How Old is the Solar System?

Transcription

1 How Old is the Solar System? Earth s crust is constantly changing due to volcanoes, erosion, and plate tectonics. So Earth rocks do not preserve a record of the early days of the Solar System. Instead, we must study other objects From the analysis of the oldest lunar rocks, we get ages around 4.4 billion years. From very primitive meteorites, the estimate for the age of the Solar System is 4.6 billion years. Where do these dates come from?

2 Elements vs. Isotopes Each atom has a nucleus of protons & neutrons surrounded by a cloud of 1 or more electrons. The number of protons in an atom defines which chemical element that atom is: All hydrogen atoms contain 1 proton. All carbon atoms contain 6 protons. All uranium atoms contain 92 protons. The periodic table is a list of all chemical elements.

3 See also Appendix D on page A-13.

4 Elements vs. Isotopes In addition to protons, each atom also contains some number of neutrons. Chemical elements are defined by # of protons. Isotopes are defined by the number of neutrons: carbon-12 has 6 protons and 6 neutrons, carbon-13 has 6 protons and 7 neutrons, carbon-14 has 6 protons and 8 neutrons. All isotopes of carbon act like carbon chemically.

5 Radioactive Isotopes Some isotopes are unstable and will break down (or decay) into another isotope: For example, carbon-14 decays into nitrogen-14. Everything contains some quantity of naturally occurring radioactive isotopes, even our bodies. The unstable isotope that decays is known as the parent isotope. The isotope that is made by decay is known as the daughter isotope. parent isotope decay daughter isotope

6 Radiometric Dating If we can identify a parent-daughter pair in a lab, we have a way of determining the time since the rock solidified. Here s how it works Each isotope has a characteristic time to decay called the half-life: The time it takes 50% of parent sample to decay carbon-14 has a half-life of 5730 years. iron-60 has a half-life of 2.6 million years. potassium-40 has a half-life of 1.3 billion years.

7 Radiometric Dating Let s start with 1 kg of parent (with a half-life 1 yr) and watch it decay into the daughter: 1 yr 1 yr 1 yr 1 yr 1 yr 1 yr 1 yr 1 kg 0 kg ½ kg ½ kg ¼ kg ¾ kg 1 / 8 kg 7 / 8 kg 1 / 32 kg 31 / 32 kg 1 / 128 kg 127 / 128 kg

8 Radiometric Dating As the rock ages, the parent decays away and the daughter increases. So the ratio of the amount of daughter to parent (known as D/P) increases. D/P tells us the number of half-lives that have passed. The length of the half-life tells the amount of time that has passed. A solid object, like a rock, will hold onto parent and daughter isotopes inside indefinitely. The only way for the history of the rock to be destroyed is if the rock melts and its atoms mix with other atoms from other rocks.

9 Radiometric Dating In reality, several parent-daughter pairs are used to confirm that the dates are consistent. A remarkably accurate and consistent history of the Solar System can be built in this way. This process can be used to date samples from Earth, the Moon, Mars, and meteorites to learn their individual histories. Plans call for samples from asteroids and the planets to be brought back to Earth for study.

10 If a rock contains equal amounts of daughter and parent isotopes, how old is the rock? A. Less than 1 half-life B. Exactly 1 half-life C. Exactly 2 half-lives D. More than 2 half-lives

11 To find the true age of a rock, you need: A. The ratio of daughter to parent isotopes B. The half-life of the parent isotope C. Both A and B D. Neither A nor B is sufficient

12 A rock contains 3 times as much daughter isotope as parent isotope (D/P = 3). If the parent isotope has a half-life of 1 million years, how old is this rock? A. 1 million years B. 2 million years C. 3 million years D. 4 million years

13 You notice an interesting rock on a hike and bring it to a lab. The lab reports that D/P = 15 for samarium-146 and neodymium-142. You find out that samarium-146 decays into neodymium-142 with a half-life of 103 million yrs. How old is your rock? A. About 100 million years B. About 140 million years C. About 400 million years D. About 1.4 billion years (1400 million years)