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This Planetary Society article explains how crater density per unit area on the lunar surface helps identify what regions are older/younger than others. It also mentions that this chronology can be used as a basis for the chronology across the rest of the solar system.

Is this methodology accurate? I understand that there aren't many alternatives at the moment, but can we really assume that regions on the Moon and (say) Mars with a similar crater density were struck at around the same time? How can we verify that these craters weren't simply formed by two temporally distant different events with a similar impact density?

Edit:

The article linked above explicitly says that crater counting on the Moon acts as basis for chronology for other bodies:

That's the lunar crater chronology: The spatial density of craters larger than a standard size versus radiometric age (we use 1 km as that standard size). This crater chronology is then scaled and used as a basis for the chronology across the rest of the solar system. When you hear someone say that something on the surface of Mars is X number of years old, chances are that's based on the lunar samples from the 1960s and 70s and the crater counting done 40 years ago.

My question is about crater couting for relative chronology between celestial bodies - is it really reliable? I guess it's the best we can do at the moment.

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    $\begingroup$ My understanding is that crater density dating is used for each celestial object individually. A higher density for a particular object, say the Moon, implies that region of the Moon is older than a region on the Moon with a lesser density. If the Moon has a crater density of say x craters per square km it doesn't mean that if Mars has a region with the same number of craters per square km the two regions were bombarded at the same time, in solar system history. It's all relative to the history of individual celestial objects. $\endgroup$
    – Fred
    May 16 at 0:40
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    $\begingroup$ @Fred That sounds like a good answer! $\endgroup$ May 16 at 9:36
  • $\begingroup$ Thanks for the reply! The article I linked above explicitly mentions that lunar chronology through crater counting helps relative dating of other celestial bodies like Mars. See my edited question above. $\endgroup$
    – olamarre
    May 16 at 21:58
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    $\begingroup$ That's whack. At best, I can think they defined a given curve (likely not an especially smooth one, as bombardment levels varied) using Lunar data & are applying that same curve to relatively compare Martian samples. I doubt the numbers are absolutely related at all, however. $\endgroup$ May 19 at 22:36

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It can be done, to a certain level of accuracy.

The question boils down to whether cratering rates are unique to individual planets, or common between planets within the solar system.

The answer is both. Some cratering was common to all the planets in the early solar system due to a large amount of solar system crossing projectiles falling in from the Oort cloud and Kuiper Belt.

However, some cratering will be due to debris left in orbit after a planet forms, and planets will have different cratering rates due to gravity differences, proximity to other gravity wells (Earth, Jupiter), and in Mars' case, proximity to the asteroid belt.

In addition, impactors from the outer solar system will be traveling at a higher velocity as they get closer to the sun, making impacts more energetic on the inner planets and changing the average size of craters from the same epoch.

All that said, if these various differences can be quantified it should be possible to build models that allow you to compare cratering rates on different bodies to match ages to some degree of accuracy.

There has been work done to determine this. For example:

Mars/Moon Cratering Rate Estimates

This article presents a method to adapt the lunar production function, i.e. the frequency of impacts with a given size of a formed crater as discussed by Neukum et al. (2001), to Mars. This requires to study the nature of crater-forming projectiles, the impact rate difference, and the scaling laws for the impact crater formation. These old-standing questions are reviewed, and examples for the re-calculation of the production function from the moon to Mars are given.

The paper gives tables of cratering rates and crater sizes between Mars and the Moon, which would allow age comparisons given the measurement of either.

Short form: Cratering rates are different between the planets, with Mars/Moon ratio being 0.6-1.2. So the number of craters per area will not be the same for similarly-aged planets/moons, but by characterizing and numerically measuring the difference in cratering rates and sizes between them, counting craters on one planet can be used to estimate age by comparing crater counts on the Moon and applying a proper transfer function to the number.

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  • $\begingroup$ Oh interesting! Good to know, thanks for the detailed answer. $\endgroup$
    – olamarre
    May 20 at 12:39
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Crater density dating is used for each celestial object individually. A higher density for a particular object, say the Moon, implies that region of the Moon is older than a region on the Moon with a lesser density. If the Moon has a crater density of say $x$ craters per million square km it doesn't mean that if Mars has a region with the same number of craters per million square km the two regions were bombarded at the same time, in solar system history. It's all relative to the history of individual celestial objects.

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    $\begingroup$ Thanks for the reply - that's what I thought too. But the article I linked above explicitly mentions how lunar crater counting acts as basis for other celestial bodies. I've updated my question above and included the quote I'm referring to. $\endgroup$
    – olamarre
    May 16 at 22:02
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In December of 2020 China's Change'e 5 brought a 3.81 lb sample from the moon. The dating of bits from that sample to about two billion years ago helps to estimate the age of celestial surfaces with a similar density of cratering. Moon rocks brought to Earth by Chinese mission fill key gaps in solar system history. So the ability to date celestial surfaces by cratering is improving. Cratering density gives us an estimate of age but it is the best that we can get from photographs without bringing back samples.

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  • $\begingroup$ Thanks for the reply! I understand how relative chronology works (and how having physical samples let us date in an absolute manner). The article I linked above explicitly mentions that lunar chronology through crater counting helps relative dating of other celestial bodies like Mars, and this is mainly what my question is about. See my edited question above. $\endgroup$
    – olamarre
    May 16 at 22:00

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