Venus, Io, Europa, Titan, and Triton have a similar problem.On almost all the other solid-surfaced planets in the solar system, impact craters are everywhere. We use craters to establish relative age dates in two ways.In fact, I have sitting in front of me on my desk a two-volume work on is not light reading, but I think that every Earth or space scientist should have a copy in his or her library -- and make that the latest edition.In the time since the previous geologic time scale was published in 2004, most of the boundaries between Earth's various geologic ages have shifted by a million years or so, and one of them (the Carnian-Norian boundary within the late Triassic epoch) has shifted by 12 million years.In the science of geology, there are two main ways we use to describe how old a thing is or how long ago an event took place. When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4.6 billion years ago, those are absolute ages.There are absolute ages and there are relative ages. We use a variety of laboratory techniques to figure out absolute ages of rocks, often having to do with the known rates of decay of radioactive elements into detectable daughter products.Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here.Look closely at the Geologic Time Scale chart, and you might notice that the first three columns don't even go back 600 million years.
On Earth, we have a very powerful method of relative age dating: fossil assemblages.The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed.Nowadays, age-dating of rocks has established pretty precise numbers for the absolute ages of the boundaries between fossil assemblages, but there's still uncertainty in those numbers, even for Earth.Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear.When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.