How would life on a planet perceive gamma radiation from a supernova explosion?

Question

There are many articles on the web about cosmic events that would wipe out life on Earth, supernova explosion being one of the most prominent ones. Gamma radiation is said to hit Earth, strip the atmosphere, lead to mutations and illnesses, and all that - without warning, as the radiation from a near-by supernova will be moving close to light speed.

What I could not find, is how life on Earth would actually perceive it?

Is it like I'm sitting in my room and typing this message and like looking out of the window: oops, the atmosphere is gone. People around start to suffocate or puke or die instantaneously.

Or is it more like: ha, weird weather today. Acid rain is falling. Next winter is too cold. People are like living in Chernobyl zone, getting sick and vulnerable. In something like 100 (1000?) years civilization has slowly come to its demise.

Answer 1

As pointed out by controlgroup, the effects are entirely dependent on how close and what type of supernova it is. If you want this event to be survivable i.e. the earths atmosphere is not completely destroyed, then the greatest danger is the breakdown of our ozone layer. From wikipedia:

According to a 2003 estimate, a type II supernova would have to be closer than eight parsecs (26 light-years) to destroy half of the Earth's ozone layer, and there are no such candidates closer than about 500 light-years.

On average, a supernova explosion occurs within 10 parsecs (33 light-years) of the Earth every 240 million years.[citation needed] Gamma rays are responsible for most of the adverse effects that a supernova can have on a living terrestrial planet. In Earth's case, gamma rays induce radiolysis of diatomic N2 and O2 in the upper atmosphere, converting molecular nitrogen and oxygen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation (mainly ultra-violet). Phytoplankton and reef communities would be particularly affected, which could severely deplete the base of the marine food chain.

That should give you a good range of distances of where to place your supernova. The after effects will be increased uv penetration from the main star to the surface. Depending on how much makes it through, you could get immediate sunburns and burnt corneas. The flora of the planet would degrade quickly and the animals would follow. My guess is within a few months, the planet would face starvation as food runs out.

Answer 2

It is barely noticeable.

There is a real world example where this happend to earth (but it was estimated to be about 5000-8000 lightyears away). The "Crab Nebula" was formed by a supernova which was detectable on earth around 1050 CE. Chinese astronomers observed and described the phenomena.

It was mentioned that a bright star was visible even during daylight.

That's about it. Disappointing ;)

Earth has not only an atmosphere to protect itself from all sorts of radiation. It also has a strong magnetic field. This forms the Van-Allen-Radiation-Belt, which deflects most of the solar wind around earth.

Furthermore, our sun also has a magnetic field, which counteracts any ionized radiation/particles incoming from outside. This is the Heliosphere.

However, the effects of the magnetic fields would not help against gamma ray bursts or other purely electro-magnetic radiation, as @MikeScott and @JoshuaZ pointed out in the comments!

Gamma-radiation can even be produced on earth inside thunderstorms, those "Gamma-Ray-Flashes" have a duration (approximating here) as long as light needs to travel the wavelength of the gamma ray in question. Recordings on earth show lengths of 0.2 to 3 milliseconds. There are about 500 per day. Another source of Gamma radiation is an atomic explosion. I would argue, the effects of such radiation is highly exaggerated in media.

However, there are sensors - namely our radio telescopes and satellites - that are very sensitive to gamma ray wavelengths (or could be affected more than anybody on earth). It was observed that there is a constant flashing of gamma radiation all over the universe. These are called "Gamma-Ray-Bursts (GRB)". It is an ongoing debate where they originate from. But some star going supernova is a good candidate.

Duration of GRBs vary from milliseconds to several hours. It is said there are a few per galaxy per million years, but given the abundance of stars, they can be observed often. The energy is thousand times or more the amount of a flash. It is hypothesized, though considered unlikely, that a GRB, directly pointed at earth, could have had caused a mass extinction event (see Late Ordovician; it mentions a distance of about 6000ly. This would be not much farther away than the Crab Nebula. The cause is supposed to be a "Hyper-Nova" though, not a regular Supernova. Seems far fetched to me).

On a general note: Remember that a supernova is a star dying. That means it is like our sun, the center of some other solar-system. The closest solar-system we know of is about 4 lightyears away (Proxima Centauri). And distances get only longer after that. To really be affected by a supernova, you would have to be in the same system as that star, probably. This might or might not turn out as in the cases you described. It surely would be devastating, if survivable at all.

But it is true, that such radiation would hit us without possibility of detecting it prior to impact. It is because we can only look "into the past" if we watch any star. The light that is on its way is not detectable, because it needs to hit a sensor (or eye) first.

Grave thought, if our sun would go supernova: Since the sun is about 8 lightminutes away, and we have satellites orbiting it, if ground control notices that it had lost contact with the satellites, the inbound supernova/radiation travelling at lightspeed is imminent to impact at the same time. No time to kiss anyone goodbye - That is if the sun would explode from one second to the next.

It is very probable that a star will send signs of a changing situation much earlier and less violently. For example by increased solar winds over a long time, or the circumference expanding a bit, then contracting again or something like that (see Betelgeuse of the Orion constellation, a red giant supposed to go supernova "sometime").

After such events it might still be millions of years until it will explode. Our understanding of the sun is not yet advanced enough to know for which signals to look in order to predict anything, though. We just started observing our sun in a level of detail that might lead to some insights one day (see SOHO satellite for example).

Answer 3

It all depends on how close it is (inverse squares law), and also on what type of Supernova. Some give rise to a gamma-ray burst, which focusses most of the gamma radiation into a very narrow beam.

An ordinary(!) supernova will be harmless if it's 500 light years away (so Betelgeuse won't do us any harm when it blows up sometime in the next 100,000 years). A gamma-ray burst hitting a planet could be catastrophic at 5000 light years distance. IIRC for an ordinary supernova, 50 light years is the distance at which it starts getting serious for life on the planet.

What's interesting is that it's not the direct effects on living organisms of the gamma radiation that is to be feared. It's the effect of ionizing radiation on the atmosphere. In an Oxygen-Nitrogen atmosphere it would create Nitrogen Oxides, which would then catalyze the destruction of the Ozone layer. The Nitrogen oxides would then form Nitric acid which would fall globally as acid rain for several decades. Also Nitrogen oxides are brown, so heat absorption in the atmosphere would be completely altered, and global climate patterns likewise.

This is not entirely dissimilar to a supervolcano eruption, except that at close range (for supernova-sized values of close) it could be much, much worse.

It's been calculated that the probability of a catastrophically close supernova during the 600 million year history of multicellular life on Earth is somewhat over 0.5 This means that one of the many mass extinction events in the fossil record might well have a Supernova as its cause.