The Origin of Our Solar System
Part 2
Aging Our Solar System
Starter
• How would you age the solar system? What
method(s) would you, or could you, use?
The Age of the Solar System
• According to Solar Nebula Theory (SNT), the
planets should be about the same age as the
sun.
• In order to test this, we can analyze the
radioactive elements found in rocks to date
them.
The Age of the Solar System
• When a rock solidifies, it incorporates known
%s of chemical elements.
• Some elements have “differing forms” which
we call isotopes.
– Isotope - each of two or more forms of the same
element that contain equal numbers of protons
but different numbers of neutrons in their nuclei.
Therefore, they have different atomic masses but
identical chemical properties.
The Age of the Solar System
• Some are radioactive and decay into other
isotopes – like our example below.
The Age of the Solar System
• Half-Life – the time it takes for half of a
radioactive substance, the parent isotope
atoms, to decay into daughter isotope atoms.
• The abundance of a radioactive element
gradually decreases as it decays, and the
abundance of daughter elements gradually
increases.
The Age of the Solar System
• There are many radioactive substances we can
use. For example:
– Potassium 40, half-life of 1.3 billion years.
– Uranium 238 decays with a half-life of 4.5 billion
years – decaying into lead 206.
– Either of these radioactive substances, along with
others like Rubidium 87, can be used a radio
active clock to date mineral samples.
The Age of the Solar System
• To get radioactive age, you need to get rock
samples into a laboratory.
– We have samples from Earth, the moon, Mars,
and meteorites.
http://www.space.com/1
4268-rare-marsmeteorite-rockstissint.html. “Rare Mars
Rocks Crashed to Earth in
July.”
The Age of the Solar System
• The oldest Earth rocks discovered are tiny zircon
crystals from Australia.
– They were formed 4.4 billion years ago.
• This does not mean the Earth is 4.4 billion years
old, but that the last time the zircon crystals
were melted, when their radioactive clocks were
reset to zero, was 4.4 billion years ago.
– Earth crust is active and rocks are constantly being
destroyed – thus resetting their radioactive clocks.
The Age of the Solar System
• The zircon crystals tell us that the Earth is at
least 4.4 billion years old.
– They give us only a lower limit to Earth’s age.
The Age of the Solar System
• Lunar rocks (brought to Earth by Apollo).
• The moon, unlike Earth, is not geologically
active.
• Many of its rocks should have survived
unaltered since the beginning of the solar
system.
• In fact, the oldest moon rocks date to 4.5
billion years ago (thus moon is at least 4.5
billion years old).
The Age of the Solar System
A Moon Rock
The Age of the Solar System
• Over a dozen meteorites found on Earth have
been chemically identified as having come
from Mars.
– Most have an age of “only” about a billion years
but one is approximately 4.5 billion years old.
– Therefore, Mars must be at least that old.
The Age of the Solar System
• Meteorites are our most important source for
determining the solar system’s age.
• Many meteorite samples have ages of 4.56
billion years old – none are older.
• 4.56 billion year old (often rounded to 4.6
billion years) is the widely accepted age for
the solar system.
The Age of the Solar System
• The sun – astronomers estimate the age of the
sun at 5 billion years.
– This is not a radioactive date because you cannot
get a sample from the sun.
– Sun-age-estimates come from helioseismological
observations and mathematical model of the sun’s
interior.
• What is helioseismology?
http://image.gsfc.nasa.gov/poetry/ask/a1138
0.html