Gamma-ray bursts (GRBs for gamma-ray bursts ) are defined as extremely energetic explosions lasting between ten milliseconds and several hours. The violence of this mechanism makes gamma-ray bursts the most energetic and luminous known electromagnetic phenomenon in the observable Universe. They most often appear during chaotic events like supernovas or gravitational collapses of massive stars. In view of the energy deployed by gamma-ray bursts, what would be the consequences if the Earth were in the trajectory of one of them?
The majority of known sources of gamma-ray bursts are several million light-years from Earth. Because the energy of a GRB is highly concentrated in the manner of a coherent beam, the probability of it being directed precisely at Earth is extremely small, but not zero. In such a case, the consequences could be devastating. Several hypotheses even suggest that some mass extinctions in the past could have been caused by such events.
Gamma rays have an average energy of about 10 44 J. They thus release more energy in a few seconds than the Sun in 10 billion years of existence. No other phenomenon in the observable Universe is known to produce so much energy in such a short time. This is an amount of energy comparable to that released by type Ib/Ic supernovae (core-collapse supernovae).
Scientists estimate that only 10% of all known galaxies are likely to harbor life as we know it. The other 90% have too dense stellar regions, and therefore too high occurrences of GRBs to present a safe environment. For galaxies located at a redshift z> 0.5, life is simply impossible with regard to their very high rates of GRBs, due to very compact stellar regions.
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So far, all gamma-ray bursts have been observed outside our galaxy and are considered harmless to Earth. Currently, satellites detect approximately one gamma-ray burst per day. The closest burst, detected in March 2014 and named GRB 980425, was at a distance of 40 Mpc (130 million light-years) in a dwarf galaxy. It was, however, less energetic than ordinary bursts and was associated with the type Ib supernova, SN 1998bw.
The Earth's atmosphere has a great capacity to absorb ultra-energetic radiation such as X and gamma rays; therefore, during the burst, these radiations would not reach truly dangerous levels on the Earth's surface. The immediate effect would rather be a brief increase (from one second to about ten seconds) in UV radiation, without however this endangering life on Earth.
The long-term effects, however, are far more dangerous. Upon striking the atmosphere, gamma radiation causes the gradual chemical conversion of nitrogen and oxygen molecules into nitrogen oxide and nitrogen dioxide. Nitrogen oxides destroy ozone molecules, with, depending on the models, a reduction in the ozone layer of between 25 and 35% (and up to 75% in the most exposed areas). This first effect could last for several years.
This decrease in ozone is sufficient to cause a dangerous increase in the UV Index at the surface. Nitrogen oxides also cause the appearance of photochemical smogs (thick brownish haze) obscuring the sky and blocking about 1% of the solar electromagnetic spectrum, leading to a decrease in photosynthesis. Also, by blocking solar radiation, smog could lead to a cosmic winter if Earth's climate is already unstable.
High nitrogen levels would cause nitric acid rain, which is toxic to some organisms, but likely far too low to present a global hazard. The real danger of a gamma-ray burst on Earth is ultimately the sudden increase in UV radiation during the burst as well as several years after. Biophysical models show that such a catastrophe would cause DNA damage 16 times greater than during normal UV exposure.
