1985 08 01 Nature - Vol 316 No 6027 - Weber

Magazine Overview

This issue of the scientific journal Nature, dated August 1, 1985, features a lead article titled "Can spores survive in interstellar space?" by Peter Weber and J. Mayo Greenberg from the Laboratory of Astrophysics, University of Leiden. The issue also includes a report on "A peculiar supernova in the spiral galaxy NGC4618" by Alexei V. Filippenko and Wallace L. W. Sargent.

Can spores survive in interstellar space?

This extensive article investigates the survival of Bacillus subtilis spores under simulated interstellar conditions, focusing on the effects of vacuum ultraviolet (UV) radiation and low temperatures (10 K).

Introduction and Background

The authors begin by discussing the timescale for the origin of life on Earth, noting that if life evolved rapidly, it might have originated from extraterrestrial sources, supporting the panspermia hypothesis. They re-examine Arrhenius's ideas on panspermia, breaking it down into four phases: ejection from a planet, interstellar transport, survival during transport, and deposition on a new planet. The study focuses on the transport and survival phases.

Interstellar transport is considered via molecular clouds, where spores could be swept along at speeds of 10 km/s, leading to passage times of 10^5-10^6 years between stars. This necessitates a survival time of 10^6-10^7 years for panspermia to be plausible. The primary destructive mechanism in space is photolysis by starlight, particularly UV photons. Spores are subjected to UV radiation, and the low temperatures (10-15 K) and high vacuum of interstellar space present significant challenges to their chemical integrity.

The three main factors hostile to microbes in interstellar space are identified as: (1) vacuum, (2) energetic photons and cosmic rays, and (3) low temperatures (mean particle temperature ≤10 K).

Experimental Method

The researchers used a laboratory facility designed to simulate interstellar conditions for studying the chemical evolution of interstellar grains. Their experiment involved irradiating Bacillus subtilis spores with UV light under controlled conditions of temperature and vacuum. They selected two strains: wild-type 168 (relatively irradiation-resistant) and TKJ 6323 (sensitized due to repair deficiencies).

The apparatus included a vacuum chamber, a cold finger maintained at 10 K or room temperature, and UV lamps (Hg and H2). The H2 lamp was used to simulate the UV flux in interstellar space, with its emission peaking in the vacuum UV (1,000-1,900 Å) and having a continuum in the far UV (2,000-3,000 Å).

Results

Inactivation Kinetics: The study found that the repair-deficient strain TKJ 6323 was significantly more sensitive to UV radiation than the wild-type strain 168, especially when irradiated on a surface.

Effect of Temperature and Vacuum: At room temperature, spores were more sensitive to UV in vacuum than at atmospheric pressure. However, when cooled to 10 K before irradiation, spores showed significantly reduced inactivation, even with fluences that would have been lethal at higher temperatures. For example, wild-type spores irradiated at 10 K and vacuum with a dose of 1×10^4 J/m² of RUV showed 25% survival.

UV Spectrum Effects: The H2 lamp, simulating interstellar UV, was used with different filters. It was found that wavelengths around 1,400 Å and 1,215 Å did not significantly inactivate spores. However, 1,600 Å light caused inactivation, particularly at room temperature. The total spectrum (vacuum UV + far UV) was more effective at room temperature than at 10 K. Surprisingly, the far UV spectrum (wavelengths >2,000 Å) accounted for most of the inactivation, suggesting that photons with energies less than 6 eV were more effective in killing spores than those with energies greater than 6 eV.

Mantle Effects: To investigate protection by interstellar ice mantles, spores were coated with a mixture of H2O:CH4:NH3:CO. These mantles protected spores from vacuum UV but not from far UV, as the simple molecules in the ice absorbed strongly in the vacuum UV but were transparent in the far UV. However, photo-processed mantle materials, which would accrete in space, are expected to absorb strongly across a wider UV range and provide effective shielding.

Discussion

The authors distinguish between damage occurring within and outside the spore core. UV light is attenuated by the spore's outer layers. Within the core, damage can include cyclobutane-type dimers, TDHT, DNA-protein crosslinks, strand breaks, and DNA adducts. At low temperatures and high vacuum, irradiation primarily induces radicals leading to TDHT, a process that might be suppressed at extremely low temperatures due to inhibited diffusion.

The study found that while the H2 lamp emits more energy in the far UV than the vacuum UV, the far UV spectrum (2,000-3,000 Å) was more effective in inactivating spores at 10 K. This suggests that the target chromophore might be different under these conditions.

Astrophysical Implications

Based on their survival data, a dose of approximately 6 kJ/m² of vacuum and far UV inactivates spores to F10 (10% survival) at 10 K. Considering the H2 lamp emits five times more energy in the far UV relative to vacuum UV compared to the interstellar medium (ISM), the F10 for spores in the diffuse ISM is estimated to be around 150 years. This is too short for panspermia.

However, two possibilities can prolong spore lifetime in the ISM: (1) protection within dense clouds where UV radiation is attenuated by several orders of magnitude, and (2) accretion of mantles of condensable matter that reduce UV penetration. Attenuation within a dense cloud (n=10^4 cm⁻³) can reduce UV radiation to at least 10^-4 times that of the diffuse ISM. Accretion of a mantle of ~0.15 µm, which takes about 1.5 × 10^5 years, can provide significant shielding.

Combining cloud attenuation and mantle effects, the biological time (F10) for spores could reach 4.5-45 Myr, which is long enough for transport between solar systems.

Conclusions

The experimental evidence suggests that in the general interstellar medium, spore survival times are only hundreds of years, insufficient for panspermia. However, within dark clouds, survival times of millions to tens of millions of years are attainable, making transport from one solar system to another probable. The authors note that the problems of spore ejection and deposition on a host planet remain unaddressed.

A peculiar supernova in the spiral galaxy NGC4618

This report details the discovery of a bright stellar object near the nucleus of the spiral galaxy NGC4618. Spectroscopic analysis revealed strong, very broad emission lines that are unusual for its location and spectral characteristics. The object is identified as a supernova, and its highly unusual spectrum suggests it may represent a new subclass of supernovae.

Recurring Themes and Editorial Stance

This issue of Nature showcases cutting-edge scientific research, particularly in astrophysics and astrobiology. The article on spore survival directly addresses fundamental questions about the origin and distribution of life in the universe, exploring the scientific plausibility of the panspermia hypothesis. The research employs rigorous experimental methods to simulate extreme cosmic conditions and provides quantitative data to constrain theoretical models. The inclusion of a report on a peculiar supernova highlights the journal's commitment to reporting significant astronomical discoveries and pushing the boundaries of our understanding of the cosmos.