November 1993

By Ivor Clarke

Drawing of Saturn on 18,10,1993. 21.30 to 22.15 UT.
R.A. 21 46' 13". Dec. -15 2' 39". Size, Saturn 17.9 arcsec. Rings 40 arcsec. Mag. 0.6.
Distance from Sun 9.795AU. 1465.46 million km

Saturn lies at present in the constellation of Capricornus and is an easy, bright cream coloured planet to observe, with the rings open at an angle of 13°.  The rings looked to be just darker than the body of the planet with the outer edge darker than the inner, Cassini’s division marked the start of the outer edge.  A sharp shadow of Saturn cut across the western side of the rings, this side was darker than the eastern side, giving a round "3D" look to the planet.  Over the north polar region there was a dusky cap. From the pole down to the tropical regions was a broad band of cloud, darker nearer the equatorial zone.  The ring system is closing up and will be just a taint line in 1995.

 by Ivor Clarke

Chapter 1 - BANG!!!

Chapter 2 - OOoooaaaahh

Chapter 3 - Crunch.


Solar Observations

By Mike Frost

A quiz to test your knowledge of astronomy - and the top 40! Some of the questions get a bit obscure but everyone should be able to manage Round 1. Question 1 - if you can't, where have you been for the last 40 years?

If you answer "David Bowie" to every question you'll get at least 4 right.

Round 1 - General

1.        Whose backing group was the Comets? (Hint - It was not Bill Giacobbini-Zinner)
2.        And whose backing was, at one stage, the Spiders from Mars?
3.        Who changed his name to a planet?
4.        Which pop star is of magnitude 0.0?
5.        Which rock guitarist has a degree in astrophysics from Leicester University?
6.        What is the astronomical connection between Kiri Te Kanawa, Gustav Hoist and Mannfred Mann?

Round 2 - Albums

Who had an album called? :-

1.        Dark side of the Moon
2.        Stars
3.        Venus and Mars
4.        Equinox
5.        Night flight to Venus

Round 3 - "Easy" Lyrics

Name the artist and the song in which the following lines appear:

1.        "I blame you, for the moonlit night,
            For the dream which died, with the Eagle's flight..."

2.        "Mars ain't the kind of place to raise your kids,
            In fact, it's cold as hell..."

3.        "Two hundred degrees, that's why they call me Mister Fahrenheit,
            I'm travelling at the speed of light,
            I wanna make a supersonic woman of you..."

4.        "Here comes a supernova,
            What a pushover, yeah..."

5.        "The man from Mars stopped eating cars
            and eating bars
            and now he only eats guitars.."

6.        "Planet Earth is blue
            and there's nothing I can do..."

Round 4 - Name the artist

With the "Guiness Book of Hit Singles" this round is dead easy to compile. Did you know Earth Wind and Fire had a number 41 hit with "Jupiter" in 1978? No. me neither! So, I've tried to keep to top 10 songs only. Who recorded the following?

1.        Invisible Sun                                           (#2,1981)
2.        Venus                                                     (#8,1970 #8,1986 #4,1990)
3.        Venus in Blue Jeans                                 (#4,1962)
4.        Under the Moon of Love                            (#1,1976)
5.        Blue Moon                                              (#1,1961 #9,1956)
6.        Moon River                                             (#1,1961)
7.        Moonlight Shadow                                   (#4,1983)
8.        Walking on the Moon                               (#1,1979)
9.        Man on the Moon                                    (#10,1992)
10.      Bad Moon Rising                                     (#1,1969)
11.      Total Eclipse of the Heart                         (#1,1983)
12.      Shooting Star                                         (#14,1978)
13.      The Northern Lights                                (#10,1978)
14.      Apollo 9                                                 (#13,1984)
15.      Telstar                                                   (#1,1962)
16.      Life on Mars                                            (#3,1973)
17.      Star                                                       (#13,1981+#11,1990)
18.      Starman                                                 (#10,1972)
19.      Calling Occupants of Interplanetary Craft   (#9,1977)
20.      I Lost my Heart to a Starship Trooper        (#6,1978)

Round 5 - Tricky Lyrics - for advanced clever clogs only

O.K. - so you've got A-Level Bowie and so far it's been a doddle. Well this little lot are from the depths of my record collection - most of them are album tracks.   Who recorded the following lyrics?

1.       "Won't you come and look through my telescope,
           The Milky Way's a fine sight tonight..."

2.       "When you see the Southern Cross for the first time,
           You understand now why you came this way..."

3.       "Joseph's face was black as night,
           The pale yellow moon shone in his eyes,
           His path was marked, by the stars of the southern hemisphere,
           And he walked all his days, under African skies..."

4.      "Sub Luna Saltamus"            (Hint - this is the only known Latin lyric for a group who had song titles in English.  French,  Spanish and Italian - in English it means "We dance beneath the moon")

5.      "Clouds and Eclipses, stain the moon and the sun...."

Who did the artist borrow the line off? (Hint - it wasn't in copyright)

Answers in the next MIRA.


By Ivor Clarke
Part 1: The Craters

A quick look at the surface

One of the first things an amateur does, when they set up their new telescope, is to look at the Moon.  After all it is easy to find, bright enough to focus on easily and looks great in any size of instrument.  At first, the beginner will soon lose their way around on this unfamiliar landscape, or should I say moonscape.   Even with a good Moon atlas it can be difficult to know which crater is which in the field of view.
The lunar surface comprises a fascinating mixture of flat 'Seas', the Mare, and what looks like a complete jumble of other features; all looking as if they have been scattered about haphazardly across the surface.  In fact, most of what you see can now be explained thanks to the Orbiter and Apollo moon missions.  These flights have answered many of the old questions about the Moon.  How it got here and where it came from, how old is it? What formed the craters and the Mare and what brand of green cheese is it made from? But they also left many more new questions unanswered and have raised new problems about our neighbour.  BUT in one area it has explained one of the major debates about the lunar craters, plus giving us leads into how the various mountains, valleys and mare were formed.
In this first article we will take a look at the craters.

Lunar Crater History

At first glance the surface seems covered in craters, but this isn't so.  Large areas of the mare are free of any sign of cratering in any size of telescope and only from close range can any of the very small craters be seen.  So what causes craters?  This problem has puzzled many generations of astronomers and fearsome debates took place in the past as each developed their own pet theory.  No one has ever seen a new crater form on the Moon so it was possible to have many wild ideas concerning their origins.   In most of the older ideas, craters were thought to be the product of volcanism.   In 1874 Nasmyth and Carpenter developed the Volcanic Fountain idea with the crater being created by a fountain of material from the central cone showered debris onto the surrounding area in a ring shape to form the walls of the crater.  This idea was taken seriously, mostly in Europe, during the 19th century and was taken as the best theory of the day.
With craters varying in size from ones just visible up to more than 300km across, it is now clear that this idea was wrong.  This effect of a volcanic fountain, forming crater rings dozens of kilometres in diameter, has never been seen on Earth in any volcanic eruption did not seem to matter or worry the authors.  On the Moon with its lower gravity and lack of any atmosphere volcanoes were expected to do things differently!

Volcanoes do create craters and cones, but these are not usually more than a kilometre or two across and so would hardly be visible from Earth, for they would be too small to be seen.  The final nail in the coffin for this theory must surely be the Voyager pictures of the Jupiter moon Io.  These show volcanoes producing fountains of material; but not forming lunar type craters.
The material returned by the Apollo and Luna missions show that almost all of the visible cratering is caused by only one thing: bombardment by meteorites and asteroids.

Impact cratering

When looking at the lunar surface you are looking at a unchanged surface many millions of years older than any on our own planet.  Indeed, nearly all of the visible surface was formed earlier than 3,900,000,000 years ago.  Since that time hardly any change has taken place over most of the Moons surface for 3 billion years.  This time span is almost equal to the oldest known rocks on Earth and goes back to the time when life was just starting to form in our warm oceans.  This is a tremendous span of time when you consider that life on Earth first crawled out of the sea onto land only about 350,000,000 years ago.
That nothing else has happened to the surface of the Moon in all of that time apart from two or three 30km impacts is amazing.  Consider the amount of movement that has taken place in that time in the Earth's tectonic plates.  For instance it has only been 300 million years since the Super Continent Gondwanaland broke up and America started to move away from Europe and Africa.  Since then there have only been one or two major impacts on our side of the Moon, the craters Tycho, Aristarchus and possible Copernicus.
Surprisingly new craters are rare nowadays, only 1 meteor greater than 10gms in weight will fall onto an area of 70,000km² in any one day.  In order to witness a crater being formed, say, 1km or more in diameter, a wait of many millions of years may be necessary.   Far more will hit the Earth for the low gravity of our satellite cannot deflect many of the incoming Earth bound meteors onto its surface.

Impact theory

When the Moon is hit by a large object, 1 to 10km in diameter travelling at cosmic velocities; anything up to 50km per second, a new crater will be formed in an few moments of extreme violence.  Craters such as Copernicus are the result of events such as this.  Impacts of this magnitude would shake the Moon causing moonquakes and landslides all across its surface.
So how is the crater formed?  The impact mechanism has been investigated on Earth by studying explosive events and through firing very high speed projectiles into various surfaces and studying the result.
On Earth there are two types of explosive phenomenon which can cause cratering.  Water can flash to steam deep in volcanoes giving pressures in this processes of up to 5 kilobar (5000 atmospheres).  This is quite enough to take the top off a mountain, as in the Mount St. Helens eruption in 1982.  A different process is in force during high energy TNT or atomic device explosions.  In these events craters are excavated by the shock-wave of the explosive process causing a pulse of instantaneous compression at ultra-high pressures propagating away from the source at several kilometres per second.  The shock front interacts with the surrounding ground, first a pulse of compression followed behind by a region of decompression, called the rarefaction phase.  The crater is formed behind the decompression region as material is accelerated upwards towards the surface.  For a brief period, pressures up to hundreds of kilobar or even megabars are generated.
On the Moon of cause, there is no water, so the craters are formed much as in high explosive cases.  As the incoming rock hits the lunar surface two shock-fronts are produced, one in the projectile and the other in the lunar surface.  A lens shape shock-compressed mass grows directly under both parties with white hot molten material being squeezed out of the edges at many times the impact speed.  The deformational stresses are at least 103 to 104 times the strength of the surface material, so it flows hydrodynamically.  This flows out around the impact point at well above escape velocity.

As the object buries itself into the Moon, more and more of it will be consumed as it slows down and the kinetic energy is turned to heat.  As the shock waves from impact reach the back of the object, they create regions of stress and rarefaction which cause a process of "jetting".  This ejects mass at high velocities with both meteor and target melting and vaporising,, braking the object into smaller pieces.
Gas de-acceleration happens as a bubble of vaporised material swells up in front of the projectile slowing down the penetration and eventually halting it.  At the end, shock-waves reflected from the rear of the incoming rock destroy it.   Kinetic energy is distributed over the target site and crater excavation proceeds at lower speeds until stress decays to the level of the strength of the target rock.  The low lunar gravity, only one-sixth of Earth, causes larger craters to be dug out of the Moon with a greater spread of ejecta around the impact sight.  Most of the ejecta from the impact is expelled at an angle of around 40°, with the finer material having the highest velocities in the first moments of impact.  As the excavation of the crater proceeds larger pieces of material are expelled at lower speeds until finally the material builds up on the rim of the crater and starts to roll back into the crater.

For most impacts on the Moon there is a splash limit of about 1.5x the crater diameter beyond which the continuous deposits don't reach.  After this distance secondary cratering can fall in groups and rows and the well known "Rays" can reach far round the surface of the Moon.

Lunar Soil

The lunar surface eventually reaches an equilibrium in the number of visible craters, after which the older craters are destroyed and covered by later ones (at the same rate as they are formed).
The layer of ejector material which covers the lunar surface is called the regolith.  This is layers of vertically non-sorted particles, in all sizes from microns to meters.  This material is made up of all types of material from many different depths mixed together to form a stable mass.  The Apollo 15 astronauts obtained a long core of material by drilled 242cm into the regolith, which revealed 42 discrete layers.   Only 5% of the regolith would have come from greater distances than 100km, while 50% would have been thrown less than 3km by the force of impact.  One other type of rock is called breccias, these are coherent rocks consisting of many fragments of other rocks fused into a whole.

In MIRA 32   Part 2:  The Mare Areas