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Rock of Ages, or Ages of Rocks

How do we know a Moon rock is4.2 Gyr old (for example?)

Age of formation of rocks isdetermined by radioisotope dating

• “all matter is composed ofatoms”

• Atoms are composed of nucleiand electrons

• Almost all of the mass (allexcept about 1 part in 2000) iscontained in the nucleus

• The nucleus is incredibly small(~1-10E-15m)

• Nuclei are composed of twotypes of particles, protons(positive charge) and neutrons(electrically neutral)

See section 7.4 for content

The world of atomic nuclei

• The construction of an atomicnucleus is indicated by itssymbol, such as 7N14 or 713N.

• The lower number is the numberof protons (atomic number), anddetermines the chemistry

• The upper number is the numberof protons and neutrons (atomicweight) and determines thenuclear physics.

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Radioactivity and radioisotopes

Isotope is the term for different nuclei withthe same number of protons (and thus

chemistry) but different numbers ofneutrons. Some isotopes are stable,

meaning they last forever, and some areunstable (radioisotopes) and decay into

“daughter nuclei” and radiation.

The radioactive decay of rubidium 87to strontium 87

Rubidium is selected in formationof minerals in place of potassium.

An atom with a rubidium 87nucleus will decay to strontium 87

How can this be used to date rocks?

The number of parent nuclei (Rb in this case) declines withtime according to a precisely expressed exponential law. Foreach Rb atom that “disappears”, a strontium atom is formed

The concept of the “half life” of aradioisotope

• The amount of time after which half of theoriginal number of radioisotopes is left.

• Ranges from 10 minutes for Nitrogen 13 to 46billion years for Rubidium 87

• By comparing the number of “parent nuclei” to“daughter nuclei”, you can determine theamount of time since the parent atoms weretrapped in the rock

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A simplified version of the idea

time

rubidiumstrontium

How can we be sure there was noStrontium 87 in the rock when it formed?

There would bemore present thanwas produced byradioactive decay,

and we wouldoverestimate the

age of formation ofthe rock.

Answer: there is another common isotope of strontium,Strontium 86, which is not the daughter product of radioactivedecay. Every Sr86 nucleus that was in the rock when itformed is still there. By measuring the abundance of Sr86, aswell as Sr87 and Rb87, we can control for the “affinity” of themineral for Strontium, and unambiguously obtain the age offormation

With some algebra, you can show that

You can thenmeasure x and yfor minerals in apresent day Moonrock, and plotthem up on agraph. Theyshould fall along astraight line(consistencycheck). If theydo, the slope ofthe line gives theage of formationof the rock.

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You can thenmeasure x and yfor minerals in apresent day Moonrock, and plotthem up on agraph. Theyshould fall along astraight line(consistencycheck). If they do,the slope of theline gives the ageof formation of therock.

By comparing the numbers of parent and daughter nucleifrom the rubidium-strontium system, and other sets ofradionuclides, scientists can determine the age of formationof lunar rocks, meteorites, and Mars rocks

Next Topic: Mercury and Venus

We follow the traditional astronomy approach of going out fromthe Sun

Where are Mercury and Venus in the SolarSystem?

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Mercury and Venus in the night sky

• Mercury is always very close to the Sunin the sky, is small in diameter, and isfarther away than Venus. This makes ita difficult object to see. The legend isthat Copernicus never saw it.

• Venus at times is the brightest object inthe sky after the Moon; you can’t missit.

Relative sizes of Mercury and Venus

3/2synchronousrotation and

the weird dayof Mercury

The Messenger spacecraft and the study ofthe planet Mercury

Launch: 2005First flyby: 2008Orbital insertion:2011

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The Messenger Spacecraft: launch andarrival

http://messenger.jhuapl.edu/the_mission/movies.html

The surface of Mercury