The 1971/1973 Mars data set acquired by the Goldstone Solar System Radar was analyzed. It was established that the seasonal variations in radar reflectivity thought to occur in only one locality on the planet (the Solis Lacus radar anomaly) occur, in fact, over the entire subequatorial belt observed by the Goldstone radar. Since liquid water appears to be the most likely cause of the reflectivity excursions, a permanent, year-round presence of subsurface water (frozen or thawed) in the Martian tropics can be inferred

Downs, G. S.

Roth, L. E.

Saunders, R. S.

Schubert, G.

English

NASA Technical Reports Server

-0 - URS:  SEASONALLY VARIABLE RADAR RZ7LECTIVITY; L. E, Roth, 6 .  S. 
D o n s  , and R. S. Saunders, Jet FropuZs i o n  Laboratory, California Inst i tute  of 
Technology, Pasadena, California 91 109; G. Sehubert , Department of E a r t h  and 
Space Sciences, University of C a l i f o r n i a ,  LQS Angeles, California 90024,  
Xeconnaissance by radar from Earth is one way of exploring our closest 
planetary neighbors, Mercuq, Venus, and Mars. Radar exploration of Hlars, 
begun in mid-sixties ( 1,2] produced a wealth of information .on the topography, 
roughness, and re f l ec t iv i ty  of the planet.  In dealing w i t h  the  rader 
reflectivity, the work discussed here i n d i r e c t l y  addresses the questlcn of 
lfquid water on Mass, a subject ta in ted  by the memory e€ the extravagant 
claims of Festival Lowell made a t  the turn of the century ( 3 ) .  We have 
analyzed t h e  1471/1973 Mars d a t a  set acquired by the  Goldstone Solar System 
Radar C 4 , 5 )  and established tlrat t h e  seasonal  var ia t ions  in radar ref l e c t i v i l y  
thought t o  occur in only one l o c a l i t y  on the planee  (the 'Solis Eacus radar 
anomaly' (611 occur, in fact, aver the e n t f r e  subequatorial belt observed by 
the Goldstone radar. Since liquid water appears t o  be the most I,kely cause 
of the  rclflectivl  t y  excursions, a permanent year-round presence of sub- 
surface water Cfsozer, OK thawed) in tlle Martian 'tropics' can be inferred. 
This conclusion is based on t h e  following ideas:  TIE ability of 
materials o r  objects t o  reflect eleet:omagnet Lc waves is expressed through a 
quantity cal led ref Icetivity. n e  magnitude of reflectivity Tanges fran 0.0 
for ideal absorbers to 1.0 for ideal reflectors. Both the common rocks and 
the water ice are poor reflectors ( r e f l e c t i v i t y  4 . 1 ) .  Liquid  water, on t h e  
other hand, is a good ref lector (reflectivity -0.81. Presence of liquid water 
in geologic targets increases their  ref lertivity. A target containing mixture 
of rocks, s o i l ,  and l i q a i d  waeer would Se more reflective than the same 
https://ntrs.nasa.gov/search.jsp?R=19850018259 2020-03-22T19:06:45+00:00Z
w: SEASONALLY v m u  RADAR REFLECTLVfTY 75 
I,. E. Roth et el. 
mhture, bt% vi th  solid ice €nstead of l i q u i d  water. Since the ref lect idty  
fs a quantity charaesteristfc Q€ a given target, the dame target examhied by 
radar at  two different oceaeions should have the same reflectivity. If the 
reflectivity varies from one expeximnt t o  the next, the target must have 
undergone a change durtng the intervening period.  This change could either Is 
internal,  involving the compositfmdphase of the target i t se l f ,  or external, 
involving the geo;letry/rexture of i t s  surface. 
During the 1971/ 1973 oppositions I the Goldstone radar scanned t h e  
Southern Hartian latitudes on almost SO mcasians. The observathns extended 
fzom the early Martian spring ta  the Martian mid-summer. Applying the notions 
of out terrestrial calendar to  Mars, the Qbstrw&tLonB vould  have started on 10 
(Martian) Aprfl and temtnated on 15 (Martian) August. Many areas were 
scanned mre than once, Intervals between some observations corresponded t o  
up a few (Martian) months. (The a n a l o e  with the terrestrial calendar is 
helpful;  it should not be taken l i teral ly ,  however. A yea. on Haxs is almost 
twice as long 88 a pear on Earth; a (Martlan) month, in the context of our 
ana!ogy, is almost twice as a long as a month on Earth, a (Yartian) week 
almost twice as long as a week on Earth.) In view of what we s a i d  before, the 
reflectlviry of an inactive, unchanging target should always be the same, 
regardless of how often or a t  what time intervals the target is seanned by 
radar. Analysis of the Goldstone data shows, however, that the Mars 
reflectivities do not conform to  the expected, steady pattern. Instead, 
starting from the lower values in the Martian apring, they tend tokard higher 
values In the Martian summer [FIR. 11. T h h  overal l  trend can be observed 
over the catlre planet. For inetance, tn Solls ;tscas/Sinai PXanvm (6)  (Fig. 
2) an avearge reflectivity of 0.Q8 vas measured on 6 (Martian) June; an 
average reflectivity of 0.11 w88 recorded five (Martian) weeks later, on 10 
(Martlen) July. ds stiX1 another example, consider the cratered highlands 
76 
FIGURE 2 
-12' 
* .IO' - 
> 
c .om- 
u 
c 
- 
- 
.os- a 
.M. 
.02' 
MARS: SEASONALLY VARIABLE RADAR REFLECTIVITY 
FIGURE 4 
14. 
L. E. Roth et al. 
. 
in Margaritifer Srnus (Fig. 3) and Aeolis (Fig. 4). Those radar scans that 
followed each other by less than about one (Martian) week (Us < 10 deg) show 
no change in reflectivity (i.e., the ratic of respective reflectivities equals 
unity). An increase in the interval of separation from about one (Msrtian) 
week to two-to-three (Martian) months is usually accompanied by a 
corresponding increase in the average ref lectivlty (the ratios of respective 
reflectivities are larger than unity). 
I 
m a t  should be the interpretation of this puzzling phenomenon? 
Discarding the unlikely case of a systematic increase in the instrumental 
bias/calibration error, we are left with two possibilities: (a) Seasonal 
variations in Mars reflectivities are due to changes taking place on the 
MARS : SEASONALLY VARIABLE RADAR REFLECTIVITY 77 
L. E. Roth e t  a l .  
planet ' s  surface.  These changeu could be related t o  tho ac t ion  of wind and 
associated with it r ed i s t r ibu t ion  of dust. Even though t h i s  mechanism cannot 
be ruled out e n t i r e l y ,  a s i g n i f i c a n t  cont r ibu t ion  does not seem t o  be 
supported by the photogeologic evidence. (b )  Seasonal changes i n  Mars 
r e f l e c t i v i t i e s  are due t o  changes taking place below the p lane t ' s  surface.  
mat is the  most probably subsurface change t h a t  could occur i n  p a r a l l e l  with 
the  progress of the Martian seasons from the e a r l y  spr ing  t o  the  mid-summer? 
It has t o  be the thawing of water ice and the emerging presence of the  l i qu id  
water as a temporary cons t i tuent  of the  Martian s o i l .  This brings u s  f u l l  
circle t o  the beginning of our s tory :  Since the l i qu id  water is the  most 
l i k e l y  cause of the observed r e f l e c t i v i t y  excursions a permament , year-round 
presence of sub-surface water (frozen o r  thawed) i t 1  the  Martian ' t rop ics '  can 
be inferred.  
The f i n a l  judgement on t h i s  cont rovers ia l  i s sue  should be suspended u n t i l  
more data  are ava i l ab le  and the r e a l i t y  of the e f f e c t s  observed i n  t he  
1971/1973 Goldstone data  is confirmed. Unt i l  then, the  p o s s i b i l i t y  of the  
presence of l i qu id  water i n  the Martian s o i l  should be t r ea t ed  as a working 
hypothesis r a the r  than an es tab l i shed  f ac t .  Mars is a d i f f i c u l t  Gbject t o  
observe; geometric considerat ions make the  subequator ia l  breas on Mars v i s i b l e  
t o  radar from Earth only during 2-3 favorable  opposit ions out of the f u l l  
cycle  of 7-8 opposit ions.  The last p a i r  of favorable  oppos;tions took place 
i n  1971/1973. The iorthcoming appar i t ions ,  favorable  ones in 198611988 and an 
intermediate one i n  1990, should make i t  possible  t o  enlarge the  Mars radar  
da ta  base and allow a new a t t e m p t  a t  solving the va r i ab le - r e f l ec t iv i ty  r idd le .  
References : 
(1) R. M. Goldstein (1965) Science 150, 1715. (2 )  G. 8. P e t t e n g i l l ,  C. C. 
Counselman, L. P. Rainvi l le ,  and I. I. Shapiro (1969) Astron. J. 74, 461. ( 3 )  
P. Lowell (1906) Mars and Its Canals, Macmillan. (4 )  G. S. Downs, R. M. 
Goldstein, R. R. Green, G. A. Morris, and P. E. Reichley (1973) 18, 
8. (5 )  G. S. Downs, P. E. Reichley, and R. R. Green (1975) Icarus  26, 273. 
(6) S. H. Zisk and P. J. Mouginis-Mark (1980) Nature 208, 735. 


Mars: Seasonally variable radar reflectivity

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