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Cosmic rays when life first emerged on Earth

posted May 29, 2013, 6:48 AM by Jeremy Drake   [ updated Oct 2, 2013, 1:10 PM ]
In the early 1970's, Carl Sagan pointed out that during the epoch when it is thought life emerged on Earth - when it was about 1 billion years old - the Sun would not have been bright enough to keep the Earth's oceans from freezing over. The problem arises because a star like the Sun gradually brightens as it ages on the main-sequence. Tracing the Sun's evolution back to when it was 1 billion years old, it should have been about 25% fainter than today. But a frozen Earth would have presented difficulties for the emergence of life and there is copious evidence for liquid water at that time. This conundrum is known as the Faint Young Sun paradox. Various solutions have been proposed, such as a much greater concentration of greenhouse gases in the Earth's atmosphere, or that the Sun has lost several percent of its mass over time - a more massive early Sun would have been brighter.  However, none of the proposed solutions is without difficulty.

About a decade ago, Nir Shaviv of the Hebrew University of Jerusalem suggested that cosmic rays might hold the key.  Cosmic rays are mostly energetic protons that have been accerlated to extremely high velocities, probably in supernova remnants. About 10,000 particles per square metre per second make it past the Sun' shielding magnetic field to the Earth's surface. Shaviv's idea was that cosmic rays help cloud formation by ionizing molecules in the Earth's atmosphere and creating condensation seeds. At early times, the Sun's magnetic field was much stronger than today, shielding cosmic rays much more effectively and leading to less cloud cover and less sunlight reflected back into space. A lower albedo would then lead to a warmer Earth, and liquid water.

We looked into what the cosmic ray flux at the Earth should be like during the Archean eon, 3.5 billion years ago when life first emerged, by modelling the shielding of the young solar wind. We used clues from young solar-like stars to construct the Sun's surface magnetic field (shown in the figure here) and rotation rate, and then computed how cosmic rays would have propagated through the solar wind as it carries the magnetic field outward, wound up by the solar rotation. We found cosmic ray fluxes at Earth were probably lower by a factor of 100 compared to the present day. The cosmic ray cloud seeding hypothesis is still very controversial - - the idea is currently being tested by the CLOUD project at CERN - but if there is a link, our study supports the idea that there could have been fewer clouds at earlier times. This work was lead by postdoc Ofer Cohen and was published in the 2012 November 20 edition of the Astrophysical Journal.