Astronomers Investigating Unknown Object That Hit the Earth In 773 AD

KentuckyFC writes "In November 2012, a group of Japanese scientists discovered that the concentration of carbon-14 in Japanese cedar trees suddenly rose between 774 AD and 775 AD. Others have since found similar evidence and narrowed the date to 773 AD. Astronomers think this stuff must have come from space so now the quest is on to find the extraterrestrial culprit. Carbon-14 is continually generated in the atmosphere by cosmic rays hitting nitrogen atoms. But because carbon-14 is radioactive, it naturally decays back into nitrogen with a half-life of about 5700 years. This constant process of production and decay leaves the amount of carbon-14 in the atmosphere relatively constant at about one part in a trillion will be carbon-14. One possible reason for the increase is that the Sun belched a superflare our way, engulfing the planet in huge cloud of high energy protons. Recent calculations suggest this could happen once every 3000 years and so seems unlikely. Another possibility is a nearby supernova, which bathed the entire Solar System in additional cosmic rays. However, astronomers cannot see any likely candidates nearby and there are no historical observations of a supernova from that time. Yet another possibility is that a comet may have hit the Earth, dumping the extra carbon-14 in the atmosphere. But astronomers have ruled that out on the basis that a comet carrying enough carbon-14 must have been over 100 km in diameter and would surely have left other evidence such as an impact crater. So for the moment, astronomers are stumped."

Re:Supernova

[The123king]

Here's a link [scientificamerican.com]

There is evidence of a SN at that time

[overlord]

Read this, already published here like a year ago (o more):
http://phys.org/news/2012-06-red-crucifix-sighting-supernova.html

Re:Why unlikely?

[aardvarkjoe]

The actual quote from the article:

The second way carbon-14 can be created in the Earthâ(TM)s atmosphere is if the Sun suddenly belched high energy particles our way. In other words, the Sun might have emitted a superflare 1000 times larger than usual which then engulfed the Earth.

There was a time when astronomers would have immediately ruled out this possibility as well. But last year, astrophysicists calculated that sun-like stars can produce superflares of this size about once every 3000 years.

The "seems unlikely" appears to be an invention of KentuckyFC, enabled by samzenpus.

Re:Carbon 14

[Anonymous Coward]

At the simplest level, you can assume that C14 is constant, but the question of is it constant or not has long ago been addressed. Using just tree rings for example, you can match up rings from past dead trees as they tend to be of similar size in years with good weather and smaller in years of bad weather, and records of wood going back thousands of years can be created. This gives a way of testing for C14 levels going back a long time, and yields corrections for dates on the order of 10% from the naive assumption that C14 levels don't change. In addition to tree rings, carbon trapped by other processes and in historical artifacts also shows close agreement, and carbon trapped in some mineral formations allow for a comparison between carbon dating and other radiocarbon dating. So there is a standard calibration graph produced by all of this to convert naive, constant C14 years into actual calibrated years (it also includes a 3% correction to the half life of C14 found in the 60s). ;

If you really want to get into the details of this, you can find information that deals with the chemical differences between C14 and C12, which is pretty slight. Depending on if a plant is C3, C$ or CAM photosynthesis based, it will absorb C14 at slightly different rates, and the absorption into the ocean is delayed from the atmosphere due to how slow it takes to mix the whole volume, and chemistry for incorporating C14 into things like shells is different than from that of photosynthesis slightly. But at that point, the differences are pretty small. You can look at the carbon isotopes in something modern, created less than a year ago like honey, and still struggle to tell if it is "real honey" made from typical C3 plants or had additional sugar from a C4 plant like corn unless it was above a 10% level.

Re:Why unlikely?

[possiblywrong]

Almost-- AC's figure of about 39% is assuming that these events occur as a Poisson *process*, so that the length of the interval between consecutive events has an *exponential* (continuous) distribution. In other words, 0.39 is the probability that this particular exponentially distributed random variable has a value less than 1500. (The Poisson *distribution*, on the other hand, is a *discrete* distribution-- in this case, non-negative integer-valued-- that in this case would describe the probability of a *number* of these events occurring within a given length of time.)