Dating Planetary Surfaces with Craters – Why There Is No “Crisis in Crater Count Dating”

This entry is in specific response to the “Crisis in Crater Count Dating” article from the Institute for Creation Research, written by David Coppedge.

How can astronomers say that Mars had recent volcanism?  Or that the surface of the moon Io is younger than 50 years?  Or that the youngest stretches of terrain on our moon’s surface dates back to about 3 billion years ago?  The answer is one of the basic tools of comparative planetology:  Impact craters.

Impact craters are ubiquitous throughout the solar system – every single solid body has craters on its surface except for the moon Io (because its surface is so young due to the incredible amounts of vulcanism).  Impact craters form when an impactor – like an asteroid or comet – hits the target surface of a planet or moon.  The impact occurs at high speed, and the final crater depth, diameter, and shape are effectively determined by the surface gravity, the mass of the impactor, and the velocity of the impactor.  Almost all impact craters are circles; only impacts at very low angles (less than 10°) will form elliptical craters.

Note:  There are craters of other origins, such as pit craters or caldera craters at the top of volcanoes.  Only impact craters are used to date surfaces, and for brevity I will only be referring to them from this point on as “craters” instead of “impact craters.”

The basic idea behind using craters as an indicator of a surface’s age is that the longer the surface is around, the more craters will form.  If an impactor were to hit a target at a rate of 1 per year, then a surface that’s 1,000,000 years old should have 1,000,000 craters.  But if that surface were to have something happen to it, like it got covered by lava, then that would erase the craters and the crater age would be set back to 0.

That’s effectively what people do in order to date the surfaces of planets or moons that are not Earth:  We count the number of craters of different sizes for a part of the surface and then compare that with the rate of impacts of that size.  This is called “crater age dating,” and it is a form of “relative age dating.”  The reason that it’s relative is that it cannot give an absolute age in years, it can only say if a surface is statistically older or younger than another surface.

To actually calibrate the number of impactors of a given size to an absolute age requires us to date the rocks within that surface.  This was one of the science results from the Apollo lunar missions – samples brought back from the moon were dated in the lab and hence an absolute age could be assigned to surfaces with a certain density of craters (number of craters per area).  This can then be extrapolated to other locations in the solar system.

Craters form in all sizes – from microcraters on airless bodies like the moon to giant basins literally 1000s of kilometers across.  In general, researchers use craters that are on the 10s of meters scale to about 1 kilometer, or a few kilometers to a few 10s of kilometers for age dating (at present, there is a general mismatch gap in what is used; this is generally because the meter-scale craters are used to date smaller, isolated surface areas whereas kilometer-scale craters are used to date much larger geologic units that cover a significant percentage of the planet or moon).

One more piece of background information is that when craters form, they send up clouds of debris, from dust-sized particles to objects up to a few percent the size of the original impactor.  These larger chunks of material are ejected outwards from the forming crater, and they may end up forming their own craters.  These are called secondary craters since they were formed as a result of the original, or primary crater.

Secondary craters are different from primary craters in the way they look because of their formation history — mainly they are much smaller and they are also shallower.  This is both because the ejected material that formed them was much smaller than the original impactor and because the velocity of the debris is much less than the original impactor, so there is significantly less energy to form the secondary crater.  Observations and computer models have shown that the largest secondary craters can only be up to ~5% the diameter of the primary crater (observations made on Earth, Moon, Mars, Mercury, and Europa), although the vast majority are much smaller than 1% of the primary.  In addition, secondary craters that form closest to the primary (within about 10 crater radii) are usually very easy to identify as secondary due to the way they look and the surrounding surface.

The point of this background it that crater age dating has been used for over 50 years, and it rests on very solid theoretical, experimental, and observational grounds.  However, you wouldn’t think that given the ICR article, “Crisis in Crater Count Dating:”  “New thinking about ‘secondary craters’ has thrown this whole foundation of comparative planetary dating into disarray.”

The article continues with misleading statements:  “One writer in Nature estimated that a single large impact on Mars could generate 10,000,000 secondaries, and that 95% of the small craters on Europa could be from fallback debris.”  You are clearly expected to infer from this that almost all craters (95%) on surfaces are secondaries by simply connecting those two phrases together.  That may actually be true.  But there is no size range stated.

Those same authors, Alfred McEwen and Edward Bierhaus, who are not mentioned in that quote wrote a paper in 2006, “The Importance of Secondary Cratering to Age Constraints on Planetary Surfaces,” in the Annual Review of Earth and Planetary Science.  I highly recommend reading it if you are interested in this subject, and it is written at a non-technical level.

In their paper, they show that yes, secondary craters do dominate planetary surfaces, but for Mars (the object of interest at the moment), the critical diameter at which secondary craters dominate is about 1 km.  Craters smaller than 1 km are likely >50% secondaries, but craters larger are >50% primaries.  And because a significant amount of age dating is done with craters larger than 1 km, there is no way that “this whole foundation of … planetary dating [is in] disarray.”

The ICR article goes on from there, and either shows the author’s ignorance of the issues or that they are simply lying:  “Without a way to reliably identify secondary craters …”  As I stated above, the majority of secondary craters that are close to the primary can be identified because of their distinct shapes and characteristics – they are shallower, they are often elongated with the long axis pointing towards the primary, they appear in clusters, and there are generally trails of them that lead back to the primary.  One could also ask the question, “If there were no ‘way to reliably identify secondary craters,’ then how could we know that there even are secondary craters?”  Granted, it does become more difficult the farther from the primary crater and the smaller the secondary, but this becomes a non-issue when using large craters.

But, you can still use small craters to date planetary surfaces.  One of the arguments used is that reference crater densities that indicate a certain age for a surface were created  without taking secondary craters into account – in other words, they have both primary and secondary craters in them.  So when using them to date a surface, it doesn’t matter if there are plenty of secondary craters there because they are in your reference, too.

Besides this, this topic is still in active debate at planetary astronomy conferences today (William Hartmann and Gerhard Neukum are two of the strongest proponents that secondary craters aren’t even an issue for sub-kilometer age dating).  In fact, I was just at a conference – the Division of Planetary Science for the American Astronomical Society held at Cornell University in Ithaca, NY (October 2008) – where Dr. Hartmann presented results which indicated that secondary cratering is not a problem at sub-kilometer diameters for age dating.

In the last crater-related point I want to address from the article, it implies that astronomers applied crater age dating from the moon to other objects “believing they knew how old the earth-moon system was.”  This is true, but it’s not true the way they imply it.  From this statement (the next-to-last paragraph of the article), you are clearly meant to think that we used crater ages from the moon to go to other bodies, but if we don’t even know how many craters equals 1 year or 100 years or 1000 years on the moon, how could we possibly know what it means on other objects?

The answer should be apparent given the background I discussed above:  Craters give relative ages, while radioactive decay dating methods give absolute ages.  We applied the relative ages from the crater densities on the moon to other bodies, so it doesn’t really matter if we don’t know how old that surface is on the moon for that exercise.  But then we can calibrate the relative scale with the absolute scale from the moon because we have independently dated its surface with a completely different method.  Therefore, the ICR article is yet again trying to mislead the reader.

The final point that I would like to address is the article’s last sentence:  “There is an important lesson here, though, for all science lovers:  question assumptions.”  (emphasis mine)

I whole-heartedly agree.  You should question assumptions.  You should try to understand why someone says what they do.  You should do your own research, your own experiments, and make your own observations.  You shouldn’t take my word for it, you shouldn’t take ICR’s word for it, you should go out and look for yourself.

Finally, you should always question someone’s assumptions, especially if they are based in an ideology:  If they start from the premise that the Bible is Truth, the literal word of an omnipotent and infallible deity, and then try to make all observations fit within that view, you should be questioning that assumption.