Mark I. Vuletic

Last updated 21 March 2008

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 the evidence is consistent with recent creation of the universe.

Analysis

Saturn and its rings(i) The most obvious thing to point out is that there is no contradiction in Saturn's rings being younger than the planet itself, so it is difficult to understand why [1] in itself should be considered problematic for science.

(ii) The next most obvious thing to point out is that evidence that indicates an age of 100 million years for Saturn's rings is manifestly not consistent with "recent" creation.

(iii) New data from the Cassini spacecraft (Esposito 2007) suggests that material in Saturn's rings is being recycled constantly. If true, this undermines the standard lines evidence that have favored the 100-million year figure. As far as the Cassini data is concerned, the rings may well be as old as the solar system. [Extras: JPL press release | UC Boulder podcast]

The Cassini spacecraftUp until now, the standard hypothesis appears to have been that Saturn's rings were formed around 100 million years ago by a collision between a comet and a large mass orbiting Saturn. Harris's contention that this could not have happened within the lifetime of the universe draws on a calculation by W.-H. Ip which allegedly showed that such a collision event would not take place in 30 billion years. 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 Ip (1988).

(i) Looking at Ip (1988), 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 upon caculations presuming 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 result in a 30-billion year time scale. But it is the 10-billion rather than the 30-billion year figure that he stands behind.

(ii) 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 x N). It should immediately be clear that the resultant number is not the number of years that has to pass before a collision event occurs.

If it is not clear, then consider the following: suppose you roll a single (fair) six-sided die once per second. What is the time scale for a roll of "2"? The way Ip calculates it, 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 you will not roll a "2" until six seconds have elapsed? Of course not; you have, in fact, a little over a 42% chance of rolling at least one "2" in the first three seconds. 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.

Importantly, Ip's calculations do call into question the view that a ring-producing collision took place only 100 million years ago; if Ip is correct, so recent a collision would be extraordinarily improbable. Scientists, however, do not take this as automatic license to jump straight to "God did it." Instead, they first look for alternate hypotheses or better data. If the Cassini data bears out, their skepticism will already have been shown to be justified, as has thus far always turned out to be the case for previous purported evidences against the scientific history of the universe.

References

L.W. Esposito et al., Moonlets and clumps in Saturn’s F ring, Icarus (2007), doi:10.1016/j.icarus.2007.10.001

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

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

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