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Saturday 5 October 2013

Why are the planets so different?

If you were to take the eight planets in our solar system and look at them as a whole, there would be two obvious groupings. The first of these are the giant planets, which are well deserving of their names - hundreds of times larger than the others and with thousands of times more mass.

image showing the planets and dwarf planets of the solar system to scale
The planets of our solar system, to scale by size. Credit: Wikimedia Commons

The spectacular contrast between the four giants (Jupiter, Saturn, Uranus and Neptune) and the other planets is fairly easy to explain. Before the planets were formed, all the material which they now contain was spread out in a giant ‘protoplanetary disc’.

artist's impression of a protoplanetary disc
Artist's impression of a protoplanetary disc.
Image credit: Wikimedia commons
The four ‘gas giants’ formed outside the ‘snow line’ of our early solar system, where it was cold enough to allow the relatively abundant volatile substances like water or ammonia to condense out. This gave the first baby planets in this region of the disc plenty of liquid and solid material to sweep up to form bigger and bigger planets in a process called accretion. This process is a runaway one: gaining more mass means more gravity means gaining more mass. And hence we get the giant planets, huge balls of gaseous material that (we think) contain some kind of rocky or metallic core at their centre. Some questions remain about their formation, particularly about their potential travels throughout the disc early in their lives, but that’s not what we’re here to look at right now.

Does Saturn have a solid core?

Instead, let’s look at the planets we swept aside earlier, otherwise known as the terrestrial planets - Mercury, Venus, Earth and Mars. These planets share many similarities, but have many striking differences as well. The terrestrial planets are all formed of essentially the same 'stuff': silicate minerals and heavy iron cores, elements that could exist as solids and liquids in the warm early inner solar system, so in essence they are all made of rock. Their exact composition varies slightly, perhaps due to the distribution of different materials in the early solar disk, but we can consider them essentially the same.

image of the four terrestrial planets to scale
The terrestrial planets close up, to scale in size. Notice how different they look at first glance.
Image Credit: Wikimedia commons

Venus, Earth and Mars also have similar proportions of volatile elements (material which become gas at comparably low temperatures, like water) - those substances that the giant planets have plenty of, but that the terrestrial planets must have collected later in their lives (exactly where from being a big question in planetary science, with cometary impacts being a likely candidate). Measurements and estimations of the volatile inventories of these three planets show them to have roughly the same proportions of things like water and carbon, even though they might be contained within different types of reservoir - in some cases the atmospheres, in others the rocks. Here we see the first striking difference. Mercury, the closest planet to the Sun, is too small and far, far too hot to retain a substantial atmosphere, and the incessant solar radiation it receives has stripped away any atmosphere that it may have once had. The lack of atmosphere means that Mercury undergoes massive temperature variations between its dark and sun-ward facing sides.

Where did the Earth get its water from?

Why the terrestrial planets are the size they are is something of a mystery. It may simply be chance - that the particular planetesimals that formed them were larger or smaller - or it may have been due to the specific distribution of material in the solar nebula. But Mercury is the smallest, and combined with being so close to the Sun, this means it has next to no atmosphere.

The surfaces of Venus, Earth, and Mars. At this scale the planets look more similar than different.
Image credits: Roscosmos, the Author, NASA

Mars is the next largest, and has a thin, but definitely present, atmosphere, mainly composed of carbon dioxide. Venus and Earth are almost identical in size, and in fact are nearly twins in many more ways, but the two most similar planets look incredibly different to a casual observer. Venus, shrouded in clouds and acid haze, is the hottest place in the solar system, with an atmosphere so thick it could crush you many times over. Compared to Earth, Venus is a hellish world, while Mars is cold and dead. But under slightly different circumstances, these three planets might have been much more similar.

Small things matter

Since Venus and Earth are almost exactly the same size, it cannot have been the strength of their gravity that caused the difference. Instead, as Venus is slightly closer to the Sun, it was probably just a bit hotter than Earth in its youth, and if it didn’t have such a thick atmosphere it probably would still only be that little bit hotter. Yet the dense, sulphurous, carbon dioxide atmosphere causes an intense greenhouse effect, heating the whole planet up to several hundred degrees. The thick atmosphere traps this heat and distributes it all round the planet, creating the nightmare environment we see today.

Although Mercury is closer to the Sun, Venus is our hottest neighbour.

False-colour image of cloud features seen on Venus by the Venus Monitoring Camera (VMC) on Venus Express.
We don't know why, but the super-hurricane force winds on Venus are getting even stronger
Image credit: ESA/MPS/DLR/IDA
An important question then is where Venus got this tremendously thick atmosphere from. In fact, it may have been that the small difference in temperature from being slightly closer to the Sun is enough to have a massively disproportionate effect. The carbonate-silicate cycle, which governs the conversion between these two types of rock, is strongly dependent on temperature. Higher temperatures tend to mean that more silicate minerals are formed, along with the release of carbon dioxide. This carbon dioxide is a strong greenhouse gas, meaning that its release will increase the temperature a little more, initiating a vicious circle that has left Venus such an unappealing place to travel to. If Venus had any oceans early in its history, their evaporation would have compounded the effect, as water vapour is also an incredibly strong greenhouse gas.

The Earth, slightly further away from the Sun and therefore slightly cooler, has undergone the opposite - most of the carbon on our planet is locked away in carbonate rocks, leaving our atmosphere relatively clear of it (though this would not have been true in certain periods of our planet’s past). In fact, we need a little bit of greenhouse warming to make our planet anything other than a frigid, ice-covered world.

Mars is at the other end of the scale. Further away from the Sun, and slightly smaller (which meant it will have cooled down quicker), it is now a cold, dead world. Its thin carbon dioxide atmosphere offers little to no warming effect, since most of it has been lost to the ravaging effects of the solar wind.

What will the Greenhouse-effect do to the Earth?

Our Earth sits in a potentially precarious place - could the input of carbon dioxide and other greenhouse gases into our atmosphere turn us one day into another Venus, or will our planet suffer the same fate as Mars and end up cold and dead?

EDIT (July 2014):
This article is now also available as an animated video on our YouTube channel, created by students at Sheffield Hallam University, through our membership of the Venture Matrix scheme.

This guest blog is a part of our World Space Week series, and was written by Adam Stevens - a research student in Planetary and Space Sciences at the Open University's Department of Physical Sciences. He is generally found simulating martian environments in support of the ESA ExoMars 2016 Trace Gas Orbiter mission but can also be found wittering away on his website or twittering away on twitter as @adamhstevens.

Over the course of the next few days, we'll be taking a closer look at some of the secrets these planets are hiding, and at the space missions that have been sent to help uncover them.

why don't all references have links?

[1] Kieffer, H. H., Jakosky, B. M., Snyder, C. W. and Matthews, M. S. (eds.) (1992) Mars, University of Arizona Press, Tuscon.
[2] Marov, M. I. A. and Grinspoon, D. H. (1998) The Planet Venus, Yale University Press.
[3] Rothery, D. A., McBride, N. and Gilmour, I. (2011) An Introduction to the Solar System, Cambridge University Press.

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