I can tell you that as Scientists, we get plenty of questions from all over, in fact learning how to field such questions and to get someone an answer (even if you don't already know!) is a big part of the job. This week, Christina, pictured above, takes some time away from her mission control work to answer some common questions in this space.
By Dr. Christina Smith
This time for my blog post, I thought I’d try a slightly different tack to usual. I often find that I get questions when in social situations about science-y or space-y or astronomy-y... things. So, I thought I’d open up the floor on social media to Solar System questions which I would then answer (or attempt to answer) in this blog post. And here we go!
P.S. In the discussion below, I have taken lots of information from papers and sources rather than pulling the information out of my own brain so the references are there in brackets and refer to the full links at the end in case you want to take a gander!
P.P.S. this was quite fun to do and I might do it again, so if you have any questions add them in the comments :)
P.S. In the discussion below, I have taken lots of information from papers and sources rather than pulling the information out of my own brain so the references are there in brackets and refer to the full links at the end in case you want to take a gander!
P.P.S. this was quite fun to do and I might do it again, so if you have any questions add them in the comments :)
Question 1: “With regards to the Solar System, what do you find to be the most fascinating mystery you would want an answer to?”
Oooooh now this is a tricky one to start with because there are A LOT of mysteries about the Solar System that I’d like answers to! Something that springs to mind given this year’s activity is the cause of Global Dust Storms on Mars. Mars has a lot of dust activity (things like dust devils, local and regional dust storms) which we see from the surface and from orbit. But once every few Mars Years (one Mars Year is around 2 Earth years ish) dust activity on the planet increases massively and becomes a “planet encircling dust event” or a Global Dust Storm – basically a massive dust storm that wraps around the planet, may not cover it all entirely depending on the storm, but it kicks dust up to 60km in altitude and takes several months to clear down to normal seasonal levels (Mars has a dusty and a clear season). Why this happens in some years and not others is a mystery, and this year had quite the doozie of a Global Dust Storm (RIP Opportunity!).
More large-scale questions that are pretty substantial mysteries (to the best of my knowledge) are things like: how did the ice giants (Uranus and Neptune) actually form? The major Solar System formation theories actually really struggle to produce planets of Uranus and Neptune’s type, yet we have not one, but two of them in the Solar System! And of course, no Solar System mystery discussion would be complete without the question of answering whether there could maybe somewhere out there in the Solar System, at some point in time, have existed some kind of life, and if not, why not?
IMPORTANT NOTE I do not mean little green men with antennae or Martians with big bulging eyes and gross, visible, exploding brains - I am referring pretty much to microbes, germs, bacteria and the like. Though of course I wouldn’t say no to discovering something more complex, it’s just this scientist is immensely skeptical of that happening! So these are really only three of a massive number of mysteries that still exist in the Solar System – if we knew everything there was to know then there wouldn’t be a significant number of scientists across the globe trying to understand phenomena across the Solar System. In summary: we still have a long way to go before we answer the mysteries of the Solar System!
Question 2: “on a recent Horizon programme [a science documentary TV show on the BBC] the suggestion was that the planets did not form near their current orbits, and that Jupiter in particular roamed around, changing all of the other planetary orbits because of its gravity. Is this accepted as a mainstream idea, and if so, where is Jupiter heading next?”
The concept that you’re alluding to here, where planets “roam around” is called “planetary migration” and yes it’s very much a mainstream concept! Jupiter and Saturn are thought to have formed first in the Solar System, migrated inwards – Jupiter to perhaps 1.5 AU (an AU is an “astronomical unit” or the average distance between Earth and the Sun) which is roughly the distance of Mars from the Sun - and then migrated outwards again. Once the gas disk surrounding the early Sun (explained a bit more in Question 3) is completely dissipated, the migration stopped (as far I know!). However, Jupiter’s orbit does evolve over time, just not in a migration-kind-of-way. For example, there’s something called precession of perihelion (closest point to the Sun) of an orbit where its orbit rotates, kind of like a spirograph:
Illustration 1: http://ccnmtl.columbia.edu/projects/mmt/frontiers/web/slideshows/mercurys_orbital_precession.html
which, for Jupiter, changes at a rate of 0.18 (ish) degrees per century. So it’s pretty slow in human terms!
Question 3: “why are the inner planets small and rocky, and the outer planets are gas giants? Is it because all of the gas from the inner planets has been blown away?”
Before I begin, a quick run down of the structure of the planets. Terrestrial planets are the rocky planets, like Earth and Mars. (Yes I know there’s liquid magma and parts of the core, but compared to the gas and ice giants – pretty rocky). Then we have the gas giants, Jupiter and Saturn. They have a solid core, surrounded by a massive envelope made up of mostly hydrogen and helium (though if you went much below the surface it isn’t actually gassy at all – it goes liquid and then metallic due to the high squishing pressures involved in being part of a giant planet). The ice giants, Neptune and Uranus, are a little different from both of the others even though they’re often lumped in with Jupiter and Saturn. Neptune and Uranus are a fair bit smaller than Jupiter and Saturn and, instead of being made up of mainly hydrogen and helium, they’re actually mostly made up of ices (water ice, ammonia ice, and methane ice). Water ice is the main component and goes from solid -> liquid -> supercritical liquid at high pressures. At the very middle is a small rocky core, similar to the ice giants. A little summary of the structures can be seen here, from the NASA Ice Giants Pre-Decadal Study Final Report, 2017:
There are two main (I emphasize main, not only) theories of how the Solar System formed. The first, and generally preferred, is called ‘core accretion’ (Pollack 1996) and the second is ‘called gravitational disk instability’ (Boss 2003). Both theories begin the same way with a very, very young Sun at the centre of a big gassy, dusty disk (named a “proto-planetary disk”). This disk is a by-product of star formation.
Core accretion is pretty much the accepted method of producing terrestrial (read: the inner rocky) planets (though maybe I should duck and cover whilst some theorists chuck things at me for saying that). This is where little bits of solid stuff in the proto-planetary disk collide, sometimes stick together, rinse and repeat. Slowly, slowly this can form something the size of a terrestrial planet. Slowly. But they don’t reach a sufficiently large size to collect up hydrogen and helium.
The gas giants (Jupiter and Saturn) could be formed by core accretion or disk instability. In the core accretion (and again, generally preferred) model, small bits of solid stuff (metals, ices) clump together to form a solid core which, when it is big enough, can then start to accrete (collect onto itself) hydrogen and helium from the disk ultimately forming a gas giant. The disk instability model (aptly named) relies on the disk, or parts of it, becoming unstable and forming clumps which eventually lead to the gas giant planets. In this case, the solid cores of giant planets would be formed from solid particles settling down through the planet to the core.
The ice giants are pretty problematic – both core accretion and disk instability can form ice giant planets (though the ability of the latter to form them has been questioned), but the issue is forming them at their current size, at their current distance, with their current structure, but before the protoplanetary disk disperses... There are a number of adaptations to the models (including migration of planets, see Question 2) that can produce ice giant type planets, but it’s still very much a watch-this-space for new observations that can constrain how and where ice giants form! Exciting times!
So to answer your question properly after rambling on about how things form, why are the inner planets rocky? Well, it comes down to temperature. The inner portions of the Solar System are hotter than the outer portions. At the time the Solar System was forming, the inner 3 AU (remember astronomical units: 1 AU is the average distance of the Earth to the Sun) was too hot to let any ices (e.g. water, methane, ammonia) condense out of the gassy envelope. Beyond this distance, called the “snow line”, ices can condense out of the nebula. Because core accretion really needs solid stuff, none of these ices were available, thus no ice giants could form. However, there are some theories that Jupiter actually formed closer to the sun where a planetesimal (baby planet) *did* reach a sufficiently large size to start collecting hydrogen and helium, and then smashed its way out through the Solar System destroying a first generation of planets before our current set could form. There are yet more theories that Jupiter formed outside the snow line, then migrated inwards, then outwards again, which has the handy result that Mars ends up teeny tiny in comparison to Earth and Venus (which is a bit difficult to get with some models).
So that’s probably far more detail than you were ever asking for, but that’s apparently just how I roll!
P.S. here are some of the articles I used to help me answer this question:
Boss 2003: Boss A. P., 2003, ApJ , 599, 577
Mousis et al., 2018: Mousis O., Atkinson D. H., Cavalié T., et al., 2018, Planet. Space Sci. , 155, 12
Pollack 1993: Pollack J. B., Hubickyj O., Bodenheimer P., et al., 1996, Icarus , 124, 62
https://link.springer.com/content/pdf/10.1007%2Fs11214-005-1945-3.pdf
https://www.aanda.org/articles/aa/pdf/2011/09/aa17451-11.pdf
https://www.nature.com/articles/nature10201
Question 4: “I remember in one of our recent chats you saying that the most difficult thing about putting people on Mars wasn’t the issue of equipment reliability etc, but ionising radiation. I can think of strategies to minimise this in flight (e.g. using fuel, water etc as a shield) so that leaves the Martian surface. So, can you quantise the problem? Is it the “general” level that is the problem or peaks associated with flares? How do these compare to our average dosage rates?”
So, to clarify, I wouldn’t say that radiation is the most difficult thing about putting people on Mars (as a one-way, possibly short term ticket), as the engineering and technological side of things is very complex. However, it is a BIG concern (IMO) if you wanted to have humans living on Mars or visiting it and surviving for a long time to tell the tale.
So first thing: why is there a lot of radiation on Mars? Well, Earth’s core produces a magnetic field (magnetosphere) that protects us from radiation from space – but Mars doesn’t really have that. The only magnetic field it has going on is imprinted in the rock from a time there was one, and this is not strong enough to deflect charged particles. There is a bit of a magnetosphere that’s formed by interactions with incoming charged particles with the atmosphere, but again it’s not like Earth’s. Also the Martian atmosphere is really really really thin – like 1% of Earth’s atmosphere thin.
Second thing: what type of radiation are we talking about? I’m talking about highly energetic particles – charged particles with a lot of energy. Most of them are protons or alpha particles (helium nuclei – what would be left if you took the electrons out of a helium atom). These come from outside the Solar System but within the Milky Way. The other kind are called Solar Energetic Particles and these are the same kinds of particles but have been ejected from the Sun in flares or massive flares called “coronal mass ejections” (think a Solar flare on steroids).
So in reference to the first part of the question: is it “general” or “flares” - both!
There has been work done calculating how much of a radiation dose would be received on Mars and in transit to Mars (e.g. Hassler 2014) based upon measurements taken with good old Curiosity (the rover exploring a crater near the equator). They found that for a 180 day trip to Mars, you’d receive a dose 100 times higher than that of the US annual average dose. Want to stay for 500 days on the surface of Mars? That’ll be about the same again: 100 times higher than the US annual average dose. So if you want to go to Mars, stay for 500 days, and come back again, that’s a big dose of radiation. If Sieverts are your thing: they estimated around 1.01 Sv for the whole trip plus stay. No thank you!
There are people working on mission designs or mission habitation complexes to make a stay like this possible, but they require lots of shielding or caves or lava tubes or something similar to offer protection.
Hassler 2014: http://science.sciencemag.org/content/343/6169/1244797
Some fun particle reading: https://helios.gsfc.nasa.gov/cosmic.html and https://ccmc.gsfc.nasa.gov/RoR_WWW/SWREDI/2014/SEP_YZheng_20140602.pdf
Question 5: “What is the latest view on which of the gas giant’s moons is most likely to harbour life of some form (like the single cell gunk that grows close to underwater volcanic vents)?
So as far as I’m aware, there isn’t an official designation of “this is where we think is the most likely place” as it’s all speculation. I can however give you my opinion! If I had to put money on it, I would hedge my bets and go with Europa and/or Enceladus. Europa is one of the Galilean moons of Jupiter. Enceladus is one of the bigger moons of Saturn. Both are ice covered and thought to have water oceans beneath their ice crusts. And life as we know it has a tendancy to follow the water...
Europa (https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/), is thought to have an iron core, then a rocky mantle and then a brine (salty) ocean followed by an icy crust. It’s not the only one of the Galilean Moons of Jupiter that is thought to have a global sub-surface (below the crust) ocean but Ganymede and Callisto’s oceans are thought to be very deeply buried (>80km, Schenk 2002). Europa’s, however, is thought to be much closer to the surface and there may be liquid water-ice mixtures in the top few km of the surface with the majority of it actually 15-25 km below the surface (Heggy 2016, Zimmer 2000). Which is cool. BUT, Europa is also super close to Jupiter and orbits every 3.5 days. It has a very elliptical (oval-shaped) orbit so the actual distance from Jupiter does vary a lot but the important thing is that it’s close enough that it is within Jupiter’s magnetosphere (the bubble created by a planet’s magnetic field interacting with the Solar wind) which, conversely to how our magnetosphere protects us from Solar radiation, causes Europa to be hammered by charged particles because Jupiter actually emits quite a lot of radiation so being close to it isn’t really a super pleasant thing (Nordheim 2017). BUT, that may only affect the top layers of Europa, so given the majority of ocean is buried under kilometers of icy crust – maybe that would still be ok. There’s a planned mission called Europa Clipper to launch in 2020 ish which has a goal of conducting recon on the moon to find out whether it could have conditions that could maybe harbor life. EXCITING.
The other moon I’ve referred to is Enceladus (https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/). Enceladus is also pretty darn icy, and also is thought to have a global ocean. It orbits Saturn every 32.9 hours and is inside one of Saturn’s rings (E ring). But the really cool thing about Enceladus is it sprays bits of its ocean out into space (a “plume” or “jet”) which we have detected through bits of its surface known as “tiger stripes” (massive fractures in the surface). Side note: recently there have been suggestions that Europa does this too – also exciting! And, when Cassini was still with us (RIP Cassini), it actually managed to fly THROUGH part of a plume and analyze the composition of the material it flew through. Scientists (Waite et al. 2006) found that it was dominated by water, with bits of carbon dioxide, methane, ammonia AND (Waite et al, 2017) molecular hydrogen (H2 or hydrogen in a state like the gas we find on Earth) in the plume. Now, there are types of life (again, think germs not jellyfish) on Earth, some of them are very ancient types of life, who feed on molecular hydrogen. Cool. But I have also seen it argued that if we are detecting this molecular hydrogen in space when it gets sprayed out then perhaps there aren’t any little alien germs out there feeding on it otherwise there would be less of it to detect. Either way, I think it’s rather exciting and one of the reasons Enceladus is on my shortlist.
Illustration 3: Enceladus plume!
Illustration 4: One theory of how these plumes occur
Heggy 2016: https://www.sciencedirect.com/science/article/pii/S0019103516307953#bib0080
Nordheim 2016 https://www.hou.usra.edu/meetings/europadeepdive2018/pdf/3039.pdf
Schennk 2002: https://www.nature.com/articles/417419a
Waite 2006: http://science.sciencemag.org/content/311/5766/1419
Waite 2017: http://science.sciencemag.org/content/sci/356/6334/155.full.pdf
Zimmer 2000: https://www.sciencedirect.com/science/article/pii/S001910350096456X
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