I had read part of Brian May’s thesis, A Survey of Radial Velocities in the Zodiacal Dust Cloud, a week or so ago, and one section of it had stuck in my mind. Here I quote it below (ellipses mine).

It is remarkable that so few earlier records are available, since…the mysterious Light of the Zodiac ought to have been very conspicuous in the dark skies of earlier civilisations, comparable in dimensions with the Milky Way, and at its brightest, more luminous. … Could it be that our view of the Zodiacal Light is highly variable? Cassini was convinced that it disappeared completely between 1665 and 1681, and this man was certainly no casual observer. I have no solution to the puzzle. In the light of Cassini’s reports, along with Jones’s observations detailed below, and the curious dearth of references pre-Cassini, I am convinced that, despite the lack of any recent confirmation, we must admit the possibility that the Zodiacal Light has not always been what it is today.

The passage in full is worth reading, but the salient detail is that the zodiacal light may be a relatively recent phenomenon.

Later in the same chapter, another note got my attention, here quoted.

One of the great ‘surprises’ in space observations, as noted by Sykes et al (2004), was that the detectors on board the space vehicle Ulysses, once past the orbit of Jupiter, began to register particle impacts from the opposite direction to that expected from interplanetary dust particles, and at high velocities, clearly indicating an interstellar component to the dust cloud, which predominates at these distances from the Sun, but has been now detected, by the Hiten detector, even at 1 AU (Grün et al 1993).

Here we see that the dust particles are interstellar, as well, meaning from outside the solar system.

My mind returned to these two details today, and it occurred to me that the latter detail easily accounts for the former if I posit the explanation that the zodiacal dust cloud originates from interstellar dust through which the solar system has moved during its galactic orbit, encountering it in much the same way the Earth encounters cometary dust during its orbit, giving rise to meteor showers.

In fact, the zodiacal dust cloud’s interplanetary dust particles (IDPs) are located asymmetrically with respect to both the ecliptic and to the circumstellar disc, and I speculate it is for this very reason. (That is to say, in reading Brian May’s summarizations of other surveys’ findings of the locations of the IDPs in our solar system, they tend to be located in particular areas, not in a smooth ring.)

When I turned to chapter four of his thesis, Brian May’s interpretations of his results already include hypotheses about IDPs flowing in from the interstellar medium, both as the solar system moves through it and as the IDPs flow inward. These were vindicated by more recent observations.

The novelty of my speculations, which I’m sharing here, lies in the recency and asymmetrical nature of the zodiacal dust cloud, and the implications it has on understanding the nature of the galaxy.

By this, I mean that although the interstellar dust throughout the galaxy cannot be observed directly, we can extrapolate from our knowledge of the zodiacal dust cloud—our knowledge of the velocities and positions of the IDPs within our solar system—to use our sun’s past as a kind of lantern to shine a light on the dark, dusty path through which we have passed over the past centuries or millennia. In other words, if the spatial distributions of unseen interstellar dust in our galaxy are uneven, we can gain insight into these distributions.

If this is possible, I foresee implications in dark matter physics because it would help refine our knowledge of the mass of the galaxy, which would in turn help narrow the parameters of dark matter.

I’ve also wondered if this has climate science implications. I’ve read about hypotheses involving climate cycles tied to phenomena from the interstellar medium, and clouds of dust may be one such phenomenon.