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On other solid-surfaced worlds -- which I'll call "planets" for brevity, even though I'm including moons and asteroids -- we haven't yet found a single fossil.

Something else must serve to establish a relative time sequence. Earth is an unusual planet in that it doesn't have very many impact craters -- they've mostly been obliterated by active geology.

If an impact event was large enough, its effects were global in reach.

For example, the Imbrium impact basin on the Moon spread ejecta all over the place.

The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. We can all agree (to the extent that scientists agree on anything) to the fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make new discoveries to improve that calibration, or estimate as best we can based on the data we have already.

To show you how this calibration changes with time, here's a graphic developed from the previous version of Fossils give us this global chronostratigraphic time scale on Earth.

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.

This is called the chronostratigraphic time scale -- that is, the division of time (the "chrono-" part) according to the relative position in the rock record (that's "stratigraphy").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.

This is called the chronostratigraphic time scale -- that is, the division of time (the "chrono-" part) according to the relative position in the rock record (that's "stratigraphy").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.The Geologic Time Scale is up there with the Periodic Table of Elements as one of those iconic, almost talismanic scientific charts.