MOON

Moons may bow to planets in terms of size, but in character they often outshine their stolid parents. The named moons of the solar system outnumber planets by more than 20 to 1, and they display a remarkable diversity. There are fully fledged worlds such as Titan, as complex as any planet. There are possible havens for life, such as the ice-crusted water world Europa. New mysteries surround even the smallest satellites, most recently the apparent flying saucers orbiting Saturn.

This year it will be four centuries since Galileo discovered Jupiter's four large satellites, at a stroke quintupling the number of moons then known to humanity.

1. IO
Pockmarked with sulphurous pits, bathed in intense radiation and shaken by constant volcanic eruptions, Io is the fiery hell of the solar system.

2. Iapetus
Even a cursory glance at Saturn's moon Iapetus reveals it to be an oddball. For one thing it is two-toned: one half is black, the other shining white.

3. Europa, Enceladus and Triton
The seemingly bleak icy surfaces of Europa, Enceladus and Triton are in fact among the most active landscapes in the solar system. They may even contain cosy habitats for living creatures.

4. Pan and Atlas
Pan and Atlas come straight from a 1950s B-movie. With a central bulge set inside a disc-like ridge, they bear an uncanny resemblance to your stereotypical flying saucers.

5. Nereid
An otherwise undistinguished satellite of Neptune, moderately lumpy and middling in size, Nereid travels on the most eccentric orbit of any moon in the solar system.

6. Titan
It is perhaps the strangest of all moons because it is so eerily familiar. The newly revealed face of Titan has the same weather-beaten features as Earth.

7. Earth's MOON
Scores of moons have been discovered within our solar system, yet Earth's companion still stands out as one of the most remarkable.

iF OUR solar system holds so many remarkable moons, then what strange satellite worlds might we find among the billions of planetary systems in the Milky Way? Perhaps there are temperate, habitable moons orbiting some giant exoplanets. We shouldn't expect to find them inhabited by intelligent life such as the furry Ewoks of Endor in Star Wars, but such moons may be among the most likely habitats for life in the universe.

On the face of it, detecting a moon around a planet orbiting a distant star seems like a spectacularly difficult task, but with a bit of luck today's technology may be able to do it. The best approach is to look for transits, in which an orbiting planet passes in front of its star, dimming the amount of light we detect on Earth. This method has already been used to find several planets, and it could indirectly reveal exomoons. As a moon orbits a planet, its gravity makes the planet move, speeding it up and slowing it down and so changing the timing and duration of transits.

The bigger the moon in relation to the planet, the bigger this effect. In one simulation, a planet with the mass of Neptune situated in the habitable zone of a star - not too hot, not too cold - was given a moon the size of Earth. This weighty moon would change the timing and duration of its planet's transits enough to be detectable by the Kepler planet-finding satellite, or even by ground-based telescopes. Such a large moon would also be able to hold onto a thick atmosphere, making it a prime spot for life.

READ MORE_____ http://www.newscientist.com/special/weird-worlds-solar-system-strangest-moons

Seven theories of everything

The "theory of everything" is one of the most cherished dreams of science. If it is ever discovered, it will describe the workings of the universe at the most fundamental level and thus encompass our entire understanding of nature. It would also answer such enduring puzzles as what dark matter is, the reason time flows in only one direction and how gravity works. Small wonder that Stephen Hawking famously said that such a theory would be "the ultimate triumph of human reason – for then we should know the mind of God".

But theologians needn't lose too much sleep just yet. Despite decades of effort, progress has been slow. Many physicists have confined themselves to developing "quantum gravity" theories that attempt to reconcile quantum mechanics with general relativity – a prerequisite for a theory of everything. But rather than coming up with one or two rival theories whose merits can be judged against the evidence, there is a profusion of candidates that address different parts of the problem and precious few clues as to which (if any) might turn out to be correct.

Here's a brief guide to some of the front runners.

String theory
This is probably the best known theory of everything, and the most heavily studied. It suggests that the fundamental particles we observe are not actually particles at all, but tiny strings that only "look" like particles to scientific instruments because they are so small.

What's more, the mathematics of string theory also rely on extra spatial dimensions, which humans could not experience directly.

These are radical suggestions, but many theorists find the string approach elegant and have proposed numerous variations on the basic theme that seem to solve assorted cosmological conundrums. However, they have two major challenges to overcome if they are to persuade the rest of the scientific community that string theory is the best candidate for a ToE.

First, string theorists have so far struggled to make new predictions that can be tested. So string theory remains just that: a theory.

Secondly, there are just too many variants of the theory, any one of which could be correct – and little to choose between them. To resolve this, some physicists have proposed a more general framework called M-theory, which unifies many string theories.

But this has its own problems. Depending how you set it up, M-theory can describe any of 10500 universes. Some physicists argue that this is evidence that there are multiple universes, but others think it just means the theory is untestable.

Loop quantum gravity
Although it hasn't had the same media exposure, loop quantum gravity is so far the only real rival to string theory.

The basic idea is that space is not continuous, as we usually think, but is instead broken up into tiny chunks 10-35 metres across. These are then connected by links to make the space we experience. When these links are tangled up into braids and knots, they produce elementary particles.

Loop quantum gravity has produced some tentative predictions of real-world effects, and has also shed some light on the birth of the universe. But its proponents have so far struggled to incorporate gravity into their theories. And as with string theory, a true experimental test is still some way off.

CDT
Causal dynamical triangulations looks pretty similar to loop quantum gravity at first glance. Just as loop quantum gravity breaks up space into tiny "building blocks", CDT assumes that space-time is split into tiny building blocks – this time, four-dimensional chunks called pentachorons.

The pentachorons can then be glued together to produce a large-scale universe – which turns out to have three space dimensions and one time dimension, just as the real one does. So far, so good, but there's a major drawback: CDT as it currently stands cannot explain the existence of matter.

Quantum Einstein gravity
This idea, proposed by Martin Reuter of the University of Mainz, Germany, takes a rather different tack.

Part of the problem with unifying gravity and quantum mechanics is what happens to gravity at small scales. The closer two objects are to each other, the stronger the gravitational attraction between them; but gravity also acts on itself, and as a result, at very small distances a feedback loop starts. According to conventional theories the force should then become ridiculously strong – which means there's something wrong with the conventional theories.

However, Reuter has come up with a way to generate a "fixed point": a distance below which gravity stops getting stronger. This could help solve the problem, and lead to a quantum theory of gravity.

Quantum graphity
All the theories above assume that space and time exist, and then try to build up the rest of the universe. Quantum graphity – the brainchild of Fotini Markopoulou of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, and colleagues – tries to do away with them.

When the universe formed in the big bang, Markopoulou says, there was no such thing as space as we know it. Instead, there was an abstract network of "nodes" of space, in which each node was connected to every other. Very soon afterwards, this network collapsed and some of the nodes broke away from each other, forming the large universe we see today.

Internal relativity
Developed by Olaf Dreyer of the Massachusetts Institute of Technology, internal relativity sets out to explain how general relativity could arise in a quantum world.

Every particle in the universe has a property called "spin", which can be loosely thought of as what happens to the particle when it is rotated. Dreyer's model imagines a system of spins existing independently of matter and arranged randomly. When the system reaches a critical temperature, the spins align, forming an ordered pattern.

Anyone actually living in the system of spins will not see them. All they see are their effects, which Dreyer has shown will include space-time and matter. He has also managed to derive Newtonian gravity from the model: however, general relativity has not yet emerged.

E8
In 2007 the physicist (and sometime surfer) Garrett Lisi made headlines with a possible theory of everything.

The fuss was triggered by a paper discussing E8, a complex eight-dimensional mathematical pattern with 248 points. Lisi showed that the various fundamental particles and forces known to physics could be placed on the points of the E8 pattern, and that many of their interactions then emerged naturally.

Some physicists heavily criticised the paper, while others gave it a cautious welcome. In late 2008, Lisi was given a grant to continue his studies of E8.

from New Scientist 04 March 2010

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