[The Apollo astronauts who visited the Moon between 1969 and 1972 remain among the most travelled humans in history, but they didn’t exactly follow the environmentalists’ maxim of “take only memories, leave only footprints”. Between them, NASA’s six successful lunar missions brought back 2,415 samples of Moon rock – totalling 380.05 kg (837.87 lbs), or about three fifths the mass of a cow. But it was worth it; not only were scientists able to confirm once and for all that the Moon wasn’t made of cheese, but detailed analysis of the samples provided significant evidence regarding how and where our closest neighbour in space actually originated.]
These days, space rocks generally only make headlines thanks to their perceived danger to human life and civilisation. However, space scientists have good reason to be seriously interested in the numerous rocky objects found throughout the solar system, as they can potentially tell us so much about its origins.
“There’s still a lot of questions about the details, but what we think happened is that the solar system formed from a huge cloud of gas with tiny dust grains mixed in,” explains Dr Marek Kukula, Public Astronomer at the Royal Observatory Greenwich. “These grains would have been kind of silicates – silicon and oxygen bonded together – similar in size to smoke particles, so really tiny, but they’re actually very important in gathering together other gases and molecules and allowing chemical reactions to form.
“These dust grains then start to bump together, they start to stick together, eventually building up from grains of dust into pebbles, into boulders, and then into mountain-sized asteroids, which then start to clump together into things we recognise as planets,” he adds. “So it really is building from the bottom up.”
However, there’s one serious problem when it comes to working out what these planetary foundations were actually like. “All of the rocks on Earth, although they ultimately come from this interstellar dust material, have been modified,” says Marek. “As the Earth formed, it heated up – those dust grains would have melted together, become a fluid, and later crystallise out in various volcanic processes. So the rocks that we have on Earth have very different composition and structure from the original dust grains.”
Helpfully, not all the rocky material from the early solar system was sucked up and locked away into the planets. “A lot of that debris remains in the form of dust grains or small rocks or even large rocks like asteroids,” Marek adds. “When meteorites fall to Earth, some of them are pretty much unmodified; they are a record of that early stage of the solar system, they’ve been in a deep freeze since then, preserving a record of what the early solar system was made of.”
However, there are many types of meteorites – remnants of space rock which have survived falling through Earth’s atmosphere. Some are now known to have originated from the Moon and Mars, shot out into space by explosive collisions. Others are much more similar in structure to Earth-type rocks, at a stage where the dust has already melted and agglomerated into something larger. It’s believed that these are fragments of larger asteroids which had their dreams of planet-hood literally shattered by a major collision.
Some space rocks may even be fragments from the numerous rocky impacts now believed to have occurred during the early days of the inner solar system – not least the collision between a proto-Earth and a Mars-sized planet, the debris from which condensed to form the Moon – incidentally, a theory significantly supported by those Apollo samples!
While all planets are believed to have rocky cores, those which formed further away from the sun’s heat were able to hold onto much more gas than the inner worlds.
Officially defined in 2006, these are space rocks massive enough for gravity to ensure a spherical or ellipsoid shape, but have not cleared their orbits of other objects.
Coming in all shapes and sizes (from 900 km down to just tens of metres across), these rocky, airless worlds are remnants of the birth of the solar system.
THE ASTEROID BELT
The majority of known asteroids (estimates vary between 1.1 and 1.9 million) perform mildly elongated orbits within this vast doughnut-like ring between the orbits of Mars and Jupiter.
COMETS & COMET NUCLEI
These cosmic snowballs from the edge of the solar system are thought to preserve significant amounts of rock and dust from the early solar system, especially in their cores.
Principally fragments and debris from comets and asteroids, meteoroids are among the smallest objects in the solar system, ranging in size from small grains to metre-wide objects.
Often called “shooting” or “falling stars”, meteor showers are the results of meteoroids glowing hot while entering the atmosphere at speeds in excess of 20km/s (12.4 miles/s).
METEORS VS METEORITES
Meteoroids which burn up in Earth’s atmosphere are meteors; any remnant reaching the ground is a meteorite: so a meteor can become a meteorite, but not all meteors become meteorites!
First published in All About Space #40.