Radioactivity in meteorites

All rocks are radioactive (and so are all bananas and all people), but meteorites are less radioactive than Earth rocks, on average

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)
1	Ordinary chondrites (88% of all stony meteorites)	780–825	42–43	12–13
2	Sayh al Uhaymir (SaU) 169 (the most radioactive meteorite)	4400	31,000	8000
3	Earth’s continental crust (range of 4 estimates)	9000–26,000	3500–7200	900–1800
4	Meteorwrongs (mean of 598 samples)	10,200	6300	2500
5	Terrestrial Granite (range of 18 diverse samples)	23,000–57,000	1500–130,000	800–63,000
6	My kitchen countertop (garnetiferous leucogranite)	47,700	16,200	2100
7	A banana (126 g) (a good source of potassium!)	3600	?, but low	?, but low
8	Humans	2000–3000	~1	~1

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
Gnos E., Hofmann B. A., Al-Kathiri A., Lorenzetti S., Eugster O., Whitehouse M. J., Villa I., Jull A. J. T., Eikenberg J., Spettel B., Krähenbühl U., Franchi I. A., and Greenwood G. C. (2004) Pinpointing the source of a lunar meteorite: Implications for the evolution of the Moon. Science, v. 305, p. 657-659.

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
Mason B. (1966) Principles of Geochemistry, Third Edition, John Wiley & Sons, New York, pp 329.

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.

**If a rock registers substantially above background on a laboratory survey meter (“Geiger counter”), then it is not a meteorite (but, see SaU 169 above for the only exception of which I am aware).**

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.)

Take-home messages

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.

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