MIRA 32
March 1994


MARE IMBRIUM BETWEEN PYTHEAS & LAMBERT



FEATURES . . . . . . .  MARE IMBRIUM BETWEEN PYTHEAS & LAMBERT
LOCATION . . . . . . .  23° N  21° W
DATE . . . . . . . . . . . 1993 / 4 / 30
TIME . . . . . . . . . . .  21.30 UT to 22.00 UT
MOON . . . . . . . . . .  8 days old
LUNATION . . . . . . .  870
COLONG . . . . . . . .  22
SEL. LIBRATION . . . -4.2 long.  6.5 lat.
OBSERVER . . . . . . . IL Clarke, Warwickshire
CONDITIONS . . . . .  Good, seeing 4/5
INSTRUMENT . . . . . 102mm Refractor, 7.4mm Plossl +1.8 Barlow = x243

The two craters LAMBERT (30km diameter) and PYTHEAS (20km) lie in the southern half of the MARE IMBRIUM.  This is the largest of all the impact basins on the moon at over 1,300km across and in area is larger then Great Britain and France combined.  All of this area has been heavily flooded with lava and is covered with many lava wrinkle ridges.  The magnitude of this impact must have been almost enough to break the moon apart.  It has been estimated that the depth of the original crater must have been hundreds of kilometres before the floor rebounded back again.  All of the Imbrium basin has been flooded many time since the original impact about 3,900 million years ago by thin sheets of lava sprouting from many fracture points around the Mare.  These successive layers give rise to the uneven surface and are an interesting sight at local sunrise and sunset as the terminator passes.  The bright spot to the north west of Lambert is the MONS LA HIRE 1500m high just catching the sun.  This is one of a number of mountain masses scattered across the Mare.






Astronomical Pop Quiz - Answers
By Mike Frost


Round 1
1.        Bill Haley
2.        David Bowie (as Ziggy Stardust)
3.        Freddie Mercury (originally Frederic Bulsara)
4.        Suzanne Vega
5.        Brian May
6.        Jupiter, bringer of Joy.  Holst wrote it as part of the "Planets" Suite, Mannfred Mann used one theme for "Joybringer", Kiri used the main theme for the Rugby World Cup tune.


Round 2
1.        Pink Floyd
2.        Simply Red
3.        Wings
4.        Jean-Michel Jarre
5.        Boney M


Round 3
1.        Tasmin Archer - "Sleeping Satellite"
2.        Elton John (+ latterly Kate Bush) - "Rocket Man"
3.        Queen - "Don’t Stop Me Now" (written by Freddie Mercury rather than Brian May)
4.        Frankie Goes to Hollywood - "Welcome to the PleasureDome"
5.        Blondie - "Rapture"
6.        David Bowie - "Space Oddity"


Round 4
1.        The Police
2.        Shocking Blue, Bananarama, Don Pedro’s Animals
3.        Mark Wynter
4.        Showawaddywaddy
5.        The Marcels, Elvis Presley
6.        Danny Williams, Henry Mancini
7.        Mike Oldfield
8.        The Police
9.        R.E.M.
10.      Creedence Clearwater Revival
11.      Bonnie Tyler
12.      Dollar
13.      Renaissance
14.      Adam Ant
15.      The Tornados
16.      David Bowie
17.      Kiki Dee, Erasure
18.      David Bowie
19.      The Carpenters 
20.      Sarah Brightman


Round 5
1.        Mike Oldfield - "Saved by a Bell" (off the album "Discovery")
2.        Crosby, Stills and Nash - "Southern Cross"
3.        Paul Simon - "Under African Skies" (off the album "Graceland")
4.        Abba - "The Piper" (off the album "SuperTrouper")
5.        Sting - "Consider me Gone" (off the album "Dream of the Blue Turtles")
           It’s originally from a Shakespeare sonnet.








THE GEOLOGY OF THE MOON
By  Ivor Clarke

Part 2:   The Mare Areas

As we saw in the last issue of MIRA, the lunar surface is a fascinating mixture of craters and the flatter, darker areas, the Mare.

These large plains give the impression of being almost flat, formless and uninteresting flooded lava fields.  This is misleading.  Before the Apollo missions some considered these areas to be dust bowls, filled many meters thick with fine meteor dust which had gathered over the aeons of time since the formation of the Moon.  Any attempt to land the Apollo LEM module on to these surfaces would be doomed to failure, with the LEM being swallowed up by dust.
This was disproved on 1966 January 31st with the Russian Luna 9 spacecraft landing in the Oceanus Procellarum and 5 months later, almost to the day, the Americans soft-landed their Surveyor 1 near the crater Flamsteed.
From the photographs returned it was obvious that the surface was covered in only a thin layer of dust, so manned flights could safely go ahead.  The material returned by Apollo eventually answered the questions of the origin and formation of the Maria.

Distribution of Maria

From the Orbiter pictures it is noticeable how much of the near side of the Moon is covered with Maria compared with the far side, about 95% compared  to 5%.  Why this is so, is a complete mystery, and no satisfactory theory has yet explained this anomaly.
The material returned by Apollo 14 showed that the Mare is composed of basaltic lava deposits and are of the order of 4,400 to 3,200 million years old.
Not all of the area of a Mare was covered in one mighty flow of lava, but rather the surface has been built-up by a number of flows, followed, sometimes millions of years later, by others.  This can be shown in an number cases by a crater count in adjacent areas with a marked increase in the number of craters in the older sections.
These crater impacts are of both primary and secondary types and can be used to date various flows.  Also visible are colour differences in various areas indicating that a different type of material has flowed over older sections, sometimes leaving traces of the underlying surface showing through.  From these it is possible to estimate the thickness of the flow and the amount of material deposited.  By 3,500 million years ago most of the circular Mare basins had been formed, and the surface would have looked familiar to us today.




Formation of the Maria

The two youngest major circular basins on the Moon are Mare Imbrium and Orientale.  Orientale in particular is relatively unmodified by later erosion.  Indeed it is possible that Orientale is one of the freshest of all impact basins (in the Solar System?).
Not until the Lunar Orbiter IV spacecraft gave us the first good full view of it, did the size and scale of this formation become apparent.  This major ring basin was not known properly for what it was, having been seen from Earth only occasionally because of its position just over the limb of the Moon on its far-side.
At over 900km in diameter, it is one of the Moons largest impacts.  It is mostly undamaged but for craters such as Kopff and Maunder at around 35km diameter and a few others and it has not been obscured by later material.  From a study of this major impact has developed the realisation that most of the features on the surface of the Moon and all of the other circular Maria must have been formed by impacts of various sizes.

Basin Formation

Craters which are less than 150km in diameter mostly display a single central mountain complex (some in a ring).  From 150km to 300km diameter, craters almost always display a central mountain system in a ring shape, but above 300km, concentric rings with out central peaks are the rule.  The outer ring of Imbrium mountains are 1300km across and rises 7km above present floor.  The original crater excavation was probably over 100km deep at the time of impact.
Up to now, 29 basins have been identified, randomly distributed over the surface of the Moon.
As the size of the impact increases, so the final diameter and shape of the crater changes depending on the underlying terrine and the force of the impact.  In all of the 300km diameter and larger events, the impacts formed large basins where molten rock from the impact pooled in the crater, forming lakes which flooded most of the space between the inner rings.
These impacts would cause large Moon-wide disturbances while seismic, and shock-wave activity would flatten large sections of the surrounding terrine.
Larger basins such as Orientale have several rings of mountains.  The innermost one is 320km; the inner Rook ring at 480km; the outer Rook ring at 620km and the Cordillera ring at 920km diameter.  Between the outer mountain rings the surface is covered by various forms of hummocky material inside the extensive blanket of ejecta.
The formation of a Mare type area proceeds similar to that of a smaller impact, with a large body impacting into cratered terrain at speeds up to 45km sec.  At impact, molten shocked material forms, lining the interior of the rapidly expanding crater.  Ejection of high velocity material from the impact, will, if its speed is greater than 2.4km sec. be expelled into space.  The slower moving material will give rise to secondary cratering with some blocks travelling many hundreds of kilometres before landing.  As ejection of increasingly lower velocity ejecta continues, most will fall within a radius of 1.5, onto the surrounding area in a continues blanket.  In the case of Orientale this would be 4 to 5km deep around the immediate crater thinning to a few meters at around 1000km to 1300km from the site.
The shock wave and ejecta from the collision would pass through the surrounding area depositing layers of semi-molten material, this would be radially striated from the impact centre, depending on the underlying topography.  As the expanding shock wave hit the sides of craters or hills in its path, it would be deflected or forced into a backwash.  In some cases material would fall taking on the appearance of a dune field at a right angle to the flow.  This can be seen in craters such as Inghirami and Riccioli (Orbiter IV HR 169 and HR172 frames) as a field of hummocky material lying within the crater, adjacent the far wall.
At other times small obstructions would cause the flow to deposit material behind the object in a herringbone pattern.  These may be caused by bow-waves as the material streamed away from the impact crater like a liquid.  A herringbone pattern can also be caused in secondary cratering by blocks of material landing simultaneously close together.



Ring Mountain Building

As the incoming body is destroyed, continued movement of ejecta by radial flow floods the final rim building a wall.  As the strength of the under-lying rock matches the decreasing pressures of the impact and the incoming body is consumed. The floor of the transient cavity, which could be well over 50km deep in a major event, will start to rebound, pushing up rings of mountain ranges like ripples from a water drop.  This process will continue outward like a wave as the rock adjusts to the compression and rarefaction processes caused by the impact.  Over a large enough area, highly shocked rock is effectively fluidized and can bend and flow producing folds and ridges.  This process pushes up a central mountain ring which in turn collapsed when gravity exceeds the internal force of the rock strength.  As the cycle proceeds, insufficient energy it will eventually "freeze" the outer rings into their present shapes and levels, while the basin will slump inwards producing large faults.
After the impact is over, molten ejecta will fall back inside the basin and pool as it settles into its present topography.  The estimated total volume of melt produced at the Orientale impact was in the order of >200,000 cubic km., while the Imbrium impact produced over 1,000,000 cubic km. of molten rock!
From the moment of impact to the last of the eject falling, causing secondary cratering and rays, would have only taken about 20 minutes.  This would significantly alter the appearance of the landscape over thousands of square kilometres obliterating the underlying surface forever.
Because of the huge amount of heat generated by such an impact, it is not hard to see why vast amounts of lava would start to flood out of the fractured floor around the impact site.




Maria Surface

It is difficult to estimate the thickness of material laid down, but it has been estimated that northern Oceanus Procellarum is thinner than 500m, but in places could be over 1km, while Mare Serenitatis is 1km to 2km deep.  These measurements are based on lunar gravity anomalies and radar soundings.  Also it is possible to see the remains of drowned craters through the thinner areas, so a estimate can be made depending on its diameter.
The mare material are grouped broadly into two sets: High or Low titanium content.  HIgh content include Mare Tranquillitatis and Serenitatis.  While low includes the Maria Procellarum, Imbrium and Fecunditatis.  The maria material has a difference composition to most of the rocks found on Earth.  This is because the Moon is very dry, the absence of hydrous minerals, causes unique materials to form such as pyroxferroite, tranquillityite and armalcolite, named after the Apollo 11 crew (this is rare, formed during the cooling of mare lavas, when the temperature falls to 1130°c it reacts with the remaining melt, remaining today in small amounts that did not completely react before lava solidified).
The Imbrium mare appears to have erupted in three episodes, mainly from near the outer mountain ring and then flowed into the centre.
Why did the lava flood these areas?  It seems that it is necessary for the Lunar crust to be thin enough for the lava to reach the surface from its magma chamber about 75km below the surface.  Because of the basin's moulding impact, in these areas the crust would be thinner, so a column of lava 60km or so high would be able to reach through the crust and pour out onto the surface.  This could explain why the near side has the mare deposits: the crust on the far side is considerably thicker.
A study of Orbiter and Apollo photography of the maria surfaces reveals very little evidence of where the source of the flows originated.  One of the problems is that Mars and Venus is mapped better than our Moon, and the relative heights of all parts of the surface are not known accurately.  Therefore it is difficult to know where a lava flow would go to from any point as the gradient and direction of slope is uncertain.
Many domes are visible in some areas, some having elongate pits on their summits.  This type of pit is also visible in flat mare and is thought to be a lava tube exit point.  In any one area there must have been many exits for the amount of flow.  There is no doubt that lava travelled great distances across the Moon, up to 1200km in the case of Mare Imbrium.  These show as lava-flow fronts (Wrinkle ridges), with lobate edges which resemble terrestrial lava flows, and formed deposits from 10m to 35m high, the largest known are 65m high as measured by shadow lengths.


Low viscosity lavas can flow great distances, 400km to 600km down gradients as small as 1 in 100 and 1 in 1000.  These lavas have a melt point of about 1400°c which allows a long run.  The later Imbrium lava flows show slight colour difference to the older fronts, with the freshest being bluer while the older flows are redder.  Apollo samples show that the bluer have a higher titanium content while the older redder have a titanium content more typical of terrestrial lavas.  This is shown well in the latest false-colour Galileo space-craft views of the Moon with the high titanium lavas of Imbrium, Crisium, Procellarum and Tranquillitatis showing up blue while the other areas show a redder / orange colour.
Across many areas of mare run long sinuous valleys (rills), resembling dry river beds, in this case ancient lava flows.  These run for many hundreds of kilometres and are probably a mixture of collapsed lava tubes formed by fast flowing turbulent lava and cracks in the surface caused by contraction of the lava sheets as they cooled.  Apollo 15 landed near Hadley Rille in July 1971 and drove the Lunar Rover along the edge collecting samples.  The rill is about 100km long, 2-3km wide and nearly 1km deep; there were obvious signs of stratification showing signs of lava-stream flows from the Moon's active Period.  Much work still needs to be done on our neighbour before we understand all of its secrets.







Hipparcos: Mission Accomplished

After more than three years of efficient and successful operations, communications with ESA's (European Space Agency) scientific satellite Hipparcos were terminated on 15th August 1993.  The Hipparcos satellite, a purely European undertaking and the first space experiment dedicated to the highly accurate measurement of star positions, distances and motions, was launched in August 1989.  Since then an enormous wealth of scientific data has been gathered by Hipparcos.
Targeted for an operational lifetime of two and a half years, it was trapped in an elliptical orbit through failure of its apogee boost motor.  But now, more than three years of high-quality star measurements have eventually been accumulated. and all of the original scientific goals of the mission have been fully accomplished.
Half of the resulting parallaxes have a formal error of better then 2 milli-arcsec, while 90% are better than 3 milli-arcsec.  Examination of the distributions indicates that the formal errors agree quite well with the external errors when they are less than 3 milli-arcsec.  This will provide highly accurate stellar positions on 120,000 stars in our local solar neighbourhood with measurements on the velocity with which these stars are moving in their orbits through the Galaxy.
During the last few months of its life, as the high radiation environment to which the satellite was exposed took its toll on the on-board systems.  Hipparcos was operated with only two of the three gyroscopes normally required for such a satellite following an ambitious redesign of the on-board and on-ground systems.  Plans were in hand to operate the satellite without gyroscopes at all and the first such "gyro-less" data had been acquired, when communication failure with the on-board computers on 24th June 1993 put an end to the relentless flow of 24,000 bits of data that have been sent down from the satellite every second since launch. Further attempts to continue operations proved unsuccessful and after a short series of tests, operations ceased 4 years and one week after launch.
Extremely accurate positions of more than one hundred thousand stars, precise distance measurements (in most cases for the first time), and accurate determination of the stars' velocity through space have been derived.  The resulting Hipparcos Star Catalogue, expected to be completed in 1996 will be of unprecedented accuracy achieving results some 10 to 100 times more accurate than those routinely determined from ground-based astronomical observatories.  A further star catalogue, the Tycho Star Catalogue of more than a million stars, is being compiled from additional data accumulated by the satellite.
These catalogues will be of enormous value in astronomers' attempts to understand and describe the properties and evolution of stars, and the dynamically motion of these stars within our Galaxy.  In the process, Hipparcos has discovered many thousands of new binary star systems; measured the precise light variations of many hundreds of thousands of stars over its operational lifetime.  It has provided an accurate and independent validation of the predictions of General Relativity.
Dr. Michael Perryman, ESA Project Scientists for Hipparcos said. "We are delighted that it has delivered substantially more than it had been originally designed for.  When our final results are published, some very interesting new insights into the nature of our Galaxy, its structure and evolution will emerge." 

From the ESA Bulletin







 
The Delta Clipper Demonstrator vehicle

The first unwinged rocket vehicle has landed on Earth.  Although this had already been done on the Moon, the first flight of the DC-X, the Delta Clipper Experimental vehicle is a new American craft that embodies a new approach to the problem of reducing the cost of access to space.  This vehicle is only aimed at demonstrating a few of the problems that are still open.  Other demonstrators may follow.  It has taken off twice from the White Sands Missile Range in the USA, on 1993 August 18 and September 11 and has landed again after a minute-long fight.  This vehicle does not incorporate stunning technological advances, but it does correspond to a notable change in approach to launch vehicle design.
Up to now launch vehicles have been performance driven.  The initial design of the first vehicles was performed independently of cost considerations, and operations cost were considered after the design was frozen.  The European Space Agency's Ariane-5 is the first European launcher the design of which started with a value analysis within a given available technology.  The following step is a purely cost-driven design.
The DC-X vehicle is a step toward the demonstration of a purely cost-driven design, defined from past experience after an analysis of the costliest lines of a vehicle budget.  The preliminary analysis led to a single-stage-to-orbit vehicle as a likely candidate for such a cost-reducing vehicle.
The DC-X vehicle is a rocket-powered single-stage-to-orbit demonstrator built by McDonnell-Douglas for the Ballistic Missile Defence Organisation (ex-SDIO, i.e. Star Wars) of the United States, aimed at solving two of the unknowns of such a vehicle.  One unknown is vertical take-off, stable atmospheric flight and landing, the other is quick and economic turnaround of such a vehicle with air-craft-like operations.  This vehicle runs on liquid hydrogen and oxygen and is propelled by 4 RL-10 motors derived from those of the Centaur stage.  The vehicle stands 15m high, 4m in diameter and has a take-off mass of 18.8 tonnes.  As a 1/3 scale mock-up of the final vehicle it is unable to reach orbit and has no payload; it is only aimed at performing the above indicated demonstrations.
The first flights which took place were one minute fights, at an altitude of 50 and 100m respectively, with a horizontal displacement of 100m.  On both fights the vehicle landed within 1m of the target point.  The flights were very successful, the only incident on the first flight being minor damage to the non fire-proofed nose-cone due to the ignition of vented hydrogen.
The nominal test plan calls for a progressive extension of the flight envelope, up to a test at 3000m altitude. A second vehicle, designated SX-2 (2/3 of the final size), will then take over and will demonstrate the required structural index by using advanced materials.  The full-size vehicle to be named DC-Y will demonstrate re-entry and payload capability.  This vehicle will start conventionally but will re-enter nose first, perform a turn-around manoeuvre and land on its rump.
The interesting point about the approach is that it relies as little as possible on radically new technologies but whenever necessary, new modes of operation or new technologies are to be demonstrated on the smallest (and least costly) vehicles that will do the job.  These demonstrations are done as early as possible in the programme, before the final vehicle has been designed in detail.  One of the key demonstrations is the application of aircraft-derived procedures to achieve low-cost ground operations.
The DC-X vehicle has been funded at a level of 60 million dollars by a two-year contract ending in August 1993.  MDDC now supports the fights until the end of 1993 by in-house funding as an interim measure awaiting setting up a full programme and the corresponding funding.