Over the years I’ve talked about the “Geology of the Rocky Planets” - Moon, Venus, Mars, Mercury + Meteorites … and of course the Earth. Here, I want to talk about new results from a spectacular NASA mission named StarDust to investigate a comet - Comet Wild2 - paying particular attention to the rocky components of the comet and the surprising new perspective they give us on the development of the early history of our Solar System that we, as amateurs, observe through our telescopes. Let’s first remind ourselves of what a comet is - and clarify what the StarDust mission was looking for.
Over the past decade, there have been 3 bright comets visible from Ottawa. Here’s Hyakutake as it appeared 1996.
Notice that it has two components: a bright head or “coma” and an incredibly extended tail
Notice that it has two components: a bright head or “coma” and an incredibly extended tail
This is comet Hale-Bopp as it appeared in 1997.
This comet clearly showed 3 components : a bright head, and two tails ... one made of gas and the other of dust particles.
Most comet tails show dust and gas components, but they’re not always as easy to separate visually as in this case.
This comet clearly showed 3 components : a bright head, and two tails ... one made of gas and the other of dust particles.
Most comet tails show dust and gas components, but they’re not always as easy to separate visually as in this case.
Most recently MacNaught, observed by many of you in early January, 2007, here imaged by Allen Marincak Of Ottawa.
Comets are commonly thought of as balls of dirty ice that warm up as they approach the Sun.
As they warm up, the ice sublimates and creates the gas tail, and the “dirt” is liberated to form the dust tail.
Comets are commonly thought of as balls of dirty ice that warm up as they approach the Sun.
As they warm up, the ice sublimates and creates the gas tail, and the “dirt” is liberated to form the dust tail.
With this understanding of comet behaviour, the StarDust mission was designed to send a probe into the tail of a comet in order to sample the dust and the gas given off by the sublimating comet head. Note the “Tennis Racquet” at the back to the probe : we’ll come back to that later. The Question here is : “Why does NASA want to investigate comets ?” To answer that question, we need to remind ourselves of a few assumptions that planetary scientists have made about the Solar System … assumptions that are now being challenged by the StarDust results !
The first assumption is that the Solar System formed 4.6 Billion years ago by the condensation of a central star and the accretion of its attendant planets from the Solar Nebula, a rotating disk of dust and gas. The Solar Nebula had a thermal gradient, with cold outer reaches beyond the Planets – which is where comets formed. The important point here is that comets are considered to contain ice and “dirt” representative of the early Solar Nebula .
The second assumption is that the Solar System is chemically zoned, and in terms of its constituent planets this is certainly true : Rock - Gas - Ice planets. The StarDust mission was designed to investigate the “dirty” stuff of comets – the “Rocky Components” if you like. This “dirt” fascinates planetary scientists because it can tell us a great deal about the early history of our Solar System.
What are those outer cold outer reaches that comets are supposed to have formed in?
Beyond Neptune : Kuiper Belt - Scattered Belt - Oort Cloud.
In theory, they contain rock/ice bodies up to dwarf planets - plus balls of “dirty” ice = comets.
Beyond Neptune : Kuiper Belt - Scattered Belt - Oort Cloud.
In theory, they contain rock/ice bodies up to dwarf planets - plus balls of “dirty” ice = comets.
That 3D image was rather complicated … here’s a plan view of the Kuiper Belt, beyond Neptune - includes Pluto now.
Scattered Belt extending out to 200 AU beyond the Kuiper Belt (blue/red).
Note the orbit of Jupiter (green) for scale
Note the orbit of Jupiter (green) for scale
So, here’s Wild2 - through the telescope ... and in someone’s overly-vivid imagination !
A more sober - perfect - image of the core of Comet Wild2 … and a sketch map of its principal surface features :
including some dance steps (I can’t tell if it’s a slow Waltz or a Quickstep !
including some dance steps (I can’t tell if it’s a slow Waltz or a Quickstep !
This is what StarDust was trying to sample : it’s not exactly solid ! These are tiny micron (1/1000 mm) size grains, vaguely stuck together as loose aggregates – “traveling sand piles” – that moved faster than the speed of a bullet (6.1 km/sec) ! How did they capture them without totally destroying them ?
Remember the Tennis racquet ?
What was it made of ?
What was it made of ?
Here’s a close-up of the aluminium frame of the Tennis Racquet that was used to capture the comet dust.
Think of it as a honeycomb with the holes filled with a special material … AeroGel.
Think of it as a honeycomb with the holes filled with a special material … AeroGel.
AeroGel is a lightweight material made of silica foam - strong enough to slow the comet dust particles to a halt – disrupting them it’s true – but without destroying the component parts
What happens when comet dust hits Aerogel ? On the left you see a test image in the lab where artificial particles where fired at AeroGel. On the right is an example of the carrot-shaped impact track in the real StarDust Tennis Racquet, where the initial impact is at the fat end in the foreground - and the dust particles explode and distribute debris along the rest of the track, with a single terminal particle at the far end of the narrow part !
So, what did StarDust find when it looked at comet dust from Wild2 ? It found two kinds of dust … called amorphous and crystalline. So what? - what does this mean in plain English - and how does this help you and me understand the early evolution of the Solar System ?
Chemically speaking, the Comet “dust” or “dirt” is very similar to what we find in rocks on Earth or in meteorites. in brief it corresponds to minerals rich in silica such as such as olivine and pyroxene, and metallic minerals such as Fe-Ni sulphides that are closely related to fools gold or “Pyrite”. However - and this is the important part - these chemical compositions come in two forms : crystalline and amorphous. What does this mean? Crystals of a given chemical composition always have same shape - Why?
If you could see individual atoms in a crystalline material, this is what you would see. In crystalline materials atoms are neatly aligned along planes in 3D space with a predictable regularity, which when you extend this to macroscopic (visible) scales is why they form reproducible and diagnostic crystal shapes. Although this image shows a very simple arrangement of atoms to illustrate the principle, sometimes they can be quite complex. This is typical of silicate minerals such as olivine, pyroxene, feldspar and quartz that are common in the Rocky Planets, in asteroids and in meteorites throughout the Solar System. Most of these formed within a limited radius of the early Sun, as determined by the temperature gradient of the early Solar Nebula. Some of this crystalline material, such as the mineral Perovskite, requires extremely high temperature conditions that would only occur very close in to the early Sun, i.e. the very innermost parts of the Solar System.
By contrast, amorphous materials have a random atomic structure – in other words there is no internal order, hence they do not form crystals at the macroscopic (visible) scale. The commonest example of this is glass – which in reality is a frozen liquid.
Stars eject enormous quantities of dust, including silicate dust, especially when they explode as super novas. Although these stellar dust grains initially form as crystalline materials with a well ordered internal atomic structure, they can hang around in inter-stellar space for quite a while before getting caught up in a second cycle of star formation such as the events that formed our Solar System 4.6 Ga ago. The problem for crystalline materials in inter-stellar space is that they are exposed to bombardment by enormous numbers of subatomic fragments called “cosmic rays” – which are not “rays” at all - that disrupt and randomise the well ordered atomic structure of crystalline materials. The resulting internal structure is called “amorphous” – or without form !
So, StarDust found inter-stellar dust from outside of the Solar System mixed with dust that can only have formed in the innermost parts of the Solar System. As you’ve probably already realised, this presents planetary scientists with one hell of a conundrum !!
How do you get crystalline dust from the innermost parts of the early Solar System into the outermost parts of the Solar System – the Kuiper Belt, the Oort Cloud and the Scattered Belt - where the ice component of comets must’ve formed? The only reasonable solution is that the dust components of the early Solar System were thoroughly stirred up, such that crystalline dust from the inner Solar System mixed with ice and amorphous dust in the outermost reaches of the Solar System. However, this leaves us with a major headache !!!
The Solar System today is clearly zoned. Planets that formed near the Sun are still near the Sun – and those that formed further away are still further away. How do you mix dust … but not planets ?
In theory - and this is my opinion - this is relatively simple : mixing of Solar System materials was size dependent. Think of a 4 cars, two 16 wheelers and 2 buses lined up on a bridge in a dust storm : the dust will get stirred and mixed but the vehicles will not change place. However, in the Solar System the big question that remains unsolved is "what was the process that mixed the fine grained dust in the first place?". Planetary scientists are still struggling with that one, so we’ll leave it to another day !
Once again, we find that while cosmologists are puzzling over the origins of the universe, and astronomers are busy looking for planets round other stars, our own astronomical neighbourhood – our own Solar System – still defies our complete understanding.
Once again, we find that while cosmologists are puzzling over the origins of the universe, and astronomers are busy looking for planets round other stars, our own astronomical neighbourhood – our own Solar System – still defies our complete understanding.