New study of solar system dust shows some is from interstellar space

Professor Michael Rowan-Robinson and Queen guitarist Dr Brian May have accurately modelled the composition of zodiac dust in the inner solar system.

Professor Rowan-Robinson is former Head of Astrophysics at Imperial, and Dr May recently completed the PhD he abandoned to become Queen’s lead guitarist.

In a new study, they conclude that 70 per cent of the dust that is found between the Sun and Mars comes from comets, 22 per cent is from asteroids and around seven and a half per cent comes from outside the solar system, dust from interstellar space.

The work is an exteion of Dr May’s PhD research, which was supervised by Professor Rowan-Robinson. In his blog in December, when the research was accepted for publication, Dr May wrote: “It's something close to my heart, because I wanted to publish this part of the research in the 1970s but was dissuaded.”

The models that Professor Rowan-Robinson and Dr May propose fit well against the recorded observations from space missions IRAS, Ulysses and COBE. Their predictions that dust from comets accounts for around 70 per cent of the material that makes up a fan-shaped cloud up to the orbit of Mars, is a lower than some previous estimates of over 90 per cent.

On top of this, they estimate that a small but significant proportion of the dust comes from beyond the solar system. Previous calculations had suggested that a combination of interstellar and asteroidal dust accounted for under 10 per cent of the dust in the inner solar system, but this new study proposes that around seven and a half per cent originates from outside the solar system while the combined figure for interstellar and asteroidal zodiacal dust is around 30 per cent.

Professor Rowan-Robinson said: “Our models explain the infrared radiation from zodiacal dust seen by the IRAS and COBE missions with unprecedented accuracy.  Besides giving a more accurate figure for the relative contributions of debris from comets and dust generated by collisions between asteroids in the main asteroid belt between Mars and Jupiter, the key new ingredient is interstellar dust flowing through the Solar System.  This is a nice confirmation of an idea put forward by Brian and his Imperial collaborators in the early 1970s, subsequently confirmed by measurements on Ulysses and other spacecraft.

“The zodiacal dust cloud, asteroid belt and cometary cloud make up the Sun’s ‘debris disk’.  Such debris disks are known now around many nearby stars.  Understanding the composition of the zodiacal dust will contribute to a better picture of how the Sun’s debris disk evolves with time.

“Despite the higher proportion of interstellar material we have estimated arriving at the inner Solar System, we think only around one per cent of the zodiacal dust arriving at the Earth would be from an interstellar source,” concluded Professor Rowan-Robinson. “These particles would then mostly burn up in the Earth’s atmosphere, but it’s nice to think a tiny fraction of the dust we sweep up from our floors might come from interstellar space.”

Learn more about this story on the Imperial Podcast.

What is zodiacal dust?

Zodiacal dust comprises tiny particles that have fallen from the trail of a comet, broken off from a collision between asteroids, or travelled from beyond our solar system. They reflect light from the Sun and this is responsible for the phenomenon of the zodiacal light, visible from dark clear sites just after dusk or before sunset as a faint glow along the zodiac – the path along which the sun orbits in the Milky Way . In the inner Solar System the dust particles have a fan-like distribution.

The cometary and asteroidal particles spiral in circular orbits and take about 10,000 years to travel from the asteroid belt to the Earth, while the interstellar particles stream through the Solar System in about 50 years.

Eventually most of the dust is destroyed as it is falls into the Sun, but the constant creation of new dust from the three sources – comets, asteroids and interstellar – replenish the fan.

Asteroids in the solar system

Between Mars and Jupiter lies the asteroid belt, containing millions of asteroids, ranging in size from a few metres to a few kilometres.   Groups of asteroids travelling in very similar orbits are known as families, and it is collisions between members of asteroid families which generates some of the zodiacal dust.