How do we know the age of the surfaces we see on planets and moons? If a world has a surface as opposed to being mostly gas and liqui astronomers have developed some techniques for estimating how long ago that surface solidified. Note that the age of these surfaces is not necessarily the age of the planet as a whole. On geologically active objects including Earth , vast outpourings of molten rock or the erosive effects of water and ice, which we call planet weathering, have erased evidence of earlier epochs and present us with only a relatively young surface for investigation. One way to estimate the age of a surface is by counting the number of impact craters. This technique works because the rate at which impacts have occurred in the solar system has been roughly constant for several billion years.

After samples were brought back from the Moon by Apollo astronauts, the techniques that had been developed to date rocks on Earth were applied to rock samples from the Moon to establish a geological chronology for the Moon.

Furthermore, a few samples of material from the Moon, Mars, and the large asteroid Vesta have fallen to Earth as meteorites and can be examined directly see the chapter on Cosmic Samples and the Origin of the Solar System.

Scientists measure the age of rocks using the properties of natural radioactivity.

Around the beginning of the twentieth century, physicists began to understand that some atomic nuclei are not stable but can split apart decay spontaneously into smaller nuclei. The process of radioactive decay involves the emission of particles such as electrons, or of radiation in the form of gamma rays see the chapter on Radiation and Spectra.

For any one radioactive nucleus, it is not possible to predict when the decay process will happen.

Earth's oldest known meteor crash site found in Australian Outback

Such decay is random in nature, like the throw of dice: as gamblers have found all too often, it is impossible to say just when the dice will come up 7 or But, for a very large number of dice tosses, we can calculate the odds that 7 or 11 will come up.

Similarly, if we have a very large number of radioactive atoms of one type say, uraniumthere is a specific time period, called its half-lifeduring which the chances are fifty-fifty that decay will occur for any of the nuclei.

Figure 2. Radioactive Decay: This graph shows in pink the amount of a radioactive sample that remains after several half-lives have passed. After one half-life, half the sample is left; after two half-lives, one half of the remainder or one quarter is left; and after three half-lives, one half of that or one eighth is left.

Dating Planetary Surfaces

Note that, in reality, the decay of radioactive elements in a rock sample would not cause any visible change in the appearance of the rock; the splashes of color are shown here for conceptual purposes only.

A particular nucleus may last a shorter or longer time than its half-life, but in a large sample, almost exactly half of the nuclei will have decayed after a time equal to one half-life.

Half of the remaining nuclei will have decayed after two half-lives pass, leaving only one half of a half-or one quarter-of the original sample Figure 2.

If you had 1 gram of pure radioactive nuclei with a half-life of years, then after years you would have.

Crater dating models make use of the characteristic population changes brought about by resurfacing events to attempt to date those events in addition to Cited by:

However, the material does not disappear. Instead, the radioactive atoms are replaced with their decay products. Sometimes the radioactive atoms are called parents and the decay products are called daughter elements. In this way, radioactive elements with half-lives we have determined can provide accurate nuclear clocks.

By comparing how much of a radioactive parent element is left in a rock to how much of its daughter products have accumulated, we can learn how long the decay process has been going on and hence how long ago the rock formed.

Table 1 summarizes the decay reactions used most often to date lunar and terrestrial rocks. When astronauts first flew to the Moon, one of their most important tasks was to bring back lunar rocks for radioactive age-dating.

Until then, astronomers and geologists had no reliable way to measure the age of the lunar surface. Counting craters had let us calculate relative ages for example, the heavily cratered lunar highlands were older than the dark lava plainsbut scientists could not measure the actual age in years.

Only inwhen the first Apollo samples were dated, did we learn that the Moon is an ancient, geologically dead world. Using such dating techniques, we have been able to determine the ages of both Earth and the Moon: each was formed about 4.

Crater dating

We should also note that the decay of radioactive nuclei generally releases energy in the form of heat. Although the energy from a single nucleus is not very large in human termsthe enormous numbers of radioactive nuclei in a planet or moon especially early in its existence can be a significant source of internal energy for that world.

For every major event in Earth's history, it seems a space rock was somehow involved. The formation of the moon?

Blame a space rock. The extinction of the dinosaurs?

Impact Cratering Dating Nathan Marsh Relative Dating Simple but not as informative Measures the crater densities (craters per square kilometer) Generally a less cratered area is younger than a more cratered area Reference points in dating The Apollo and Luna missions analyzed specific dates on certain areas of the moon Late Heavy Bombardment craters Gy Light-toned . Jan 22,   Using a method called uranium-lead dating - which can reveal a mineral's age based on how many uranium atoms have decayed into lead - the team determined that the crater was formed roughly Author: Brandon Specktor. View Lab Report - Crater Dating Lab from PHY at Northern Virginia Community College. Nicholas Spiegler Category Crater Counting Lab Lab Write Up Crater counting is a method for estimating the.

Space rock. The eventual total annihilation of our planet?

That'll probably be a space rocktoo. Humans, please don't prove the scientists wrong on that one. Despite Earth's long history of getting smacked by space rocks, evidence of those collisions can be very hard to find; even the largest impact craters vanish over time due to erosion and tectonic activity, taking the best reminders of Earth's past with them.

Now, however, researchers in Western Australia believe they've found the single oldest impact crater ever detected, dating to roughly 2.

Massive Crater Discovered Under Greenland Ice

In a new study published Jan. Today, all that's visible of the once-enormous crater is a small red hill at the area's center, known as Barlangi Hill.

According to the researchers, the minerals inside that hill hold valuable information about the impact's age. Related: Crash! That means its rocky innards hold mineral grains that were smashed, melted and eventually recrystallized by the ancient impact.