Tikalon Blog by Dev Gualtieri

GRB Extinctions

May 17, 2021

It's difficult to find a science fiction film that doesn't have human extinction as its narrative, typically by meteor impact or nuclear war. One that's not among these many B-movies is the 2008 film, The Day the Earth Stood Still.[1] It has self-replicating machines unleashed to turn humans into gray goo.

A gamma-ray burst (GRB) is far more efficient at destroying human life than Godzilla[2] and Skynet[3]. Diaspora, a 1997 science fiction novel by Greg Egan (b. 1961) imagines a GRB destroying the atmosphere of a future Earth leading to mass extinction.[4] While the novel is fiction, the possibility of such an event is very real, but fortunately very small. The probability is small, since we live in a low density region at the outskirts of our Milky Way Galaxy.

Artist's conception of a gamma-ray burst. Initially, a narrow beam (white) of gamma rays is emitted, followed by a wider beam (purple). (Still frame from a NASA/Swift/Cruz deWilde animation. Click for larger image.)

Although satellite observatories detect about one GRB each day, all these originate outside the Milky Way galaxy and they pose no danger. Closer to Earth, GRBs that have a potential to affect life on Earth occur about once every five million years. That means that life on Earth has experienced their affects about eight hundred times. The Ordovician–Silurian extinction events in which 85% of marine species were extinguished about 450 million years ago may have been caused by a GRB.

Earth has been hit by radiation from three intense GRBs in this century. The first, on December 27, 2004, was from SGR 1806-20 at 50,000 light year's distance.[5-6] This GRB ionized Earth's atmosphere down to about 50,000 feet (20 kilometers), nearly down to the level of commercial aircraft.[5-6] This gamma ray event increased the density of ions at a 60 kilometer altitude by a huge factor, from 0.1 to 10,000 free electrons per cubic foot.[6]

On April 27, 2013, Earth was assaulted by another intense gamma ray burst, designated GRB 130427A at a distance of about 3.8 billion light years.[7] This gamma ray event had an intensity reaching 95 billion electron volts (eV).[7] For comparison purposes, thermal energy (k_BT) at room temperature is about 0.025 eV. Fortunately, the radiation from this burster was easily absorbed by Earth's atmosphere.

An associated optical flash from GRB 130427A was strong enough to be observed with binoculars, and its position was nearly coincident with a galaxy designated SDSS J113232.84+274155.4. A burst comparable to GRB 130427A has a probability of less than 1 in 10 million of occurring in our galaxy.[7] The most recent energetic GRB, named GRB 190114C, was in November 2019. This GRB produced gamma-rays with the highest energy ever observed, about a tera (10¹²) electron volts.[8]

In the 1950s, physicist, Enrico Fermi (1901-1954), wondered why there is no evidence of an extraterrestrial civilization when he asked during luncheon conversation at Los Alamos National Laboratory, "Where are they?" In our Milky Way Galaxy, there should be many civilizations much more advanced than we are, and these could at least have sent automated exploratory spacecraft our way. This problem is known as the Fermi paradox, and one conjectured reason for the absence of space aliens is that GRBs eventually destroy these civilizations.

Extraterrestrials might be hesitant to visit us when they could potentially expect a reception like this.

(A color-modified Wikimedia Commons image from the Open Clipart Library, traced from Captain Science #5, art by Joe Orlando and Wally Wood.)

The high probability (95%) of lethal bursts within four kiloparsecs (kpc) of our galactic center makes that region inhospitable to life.[9] Only at the periphery of the Milky Way, more than 10 kpc from the center, does the probability of life-extinguishing bursts drop below 50%.[9] The Milky Way Galaxy will experience a 10⁴⁰ eV gamma ray burst about every billion years, and smaller bursts occur more often.[11] Gamma ray bursts would have prevented life in nearly every galaxy formed in the first five billion years of the universe, and the The Milky Way could be among only 10% of all galaxies that can sustain complex life for a considerable period.[11]

A team of Italian astronomers has recently concluded that until about six billion years ago the region at the outskirts of the Milky Way was the safest place for the development life, since it was relatively sheltered from supernova explosions and gamma-ray bursts.[12-13]. In the last about four billion years, this safe region shifted to between 2 and 8 kpc from the center.[12] The research team had members from the Università dell'Insubria (Como, Italy), the Osservatorio Astronomico di Brera (Merate, Italy), INFN Sezione Milano–Bicocca (Milano, Italy), and the Università Milano–Bicocca, (Milano, Italy).[12-13] Their analysis is published in Astronomy & Astrophysics.[12]

The first 1000 GRBs detected by NASA's Swift spacecraft.

The first thousand gamma ray bursts (GRBs) detected by NASA's Swift observatory. The plane of our Milky Way Galaxy runs across the center. The GRBs are color coded by year. (NASA image from the Goddard Space Flight Center and the Two Micron All-Sky Survey/J. Carpenter, T. H. Jarrett, and R. Hurt (modified). Click for larger image.)

The researchers developed a model of habitability over time as a function of the location from the galactic center of transient astrophysical events (GRBs and supernova explosions).[12] The model shows a high probability that such events are abundant around certain star types.[12] At our Solar System's distance from the galactic center, the model estimates that there has been at least one lethal GRB in the last 500 million years, so it's consistent with the hypothesis that a GRB caused the Ordovician–Silurian extinction about 450 million years ago.[12-13] The reassuring conclusion for us is that "the worst seems to be over."[13]

As study co-author, Giancarlo Ghirlanda, of the INAF explains,

"Supernovae are more frequent in star-forming regions, where massive stars are formed... GRBs, on the other hand, prefer star-forming regions that are still poorly engulfed by heavy elements. In these regions, massive stars that are formed by metal-poor gas lose less mass during their life due to stellar winds. Therefore, these stars are able to keep themselves in rapid rotation, a necessary condition to be able to launch, once a black hole has formed, a powerful jet."[13]

A supernova emits as much energy as the entire Milky Way emits in a few hours, and a GRB emits in 10 seconds what the Milky Way emits in a century.[13] A GRB within 3300 light years of Earth would destroy our planet's ozone layer, expose surface organisms to the Sun's ultraviolet radiation to cause their extinction.[3] It would also produce nitrogen compounds in the atmosphere that would reduce insolation and cause global cooling.[13]

GRB Extinctions

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