What happens to messages sent using instant messaging programs? Is all data lost forever when a conversation is terminated or is stored somewhere and retrievable – and if so by whom?G. B. Lyons, Newcastle-upon-Tyne

About 50 years ago, Lake Erie froze and became covered with a glassy sheet of ice about 50 millimetres thick. As an onshore wind rose, I heard the lake sing with a pure, mid-range, organ-like tone, which rapidly intensified to an almost intolerable level with no visible motion in the lake. After a few minutes the ice fractured and the pure tone was replaced by cracking, echoing at about the same pitch and, abruptly, by the atonal roar of shattering ice. Has anybody else witnessed such an event? How was the tone created?Charles Sawyer, Camptonville, California, US

If you melt fresh snow or clean freezer ice, they give off a distinctive perfume, also detectable as a flavour, and one I find pleasant - a reminder of eating snow as a child. If the odour and taste are not the product of particular gases absorbed from the atmosphere, what are they?Graham Cox, Whitstable, Kent, UK

Below is a series of lunar eclipse pictures shot on a Sony DSC F717 digital camera near my home between 3 and 4 March 2007. I have made a composite picture of the photographs. I wondered if anybody might be able to explain the geometry of the change in angle of the approach and receding shape of the shadow. There was no change in the angle of the image while the positioning of this composite shot was being createdBill Richmond, Wimborne, Dorset, UK

I stood at the foot of a lighthouse, on a 15-metre high cliff, looking out to sea. It was a generally clear night with a light sea mist and visibility was around 6 kilometres. The beam of light, which was rotating clockwise overhead, appeared to me to be coming from a phantom lighthouse on or near the horizon, and rotating anticlockwise (it was in time with the actual beam). Any explanations?tbc

Can anybody help us explain what caused the phenomenon in this photograph? It was taken on the Sands of Forvie nature reserve in north-east Scotland. The weather was dry, though there had been rain the day before. The photograph shows the lip and face of a small mobile sand dune. The slope of the dune face was approximately 45 degrees and the sand was dry at the time the photograph was taken. The image shows a section about 1 metre wide. The whole pattern extended about 20 metres.Martyn and Margaret Gorman, Newburgh, Fife, UKThe following answers were selected and edited by New Scientist staff. You can add your replies in the comments section below.Moisture from the rain retained by the striated slip face of the dune would have given it a firmer consistency than the sand on the top, which would have dried out more quickly. What happened then would have been quite startling for anyone lucky enough to witness it.The loose dry sand on the dune top would have started to move, pushed by gentle winds. As it spilled over the edge, the sand cut this series of parallel gullies, creating the herringbone pattern. The uniform spacing of the gullies is related to the even flow of the wind dynamics on the edge of the dune. Gullies would not have been able to form in the dry loose sand on the top of the dune.At the dune crest, erosion would be occurring along a hemispherical front (as you can see from the curved tops of dunes in a desert) as loose sand fell into the gullies below. This sand would have flowed down to the bottom of the slope to form a fan or ramp at the foot of each gully (visible at the base of the photograph). Once started, the whole process would have taken only a few minutes to complete. The gullies and ridges are very fragile and would soon be buried under sand as the dune face moved forward. The photographer was very lucky to see them.I have seen this phenomenon on a grander scale in Arabian and African deserts where holes up to 6 metres deep have been dug in sandy soil. There, surface soil with some structure and a moderately hard consistency lies on top of poorly structured sand. The hard surface stops the face from collapsing but the whole system is inherently unstable and dangerous to observe from inside the pit.A steady supply of drifting sand often blows over the rim of the pit, eroding the edge of the surface soil into intricate patterns, picking out minute differences in its constituents and etching out old roots and so on. The loose material produced by erosion feeds into a network of fragile gullies cut into the underlying sands. Where there are harder layers lower down, sand will flow down the first gully, cascade over the next hard layer as a "waterfall", race down another gully and then broaden out to form a fan at the base of the slope. Such a flow can be maintained for minutes on end.This type of flow produced the pattern that is seen in the photograph, and is beginning to be recognised as of considerable significance in the geological record, according to Brian Turner of the University of Durham, UK, who has observed similar particle flows in sand ramp deposits in Jordan. Allan Treiman of the Lunar and Planetary Institute in Houston, Texas, has argued that similar examples on Mars are the result of "dry granular flows".Gullies observed recently on Mars were attributed to the action of water (New Scientist, 15 January 2005, p 36, and 23 March 2006, p 19). I believe this is way off mark. Having seen the particle-flow process in action on many occasions, I believe the Mars gullies are far more likely to have been formed by dry granular flow closely related to that shown in the sand-dune photograph.R. Neil Munro, Dirleton, East Lothian, UK

Two questions, possibly about the same phenomenon. EdOn Tuesday 4 September at about 2030 GMT I noticed what appeared to be a planet in the northern sky. Since this is impossible I looked closely at it. Shortly after I began watching, its brightness dropped from an approximate -2 (about that of Sirius, the brightest star in the sky) to about +6 (the faintest visible with the naked eye) over the course of a minute or so. What could it have been? For the astronomers out there, the location in the sky was approximately: right ascension 7, declination +60.Mike Ruddock, Evesham, UKAt about 2200 GMT on 4 September, we saw two bright orange/red stationery round objects in the north-western sky. They looked like very bright, Mars-like planets, but appeared three times as wide in the sky. One object was located diagonally below the other, at an angular distance of 2 degrees.

They came towards us very slowly before flying, with increased acceleration, in a south-easterly direction, becoming fainter and smaller. No sound came from them, and they kept the separation distance at all times. The whole spectacle lasted for about 15 minutes.

Our area is regularly visited by aircraft from the air bases at Mildenhall and Lakenheath, so we are no strangers to night-flying exercises. What we found very strange is that, unlike the navigation lights of most aircraft, which are bright white, these were orange/red. Also, there were no flashing lights, which are seen on all civil and military aircraft.

The altitude they were flying at was difficult to judge but we would say about the same as most of the 'regular' night-flying aircraft.

Does anyone have any suggestions as to what was going on? Also, have similar objects been observed?

Ithel and Omara Williams, Waterbeach, Cambridge

How much force would be required to stop the world spinning? If you use, for example, the engines of the space shuttle to do it, how long would it take? And what would be the effect on the planet, in particular the weather and the tides?Stephen Frost, Richmond, Surrey, UK

If electricity can be generated by moving a coil through a magnetic field, why do we not launch large coils into orbit? As they circled Earth within the planet’s magnetic field, the energy could be sent down from the coils via microwave: even people in remote areas could receive it. I suspect that this system would cost much less than energy generation does at present. So tell me, what flaws in this scheme would prevent me from picking up my Nobel prize?Robert Burns, London, UK