Interstellar ^{60}Fe on the Surface of the Moon

A possible direct exposure of the Earth to the cold dense interstellar medium 2-3 Myr ago

Cold, dense clouds in the interstellar medium of our Galaxy are 4-5 orders of magnitude denser than their diffuse counterparts. Our Solar System has most likely encountered at least one of these dense clouds during its lifetime. However, evidence for such an encounter has not been studied in detail yet. Here we derive the velocity field of the Local Ribbon of Cold Clouds (LRCC) by modelling the 21 cm data from the HI4PI survey, finding that the Solar System may have passed through the LRCC in the constellation Lynx 2-3 million years ago. Using a state-of-the-art simulation of the heliosphere, we show that during the passage, the heliosphere shrinks to a scale of 0.22 au, smaller than the Earth's orbit around the Sun. This would have put the Earth in direct contact with the dense interstellar medium for a period of time and exposed it to a neutral hydrogen density above 3,000 cm-3. Such a scenario agrees with geological evidence from 60Fe and 244Pu isotopes. The encounter and related increased radiation from Galactic cosmic rays might have had a substantial impact on the Earth's system and climate.

Future Exploration of the Outer Heliosphere and Very Local Interstellar Medium by Interstellar Probe

A detailed overview of the knowledge gaps in our understanding of the heliospheric interaction with the largely unexplored Very Local Interstellar Medium (VLISM) are provided along with predictions of with the scientific discoveries that await. The new measurements required to make progress in this expanding frontier of space physics are discussed and include in-situ plasma and pick-up ion measurements throughout the heliosheath, direct sampling of the VLISM properties such as elemental and isotopic composition, densities, flows, and temperatures of neutral gas, dust and plasma, and remote energetic neutral atom (ENA) and Lyman-alpha (LYA) imaging from vantage points that can uniquely discern the heliospheric shape and bring new information on the interaction with interstellar hydrogen. The implementation of a pragmatic Interstellar Probe mission with a nominal design life to reach 375 Astronomical Units (au) with likely operation out to 550 au are reported as a result of a 4-year NASA funded mission study.

The New Astronomical Frontier of Interstellar Objects

The upcoming commencement of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will greatly enhance the discovery rate of interstellar objects (ISOs). 'Oumuamua and Borisov were the first two ISOs confirmed in the Solar System, although the first interstellar meteor was detected earlier. We explore the intriguing mass budget of ejected planetesimals implied by the detections of 'Oumuamua and Borisov and explore the expected abundance of ISOs as a function of size in the solar neighborhood. Specifically, we find that a significant fraction of stellar mass must go toward producing ISOs and that ISOs outnumber Solar System objects in the Oort cloud. We consider signatures of ISOs colliding with Earth, the Moon, and neutron stars, as well as the possibility of differentiating ISOs from Solar System objects in stellar occultation surveys, and we show that these methods are observationally feasible. We introduce a test for dynamical anisotropy that is capable of determining the typical ejection speed of ISOs from their parent stars. Finally, we predict a new population of dynamically distinct ISOs originating from stars in the Galactic halo. One of the two branches of the newly established Galileo Project¹ seeks to learn more about the nature of ISOs like 'Oumuamua by performing new searches and designing follow-up observations.

How accurate are half-life data of long-lived radionuclides?

We have consulted existing half-life data available in Nuclear Data Sheets for radionuclides with Z < 89 in the range between 30 and 108 years with emphasis on their uncertainty. Based on this dataset, we have highlighted the lack of reliable data by giving examples for nuclides relevant for astrophysical, environmental and nuclear research. It is shown that half-lives for a substantial number of nuclides require a re-determination since existing data are either based on one single measurement, are contradictory or are associated with uncertainties above 5%.

The Dyadic Radionuclide System 60Fe / 53Mn to Distinguish Interstellar from Interplanetary 60Fe

The discovery of live 60Fe in a deep-sea crust with proposed interstellar origin followed by evidence for elevated interplanetary 3He in the same crust raised the question on how to unambiguously identify the true production site of the identified 60Fe. Here, we show the implementation of the dyadic radionuclide system 60Fe / 53Mn to serve as a tool for the identification of surplus interstellar 60Fe over interplanetary production. The recent updates in experimental 60Fe and 53Mn data from iron meteorites as well as in production rate models confirm the validity and robustness of this dyadic system for future applications.

60Fe and 244Pu deposited on Earth constrain the r-process yields of recent nearby supernovae

Half of the chemical elements heavier than iron are produced by the rapid neutron capture process (r-process). The sites and yields of this process are disputed, with candidates including some types of supernovae (SNe) and mergers of neutron stars. We search for two isotopic signatures in a sample of Pacific Ocean crust-iron-60 (⁶⁰Fe) (half-life, 2.6 million years), which is predominantly produced in massive stars and ejected in supernova explosions, and plutonium-244 (²⁴⁴Pu) (half-life, 80.6 million years), which is produced solely in r-process events. We detect two distinct influxes of ⁶⁰Fe to Earth in the last 10 million years and accompanying lower quantities of ²⁴⁴Pu. The ²⁴⁴Pu/⁶⁰Fe influx ratios are similar for both events. The ²⁴⁴Pu influx is lower than expected if SNe dominate r-process nucleosynthesis, which implies some contribution from other sources.

The lunar surface as a recorder of astrophysical processes

The lunar surface has been exposed to the space environment for billions of years and during this time has accumulated records of a wide range of astrophysical phenomena. These include solar wind particles and the cosmogenic products of solar particle events which preserve a record of the past evolution of the Sun, and cosmogenic nuclides produced by high-energy galactic cosmic rays which potentially record the galactic environment of the Solar System through time. The lunar surface may also have accreted material from the local interstellar medium, including supernova ejecta and material from interstellar clouds encountered by the Solar System in the past. Owing to the Moon's relatively low level of geological activity, absence of an atmosphere, and, for much of its history, lack of a magnetic field, the lunar surface is ideally suited to collect these astronomical records. Moreover, the Moon exhibits geological processes able to bury and thus both preserve and 'time-stamp' these records, although gaining access to them is likely to require a significant scientific infrastructure on the lunar surface. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.

Inference of the Local Interstellar Spectra of Cosmic-Ray Nuclei Z ⩽ 28 with the GalProp-HelMod Framework

Composition and spectra of Galactic cosmic rays (CRs) are vital for studies of high-energy processes in a variety of environments and on different scales, for interpretation of γ-ray and microwave observations, for disentangling possible signatures of new phenomena, and for understanding of our local Galactic neighborhood. Since its launch, AMS-02 has delivered outstanding-quality measurements of the spectra of p ¯ , e ±, and nuclei: 1H-8O, 10Ne, 12Mg, 14Si. These measurements resulted in a number of breakthroughs; however, spectra of heavier nuclei and especially low-abundance nuclei are not expected until later in the mission. Meanwhile, a comparison of published AMS-02 results with earlier data from HEAO-3-C2 indicates that HEAO-3-C2 data may be affected by undocumented systematic errors. Utilizing such data to compensate for the lack of AMS-02 measurements could result in significant errors. In this paper we show that a fraction of HEAO-3-C2 data match available AMS-02 measurements quite well and can be used together with Voyager 1 and ACE-CRIS data to make predictions for the local interstellar spectra (LIS) of nuclei that are not yet released by AMS-02. We are also updating our already-published LIS to provide a complete set from 1H-28Ni in the energy range from 1 MeV nucleon-1 to ~100-500 TeV nucleon-1, thus covering 8-9 orders of magnitude in energy. Our calculations employ the GalProp-HelMod framework, which has proved to be a reliable tool in deriving the LIS of CR p ¯ , e -, and nuclei 1H-8O.

60Fe deposition during the late Pleistocene and the Holocene echoes past supernova activity

Nuclides synthesized in massive stars are ejected into space via stellar winds and supernova explosions. The solar system (SS) moves through the interstellar medium and collects these nucleosynthesis products. One such product is 60Fe, a radionuclide with a half-life of 2.6 My that is predominantly produced in massive stars and ejected in supernova explosions. Extraterrestrial 60Fe has been found on Earth, suggesting close-by supernova explosions ∼2 to 3 and ∼6 Ma. Here, we report on the detection of a continuous interstellar 60Fe influx on Earth over the past ∼33,000 y. This time period coincides with passage of our SS through such interstellar clouds, which have a significantly larger particle density compared to the local average interstellar medium embedding our SS for the past few million years. The interstellar 60Fe was extracted from five deep-sea sediment samples and accelerator mass spectrometry was used for single-atom counting. The low number of 19 detected atoms indicates a continued but low influx of interstellar 60Fe. The measured 60Fe time profile over the 33 ky, obtained with a time resolution of about ±9 ky, does not seem to reflect any large changes in the interstellar particle density during Earth's passage through local interstellar clouds, which could be expected if the local cloud represented an isolated remnant of the most recent supernova ejecta that traversed the Earth ∼2 to 3 Ma. The identified 60Fe influx may signal a late echo of some million-year-old supernovae with the 60Fe-bearing dust particles still permeating the interstellar medium.

Supernova-Produced ^{53}Mn on Earth

For the time period from 1.5 to 4 Myr before the present we found in deep ocean ferromanganese crusts a ^{53}Mn excess concentration in terms of ^{53}Mn/Mn of about 4×10^{-14} over that expected for cosmogenic production. We conclude that this ^{53}Mn is of supernova origin because it is detected in the same time window, about 2.5 Myr ago, where ^{60}Fe has been found earlier. This overabundance confirms the supernova origin of that ^{60}Fe. For the first time, supernova-formed ^{53}Mn has been detected and it is the second positively identified radioisotope from the same supernova. The ratio ^{53}Mn/^{60}Fe of about 14 is consistent with that expected for a SN with a 11-25  M_{⊙} progenitor mass and solar metallicity.

The quest for AMS of 182Hf – why poor gas gives pure beams

The long-lived radioisotope 182Hf (T1/2 = 8.9 Ma) is of high astrophysical interest as its potential abundance in environmental archives would provide insight into recent r-process nucleosynthesis in the vicinity of our solar system. Despite substantial efforts, it could not be measured at natural abundances with conventional AMS so far due to strong isobaric interference from stable 182W. Equally important is an increase in ion source efficiency for the anions of interest.

The new Ion Laser InterAction Mass Spectrometry (ILIAMS) technique at VERA tackles the problem of elemental selectivity in AMS with a novel approach. It achieves near-complete suppression of isobar contaminants via selective laser photodetachment of decelerated anion beams in a gas-filled radio-frequency quadrupole (RFQ) ion cooler. The technique exploits differences in electron affinities (EA) within elemental or molecular isobaric systems neutralizing anions with EAs smaller than the photon energy. Alternatively, these differences in EA can also facilitate anion separation via chemical reactions with the buffer gas.

We present first results with this approach on AMS-detection of 182Hf. With He +O2 mixtures as buffer gas in the RFQ, suppression of 182WF5− vs 180HfF 5− by >105 has been demonstrated. Mass analysis of the ejected anion beam identified the formation of oxyfluorides as an important reaction channel. The overall Hf-detection efficiency at VERA presently is 1.4% and the W-corrected blank value is 182Hf/180Hf = (3.4 ± 2.1)×10−14. In addition, a survey of different sample materials for highest negative ion yields of HfF 5− with Cs-sputtering has been conducted.

Origin of Recent Interstellar 60Fe on Earth

Over the last 20 years, evidence for a 2 Myr old supernova 60Fe influx onto Earth was provided by several authors. For the first time, independent investigations of samples from two different geological archives yielded conclusive data for a further, much younger 60Fe influx onto Earth. The origin of this influx is currently unclear because of the limited data available, the lack of consistent astrophysical models and a gap in the data between 50 kyr and 1 Myr. Possible astrophysical scenarios will be discussed with respect to the different influx patterns from different sources and a measurement to close the gap will be proposed.

Interstellar ^{60}Fe in Antarctica

Earth is constantly bombarded with extraterrestrial dust containing invaluable information about extraterrestrial processes, such as structure formation by stellar explosions or nucleosynthesis, which could be traced back by long-lived radionuclides. Here, we report the very first detection of a recent ^{60}Fe influx onto Earth by analyzing 500 kg of snow from Antarctica by accelerator mass spectrometry. By the measurement of the cosmogenically produced radionuclide ^{53}Mn, an atomic ratio of ^{60}Fe/^{53}Mn=0.017 was found, significantly above cosmogenic production. After elimination of possible terrestrial sources, such as global fallout, the excess of ^{60}Fe could only be attributed to interstellar ^{60}Fe which might originate from the solar neighborhood.

Production and characterization of 60Fe standards for accelerator mass spectrometry

Accelerator Mass Spectrometry (AMS) is one of the most sensitive analysis techniques to measure long-lived radionuclides, reaching detection limits for isotopic ratios down to 10-15-10-16 in special cases. Its application portfolio covers nearly every field of environmental research, considering processes in the atmosphere, biosphere, hydrosphere, cryosphere, lithosphere and the cosmosphere. Normally, AMS measures the content of isotopes in comparison to a validated standard. However, in some cases like for example 60Fe, well characterized standard materials are difficult to produce due to the extreme rareness of the isotope. We report here on the manufacturing of a set of 60Fe standards, obtained by processing irradiated copper from a beam dump of the high-power proton accelerator (HIPA) at the Paul Scherrer Institute (PSI). The isotopic ratios of the standards have been adjusted via a dilution series of a master solution, isotopic content of which has been characterized by Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS). In total, we produced three samples with isotopic ratios of 1.037(6)·10-8, 1.125(7)·10-10 and 1.234 (7)·10-12, respectively. The latter had already been applied in three pioneering AMS studies investigating the remaining signal of injected matter of nearby super novae explosions in sediment archives.

Hypothesis: Muon Radiation Dose and Marine Megafaunal Extinction at the End-Pliocene Supernova

Considerable data and analysis support the detection of one or more supernovae (SNe) at a distance of about 50 pc, ∼2.6 million years ago. This is possibly related to the extinction event around that time and is a member of a series of explosions that formed the Local Bubble in the interstellar medium. We build on previous work, and propagate the muon flux from SN-initiated cosmic rays from the surface to the depths of the ocean. We find that the radiation dose from the muons will exceed the total present surface dose from all sources at depths up to 1 km and will persist for at least the lifetime of marine megafauna. It is reasonable to hypothesize that this increase in radiation load may have contributed to a newly documented marine megafaunal extinction at that time.

Ultratrace Determination of 99Tc in Small Natural Water Samples by Accelerator Mass Spectrometry with the Gas-Filled Analyzing Magnet System

In the frame of studies on the safe disposal of nuclear waste, there is a great interest for understanding the migration behavior of ⁹⁹Tc. ⁹⁹Tc originating from nuclear energy production and global fallout shows environmental levels down to 10⁷ atoms/g of soil (∼2 fg/g). Extremely low concentrations are also expected in groundwater after diffusion of ⁹⁹Tc through the bentonite constituting the technical barrier for nuclear waste disposal. The main limitation to the sensitivity of the mass spectrometric analysis of ⁹⁹Tc is the background of its stable isobar ⁹⁹Ru. For ultratrace analysis, the Accelerator Mass Spectrometry (AMS) setup of the Technical University of Munich using a Gas-Filled Analyzing Magnet System (GAMS) and a 14 MV Tandem accelerator is greatly effective in suppressing this interference. In the present study, the GAMS setup is used for the analysis of ⁹⁹Tc in samples of the seawater reference material IAEA-443, a peat bog lake, and groundwater from an experiment of in situ diffusion through bentonite in the controlled zone of the Grimsel Test Site (GTS) within the Colloid Formation and Migration (CFM) project. With an adapted chemical preparation procedure, measurements of ⁹⁹Tc concentrations at the fg/g levels with a sensitivity down to 0.5 fg are accomplished in notably small natural water samples. The access to these low concentration levels allows for the long-term monitoring of in situ tracer tests over several years and for the determination of environmental levels of ⁹⁹Tc in small samples.

Limits on Supernova-Associated ^{60}Fe/^{26}Al Nucleosynthesis Ratios from Accelerator Mass Spectrometry Measurements of Deep-Sea Sediments

We searched for the presence of ^{26}Al in deep-sea sediments as a signature of supernova influx. Our data show an exponential dependence of ^{26}Al with the sample age that is fully compatible with radioactive decay of terrigenic ^{26}Al. The same set of samples demonstrated a clear supernova ^{60}Fe signal between 1.7 and 3.2 Myr ago. Combining our ^{26}Al data with the recently reported ^{60}Fe data results in a lower limit of 0.18_{-0.08}^{+0.15} for the local interstellar ^{60}Fe/^{26}Al isotope ratio. It compares to most of the ratios deduced from nucleosynthesis models and is within the range of the observed average galactic ^{60}Fe/^{26}Al flux ratio of (0.15±0.05).

Photobiological Effects at Earth's Surface Following a 50 pc Supernova

We investigated the potential biological impacts at Earth's surface of stratospheric O3 depletion caused by nearby supernovae known to have occurred about 2.5 and 8 million years ago at about 50 pc distance. New and previously published atmospheric chemistry modeling results were combined with radiative transfer modeling to determine changes in surface-level solar irradiance and biological responses. We find that UVB irradiance is increased by a factor of 1.1 to 2.8, with large variation in latitude, and seasonally at high-latitude regions. Changes in UVA and PAR (visible light) are much smaller. DNA damage (in vitro) is increased by factors similar to UVB, while other biological impacts (erythema, skin cancer, cataracts, marine phytoplankton photosynthesis inhibition, and plant damage) are increased by smaller amounts. We conclude that biological impacts due to increased UV irradiance in this SN case are not mass-extinction level but might be expected to contribute to changes in species abundances; this result fits well with species turnover observed around the Pliocene-Pleistocene boundary. Key Words: UV radiation-Supernovae-Ozone-Radiative transfer. Astrobiology 18, 481-490.

Transfer reactions for nuclear astrophysics

Direct measurements of cross sections at stellar energies are very challenging - if at all possible. This is essentially due to the very low cross-sections of the reactions of interest (especially when it involves charged particles), and/or to the radioactive nature of many key nuclei. Direct measurements with charged particles are often performed at higher energies and then extrapolated down to stellar energies using R-matrix calculations. However, these extrapolations are delicate because of the possible existence of unobserved low-energy or sub-threshold resonances. In order to bypass the difficulties related to direct measurements, indirect methods such as transfer reactions are used. These experiments are usually performed at higher energies and their conditions are relatively less stringent than in direct measurements. However, these methods rely on theoretical models for which the input parameters may be an important source of systematic uncer-tainties and thus need to be determined carefully. In this manuscript, a short overview on the difficulties related to direct measurements will be given as well as a description of thetransfer reaction method and the theoretical concept behind. Finally, the method will be illustrated through two recent performed studies.

A Supernova at 50 pc: Effects on the Earth's Atmosphere and Biota

Recent 60Fe results have suggested that the estimated distances of supernovae in the last few million years should be reduced from ∼100 to ∼50 pc. Two events or series of events are suggested, one about 2.7 million years to 1.7 million years ago, and another about 6.5-8.7 million years ago. We ask what effects such supernovae are expected to have on the terrestrial atmosphere and biota. Assuming that the Local Bubble was formed before the event being considered, and that the supernova and the Earth were both inside a weak, disordered magnetic field at that time, TeV-PeV cosmic rays (CRs) at Earth will increase by a factor of a few hundred. Tropospheric ionization will increase proportionately, and the overall muon radiation load on terrestrial organisms will increase by a factor of ∼150. All return to pre-burst levels within 10 kyr. In the case of an ordered magnetic field, effects depend strongly on the field orientation. The upper bound in this case is with a largely coherent field aligned along the line of sight to the supernova, in which case, TeV-PeV CR flux increases are ∼104; in the case of a transverse field they are below current levels. We suggest a substantial increase in the extended effects of supernovae on Earth and in the "lethal distance" estimate; though more work is needed. This paper is an explicit follow-up to Thomas et al. We also provide more detail on the computational procedures used in both works.

A Possible Role for Stochastic Astrophysical Ionizing Radiation Events in the Systematic Disparity between Molecular and Fossil Dates

Major discrepancies have been noted for some time between fossil ages and molecular divergence dates for a variety of taxa. Recently, systematic trends within avian clades have been uncovered. The trends show that the disparity is much larger for mitochondrial DNA than for nuclear DNA, also that it is larger for crown fossil dates than stem fossil dates. It has been argued that this pattern is largely inconsistent with incompleteness of the fossil record as the principal driver of the disparity. A case is presented that, given the expected mutations from a fluctuating background of astrophysical radiation from such sources as supernovae, the rate of molecular clocks is variable and should increase back to a few million years, before returning to the long-term average rate. This is a possible explanation for the disparity. One test of this hypothesis is to look for an acceleration of molecular clocks at 2 to 2.5 Ma due to one or more moderately nearby supernovae known to have happened at that time. Another is to look for reduced disparity in benthic organisms of the deep ocean. In addition, due to the importance of highly penetrating muon irradiation, the disparity should be magnified for megafauna. Key Words: Extreme events in Earth history-Molecular clock-Radiation physics-Evolution. Astrobiology 17, 87-90.

Time-resolved 2-million-year-old supernova activity discovered in Earth's microfossil record

Massive stars ([Formula: see text]), which terminate their evolution as core-collapse supernovae, are theoretically predicted to eject [Formula: see text] of the radioisotope (60)Fe (half-life 2.61 Ma). If such an event occurs sufficiently close to our solar system, traces of the supernova debris could be deposited on Earth. Herein, we report a time-resolved (60)Fe signal residing, at least partially, in a biogenic reservoir. Using accelerator mass spectrometry, this signal was found through the direct detection of live (60)Fe atoms contained within secondary iron oxides, among which are magnetofossils, the fossilized chains of magnetite crystals produced by magnetotactic bacteria. The magnetofossils were chemically extracted from two Pacific Ocean sediment drill cores. Our results show that the (60)Fe signal onset occurs around 2.6 Ma to 2.8 Ma, near the lower Pleistocene boundary, terminates around 1.7 Ma, and peaks at about 2.2 Ma.

TERRESTRIAL EFFECTS OF NEARBY SUPERNOVAE IN THE EARLY PLEISTOCENE

Recent results have strongly confirmed that multiple supernovae happened at distances of ∼100 pc, consisting of two main events: one at 1.7-3.2 million years ago, and the other at 6.5-8.7 million years ago. These events are said to be responsible for excavating the Local Bubble in the interstellar medium and depositing 60Fe on Earth and the Moon. Other events are indicated by effects in the local cosmic ray (CR) spectrum. Given this updated and refined picture, we ask whether such supernovae are expected to have had substantial effects on the terrestrial atmosphere and biota. In a first look at the most probable cases, combining photon and CR effects, we find that a supernova at 100 pc can have only a small effect on terrestrial organisms from visible light and that chemical changes such as ozone depletion are weak. However, tropospheric ionization right down to the ground, due to the penetration of ⩾TeV CRs, will increase by nearly an order of magnitude for thousands of years, and irradiation by muons on the ground and in the upper ocean will increase twentyfold, which will approximately triple the overall radiation load on terrestrial organisms. Such irradiation has been linked to possible changes in climate and increased cancer and mutation rates. This may be related to a minor mass extinction around the Pliocene-Pleistocene boundary, and further research on the effects is needed.

Observation of the ⁶⁰Fe nucleosynthesis-clock isotope in galactic cosmic rays

Iron-60 ((60)Fe) is a radioactive isotope in cosmic rays that serves as a clock to infer an upper limit on the time between nucleosynthesis and acceleration. We have used the ACE-CRIS instrument to collect 3.55 × 10(5) iron nuclei, with energies ~195 to ~500 mega-electron volts per nucleon, of which we identify 15 (60)Fe nuclei. The (60)Fe/(56)Fe source ratio is (7.5 ± 2.9) × 10(-5) The detection of supernova-produced (60)Fe in cosmic rays implies that the time required for acceleration and transport to Earth does not greatly exceed the (60)Fe half-life of 2.6 million years and that the (60)Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, ⪍1 kiloparsec. A natural place for (60)Fe origin is in nearby clusters of massive stars.