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← Life on Alien Planets May Not Require a Large Moon After All

Life on Alien Planets May Not Require a Large Moon After All - Comments

KenChimp's Avatar Comment 1 by KenChimp

Now, I'm no astrophysicist (I began my collegiate career as one, but switched when I became enamored of computer technology), but I've never understood the charge that Earth's rotation would have wild swings without the moon due to Jupiter's gravitational pull.

Mars has two tiny asteroids for its paltry display of moons. Venus has none. We don't find Mighty Jupiter reaching out his arrogant hands to "perturb" the rotational stability of those inner planets. Not to the degree claimed by some scientists that the Earth would experience.

Thu, 01 Dec 2011 13:46:07 UTC | #894695

78rpm's Avatar Comment 2 by 78rpm

Just a semantic objection I have: Why is any planet other than Earth called "alien?" Even its conjectured life minding its own business there is called alien. We don't use this word for other countries. Swaziland is not an alien country, and its inhabitants are not aliens. An alien is an outsider; in fact, if I moved to Swaziland, I would be an alien, as would a person from Swaziland be if he or she came to my country, until gaining citizenship.

Thu, 01 Dec 2011 14:51:47 UTC | #894707

mysticjbyrd's Avatar Comment 3 by mysticjbyrd

Sure we do, illegal aliens is a term you will hear frequently in the US.

Thu, 01 Dec 2011 15:06:59 UTC | #894712

78rpm's Avatar Comment 4 by 78rpm

Yes, they are aliens, illegal or legal, because they are in a country in which they are not citizens. But the country they are from is not alien, and they would not be aliens if they had stayed there. (Now I will go away and quit with the fine points.)

Thu, 01 Dec 2011 15:16:19 UTC | #894716

Alan4discussion's Avatar Comment 5 by Alan4discussion

Lissauer and his team conducted a new experiment simulating a moonless Earth over a time period of 4 billion years. The results were surprising – the axis tilt of the Earth varied only between about 10 and 50 degrees, much less than the original study suggested. There were also long periods of time, up to 500 million years, when the tilt was only between 17 and 32 degrees, a lot more stable than previously thought possible.

"only between 17 and 32 degrees" is a 15° variation compared with a 2.4° change in the tilt of the present Earth's axis is large.
50 degrees is a damned big polar region to go freezing dark for months at a time, and to bake in continuous daylight for months more, while a lot closer to the equator. (Years if further away from a larger star in a longer orbit.) - Possibly raising local temperatures above the boiling point of water. A range of variation from 10 to 50 degrees is also large (40°) compared to Earth at present (2.4°),

*(Calendar Wiki see link below) - Presently the Earth is tilted at 23.44 degrees from its orbital plane, roughly half way between its extreme values.

*The Earth's rotation axis wobbles, causing a slow 2.4° change in the tilt of the axis (obliquity) with respect to the plane of the Earth's orbit. The obliquity variations are roughly periodic, with a period of approximately 41,000 years. When the obliquity increases, the amplitude of the seasonal cycle in insolation increases, with summers in both hemispheres receiving more radiative flux from the Sun, and the winters less radiative flux. As a result, it is assumed that the winters become colder and summers warmer.

The other major issue is tidal drag of a large moon (in addition to solar tides) over an extended period, generating heat driving plate tectonics, which in turn maintains the recycling of carbon, sulphur, calcium and other elements, stabilising the atmosphere. Tidal drag is also a key part of churning oceans and mixing eroded/dissolved chemicals.

But if the new research results are right, the dependence on a large moon might not be as important after all. “There could be a lot more habitable worlds out there,”

There may well be some, but with reduced chances and suitability for life arising. The suggestions seem somewhat speculative.

The issue of Earth's orbital and axial stability is described accurately in:
Milankovitch cycles - http://en.wikipedia.org/wiki/Milankovitch_cycles

Calendar Wiki - http://calendars.wikia.com/wiki/Milankovitch_cycles

Thu, 01 Dec 2011 15:43:52 UTC | #894718

SheilaC's Avatar Comment 6 by SheilaC

I was always rather puzzled by the idea that you have to keep the tilt so stable for intelligent life to develop. I can see it would help, but given all the stuff that Earth-based life survives, I wouldn't be all that surprised to hear of a planet with intelligent life migrating on a massive scale, or hybernating, or just really, really good at coping with temperature swings.

Thu, 01 Dec 2011 16:54:13 UTC | #894746

Alan4discussion's Avatar Comment 7 by Alan4discussion

Comment 6 by SheilaC

I wouldn't be all that surprised to hear of a planet with intelligent life migrating on a massive scale, or hibernating, or just really, really good at coping with temperature swings.

Life needs to evolve quite a lot before it is capable of migrating or hibernating.

The problem with planetary temperature swings is the size of the swing and the potential range of temperatures! Earth's water and atmospheric recycling regulates temperatures to a narrow range.

  • The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K (+467°C, +872°F), while the pressure is 93 bar.

  • At the surface, Saturn's moon Titan's temperature is about 94 K (-179°C -290 F).

  • The temperature range tolerated by Earth life is known. That is how hospitals use heat to sterilise equipment, and food companies use freezing to prevent bacterial growth.

    Thu, 01 Dec 2011 20:29:48 UTC | #894820

    Net's Avatar Comment 8 by Net

    gawd, i hate that word, "alien"! it's so science fiction; so hollywood! what's wrong with "extra terrestrial" or words to that effect?

    Thu, 01 Dec 2011 23:13:47 UTC | #894879

    DavidMcC's Avatar Comment 9 by DavidMcC

    Comment 7 by Alan4discussion

    Life needs to evolve quite a lot before it is capable of migrating or hibernating.

    True, but 500MYa is plenty of time to "evolve quite a lot". Also, I doubt that anyone is denying that a large stabilising moon HELPS, it is merely emphasising that some kind of life is likely to surive for a while even without the moon, although it may well be more restricted in lattitude than without the moon. Perhaps it is a matter of how much of "that belief is being called into question".

    Fri, 02 Dec 2011 09:10:31 UTC | #894964

    Reckless Monkey's Avatar Comment 10 by Reckless Monkey

    I think the problem here is that we are trying to imagine life like us. So, there is a bias towards this in speculation about life away from Earth. This is fair enough as it makes sense to look for what you know already works, however if you examine the scope of life that has existed on this planet so far it's pretty broad. I'm quietly confident (with no real evidence) that life might be much more likely than we think. Intelligent life however is another thing altogether.

    Fri, 02 Dec 2011 09:43:13 UTC | #894971

    Alan4discussion's Avatar Comment 11 by Alan4discussion

    Comment 9 by DavidMcC

    I agree that it reduces the probabilities rather than eliminates them entirely, but as I said @5, I see tidal heating (in proportion to the rate of spin and size of the moon), maintaining plate tectonics and atmospheric recycling for climate stability, as being as important as axial and seasonal temperature stability. Rapid cessation of these processes could easily reduce the probabilities to nil with massive rapid toxic and atmospheric pressure changes with greenhouse effects.

    Fri, 02 Dec 2011 11:17:30 UTC | #894994

    DavidMcC's Avatar Comment 12 by DavidMcC

    Rapid cessation of these processes could easily reduce the probabilities to nil.

    Yes, but what matters is the integrated probablity over the galaxy/universe/whatever that matters, and this does not go to zero, it is only reduced, as we already agree, at least for life of some kind for some period. The galactic probabilty for technological life without a large moon could go essentially to zero, I suppose, but that is a different matter.

    Fri, 02 Dec 2011 11:30:25 UTC | #894998

    DavidMcC's Avatar Comment 13 by DavidMcC

    ... Also, tidal heating and plate tectonics do not have to come from a large moon, though that certainly helps. A not entirely insignicant proportion of the Earth's tides is solar in origin, not lunar, as you know. On other potential life-supporting planets, it could be entirely solar. There is no neodarwinian-style competition between life-supporting planets that would cause even a small reduction in probabilty to cause those that are less efficient to "go extinct".

    Fri, 02 Dec 2011 11:42:38 UTC | #895002

    Alan4discussion's Avatar Comment 14 by Alan4discussion

    Comment 13 by DavidMcC

    ... Also, tidal heating and plate tectonics do not have to come from a large moon, though that certainly helps. A not entirely insignificant proportion of the Earth's tides is solar in origin, not lunar, as you know. On other potential life-supporting planets, it could be entirely solar.

    I think we have looked at this before in earlier discussions. Lunar heating is a very significant proportion on Earth.

    One of the complexities is, that if a planet is near enough to its star for strong solar tides, synchronous tidal locking will quickly reduce its rotation, with serious effects on day length and diurnal temperature variations.

    If it has not (like Earth) had the initial boost to its heat from a giant collision and on-going additional Lunar tidal turbulent heating in its liquid mantle layers, the tectonic atmospheric recycling will quickly die out as the crust becomes thick, solid and continuous, like that of Mars and our Moon. Tides are also greatly affect oceans, their chemistry and the weather they generate.

    There are a lot of complex calculations on Earth's internal heat balance around page six on this link but it gets a bit beyond posting in a debate.

    Fri, 02 Dec 2011 15:47:46 UTC | #895050

    DavidMcC's Avatar Comment 15 by DavidMcC

    ... Due to a keyboard mis-hit, my last post was only half complete. I will start again, as I don't yet have edit access to it.

    Alan, I think you are thinking of planets with technological life, rather than those with single-celled life. It should be possible for single-celled life to develop quickly enough even without a large moon, especially if the planet is more massive than the earth, as that makes it more stable against disturbance from the rest of the solar system. Further, it isn't necessary for conscious migration to evolve in order to avoid extinction by rotational instability - bacteria can blow in the wind (they have been found to cross the Atlantic ocean from the Sahara to the Carribean, for example), and may even evolve cysts, like amoebae if subjected to periodic drying that isn't initially so severe as to drive them to extinction immediatedly. Having said that, the massive tides caused by the way our moon was formed certainly helped with abiogenesis. But you are extrapolating from our system to the entire, diverse universe, and claiming that the moon is essential, as opposed to something that speeded up abiogenesis and evolution. I don't think there is sufficient reason to claim it is essential for any kind of planet in any kind of solar system.

    Mon, 05 Dec 2011 11:55:49 UTC | #895829

    DavidMcC's Avatar Comment 16 by DavidMcC

    Comment 14 by Alan4discussion

    If it has not (like Earth) had the initial boost to its heat from a giant collision and on-going additional Lunar tidal turbulent heating.

    Most internal heat-boosting collisions would surely not be of the moon-forming kind, and it is by no means clear to me that the extra heating due to the moon is necessarily crucial to abiogenesis. I suspect that you are just guessing.

    Mon, 05 Dec 2011 12:37:37 UTC | #895837

    DavidMcC's Avatar Comment 17 by DavidMcC

    ... For example, a planet whose crust happens to contain more of the essential trace elements for life than does the earth's, and which is more massive than the earth might not need the extra help from a large moon.

    Mon, 05 Dec 2011 12:47:26 UTC | #895840

    Alan4discussion's Avatar Comment 18 by Alan4discussion

    Comment 15 by DavidMcC

    Having said that, the massive tides caused by the way our moon was formed certainly helped with abiogenesis. But you are extrapolating from our system to the entire, diverse universe, and claiming that the moon is essential, as opposed to something that speeded up abiogenesis and evolution. I don't think there is sufficient reason to claim it is essential for any kind of planet in any kind of solar system.

    Not essential, but the two main methods of topping up the initial heat from planetary formation are radioactive decay and tidal drag. The importance of our large moon to tidal drag should not be underestimated. It should also be noted that on the early Earth when the day length was around 6 to 10 hours and the Moon in a very much lower orbit, the tidal effects would have been massively greater.

    Comment 13 by DavidMcC-... Also, tidal heating and plate tectonics do not have to come from a large moon, though that certainly helps. A not entirely insignicant proportion of the Earth's tides is solar in origin, not lunar, as you know.

    The gravitational attraction of the Sun on the Earth's oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides. - http://en.wikipedia.org/wiki/Moon

    As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down.[96] Measurements from lunar ranging experiments with laser reflectors left during the Apollo missions have found that the Moon's distance to the Earth increases by 38 mm per year[97] (though this is only 0.10 ppb/year of the radius of the Moon's orbit). Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year,

    We are talking about energy on a scale which moves planets here.

    Without these external forces, the ocean's surface would simply exist as a geopotential surface or geoid, where the water is pulled by gravity without currents or tides. The strong gravitational fields of the Moon and the Sun result in significant changing gravitational fields causing tides to rise and fall. Because the Sun is farther away than the Moon, the Sun's pull on the tides is about 40% as strong as the Moon's. Both gravitational and tidal forces weaken as distance increases. Gravitational forces are inverse to the square of the distance and tidal forces are inverse to the cube of the distance. - http://marinebio.org/oceans/currents-tides.asp

    Both the Earth and the Moon revolve around the one center of both of their masses. This common point is called the barycenter and is located at about ¾ of the radius from the Earth's center. Although the Moon is the Earth's only satellite, the Earth moves slightly when they both orbit the barycenter. In addition, the barycenter is not always in the same place due to the elliptical rotation of the Moon.

    The absence of the Moon would result in a reduction on present levels of tidal pull, to slightly over a third and a substantially greater % reduction on early Earth, when the Moon was in a very low orbit, would have been very significant, particularly on erosion, dissolved minerals and gases, currents, and mixing in water.

    Mon, 05 Dec 2011 19:42:24 UTC | #895916

    DavidMcC's Avatar Comment 19 by DavidMcC

    Comment 18 by Alan4discussion

    The absence of the Moon would result in a reduction on present levels of tidal pull, to slightly over a third and a substantially greater % reduction on early Earth, when the Moon was in a very low orbit, would have been very significant, particularly on erosion, dissolved minerals and gases, currents, and mixing in water.

    Sure, and this probably means that the moon helped the earth a lot to have abiogenesis. However, the composition of the earth is not ideal for the same, so other planets in other systems might not need this assistance. Perhaps the sun's heat alone might be sufficient to generate the weather necessary for abiogenesis on a suitable planet.

    BTW, I didn't make my previous posts on the basis of ignorance of the law of gravity.

    Tue, 06 Dec 2011 10:20:17 UTC | #896106