Lunar surface erosion evidence - implications of Ranger moon picture

Gold, T.

English

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(NASA C R  OR TMX OR AD N U M B E R )  
CORNELL UNIVERSITY 
Center f o r  Radiophysics und Space Research 
ITHACA, N.Y. 
https://ntrs.nasa.gov/search.jsp?R=19650019853 2020-03-17T02:02:15+00:00Z
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CENTER FOR RADIOPHYSICS AND SPACE RESEARCH 
C o r n e l l  Universi ty  
I thaca ,  New York 
August 11, 1964 
CRSR 176 
THE IMPLICATIONS OF THE RANGER MOON PICTURES*% 
T. Gold 
* Work a t  Cornel l  on l u n a r  problems i s  i n  p a r t  supported by 
Nat lonal  Aeronautics and Space admin i s t r a t ion  Grant NsG-382 
P r i o r  t o  t h e  Ranger p i c t u r e s  t h e  information about t h e  small-scale 
s t r u c t u r e  of t h e  lunar su r face  had been l i m i t e d  t o  t h a t  ob ta inab le  by t e l e 7  
scope and r ada r  observa t ions ,  
given information down t o  a s c a l e  of about one k i lometer ,  and had cie;ti-ly 
3- t h a t  c r a t e r s  e x i s t e d  i n  a l l  s t ages  of " f reshness ,  
'mid be judged o l d  by t h e  c r i t e r i o n  of over lap  by o t h e r s ,  were gener.;lly 
seen t o  be smoother, l e s s  h igh  f o r  a given diameter ,  and o f t e n  had a 
r a t h e r  smoot,h i n t e r i o r ,  Young c r a t e r s  had apparent ly  sharp  r i m s  and 
s t e e p ,  rugged f e a t u r e s ,  
concluded t h a t  an e ros ion  process w a s  a c t i v e  t h a t  cont inuously degr:tded 
t h e  f e a t u r e s  much as eros ion  does on e a r t h ,  This  posed t h e  ques t ion  of 
Earthbound t e l e scope  observa t ions  had 
11 Craters t h ' E  
From the  cons is tency  of t h i s  e f f e c t  it had been 
a mechanism f o r  t r a n s p o r t a t i o n  i n  t h e  absence of agencies  t h a t  erode on 
e a r t h ,  
r e s t r i a l  ones; one micron pe r  year removal from high ground and deposi-  
t i o n  on low would s u f f i c e  t o  c r e a t e  t h e  observed e f f e c t s  i n  a l l  of geo- 
l o g i c  t ime,  This  could s t i l l  lead  t o  depos i t s  fou r  k i lometers  deep. 
I t  is very d i f f i c u l t  t o  judge such very s l i g h t  processes  i n  unusual con- 
d i t i o n s ,  However no processes  could be envis ioned t h a t  would t r a n s p o r t  
l a r g e  p i eces  of aoqk except t h e  impact explosions,  
v e r t  much m a t e r i a l  i n t o  f i n e  powder, and even wi th  t h e i r  a c t i o n  one 
could not  contemplate a l a r g e  f r a c t i o n  of t h e  depos i t  on low ground beiqg 
i n  any o t h e r  s t a t e  than  d u s t ,  
t o  t h i s  process  a r e  t o o  low i n  view of t h e  number of s m a l l  c r a t e r s  seen, 
and an a d d i t i o n a l  process  i s  requi red ,  Seve ra l  p rocesses  can be sug- 
ges t ed  t h a t  may be effect&%? i n  t r a n s p o r t i n g  material downhil l ,  bu t  only 
i f  it i s  i n  f i n e l y  divideid form, This  w a s  t h e  o r i g i n  of t h e  sugges t ion  
t h a t  rock-dust w a s  t h e  major cons t i t uen t  of t h e  l u n a r  s u r f a c e ,  and t h a t  
?-,, reg ions  had accumulations of it down t o  t h e  depth of t h e  o r i g i n a l  
The requi red  r a t e s  are of course very low compared wi th  t e r -  
Those, however, con- 
However t h e  r a t e s  t h a t  could be a sc r ibed  
-2- 
f e a t u r e s ,  judged i n  some cases  two o r  t h r e e  ki lometers  jeep, 
The f a c t  t h a t  deposit ;  6f dus t  may be ki lometers  deep m:st zct 
be taken t o  mean t h a t  t h e  rcater ia l  maintains  t h e  consis tency of a f i n e  
powder a t  g r e a t  depths,  no more than it does on t h e  e a r t h ,  
o r i g i n a l l y  made of f i n e  p a r t i c l e s  may be i n  va r ious  s t a g e s  of cementation, 
The agencies  causing cementation of sediments on e a r t h  are n o s t l y  d i f -  
f e r e n t  from those  t h a t  could be a c t i v e  on t h e  moono There vacuum weld- 
ing, may be very important e s p e c i a l l y  f o r  p a r t i c l e s  t h a t  are pressed 
t o g e t h e r  f o r  such long per iods of tim t h a t  s o l i d  s ta te  d i f f u s i o n  i s  a 
s i g n i f i c a n t  e f f e c t ,  Close t o  t h e  s u r f w e ,  e f f e c t s  of s p u t t e r i n g  and of 
evaporat ion and condensation can also assis t  in cementing g r a i n s  toge the r ,  
The d i scuss ion  of t h e  mechanical properties of' s i x h  sediments i s  c l e a r l y  
very important f o r  f u t u r e  moon explorat ion,  and measurements of t h e s e  
must r e c e i v e  a high p r i o r i t y  i n  f u t u r e  ~ o o n  mi.sionsc But it i s  import- 
a n t  t o  keep sepa ra t e  t h e  discussiori of t h e  o r i g i n  of t h e  nater ia l  and 
i t s  p resen t  mechanical p rope r t i e s  -- a point  which has  been confused i n  
t h e  l i t e r a t u r e  and i n  popl.:lar writings where I t  was cfter, implied t h a t  
i f  t h e  l o w  ground i s  f i l l e d  with dus t  sedixent, t hen  it will be loose 
and s o f t  t o  some g r e a t  depth,  This  i s  no nore a d i r e c t  impl.ication 
t h e r e  than  it would be i n  t h e  Mississ ippi  IxisPn, Bevertheless ,  such 
an o r i g i n  raises t h e  quest ion of  t h e  bear ing s t r e n g t h  of t h e  su r face  f o r  
exp lo ra t ion  p r p o s e s  while an igneorrs ?rig' n w o u 3  hsve implied alinost 
i n e v i t a b l y  an adequate s t r eng th ,  
Sediments 
Erosion and a sedimentary o r i g i n  f o r  t h e  mare rnaterisl  was also 
implied by a d i f f e r e n t  observation, It could t-c seen t h a t  almost with- 
c ~ t  7:xception c r a t e r s  t h a t  f e l l  p a r t l y  cn highland and p a r t l y  r)ri mare 
z,,;*mnd had t h e  mare po r t ion  much more bt.avily eroded, acd o f t e n  it 
- -  
had disappeared altoget:i-er. i'ron th i s  t i d C . i i  i v e : ,  L i - ( d  ' , $ i i t  ~2 a 
material was gene ra l ly  more a ? l = r ~ p t j ~ ~ l e  t o  rosicin, m t f  t l l ~ ' ~ ;  i'nre +_k.,t 
most l i k e l y ,  it was not as sc; id  a x ~ f e r i n l  3 s  r i i i ~ h  'rr t-13 n f  t h e  *I  
lands., 
and t h e  argument i s  independent o f  t k ~ i : .  layers tha t  may in f luen .?  the 
r a d i o ,  thermal o r  o p t i c a l  surface properties< 
One i s  t h e r e  concerrwd w i t h  t h i c k n ~ n ? e s  of UT t g  1,hree Bi : rm~tera ,  
On a much smaller s c a l e  the radar evidence had  tlerro?stratej-i ! h t  
t h e  Lunar ground w a s  remarkably smooth on t h e  scale of t h e  wavelengths 
used when t h e s e  were longer  than about t.k8ree cwtirr le ters  (1%2)- F>>r 
shorter wavelengths t h e  radar s c a t t e r  zissumed t h e  charnct e r  appropriatta 
t o  a rough su r face ,  dernonstratinE t h a t  i r r e g r : l n r i t i e s  of t h a t  s i z p  
cover most of t h e  surface,  A t  70 CIC. wvf , le r .g th ,  vhere t h e  most. deta i led  
observat ions were made, t h e  majori ty  cf t h e  gr'eutid appexrs smooth and 
t h e  proport ion of ground r.overed with irr .eGul:ir l t l+hs t)n *ka t  s c a l e  has 
been est imated,  by an argument. ' h s t  i n v o l v e d  w r t a i n  n d d j t i o n a l  but, 
p l a u s i b l e  assumptions, as approximat "ly f ive pes cent  ( 3 )  I 
This  had implied something o t h e r  T m i i  rock on :t depth scale 
of  at ?east. a f e w  meters,  for. i t  srerned jn?ori:?-i   at le I1r:tt. 4iirh ;.I , r i (  rxth 
rock su r face  could be generated or irLr,in"slried i n  t k e  ;>relsmc'e of priiruJry 
and secondary bombardment with Keteori t e ;  
voked t o  smocth over a!? the  small impact c r a t r P j  exccyt t h p  nL<iatg rz- 
cent  onese Uneroded craters l a r g e r  than  a T 's,c- ueter arr->ss ;in? rough 
enough t o  s c a t t e r  diffusely would 17c F*;I t.2 UbC'i j ) j '  nki2ut fivr per cent f f  
t h e  ground, on t h e  assumption t1a:tt ns o the r  type; of rough feature; 
:is'? commoIlo 
P g a i n  ~ - ~ * a : , i ~ ~ f l  h a r i b:en fr l -  
The - Y  =!lent: Ranger pict,ures t a t e  vr-n d+: i:ly conf j r m e 3  + ! A I -  se  
pr,iTt,s, F i r s t l y ,  it. f 3  s t r i k i n g  t h s +  ' r ~  i n T . e r x x 1  of ji1.e 
-4- 
or r e s o l u t i o n  from 1000 f t ,  -- the  best v i s u a l  t e l e scope  resolutiora .-- 
t o  3 f t o ,  t h e  h ighes t  Ranger r e s o l u t i o n ,  t h e r e  i s  no widespread new type  
of f e a t u r e  t o  be seen, Round c r a t e r s  seem s t i l l  t h e  predominant LZ- 
f a c e  i r r e g u l a r i t y ,  There i s  nothing t h a t  could be regarded as s igns  of 
l a v a  flow; t h e r e  a r e  no boulder f i e l d s  o r  rock s u r f a c e s  of angultLs shapes 
except i n  very s m a l l  a r e a s ,  The ground i s  not  covered with cracks os  
f a u l t s  as occurs on a l l  rock su r faces  on t h e  e a r t h ,  
-.;th any terrestr ia l  rock d e s e r t  t h e  ground i s  very f e a t u r e l e s s ,  
s m d  desert would provide a b e t t e r  approximation except t h a t  rounded 
c r a t e r  shapes are s u b s t i t u t e d  f o r  t h e  usua l  shapes t h a t  r e s u l t  from 
wind and water erosion.  A few f e a t u r e s  o t h e r  t han  c r a t e r s  are w e n  -- 
g e n t l e  r i d g e s  and some unevenness on t h e  n e a r l y  f l a t  ground -- h u t  
Though very i n t e r e s t i n g  t h e s e  f e a t u r e s  are c e r t a i n l y  very minor i n  t h e  
sense of t h e  area covered bj them, Since t h e r e  is  no new type of  wide- 
spread f e a t u r e  t h a t  might have inva l ida t ed  t h e  i n t e r p r e t a t i o n  of t h e  
radar r e s u l t s ,  it i s  c lear  t h a t  t h e  p i c t u r e s  with a d e f i n i t i o n  of n f e w  
f e e t  and t h e  i n t e r p r e t a t i o n s  placed on t h e  70 cm radar measurements had 
50 be i n  c l o s e  agreement, 
of t h e  agreement w i l l  i nc rease  t h e  r e l i a n c e  on r a d a r  data i n  f u t u r e ,  
11ost of t h e  ground i s  seen t o  be smooth o r  possessing only very rounded 
r r a t e r s  t h a t  would not  d i f f u s e  70 cm waves, 
i s  covered with sharp-edged s t eep  s ided  c r a t e r s  t h a t  c e r t a i n l y  t r i l l  d i f -  
f G s e  70 cm wavesc The d i f f e r e n t i a t i o n  i n t o  two types  of ground r a t h e r  
t han  a continuous spectrum of  i r r e e u l a r i f y  had been suggested by t h e  
By compariscn 
A 
This i s  indeed t h e  case ,  and t h e  exce l l ence  
A f e w  p e r  cent  of the ground 
J :sr observat ion of s i g n a l  s t r eng th  versus  range, which showed an 
s’rrup’ change of g r a d i e n t ,  One can now s e e  t h a t  t h i s  i s  indeed Eerely 
+’- - q- i f fe ren t ia t ion  seen on high d e f i n i t i o n  pictures between ‘9~e’rdp1 c r a t e r s  
-5- . 
, 
and e l l  t h e  r e s t ,  
The Ranger p i c t u r e s  leave no doubt t h a t  e ros ion  i s  a x a j o r  
e f f e c t  i n  shaping the lku r face ,  
make t h e  g r e a t  number of shallow and g e n t l y  rounded c r a t e r  p r o f i l e s  o the r  
than  s t a r t i n g  wi th  s t e e p ,  sharp-edged c r a t e r s  l i k e  t h e  "new" ones seen  
and l e t t i n g  e ros ion  degrade then;, For c r a t e r s  a few meters o r  t e n s  of 
meters ac ross  t h e  e ros ion  process must be very fast compared wi th  t h e  
geo log ica l  t ime s c a l e ,  i f  t h e  same process  i s  t o  have had t h e  d iseern-  
ab le  e f f e c t s  on c r a t e r s  t e n s  of ki lometers  i n  s i z e  i n  t h e  age of t h e  
moono 
f o r  t h e  explana t ion  of t h e  l a r g e  f e a t u r e s ,  would des t roy  a t e n  meter 
c r a t e r  t h a t  i s  i n i t i a l l y  t h r e e  meters deep i n  about t h r e e  mi l l i on  yea r s ,  
Assuming t h a t  t h e  e ros ion  rate shows up when a l l  g r a d i e n t s  have become 
low, such a c r a t e r  would s t i l l  remain v i s i b l e  f o r  longer ,  but  as a 
shallow one with rounded p r o f i l e ,  
No e f f e c t  could be suggested t h a t  would 
Erosion rates of t h e  order of one micron p e r  yea r ,  as needed 
It i s  not our p re sen t  purpose t o  d i scuss  poss ib l e  mechanisms f o r  
e ros ion ,  but  only t o  d i scuss  the  evidence f o r  i t s  occurrence,  However, 
s i n c e  one i s  not incl iDed t o  accept even t h e  s t r o n g e s t  evidence when 
it p o i n t s  towards an inconceivable occurrence,  it should be mentioned 
t h a t  we can see  no reason t o  d iscard  t h e  suggest ion of dus t  t r anspor t a -  
t i o n  downhill  by t h e  combined ac t ion  of micrometeori tes  l i b e r a t i n g  p a r t i -  
c l e s  from t h e i r  su r f ace  adhesion, and e l e c t r o s t a t i c  e f f e c t s  opposing 
t h e i r  immediate r e t u r n  t o  the  sur face ,  t hus  making f o r  a tendency t o  
g l i d e  f o r  some way and g r e a t l y  enhancing t h e  e f f e c t i v e  downhill c reeps  
beyond t h a t  which mere s t a t i s t i c s  of repea ted  random s c a t t e r i n g  would 
aehiey e ( 4 )  
I t  appears then t h a t  a few p e r  cen t ,  perhaps five, of t h e  moon's 
-6- 
su r face  has 
t h r e e  m i l l i o n  yea r s ,  
m e t e o r i t i c  i n f a l l  rate,  bu t  not without an assumption regarding t h e  sur- 
f ace  s t r u c t u r e  and thus  of t h e  s ize  of me teo r i t e  r equ i r ed  f o r  t h e  forma- 
t i o n  of a given s i z e  c r a t e r ,  I n  a loose ly  cemented dus t  s u r f a c e  of much 
less  than t h e  compact dens i ty ,  c r a t e r s  w i l l  be much l a r g e r  for a given 
me teo r i t e  s i z e  than  they  would be i n  s o l i d  rocko  This  d i f f e r e n c e  w i l l  
be l a r g e s t  f o r  very s m a l l  c r a t e r s  where s t r u c t u r a l  s t r e n g t h  r a t h e r  t han  
g r a v i t y  determines t h e  s i z e ,  while f o r  very l a r g e  c r a t e r s  t h e  d i f f e r e n c e  
w i l l  reduce t o  t h a t  r e s u l t i n g  from t h e  dens i ty  f a c t o r  a lone,  
been covered with c r a t e r s  l a r g e r  than one meter I n ,  say,  
From t h i s  one may d e r i v e  an o rde r  of mzgnitude of 
The f a c t  t h a t  c r a t e r  formation proceeds more r e a d i l y  than  i n  
s o l i d  rock i s  a l s o  displayed by t h e  appearance of t h e  mul t i t ude  of s m a l l  
secondary c r a t e r s  seen on t h e  Rmger p i c t u r e s .  There it i s  c e r t a i n  
t h a t  impact speeds are low, w e l l  below lunar o r b i t a l  speed, Nevertheless 
t h e  g r e a t  ma jo r i ty  of t h e s e  are a l s o  c i r c u l a r  o r  only s l i g h t l y  deformed, 
This  must mean t h a t  t h e  energy l i b e r a t e d  even at speeds of less than 
one ki lometer  p e r  second i s  s t i l l  enmgh t o  excavate a c r a t e r  t h a t  i s  
’ery l a r g e  compared t o  t h e  p r o j e c t i l e ,  
t h i s  would no t  be t h e  case ,  and t h e  c r a t e r s  would mostly appear as 
impr in t s  of t h e  sha.pes of t h e  p r o j e c t i l e s  o r  scars t o r n  by them, 
softer materials t h e  e f f e c t s  of any l i b e r a t i o n  of gas a t  impact would 
be more s i g n i f i c a n t .  
i n g  t o  c r e a t e  t h e  same gene ra l  appearance as luna r  secondary c r z t e r s  by 
means of va r ious  p r o j e c t i l e s  s h o t  i n t o  d i f f e r e n t  t e r res t r ia l  su r faces ,  
I n  rock o r  any f i r m  material. 
I n  
One may obtain more infsrmztion on t h i s  by attempt- 
The Ranger p i c t u r e s  thus  app2ar t o  skior? Tcstly a uniforr., f i ne -  
g r z i t e d  m a t e r i a l  of low s t r u c t u r a l  s t r e n g t h  n t ? a  t h e  s u r f a c e  and i n  t h e  
fi -st f e w  meters depth,  They show no h i n t  f any t r a n s i t i o r ,  t o  a aif-  
. -7- 
f e r e n t  m a t e r i a l  below, such a s  a change i n  t h e  appearance of deeper 
c r a t e r s  o r  an occas iona l  outcrop of something looking l i k e  rock,  I t  
i s  t h e r e f o r e  most l i k e l y  t h a t  one i s  see ing  t h e  same type  of m a t e r i a l  
a t  a l l  t h e  c'epths excavated by the  c r a t e r s ,  bu t  very probably i n  progres- 
s i v e l y  g r e a t e r  compaction and cementation a t  t h e  g r e a t e r  depths ,  
The Ranger p i c t u r e s  have c l e a r l y  s t rengthened t h e  case  f o r  dus t  
as t h e  main c o n s t i t u e n t  of t h e  lunar lowlands by not  showing any rock 
formations,  There i s  no case f o r  d i scuss ion  of a two-layer model, 
What s t r u c t u r a l  s t r e n g t h  can be a t t r i b u t e d  t o  dus t  sedimentat ion a t  var- 
i o u s  depths cannot be judged very w e l l  u n t i l  impact probe experiments 
have been c a r r i e d  out ;  but  without any c l e a r  s i g n s  of f i r m  rock t h e  pic-  
t u r e s  must l e a d  t o  more concern about sinkage on impact o r  dus t  blowing 
i n  rocke t  exhausts  i n  f u t u r e  operat ions on t h e  luna r  surface,, 
8 
, 
-8- 
REFERENCES 
1, Evans, J,  V., p. 429 of Physics and Astronomy of t h e  Moon, Kopal, 
ed , ,  Academic P r e s s ,  New York (ag62), 
2. P e t t e n g i l l ,  G ,  and Henry, J, (19601, p. 519 of The Moon, Kopal and 
K k h a i l o v ,  ed , ,  publ, 1962 by Acadezic P r e s s ,  London and New York 
(1960) 
3. Gold, T o ,  CRSR No, 156 and paper presented  at the American Geo- 
p h i s i c a l  Union meeting , April, 1964 e 
40 Gold, T, , (1960) p,  433 of The Moon, Kopal and Mikha.ilov, ed,, 


The implications of the ranger moon pictures

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