Background

Creationist David M. Harris (1990) argues that Saturn’s rings are problematic for “evolutionists” for two reasons:

[1] Saturn is believed to be billions of years old, but the present condition of its rings means they can’t be more than 100 million years old.

[2] The universe is believed to be about 15 billion years old, but the circumstances which might form Saturn’s rings could not possibly happen in this time.

He and other creationists (e.g. Snelling 1990) also argue that what we know about Saturn’s rings is consistent with recent creation of the universe.

The short answer

The age of the rings of Saturn is still not settled. Estimates based on the latest data place the age of the rings at anywhere from 10 million to several billion years old. Even the lower bound here is inconsistent with young earth creationism.

The longer answer

I. The age of Saturn’s rings

(a) The most obvious thing to point out is that there’s no reason why a planet can’t be older than its rings, so it’s difficult to understand why Harris thinks his first point poses a problem for science.

(b) Data from the Cassini spacecraft (which finished its mission in September 2017) initially was interpreted as indicating that the rings are anywhere between 10 and 100 million years old (Iess et al. 2019). That interpretation has been disputed by other researchers who still estimate an age of around 4 billion years (Crida et al. 2019). Even the lower bound here is not consistent with creationist timescales.1

II. Could Saturn’s rings have formed naturally?

The standard hypothesis at the time Harris wrote was that Saturn’s rings were formed by a collision between a comet and a large mass orbiting Saturn. Harris’s contention that “the circumstances which might form Saturn’s rings could not possibly happen” purports to be based on a calculation by W.-H. Ip that shows that such a collision would not take place in 30 billion years.

(a) Collision between a comet and a large mass orbiting Saturn is not the only way under consideration by which the rings could have formed. For instance, as Nature reports, they “may have been formed from the death of an early Titan-sized moon whose upper layers were ripped off as it spiraled into the infant Saturn” (Lovett 2010). None of this is settled, so discussing Ip’s calculation may be completely academic; for those who are interested, however, I do so anyway.

(b) I do not have access to the Sky and Telescope article upon which Harris relies for his information, but I presume the calculation ultimately derives from a more scholarly article in Astronomy and Astrophysics (Ip 1988). Looking at that article, we see that Ip’s actual best estimate for the time scale of the necessary collision was around 10 billion years (he initially said around 5.5 billion, but added a correction in proof). This estimate was based on calculations that assumed that the target of the impact had a material strength equivalent to that of solid ice. Ip goes on to point out that a material strength ten times higher than this would imply a 30-billion year time scale, but he stands behind the 10-billion year figure. So the time scale Harris cites is off by 20 billion years.

(c) It is important to understand what Ip means when he talks about the time scale of a ring-forming collision. Harris takes a 30-billion year time scale to mean that “such a ring-forming collision would not happen in 30 billion years” (Harris 1990, bold in original). However, this cannot be what Ip meant. Let \(P\) be the probability per comet of an impact sufficient to produce the rings of Saturn. Let \(N\) be the number of new comets entering the solar system per year. Then, according to Ip, the time scale of a ring-forming collision is given by \( 1/ (P \cdot N) \). This number is not the number of years that has to pass before a collision event occurs.

If that is not immediately clear to you, then imagine rolling a fair six-sided die once per second, and ask yourself what the time scale is for a roll of “2.” Using Ip’s formula, you would take the probability of a “2” per roll ( \(P = 1/6 \) ), multiply it by the number of rolls per second ( \( N =1\) ), and then take the reciprocal of the resultant number. So, in this example, the time scale for a roll of “2” turns out to be six seconds. Does this mean that you will not roll a “2” until six seconds have elapsed? Of course not—you probably know this intuitively. In actuality, you have a little over a 42% chance of rolling at least one “2” in the first three seconds.2 So Harris is mistaken: not even a time scale of 30 billion years (even if that were Ip’s preferred time scale) would show that a ring-producing collision could not have occurred within the history of the universe. With a 10-billion year time scale a collision event 4.6 billion years ago, at the formation of the solar system, does not strain credulity.

Ip’s calculations do suggest that a collision between a comet and a moon of Saturn within the last 100 million years would be very improbable. But worries about improbabilities have to put in their appropriate context. We do know that cosmic collisions continue to occur—some of us are old enough to remember Shoemaker-Levy 9 doing a bullseye on Jupiter—while no one has seen a disembodied spirit conjure planetary rings out of nothing by intoning a few words of Hebrew (plus the mechanism behind this is a tad unclear). Scientists will not be embracing “God did it somehow through magical ghost powers” as their hypothesis of choice any time soon.

Notes

1 In an earlier draft, I mentioned that some early Cassini observations suggested that the rings were ancient (see Esposito 2007). Those observations have been superseded by the measurements reported in Iess 2019. But the interpretation of those measurements by Iess 2019 is disputed by Crida 2019.

2 If you’re interested, the probability of rolling a given number at least once in \( n \) rolls of a fair die is \( 1 – (5/6)^n \).

References

Crida A et al. 2019. Are Saturn’s rings actually young? Nature Astronomy 3:967–970.  https://doi.org/10.1038/s41550-019-0876-y

Esposito LW et al. 2007. Moonlets and clumps in Saturn’s F ring. Icarus. doi:10.1016/j.icarus.2007.10.001

Harris DM. 1990. How old are Saturn’s rings? Creation 12(4):40-41.

Iess L et al. 2019. Measurement and implications of Saturn’s gravity field and ring mass. Science. doi: 10.1126/science.aat2965

Ip W-H. 1988. An evaluation of a catastrophic fragmentation origin of the Saturnian ring system. Astron. Astrophys. 199:340-342.

Lovett R. 2010. Saturn’s rings formed by destruction of giant moon. Nature. doi:10.1038/news.2010.515

Snelling AA. 1997. Saturn’s rings short-lived and young. TJ 11(1):1.