Science Education Standards: Earth and Space Science -- Content Standard D

Earth in the Solar System

• The Earth is the third planet from the sun in a system that includes the Moon, the sun, seven other planets and their moons, and smaller objects, such as asteroids and comets.

• Most objects in the solar system are in regular and predictable motion. Those motions explain such phenomena as the day, the year, phases of the Moon, and eclipses.

Short Description: A list of the ages of the oldest lunar rock samples is grouped into families with about the same average ages to estimate the age of the lunar mare.

Apollo astronauts recovered over 840 pounds of lunar rocks, and during the last 30 years, these have been carefully studied to find out which features came first, and the ancient history of the lunar surface including its formation. The table below shows the ages (in millions of years: Myr) of some of the mineral specimens determined by geologist using various radioisotope methods applied to the different rock samples.

Problem 1 - What is the average age of all of the samples in the table in millions of years?

Problem 2 - What is the average age of all of the samples in the table to the nearest 0.1 billion years?

Problem 3 - Order the samples from oldest to youngest age. About how many different groups can you identify in terms of their similar ages, and what is the average age of each group of samples in billions of years?

Problem 4 - To the nearest 100 million years, about how old are the lunar mare (dark regions on moon - basalts) compared to the mountainous highland regions (KREEP, anorthosites, zircons and plagioclase) in billions of years?

Answer Key

Problem 1 - Answer: Add the 16 numbers in the last column together and divide by 16 to get 63,949 million years/16 = 3,997 million years.

Problem 2 - Answer: Add the 16 numbers in the last column together and divide by 16 to get 63,949/16 = 3,997 million years, which equals 3.997 billion years which needs to be rounded to 4.0 billion years.

Problem 3 - The samples ordered from oldest to youngest age:

About how many different groups can you identify in terms of their similar ages, and what is the average age of each group of samples in billions of years? (One possibility is shaded n the PDF.)

Oldest Group: (4.56+4.47+4.42+4.418+4.331)/5 = 4.4 billion years.

Middle Group: (4.29+4.15+4.09+3.98+3.9+3.85)/6 = 4.0 billion years

Young Group: (3.79+3.75+3.5+3.25+3.2)/5 = 3.5 billion years.

Problem 4 - To the nearest 100 million years, about how old are the lunar mare (dark regions on moon - basalts) compared to the mountainous highland regions (KREEP, anorthosites, zircons and plagioclase) in billions of years?

Answer: The lunar basalts are mostly in the younger group with an age of 3.5 billion years. The highland samples have ages (4.56 + 4.47 + 4.42 + 4.418 + 4.331 + 4.290 + 4.090)/7 = 4.37 billion years, or rounded to the nearest 0.1 you get 4.4 billion years.

Note: The highland materials sampled by the Apollo astronauts are older than the mare samples by nearly 1 billion years, and represent the ancient, and very old, original crust of the moon before volcanism filled the mare with lava during the first billion years after lunar formation.