiation would likely compromise any such analysis (see Section 4.3). [Figure: see text] MAJOR FINDING SS-7: The use of heat or γ-irradiation sterilization should be avoided for subsamples intended to be used for organic biosignature investigations (for extinct or extant life). Studies of organic molecules from extinct or extant life (either indigenous or contaminants, viable or dead cells) or even some organic molecules derived from abiotic chemistry cannot credibly be done on subsamples that have been sterilized by any means. The concentrations of amino acids and other reduced organic biosignatures in the returned martian samples may also be so low that additional heat and/or γ-irradiation sterilization would reduce their concentrations to undetectable levels. It is a very high priority that these experiments be done on unsterilized subsamples inside containment (see Section 4.4). FINDING SS-8: Solvent extraction and acid hydrolysis at ∼100°C of unsterilized martian samples will inactivate any biopolymers in the extract and would not require additional heat or radiation treatment for the subsamples to be rendered sterile. Hydrolyzed extracts should be safe for analysis of soluble free organic molecules outside containment and may provide useful information about their origin for biohazard assessments; this type of approach, if approved, is strongly preferred and endorsed (see Section 4.4). FINDING SS-9: Minerals and amorphous materials formed by low temperature processes on Mars are highly sensitive to thermal alteration, which leads to irreversible changes in composition and/or structure when heated. Exposure to temperatures above MSR Campaign-Level Requirements for sample temperature, up to and including sterilization temperatures, has the potential to alter them from their as-received state. Sterilization by dry heat at the proposed sterilization temperatures would lead to changes in many of the minerals that are most significant for the study of paleoenvironments, habitability, and potential biosignatures or biosignature hosts. It is crucial that the returned samples are not heated to temperatures above which mineral transitions occur (see Section 5.3). FINDING SS-10: Crystal structure, major and non-volatile minor element abundances, and stoichiometric compositions of minerals are unaffected by γ-irradiation of up to 0.3-1 MGy, but crystal structures are completely destroyed at 130 MGy. Measurements of these specific properties cannot be acquired from subsamples γ-irradiated at the notional 1 MGy dose-they are sterilization-sensitive (see Section 5.4). FINDING SS-11: Sterilization by γ-irradiation (even at sub-MGy doses) results in significant changes to the redox state of elements bound within a mineral lattice. Redox-sensitive elements include Fe and other first-row transition elements (FRTE) as well as C, H, N, O, P and S. Almost all minerals and naturally occurring amorphous materials that formed under habitable conditions, including the ambient paleotemperatures of Mars' surface or shallow subsurface, contain at least one of these redox-sensitive elements. Therefore, measurements and investigations of the listed properties of such geological materials are sterilization sensitive and should not be performed on γ-irradiated subsamples (see Section 5.4). FINDING SS-12: A significant fraction of investigations that focus on high-temperature magmatic and impact-related processes, their chronology, and the chronology of Mars' geophysical evolution are sterilization-tolerant. While there may be a few analyses involved in such investigations that could be affected to some degree by heat sterilization, most of these analyses would not be affected by sterilization involving γ-irradiation (see Section 5.6). MAJOR FINDING SS-13: Scientific investigations of materials containing hydrous or otherwise volatile-rich minerals and/or X-ray amorphous materials that formed or were naturally modified at low (Mars surface-/near-surface) temperature are sterilization-sensitive in that they would be compromised by changes in the abundances, redox states, and isotopes of CHNOPS and other volatiles (e.g., noble gases for chronometry), FRTE, and Ce, and cannot be performed on subsamples that have been sterilized by either dry heat or γ-irradiation (see Section 5.7). MAJOR FINDING SS-14: It would be far preferable to work on sterilized gas samples outside of containment, if the technical issues can all be worked out, than to build and operate a large gas chemistry laboratory inside containment. Depending on their reactivity (or inertness), gases extracted from sample tubes could be sterilized by dry heat or γ-irradiation and analyzed outside containment. Alternatively, gas samples could be filtered through an inert grid and the filtered gas analyzed outside containment (see Section 6.5). MAJOR FINDING SS-15: It is fundamental to the campaign-level science objectives of the Mars Sample Return Campaign that the SRF support characterization of samples returned from Mars that contain organic matter and/or minerals formed under habitable conditions that include the ambient paleotemperatures of Mars' surface or subsurface (<∼200°C)-such as most clays, sulfates, and carbonates-in laboratories on Earth in their as-received-at-the-SRF condition (see Section 7.1). MAJOR FINDING SS-16: The search for any category of potential biosignature would be adversely affected by either of the proposed sterilization methods (see Section 7.1). MAJOR FINDING SS-17: Carbon, hydrogen, nitrogen, oxygen, sulfur, phosphorus, and other volatiles would be released from a subsample during the sterilization step. The heat and γ-ray sterilization chambers should be able to monitor weight loss from the subsample during sterilization. Any gases produced in the sample headspace and sterilization chamber during sterilization should be captured and contained for future analyses of the chemical and stable isotopic compositions of the evolved elements and compounds for all sterilized subsamples to characterize and document fully any sterilization-induced alteration and thereby recover some important information that would otherwise be lost (see Section 7.2). This report shows that most of the sterilization-sensitive iMOST measurement types are among either the iMOST objectives for life detection and life characterization (half or more of the measurements for life-science sub-objectives are critically sterilization sensitive) or the iMOST objectives for inferring paleoenvironments, habitability, preservation of potential biosignatures, and the geologic context of life-science observations (nearly half of the measurements for sub-objectives involving geological environments, habitability, potential biosignature preservation, and gases/volatiles are critically sterilization sensitive) (Table 2; see Beaty et al., 2019 for the full lists of iMOST objectives, goals, investigations, and sample measurement types). Sterilization-sensitive science about ancient life on Mars and its relationship to its ancient environment will be severely impaired or lost if the samples collected by Perseverance cannot be analyzed in an unsterilized condition. Summary: ○The SRF should have the capability to carry out or otherwise support scientific investigations that are sensitive to both PPO-provided sterilization methods. ○Measurements of most life-sciences and habitability-related (paleoenvironmental) phenomena are sensitive to both PPO-provided sterilization modes. (Major Finding SS-7, SS-15, SS-16 and Finding SS-1, SS-3, SS-4, SS-5, SS-6, SS-9, SS-11, SS-13) If subsamples for sterilization-sensitive measurement cannot be deemed safe for release, then additional contingency analytical capabilities are needed in the SRF to complete MSR Campaign measurements of sterilization-sensitive sample properties on unsterilized samples in containment (Figure SE1, below). ○Measurements of high-temperature (low-volatile) phenomena are tolerant of both PPO-provided sterilization modes (Finding SS-12). Subsamples for such measurements may be sterilized and released to laboratories outside containment without compromising the scientific value of the measurements. ○Capturing, transporting, and analyzing gases is important and will require careful design of apparatus. Doing so for volatiles present as headspace gases and a dedicated atmosphere sample will enable important atmospheric science (Major Finding SS-14). Similarly, capturing and analyzing gases evolved during subsample sterilization (i.e., gas from the sterilization chamber) would compensate for some sterilization-induced loss of science data from volatile-rich solid (geological) subsamples (Finding SS-14, SS-17; other options incl. SS-8).

Spectrally blue hydrated parent body of asteroid (162173) Ryugu

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0-3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu's parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570-670 K (300-400 °C), suggesting that Ryugu's parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2-2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.