Wednesday, July 15, 2015

Pluto: Because science ...

As discussed a little in my previous blog, the last few weeks have been really exciting to watch the New Horizon's probe get closer.

Let's start by showing you something - at the start of the year, this was as good as it got for Pluto - our image taken through the Hubble telescope ...

For all the amazing optics Hubble offers, it's not substitute for being up close and personal.  What has been amazing for me is to revisit much of the astronomy I learned whilst a graduate at the University Of Sheffield, and look at ways to apply the lectures of Professors Fred Combley and David Hughes to what I was seeing.  When we visited Neptune, I was just about to start studying with them, and hence I saw this planet only through their explanations.  But Pluto was mine to look at and make observations!

I'm also aware from discussions in my office, how very much I take what I know for granted, so this is an opportunity to give a quick lesson in science, to talk about how what we've seen in Pluto on the one hand confirms a lot of our models, but also asks so many new questions to a scientifically inquiring mind.

How our Solar System was formed

Actually to be honest, we don't know for certainty.  But what we have is a model which fits as many of the facts that we have - that essentially is what science is.

We believe that all star systems started out like this - a huge nebula of gas, which we see throughout the galaxy.

Typically these clouds are made of about 80% hydrogen, about 19% helium (the most abundant elements in the galaxy), and less than 1% is "everything else".

But gas clouds don't stay as gas clouds forever - even in the example above you can see that it's irregular, meaning some bits are more dense than others.  And this is where a phenomenon called accretion comes into play.  When the nebula is evenly distributed, gravitation (which is caused by the mass of an object) is pulling in all directions.  But once you have a clump of higher density, then this piece starts acting more like a point source, and starts to have higher gravitation.  This means it pulls in more gas from around it ... which means it gets heavier ... which means it's gravity increases ... which means it pulls in more material ...

So a lot of material from the nebula is pulled in, most goes to the star, but some actually accretes itself into other objects orbiting around the forming star.  This left over material forms the solar system as we know it.

All the planets in our solar system - and indeed our Sun - are made from the same source material.  So why do they seem so different?

One factor as we've talked about is gravitation from the Sun.  The Sun is the largest object in our solar system, and indeed the closer you'd have gone towards the Sun, the denser the cloud these objects would have formed from would be.

This presents us with a problem though - if this is true, then logically the cloud which would form Mercury would be the most dense, and therefore Mercury should be far and away the largest planet in the solar system (instead of Jupiter).  But it's not.

Clearly there's a second factor - which is the Sun's heat and it's solar wind.  When we look at spectral lines from the Sun, we have a good idea of the composition of the gas cloud we formed from.  The closer we are to the Sun, the hotter it is and the stronger the solar wind, especially once the Sun started becoming large enough to burn hydrogen and operate as what we know as a "main sequence star".

All this would mean the closer you were to the Sun, the more volatile lighter elements would be - especially gasses.  These would be blown outward by a combination of radiation from the sunlight and the solar wind (think much of how steam leaves a kettle).  What would be left behind is the heavier elements, the iron and silicates especially.  This is why planets like Mercury, Venus, Earth and Mars are called "the terrestrial planets" being made of mainly rocks and iron.

But as we've said, the further you get from the Sun, the colder it gets.  And at just beyond the asteroid belt, something odd starts to happen.  Gasses become that much less volatile and even "sticky", starting to clump together (and generally acting less like you'd expect a gas to behave).  When you combine "sticky gasses" with that phenomenon of greater density the closer you get to the Sun, you create a perfect storm, and something like Jupiter happens.

Jupiter is an oddity - it's 2.5 more massive than all the other planets of the solar system put together.  It's composed primarily though of hydrogen - the lightest element in the Universe.  And yet in Jupiter it's also the most dense planet.  The hydrogen of Jupiter thanks to the enormous gravitational pressure of the planet is acting in a way we just don't expect of hydrogen on Earth!

Temperature is everything - water and ice are the same substance, but one can be sculpted significantly more than the other.

As we move from Jupiter through the other "gas giants", we get the laws of diminishing returns, each planet smaller than the last (although Neptune and Uranus are almost on a parity).  Beyond Neptune the science says that the density of cloud that could form planets would never be able to form anything significant - or so our lectures in the 1990s went.

And they were right ... but just because we would not see anything significant, does not mean we'd see nothing.  Indeed, thanks to Hubble, we've started to detect a whole load of what we now call proto-planets which form what we now call the Kuiper belt.

This band seems analogous to the asteroid belt - but where that is made from terrestrial iron and silicates, we postulate that the Kuiper belt if made of frozen gasses and ices.  And much as Jupiter dominates the so called "gas giants", Pluto dominates this group of what we could call "ice dwarves".

Here comes New Horizons

Before we look at Pluto's pictures, let's talk about what we knew before this month,

  • We knew that Pluto had several satellites, including Charon
  • We had a good idea of Pluto's mass, thanks to the orbit with Charon
  • We had an idea of the size of Pluto (although it's turned out to be a bit more massive than expected, but not by much)

Knowing it's mass and knowing it's size is a good start, as it allows us to work out it's density.  And knowing it's density we can make a good guess at it's composition just by comparing with other planets.  We expect from Pluto for it to be "mainly ice with some rock".

Okay - we've gone on enough - time to include a picture ...

This is a "false colour" picture of Pluto and Charon.  It's false colour because as Sunlight is about 0.2% that of at Earth, it's similar to trying to work out the colour of something via moonlight.  Thankfully the onboard Lorri camera is set for such conditions.

Let's start by talking to that colour - it's red, mainly red.  Having spent time with Professor David Hughes red makes me think rust and iron oxides.  More than likely, given the density, some kind of iron oxide within ice.

So what of the white heart-shaped band which has so captured the public's imagination?  Well it doesn't really fit.

To me, it's baffling.  I think we're looking at a mainly red world, with an ice patch.  I say think because I can see cratering on the red part of Pluto, which suggests it's older.  So where did this white patch come from?

It could be Pluto turning itself inside out, and the outflow of a massive ice volcano.  The cleaner white ice feels clearly younger than the red patches which dominate.  One way to be sure is to from radar modelling.  Whichever area has "lower altitude" is likely to be the elder area (on the Earth volcanic flow always goes over exiting flow).

What has fascinated me looking at Pluto and Charon next to each other ...

Pluto looks mainly red, with a white patch, and Charon mainly white with a red patch?  Is it possible that there was some kind of low-speed collision between a white Charon and a red Pluto, which would allow for transfer of material between the two?  I'm kind of reminded of a low speed "fender bender" bumping between two cars where paint from each car is transferred to the other.  Spectroscopic analysis over how similar these patches are on the two different worlds will either dispel or add fuel to this idea.

Likewise there's the mystery of the cratering.  We see craters on Pluto, but not many of them.  Our own Moon is heavily cratered, with some craters such as this star shape, the impact is extreme enough for material to almost reach the lunar escape velocity, with deposits over almost a quarter of the surface ...

We see similar phenomenon on many moons around the gas giants, but around such large, massive worlds which are gravitational magnets, that's not too surprising.

Pluto is about 20% the mass of the Moon, so any similar collisions should be as dramatic (if not more so), with material being blow around the globe.  But although we see cratering, so far we've not seen those tell-tall lines radiating from impacts.  Are impacts less frequent and of less energy being so far out?  Well, maybe more detailed images of the planet can answer that.

And then again there's the atmosphere.  The ability for a body to retain an atmosphere is proportional to it's mass - however although Plutos atmosphere is minuscule compared to Earth (no surprise there), it's significantly larger than the much more massive Moon, despite being much smaller in mass.

Again, one factor is believed to be temperature.  The Moon is (in cosmic terms) the same distance from the Sun as us (when it's facing the Sun).  Heat means energetic gas particles which would quickly leave the low gravity of the Moon.  Even though Pluto is much smaller, the gas particles there are less energetic, so less likely to be able to reach escape velocity.

Now add in Pluto's odd orbit, meaning at times it's closer to the Sun than others.  It's possible that some of it's ices could boil or "sublimate", venting gas which would become an atmosphere.  We see similar phenomenon in comets, which too are part-time residents of the Kuiper belt.

Have you noticed so far how every observation we've made so far leads to more questions, and things we can test and check?  This is the nature and fun of science - we answer questions, but in doing so propose new questions.  It's also one of the reasons I find testing to be such a fun discipline to be part of - good testing is good exploring, solving questions, asking new questions.

Stay tuned for more information from New Horizons as it's downloaded (it will take 16 months to download data from the probe) here.  And a special shout-out to friends Dan and Gabrielle who I've enjoyed sharing this journey with.


  1. Great explanation! But you need to replace nearly every "it's" with "its".

    1. Looks kind of 50:50 to me, but thanks for pointing it out. Of course grammar oops like this formed part of a previous blog ...

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