Dear Member,
The Annual General Meeting took place on June 21st. The President once again reported that the Society had experienced another very fruitful year, largely due to the publicity gained from the BBC Stargazing Live programme and the Society being featured in the November edition of The Sky at Night. The Observatory had its busiest year ever evidenced from the Fund Box donations, breaking all previous records. The President thanked all those members who contributed to this success. However, at last year's AGM the Hon. Treasurer warned that our expenditure exceeded our income and that subscription rates would have to be increased. We are all aware that the costs of almost everything are increasing. Postage, printing, venue hire charges, lecturers' travel expenses and insurance costs have all risen and if we are to remain financially sound our subscription fee will also have to be increased. It was pointed out that compared with other societies, our subscription was low and had remained low for a long time. The Hon. Treasurer proposed that the subscription rates should be as follows, coming into force on 1st of October 2012:
Ordinary Members: £15 reduced to £14 if paid by Banker's Order
Junior Members (under 18 & full time students): £5
Country Members (over 30 miles distant): £5
Family Membership £23 reduced to £20 if paid by Banker's Order
Group Membership: £35
These rates were designed to keep the subscription for junior members and students as low as possible. Those members currently paying by Banker's Order are urged to inform their Bank of the changes before the 1st October or to fill in the form accompanying this Newsletter and return it to the Hon. Treasurer as soon as possible. I am sure you will agree that these new rates still represent excellent value for money.
Prior to the AGM, the Hon. Secretary had received no new nominations for Council and as there were no retirements your council elected at the AGM is as follows:
| General Secretary | Dr. Julie Atkinson | |
| Treasurer & Membership Secretary | John Tennant | |
| Programme Secretary | Jim Brightwell | |
| Ordinary Members (max 5) | Martin Williams, Julia Daniels, Peter Stern, Roger O'Brien and Dr. Kevin Devine | |
The programme card for the 2012-2013 session is enclosed with this newsletter. Once again our Programme Secretary has managed to produce a series of interesting lectures. The first in the series will take place on September 20th when Dr. Kevin Devine (London Metropolitan University) will talk on the subject: The Missing Organic Molecules on Mars. I look forward to seeing you at the meeting.
STOP PRESS: Just two days after the programme cards were printed, we received news that the speaker booked for the October talk has cancelled. A replacement lecture has now been arranged:- Andy Overall, Group Leader of the London Fungus Group affiliated to the British Mycological Society will talk on:- The Fungi of Hampstead Heath.
Peter R Wallis
Nowadays we know a bit more about the Moon than did Patrick Moore when he argued that the craters were of volcanic origin. It is now generally agreed that they were caused by the impact of asteroids. At least 30 lunar craters have diameters greater than 300 km. The largest is Aitken near the South Pole, at 2,500 km diameter and another, Mare Imbrium, is 1145 km diameter; such enormous craters are called "basins". The still limited exploration of the Moon makes it difficult to date them all but, from the analysis of shock-melted ejecta collected in the Apollo missions, they appear to be clustered between 4.1 and 3.8 billion years ago [1].
But if the Moon was battered by asteroids, what happened to the Earth? It is estimated that the much larger size and mass of the Earth would result in its being hit some 17 times as often; so where are the craters? Well everybody now knows of one impact quite recently (in geological terms) some 65 million years ago at the Cretaceous/Palaeogene boundary which is associated with the mass extinction that included the dinosaurs. The relevant crater is 180 km in diameter at Chicxulub in Mexico; the asteroid that caused it is estimated to have had a diameter of 9 to 14 km. We now know another crater of 100 km in diameter at Popigai 35 million years ago.
But when we look for older craters more comparable to those in the lunar history, we find them at less than half the age of the Earth. Two of the oldest are Sudbury in Canada of 250 km diameter and 1.85 billion years age and Vredefort in S Africa of 300 km diameter and 2.02 billion years age. The difficulty is that the surface of the Earth, unlike the surface of the Moon, is subject to continual erosion, tectonic plate movements and subduction. On the basis of the Moon's record, there must have been some 40 impact basins larger than 1000 km and hundreds of craters larger than 300 km at 3 to 4 billion years ago. It is probably impossible to find any craters from this age as there very few rocks still remaining. However there is another technique to reveal at least some of such past calamitous events.
When massive asteroids strike the Earth at velocities of 10 or 20 km per second they generate a mass of vapourised rock, comparable in mass to that of the impactor, which is ejected beyond the atmosphere; after a time it cools and falls back as a layer of "spherules". For impactors of more than 10 km diameter this will be a global layer. The 65 million years ago Chicxulub impact produced such a global layer of several mm or cm thickness, which was actually an important confirmation of the event hypothesised earlier by Alvarez et al. Because the layer is global it is more likely to survive erosion and tectonic events which would destroy the more local record of a crater.
A recent paper [2] has studied 14 known spherules layers. The authors reach the conclusion that impact velocity is the main determinant of the size of the spherules but that the size of the impactor can be estimate from the thickness of the layer, provided that it is global. Additional confirmation that the layers have an extra-terrestrial origin is shown by anomalous levels of iridium present and chromium isotope measurements. The authors regret that so far only two regions have been systematically searched for Pre-cambrian spherule layers (Pilbara Craton in Western Australia and the Kaapvaal Craton in S Africa, respectively 2.5 and 3.3 billion years ago); this only covers a tenth of Earth's impact history, so much more needs to be examined to get a complete picture. Nevertheless they offer an argument that the flux of impacts was higher at that time.
There is an existing theory to explain the lunar record of few impacts prior to 4.1 billion years ago and a reduced flux after 3.8 billion years ago. This is the hypothesis of a "Late Heavy Bombardment". This model suggests that the gas-giant planets Jupiter and Saturn were originally in slowly evolving near circular orbits. In this phase only a few asteroids would be dislodged, probably by Mars, from the asteroid belt between Mars and Jupiter. Then, about 4.1 billion years ago, the gas giants Jupiter and Saturn reached a 2:1 resonance, in which Jupiter's orbital period became equal to one half of Saturn's. This disturbed their orbits and eccentricities, also causing many asteroids to move into planet-crossing orbits . This is called the Late heavy Bombardment (LHB), lasting until 3.8 billion years ago.
A recent paper [3] extends this model. The authors suggest that the inner boundary of the asteroid belt was originally closer at 1.7 AU instead of the present 2.1 AU, set by a secular resonance. Their dynamic model shows that over about 4 billion years the inner belt between 1.7 and 2.1 AU would have been strongly destabilised by changes in the orbits of Jupiter and Saturn. These would impact at greater speeds of 20 km/sec than the asteroids of the outer belt and extend its period, virtually eliminating all the inner belt. They estimate that 15 plus or minus 4 basin-forming impactors would have struck Earth between 3.7 and 2.5 billion years ago. They comment, "The terrestrial consequences of these mammoth Archaean events have yet to be explored, but we suspect that they may have affected the evolution of life and our biosphere in profound ways".
1 The age of the Earth is now believed to be 4.56 billion years and the Moon itself was formed early in this period. Back
2 Johnson, B.C. and Melosh, H. J, "Impact Spherules as a record of an ancient heavy bombardment of the Earth", Nature, 3rd May 2012. Back
3 Bottke, W. F. Et al, "An Archaean Heavy Bombardment from a destabilised extension of the asteroid belt", Nature, 3rd May 2012. Back
Doug Daniels
I well remember attending meetings of the BAA way back in the late 1950's when arguments raged concerning the origins of the lunar craters. There were two distinct 'camps' those who supported the notion that the craters were formed by volcanic activity and those who believed that they were caused by impacts. Debates could become quite 'heated', particularly by the volcanologists! It was pointed out that many craters featured a central peak, surely this would not result from an impact. There were also numerous craters like Plato with distinctly flat interiors and other features such as the Sinus Iridium that exhibited obvious encroachment by flowing lava. Many of the lunar maria appeared roughly circular and featured wrinkle ridges indicative of flowing lava. Then there were the numerous crater chains and 'domes'; how might they be explained by impacts? Personally, I wondered if both processes might have been involved.
It should be remembered that at that time, all our knowledge concerning the lunar surface came from visual observations and drawings mostly made by dedicated 'amateurs'. The major observatories totally ignored the Moon and the best photographs obtained by professionals using the world's largest telescopes were inferior to images obtainable today with a 10-inch telescope!
It was argued that if the lunar craters were caused by impacts then surely the Earth would also have suffered a similar fate and where was the evidence for this? There was of course the single well known example, the Barringer meteor crater in Arizona and perhaps the report of the Tunguska event in Siberia; but information on this was scant because of secrecy during the cold-war between the USSR the USA and NATO. In any case, the Earth is a dynamic planet and the oceans, continental drift, active volcanism, weathering and erosion would have obscured much of the evidence of past bombardment.
The arguments raged on but resolution finally came when space probes were dispatched. Mariner showed the surface of Mars to be highly cratered, Mercury looked very much like our Moon and even Jupiter and Saturn's moons were peppered with impact craters. It soon became clear that the early solar system had been a very violent place indeed and it threw new light on the complex processes involved in the early stages of planetary formation. Apart from the impacts of asteroids, comets came under suspicion as well. Perhaps the Earth's oceans were the result of impacting comets delivering vast amounts of water in the form of ice from the cold depths of the outer Solar System.
Recent data has shown that some of the lunar craters that lay in perpetual darkness close to the Moon's poles do indeed contain ice and there is mounting evidence for the existence of salty oceans beneath the icy crusts of Jupiter's satellites Europa and Ganymede. Water appears to be essential for the existence of life so it may suggest that during the Solar System's early history, the periodic bombardment by asteroids comets and meteorites may have been vital factors in establishing life on Earth and perhaps even elsewhere in the Solar System. We may not have to wait too long for answers if the proposed mission by the European Space Agency (ESA) goes ahead in 2022. The mission called JUICE ( JUpiterICE) will take a closer look at Ganymede and Europa looking for conditions favourable for the establishment of life – we live in interesting times.
Doug Daniels
2012 – Yet another disappointing summer, the first half was cool and very, very wet. It was in stark contradiction to the prophecies made in early spring that warned of imminent drought accompanied by the traditional 'hose-pipe ban.' Reporters were filmed standing in dried-up river beds and on the banks of depleted reservoirs. Farmers were shown bemoaning crop failure and predicting future 'dust bowl' conditions in East Anglia, pointing out that food prices were bound to rise as a result. But just a few weeks later, the rivers were in full spate and bursting their banks, the reservoirs were filling up nicely and by early July we ended up with warnings of potential flooding in many areas – the farmers warned of biblical floods in East Anglia destroying their crops and pointed out that food prices were bound to rise as a result.
Once again the jet stream got stuck across the southern half of the country and refused to move northwards until late July – no doubt due to climate change caused by mankind's industrial activities. Really? I have recently been reading Arthur Bryant's book on Samuel Pepys, the 17th century diarist and it contained the following descriptions from Pepys' epic work.
In 1662, Pepys decided to improve his dwelling by adding another storey to his house. In mid July 1662 the workmen removed the roof. By July 15th every tile had been lifted – "And then was seen the sad fate that befalls the man who ventures to uncover his house on St. Swithin's day" – it rained unceasingly for the following week. And then again in 1663 we find Pepys out of sorts because of the interminable wet summer "It scarcely ceased to rain for four months" he wrote. It goes to show that the jet stream was probably misbehaving in the 17th century just as it does in the 21st – long before mankind's industrial activities could have had any noticeable effect upon it. Could there be another explanation for these periods of unseasonable weather?
Between 1645 and 1715, the Sun produced very few sunspots and the period suffered lower than average global temperatures. This period has been described as the 'Maunder minimum' named after the Victorian solar observer E.Walter Maunder who described it. Maunder built on the earlier work of Gustav Sporer and published a paper in 1894 concerned with the latitudes at which sunspots appeared within the Sun's 11 year magnetic cycle. He produced the famous 'butterfly diagram' to illustrate the effect. Coincidentally, Maunder was the driving force behind the founding of the British Astronomical Association and was the first editor of its Journal.
Records also show that in 1770 the winter was so cold that the Thames froze – a period often referred to as 'the little ice age'. In all there are three identifiable historic 'cold snaps' in 1650, 1770 and 1850 perhaps occasioned by volcanic activity and/or the effects of El Nino or perhaps decreased solar output, or even a combination of all three. The extended period of bad weather at the end of the 18th century and the resultant poor harvests and famine that it caused in Europe, led to civil unrest and may even have been the root cause of the French Revolution.
The most recent solar minimum of 2008-2009 featured a deeper and a more prolonged solar minimum than had been observed for almost a century. In 2008 there were no sunspots on 266 out of 365 days. There was a 20% lowering of solar wind pressure, a 12 year low in solar radiance .02% lower in visible wavelengths and 6% lower in UV and a 55 year low in solar radio emissions.[*] One wonders if this could perhaps have had any bearing on our recent unpredictable weather patterns? Apart from the 11 year sunspot cycle, the Sun has other long term cycles that we do not as yet fully understand because we have not been studying them for long enough. Such cycles are almost certainly bound to affect the Earth's climate and influence our seemingly unpredictable weather.
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Last updated 27-Jan-2018