Three chemical elements are naturally radioactive and occur in all earth rocks and all meteorites – K (potassium), Th (thorium), and U (uranium). Any rock from any planet will contain at least one atom of these elements so, technically, all rocks are radioactive, it is just a matter of degree. On average, however, meteorites are considerably less radioactive that typical Earth rocks.
Concentrations of radioactive elements in meteorites, earth rocks, bananas, and humans
|Substance||K (ppm)||Th (ppb)||U (ppb)|
(88% of all stony meteorites)
|2||Sayh al Uhaymir (SaU) 169|
(the most radioactive meteorite)
|3||Earth’s continental crust|
(range of 4 estimates)
(mean of 598 samples)
(range of 18 diverse samples)
|6||My kitchen countertop|
|7||A banana (126 g)|
(a good source of potassium!)
|3600||?, but low||?, but low|
|ppm = parts-per-million (μg/g), ppb = parts/billion (ng/g)|
|1. Ordinary chondrite data from|
Wasson J. T. and Kallemeyn G. W. (1988) Compositions of chondrites, Philosophical Transactions of the Royal Society of London A, v. 325, p. 535-544.
2. SaU 169 data from
Korotev R. L, Zeigler R. A., Jolliff B. L., Irving A. J., and Bunch T. E. (2009) Compositional and lithological diversity among brecciated lunar meteorites of intermediate iron composition. Meteoritics & Planetary Science, v. 44, p. 1287-1322.
Dr. Hofmann told me that SaU 169 did, in fact, register as radioactive with a laboratory survey meter. Most of the radiation detected was from Th and U decay products.
3. Earth’s continental crust data from
Weaver B. L. and Tarney J. (1984) Physics and Chemistry of the Earth (Pollack H. N. and Murthy V. R., eds.), v 15, Pergamon, Oxford, p. 39-68.
Taylor S. R. and McClennan S. M. (1985) The Continental Crust: Its Origin and Evolution, Blackwell Sci. Publ., Oxford, p. 312.
Wedepohl K. H. (1995) Geochimica Cosmochimica Acta v. 59, p. 1217-1232.
|4. Terrestrial granite data from|
Govindaraju K. (1994) 1994 compilation of working values and sample description for 383 geostandards. Geostandards Newsletter, v. 18, p. 1–158.
|5. My kitchen countertop data from my laboratory|
|6. Meteorwrongs. I encourage people to obtain a chemical analysis of their rock if they really want to know if it is a meteorite. This row contains average concentrations for 598 rocks for which people have sent me the data and there were data for all 3 elements.|
|7. Banana data from https://www.chiquita.com/fruits/bananas-class-extra/ |
I can find no data for Th and U concentrations in bananas, but they must be <1 ppb.
|8. Human data from several internet sources.|
Potassium and bananas
There are many websites on the Internet that discuss radioactivity and bananas. This is another one.
Anybody who got an A in their high school chemistry course could do this calculation. Chiquita.com says that a medium banana weighs 160 g and contains 0.451 g of K (potassium). That is 0.0115 moles of K or 6.95 x 1021 atoms of K in a banana. For every 1 million (106) atoms of K in anything on earth, 932,580 are atoms of the non-radioactive 39K isotope, 118 are atoms of the radioactive 40K isotope, and 67,300 are atoms of the non-radioactive 41K isotope. So that works out to 8.2×1017 (0.8 billion billion) atoms of radioactive 40K in a typical banana.
The half-life of 40K is 1.25 billion years (1.25×109 years). That means that 1.25 billion years ago there was 2 times as much 40K in everything on Earth as there is now, 2.5 billion years ago there was 4 times as much, and 3.75 billion years ago there was 8 times as much. Life first appeared on Earth about 3.75 billion years ago. The world was more radioactive then, but life survived and even thrives.
Anyone (I hope!) who has taken a freshman college course in chemistry or physics could easily calculate that in a medium banana there are 860 decays per minute of 40K atoms. Humans are bigger than bananas, but contain about the same concentration of potassium. Assuming 2500 ppm K in a healthy 70-kg (154 lb.) human being (table above), there are 335,000 decays per minute of 40K atoms happening continuously. (After writing this, I discovered another source, probably using a different K concentration for the human body, that obtained 266,600 decays per minute.)
- We are all radioactive.
- Life on this planet emerged and evolved when the Earth was more radioactive than it is now.
- Potassium is an essential element for life; without it we would die.
- Do not worry about radioactive meteorites or bananas.