The changing radiation environment and planetary conditions for life

  1. Brian Thomas

    An important factor for life on any planet over the long term is the changing radiation environment. One-off events like nearby supernovae and gamma-ray bursts are clearly important; it’s virtually guaranteed that life on any planet will be subjected to several such events in a few billion year history. Longer term changes to the cosmic ray flux are also important to consider.

  2. Adrian Melott

    The character of the planet will be extremely important here. Life on Earth is partially shielded from the effects of radiation fluctuations by its thick atmosphere. Mars would be very different, with the surface exposed to cosmic-ray primaries. Enceladus should, on the other hand, be almost completely impervious to radiation damage beneath the ice layer. Even muons can only penetrate about 1 km.

  3. Alan P Boss

    In addition to the points that Adrian mentions, one might also imagine life originating deep underwater around the hot smokers found at mid-ocean ridge spreading centers. While perhaps not as conducive as more favored environments closer to the surface, one could imagine partially-water worlds that evolve life in spite of a potentially hostile surface environment.

  4. Adrian Melott

    So, in deep water worlds, will the rate of evolution be slowed due to a lack of radiation-induced mutations? Or is the rate of evolution only weakly coupled to the rate of mutation? Additionally, I would expect that the radiation environment would be considerably more stable in such worlds.

    One can also imagine places where the radiation environment is not only more intense, but also more variable—again taking Mars as an example.

  5. Adrian Melott

    All considerations suggest that the cosmic ray flux at the Earth has changed greatly over time. There are a variety of mechanisms by which this changing background may have influenced life on Earth. NASA can contribute to understanding this by research into the cosmic ray flux from various sources and to increased understanding of the effects on life. The ambient cosmic-ray intensity and spectrum is also relevant to possible extraterrestrial life, including sites like Mars (highly exposed) and Enceladus (strongly shielded). Strong fluctuations in cosmic ray intensity are expected over all timescales from decades up to hundreds of Myr.

    Effects on the Biosphere Nearly all cosmic rays incident upon the Earth interact in the upper atmosphere, giving rise to air showers. There are two primary mechanisms by which they may influence life on the surface: (1) Muons may reach the surface in abundance, and penetrate of order 1 km into the ocean. They already contribute about a sixth of the penetrating radiation dose on the surface. Therefore, fluctuations in primaries over a GeV will produce major changes in radiation dose in the biosphere. This will introduce changes in the mutation rate and may be lethal to some organisms. In the case of a nearby supernova, order of magnitude increases are likely on a timescale of ~ 1 Myr. Shorter term changes in cosmic ray intensity have been tied epidemiologically to cancer rates. The production of muons is known to depend strongly on the energy of primaries, and so expected fluctuations up to the PeV scale from supernovae are important. (2) All cosmic rays are energetic enough to ionize the atmosphere and to dissociate the N2 molecule. Ionization of the atmosphere will increase the lightning frequency, and may (controversially) increase cloud cover. These effects are possible contributors to climate change. It may or may not be coincidental that 60Fe signatures of a moderately nearby supernova 2.5 Myr ago are coincidental with the onset of the Pleistocene and a switch to repeated glaciation. Ionization and dissociation of atmosphere molecules also leads to the formation of oxides of nitrogen which are normally present at low abundance. These compounds catalyze the destruction of stratospheric ozone, removing the primary shield against damaging solar UVB. UVB is absorbed strongly by DNA and protein molecules, leading to damage including that leading to mutation and cancer. Life on the surface of Mars would be directly impacted by primaries, and so would have to be extremely radiation-hardy. By contrast, subsurface life would only be affected by muons—at a level lower than terrestrial life must endure. Life in a subsurface ocean covered by ice, such as Enceladus, or a thick atmosphere like Titan, would be nearly completely shielded from external radiation, which might slow the rate of evolution. However, Titan might experience increased deposition of polymers on the surface during enhanced radiation episodes. Variation of cosmic ray flux due to discrete sources There are strong theoretical reasons and emerging evidence that the Earth has experienced episodic increases in cosmic ray flux in the past. With the Fermi detection of gamma-ray emission from supernova remnants, they are confirmed as the primary source of galactic cosmic rays. Supernovae, even at moderate distances, can produce terrestrial rises in cosmic ray flux lasting up to 1000 yr; the Sun itself appears to have generated moderate proton events within historical times, with the detection consistent with the Kepler mission detection of superflares on sunlike stars. Other astrophysical transients are relevant at longer timescales. Important questions in cosmic ray astrophysics  What is the spectrum of cosmic rays accelerated inside a supernova remnant?  How does the spectrum change as they escape the remnant and are diffused through the galaxy?  Can the baryonic loading of gamma-ray burst fireballs be constrained sufficiently to eliminate GRB cosmic ray effects as a serious terrestrial hazard?  Can the isotropy of high-energy cosmic rays give further clues into their origins, and by implication possible effects on the Earth?  What is the range of spectra of Solar Proton Events, and are there upper limits on their total energy?  What is the correlation between such SPEs and optical and/or X-ray Solar flares?  Are some solar proton events initiated by comets falling into the Sun?  Will further research confirm consistency between flare rates on the Sun and other G stars?  Can damage rates from muons be determined, so that their precise biological effectiveness can be characterized? This will be important for humans living in shelters on the Moon or Mars.  Are there geoisotope constraints on the mean muon fluence at the Earth’s surface?  How does the cosmic ray intensity and spectrum vary with time at the Earth and throughout the Galaxy?  How can new and existing geophysical signatures of cosmic ray impact help in reconstructing the history of cosmic ray variability on Earth? This is a question of importance to the high-energy astrophysics, geophysics, and biology communities).

  6. Alan P Boss

    Adrian makes a number of good points in his latest entry, including the fact that the ambient radiation environment is of immense importance for human space flight. The Roadmap should consider the possibilities for synergy on such research between NASA’s SMD (science) and HEOMD (human exploration and operations).

  7. Chris Reinhard

    Hi all!

    Perhaps a little out of my depth here, but I’ve been interested for some time now in the notion of understanding the radiation environment in Earth’s early surface oceans – how do the spectral features/intensity of radiation (bearing in mind stellar evolution models for our own star), physical/chemical features of the atmosphere and the optical properties of seawater combine to constrain photosynthetic life in the early surface ocean?

    Can the combined controls on surface ocean radiation environment be expected to exert a strong influence on the evolution of pigment systems in early photosynthetic organisms? Or do the effects of ocean chemistry override these constraints (say, via electron donor availability)?

    I would be keen to get some takes on this from people who have thought much more deeply about this issue than myself.