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.

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.)