A quick aside about the planet Mercury:
Next year, the European and Japanese space agencies are working together to launch BepiColombo a major mission to Mercury, rather surprisingly due to reach the planet after a seven-year voyage. This was at first a little confusing as Mercury, orbiting as it does within our own path round the sun, can be reached in less than half a year. Apparently the reasoning behind that thought is not obvious, but if you think about it, it must be so. Since it takes Earth a year to go round the sun and Mercury is rarely more distant from us than when it’s at its furthest point from the sun and we’re on exactly the opposite side of the sun, and since the closer an object orbits to the sun the faster it moves, the length of the longest direct path between us and Mercury has to be shorter than half our orbit and the space probe would have to be moving faster than Earth does, so it has to take less than six months. Therefore I was puzzled by this at first. It turns out BepiColombo is going to approach Venus, I think, three times and Mercury six times, so clearly it can’t be describing anything like a simple portion of an ellipse.
Mercury and Venus have a few things in common. They are the two inferior planets. This is not a reference to them being a bit naff compared to the others. What it means is that they are both “below” us with respect to the sun. Mars and the rest, by contrast, are referred to as the superior planets because from the perspective of the sun they’re “above” us. This is not strictly true because they’re all orbiting, but I hope you get my point.
I’m about to go on about orbital dynamics, so be warned.
The German mathematician Johannes Kepler, 1571-1630 aware that his contemporary model of the solar system seemed to be completely wrong as it involved epicycles, the idea that as well as orbiting the sun, the planets made their own little circles as they moved around it, simplified the system and came up with his three laws of planetary motion, which are as follows:
- The planets move in ellipses with the sun at one focus.
This could be seen on the English pound note, except that the sun is placed erroneously at the centre of the orbits:
Maybe this was done to foil potential astro-savvy counterfeiters, who knows?
2. Over equal periods of time, a planet sweeps out an equal area. This is quite difficult to express clearly, but can be illustrated with this diagram:
This is an exaggeratedly elliptical orbit of a planet with the sun at one focus (as opposed to the centre). Suppose it takes a week for the planet to move from the position in its orbit from the lower to the upper point in the blue sector. It will also take a week to move through the red sector. Moreover, the area of both sectors is the same. This means that planets move more quickly the closer they are to the sun.
3. I find Kepler’s third law the most interesting. I’ll state it first, then explain it in “normal” language: The square of the sidereal period is directly proportional to the cube of the semi-major axis.
This is not as complicated as it sounds. The “semi-major axis” is simply the average distance from the sun. It can be clearly illustrated using Saturn. Gratuitous image of Saturn:
Saturn is, rather helpfully, about ten times the distance of Earth from the sun, and the cube of ten is of course a thousand. Since thirty times thirty is nine hundred, it can be expected that Saturn takes about thirty years to orbit the sun, and this is in fact the case.
Back to missions to Mercury and Venus. Mercury is about 40% Earth’s distance to the sun and Venus is roughly 70%. Therefore spacecraft travelling to Mercury or Venus are effectively describing half of an orbit averaging the average of the sizes of the two orbits, in other words 70% and 85% of ours respectively. Doing the maths, an orbit of 70% of ours is about the same as that of Venus, which takes 225 days to orbit the sun, so it ought to take a maximum of around 123 days to get to Mercury. and 143 days to reach Venus. This is somewhat surprising because it takes longer to get to Venus than it does to reach Mercury, and this means in fact that of all planets in the solar system Mercury could be the quickest to reach even though its maximum distance from here is greater than that of Venus.
I mentioned earlier that Mercury and Venus have a fair amount in common. Both have very long “days”. Mercury’s day lasts fifty-six of ours and Venus has a day longer than its actual year at 243 of them. Both of them along with Earth are of about the same density, five and a bit times that of water. Also, both of them orbit almost upright compared to the sun, meaning that there is practically no variation in day length, and there wouldn’t be even if they rotated at the same speed as Earth either.
Which brings me to:
Stripy Blue Marbles
Here is a fairly well-known false-colour image of Venus in ultraviolet light. Given the colours, it looks remarkably like Earth although this is misleading because the choice was made to colour it blue and white, like us, and to the naked eye Venus looks more like this close up:
Now look at this photo of our own planet, as would be seen by a human eye:
This is a view over the Pacific Ocean, revealing how close to an ocean planet Earth really is. It’s almost possible to line up a globe at that distance so that no landmasses at all are visible, only the ocean and various islands, and from somewhat closer up it is easily feasible because a substantial part of that hemisphere would be hidden.
Although the first picture of Venus is false colour, it still reveals the fact that the swirls of clouds there are of a particular shape and that differently-composed clouds separate into streaks. The “blue” streaks may be a mixture of sulphuric acid and ferric chloride, the latter chemical being used to treat sewage. That part of the Venusian atmosphere sounds to me like it might be quite useful for unblocking toilets, except that it isn’t that concentrated.
The relevant contrast between the two images is that whereas the clouds on Venus form streaks running roughly east-west interspersed with other clouds, the ones on Earth are much more swirly, indicating a more turbulent atmosphere at that level. There is a reason for this.
It’s been theorised that ocean planets are in fact very common in the Universe. All that’s needed, ultimately, is for the most common compound in the Universe, water, to accumulate in a fairly large lump at an appropriate distance from an appropriate star. Moreover, once that has happened the atmosphere is likely to become high in oxygen without any biological activity simply because the radiation from the star will then break the water up into hydrogen and oxygen. On smaller such planets, the hydrogen will then float up into space and the ultraviolet from the sun will generate an ozone layer from the remaining oxygen. However, such a planet could well be lifeless because there wouldn’t be enough of any other elements for life “as we know it” to begin. The kind of substances which might form on such a planet would be water, hydrogen peroxide and ozone, and whereas life clearly does well with water, it can’t really be expected to exist just as water itself. Such ocean planets could simply be balls of highly pressurised ice covered completely in fresh water with oxygen atmospheres, although like us there would be clouds and rain.
The English interwar religious communist science fiction author and academic philosopher Olaf Stapledon supposed that ocean planets would in fact be very common, and were likely to form when the gravity was able to attract a lot of water but was also too high for there to be enough land of such an altitude to stick out of the ocean, meaning that there would either be islands or no land at all rather than continents, at least above sea level. On the other hand, such worlds could be rocky. It’s just that all rocks would be on the sea bed. An intermediate kind of world has also been suggested where there is a rocky core covered by a thin layer of ice at the bottom of a deep ocean, with deep sea vents and active volcanoes providing other elements and thereby making life possible. Finally, there could simply be Earth-like worlds with single continental plates like the Pacific, without enough geological activity to balance the erosion of land and a consequential global ocean.
Whatever the cause, ocean planets are probably very common, and one of their shared features is that because they are almost completely covered in water, their climate is likely to be fairly samey all over. It takes a lot of energy to heat water up, once warm, water takes a relatively long time to cool down, and because it’s a liquid, warm and cold water tend to mix together. On a planet with no land, nothing blocks this effect, so temperature variations on such a planet would be small. However, certain things could interfere with this. For instance, if an ocean planet was like Uranus and it orbited a sun-like star at the same distance as Earth, each hemisphere would spend half the year in daylight and the other half in darkness, and such a planet could do such things as have boiling oceans at one end and frozen ones at the other, which would swap regularly. However, this is a fairly extreme case. Another possibility is a planet with an orbit elliptical enough for it to cross the entire habitable zone of its star every half year, and this planet too would be stormy and turbulent. Yet another is that the planet would simply be quite small and have a thin atmosphere, though still thick enough for liquid water to exist on its surface, which could lead to greater temperature variations between the poles and equator, although smaller planets are probably less watery anyway because they’d have larger variations in altitude and less gravity to attract or hold on to their water, so such a planet is quite unlikely. However, even in these cases the conditions would have to be a lot more extreme to make a planet uninhabitable compared to one with more evenly mixed land and water on its surface.
Some of the planets in our own solar system have a marked axial tilt whereas others rotate almost “upright” compared to the sun. Mercury, Venus and Jupiter are in the latter category, whereas Earth, Mars and Saturn are in the former. Uranus is really extreme and roughly rotates “on its side”, giving most of the planet a forty-two year day and a forty-two year night, except for a narrow zone at the equator. If it’s assumed that our solar system includes a typical distribution of axial tilts, it looks like there’s a roughly three in eight chance of an ocean planet rotating roughly “vertically” with little variation in day and night length, and along with that little seasonal change.
Now imagine such a planet which is on the margins of human habitability. It has a gravitational pull around 30% greater than ours, hardly any axial tilt and is studded with the occasional low-lying island, rather like Polynesia. Assume also that that planet rotates at about the same rate as Earth and that it has plankton which release enough oxygen into the atmosphere for us to breathe. It has a roughly circular orbit like ours too.
Such a planet would have quite monotonous weather, with the exception that any hurricanes which developed would last a very long time. Hurricanes are fed by the ocean and start to abate as soon as they make landfall. Also, it’s possible that there would be very high winds at sea level due to the relatively rare interruption of land. A wind could easily blow right round the planet with nothing to stop it at all. On Earth this can only currently happen in the Southern Ocean although in prehistoric times there was an equatorial ocean of this kind. The same would apply to ocean currents to some extent, although islands could possibly deflect these.
You could, in other words, expect stripes I think. Winds would clear the atmosphere of cloud in some places and not others and there would be a tendency for certain places to be almost permanently cloudy and for others to be mainly sunny. It would look roughly like a blue and white version of Jupiter:
(only much better-looking than this rather ropy illustration).
I might be ignoring possible chaotic effects brought on by the butterflies of little islands of course. There are streams of clouds leeward of oceanic islands, for example, which for all I know might add up to a major effect. However, given that the atmosphere is denser, the clouds are likely to be higher, and therefore further above the lowlying land.
In other words, if we ever leave this solar system, one thing we can expect to see, I think, is stripy marbles.