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Q Was there ever running water on Mars? And is there any now?
Based on orbital photography, we believe that Mars once had an atmosphere thick enough to sustain liquid water on its surface. We have photographed valleys on Mars that appear to have been cut by running water; the Mars Pathfinder spacecraft touched down in one. In fact, billions of years ago Mars may even have had oceans. Today, its atmospheric pressure is only 1/100th that of Earth's - so any liquid water on the surface instantly turns to vapour. The atmosphere must have been lost over millions of years. Exactly how is not clear, though Mars's low gravity means that it can't hold on to a thick atmosphere anyway. However, there is believed to be a layer of permafrost - permanent ice - just a few metres below the surface of Mars at mid-latitudes.
Q How do muscles work?
The mechanism that allows muscles to move is very complicated. Basically, each muscle is made up lots of fibres, each of which contains lots of individual cells, all long and thin. Each cell is a similar collection of tiny, long fibres lying next to each other.
These fibres are of two types. One has bits sticking out of it, along its whole length, which look a bit like the oars of a long-boat. The other is basically smooth, but with regular pits. When the muscle is still, the "oars" rest inside the "pits". When a nerve signal passes from the brain to, say, the arm, to reach up and scratch your nose, chemicals are released at the nerve ending and set off a complicated chain reaction inside the muscle. The end result is that the "oars" in one set of fibres move up and push into the next set of "pits", causing the muscle to contract.
Q Do tectonic events on one side of the earth create geographical features on the other side?
Broadly, no. Seismic waves do travel through the earth's crust and mantle, often to great distances, but there is no evidence that they are strong enough to create a new geographical feature. Recent observational evidence also suggests this could not happen. Earthquakes regularly occur at exactly the opposite side of the earth to seismic stations (for example in Tamanrasset Africa and in Bogota, Colombia) but the instruments record no unusual ground motion. Since they are sensitive to displacements of the order of thousandths of a millimetre, there is no observational evidence that permanent features could be formed half way round the earth.
Q Sometimes you can see a halo of light around the Moon. What causes this?
There are two possible explanations. It is either a corona - a round patch of brightness, two or three times the diameter of the Moon, often with a hint of colour around its edge. This is caused by diffraction of moonlight by tiny drops of water in a thin veil of misty cloud, usually at low levels in our atmosphere. A corona often precedes a warm front, when the thin veil of cloud thickens and produces drizzle, and then possibly heavy rain.
Alternatively, there is an effect that is, in fact, known as a "halo". This is a much larger, bright circle around the Moon (radius 22 degrees) which is only bright at its outer edge. The halo is caused by reflection of moonlight from ice-crystals in a thin veil of high-level, cirrus-type cloud. A halo is often seen in spells of fine weather.
(Left: Moon eclipse)
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