Ground based images obtained hourly by seven observatories are used to study Martian phenomena. Maps of global dust storms show the degree of activity of the storm depends both on the region and on the time of day. Statistical analysis of regional contrast variations on the images supports the opinion that the contrasts in the brightness of the light and dark areas depends on the Martian season. Residual differences may be due to a phase angle dependence. Diagrams confirm the earlier finding that there is a systematic trend of regional contrast with the time of the Martian day and that the afternoon is not symmetric with the morning. The dissipation of Martian polar caps is also discussed

Baum, W. A.

English

NASA Technical Reports Server

W i l l i a u  A. Barn 
Planetary Research Center 
Flagstaff, Arizana 86001 
The purpose of this paper is t o  give an updated report on several 
recent findings concerning &TS obtained frun studies nm in progress 
a t  the Planetary Research Center of LarJlell Observatory. 
extensive use has been made of gramd-based images odtained almost 
In this work, 
hourly by seven observatories c w r a t i n g  under the Internatimal 
Planetary Patrol Program (Baum, 1973). 
L. J. Martin has recently caqleted a detailed ham-by-han mapping 
of the f i r s t  twenty days of the 1971 global dust storm, and Figure 1 
shows an interesting behavior that we were not so nu& aware of at the 
time of Capen and Martin's c1971, 19724 original reports a l i t t le mer 
a year ago. 
storm-brightened areas a t  two-hour intervals on the 11th day of the 
In Figure 1 you see, for illustration, the uutline of 
stonn. Each small map represents approximately the area visible fran 
Earihat that hour, with the morning terminator at the left-hand edge 
and the afternoon limb a t  the right-hand edge. The solid line identifies 
the central meridian, while the dashed line represents where it is noon 
on the planet. 
I f  the dust storm were! associated mainly with particular regions 
m Mars where the dust has been stirred up, evolving gradually in  the 
course of weeks, its shape m the map would appear t o  change cmly a 
( l iASA-CR-13BCO7)  RESULiS  OF CULL ENT H A R S  N 74- 2 04 6 9 
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l i t t le i n  the course of are day. Figure 1 clearly shaws that it does 
not behave that way. Nor a u l d  the apparent changesAb explained as 
of shape and positim be easily 
sytypw sane kind of aptica e ect. The degree of activity of the dust stonn 
A 
depends both m the regim and m the tinre of day. 
We find, i n  fact, that the dust s tom seems t o  be locally regenerated 
in t h i s  manner about midday e3ch day during its devlelclping stages. 
These maps for two-hour intervals can be sqerinposed upcm me another 
t o  produce a single map describing the day's progress of the s t o n ,  
and this  can be dme for each day of the storm. 
such maps representing the first ten days of the storm. The contarrs 
shclw outlines of the active areas at two-ham intrvals, and the nuhers 
Figure 2 shws ten 
labelled on them indicate the time of day a t  0' longitude. 
words, the nunbers are approximately a Martian equivalent of Greenwich 
In. other 
Mean Time. A very similar set of maps has been made for days 11 through 
20. After the 20th day, the acannulated general haze had becane so 
dense that the daily pattern of regional dust generaticm became too 
difficult  t o  map. This kind of ham-to-hour history of the storm would 
have been totally inpossible without a ccmplete network of observatories 
participating fu l l  time in the Patrol Program. 
Figure 3 SLEW up the regions that were active during the developing 
stages of the stom. .he main core and the secondary core'include the 
great bowls of Hellas an? Argyre, and extend wstward from them. Of 
the five regions identified as recurring bright spots, the me a t  the 
r i n h t  is centered m the great depress'.m just east of Syrtis Major, 
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the me near the center of the map also lies in  a regim of 
elevation, and the small one at  about 70' lmgitude. in the vicinity 
law 
of Melas Lacus straddles the enoll~us equatorial canm discovered by 
Mariner 9. Hatever, the m i n i n g  two recurring bright spots do not 
seem t o  be associated with musual topography. 
Neither does the in i t ia l  clad, but as Figure 4 by Capen and 
Martin (1972b) shaws, the three best recorded dust stonas of the Martian 
perihelion season have a l l  started at about the same place. In additim 
t o  the remarkable coincidence of positicm of these three in i t ia l  c l a d s ,  
they share another musual trait, namely, the suddenness and intensity 
of their onset. 
evidence of any unusual activity on .September 21st, but early the 
follrrwing morning as Noachis emerged fma the morning terminator, it 
was brighter and whiter than any features of the storm during the days 
that f o l l m d .  What would trigger such a vigorous onset? And why so 
early in the day, possibly even before d m ?  I f  condensates play a 
- 
In Septenhr 1971, for example, there was no visible 
role in  making it rather white, why mly at the site of the in i t ia l  
cloud? 
There is an entirely different kind of observation that may be 
evidence for persistent dust activity t h g h m t  the Martian year. 
Figure 5 shows *data obtained by Thanpsm (1973) fran the statistical 
analysis of regional contrast variations m mr Patrol photographs. 
Historically, this is ~e old ?blue clearing'' phemanenm, which we 
find is not limited t o  blue and which we do not believe has anything 
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to do with dilution of cmtrast  by a widespread overcast. Thaapsm 
has measured the cmtrast of faur selected features in blue light m 
about 6,000 image sets fmn 1969 and 1971. In Figure 5 the ordinate 
represents the brightness of neighboring light areas divided by the 
brightness of the dark feature indicated, and the data are plotted 
against the planetocentric largitude of the SUI, Ls. This diagram 
spans about me-third of a Martian year, starting with Martian "September" 
and extending through Martian l%c&r.ll Data for 1971 considerably 
werlap those for 1969 and seem t o  shaw sum degree of qualitative 
agreement, so we think that the contrasts of these features have at 
least scum dependence 011 the Martim season. Residual differences 
may be due to  a phase angle dependence. 
The most interesting feature of Thanpsam's analysis is illustrated 
in Figure 6. These data pertain t o  the Nilokeras region during a 
particular time interval. Similar diagrams exist for other time 
intervals and also for the Syrtis Major region. These diagrams cmfinr 
our earlier finding that them is a systematic trend of regimal cmtrast 
with t ime of the Martian day and that the a f t e m m  is not symnetric 
with the morning. 
If we assme that the cmtributian of the surface can be represented 
by a symmtric Minnasrt fbc t i cn  having slightly differe9t coefficients 
for light and dark areas, we are then left w i t h  the non-symnetric 
residual ccntribution shown in Figurn 7. Us think it is reasonable 
t o  assune that this cmtributian is due t o  the atmosphere. The three 
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curves in Figure 7 repnsent three different time h t e m l s - - e a r l y ,  
middle, and late--during the 1971 Mars apparition. Ihe agreement of 
the three cums shaws that the apparent difference between morning 
and aftemom has nothing to do with any phase angle effect. I t  is 
really due to  sanething going <n on Mas. 
The aftemom upturn illustrated here is an order of magnitude 
greater than stat is t ical  uncertainties in the data. It amounts to  
a daily rise of 88 betwen noon and 4 p.m. local Martian time, and 
it is occurring when Mars is supposedly ' homl"  and no actual &t 
storm is i n  progress. We interpret this as mainly a rise in the 
brightness of the light areas, with less change (perhaps none) taking 
place in the dark areas. 
that is regenerated each afternom, one can show that it may limit 
the horizontal visibil i ty range of Mars landers i f  they are put dawn 
If  this prienanenan is due t o  a p m d  haze 
in light areas. 
The third finding (Barn and Martin, 1973) t o  mention in this paper 
ccmcerns the Martian polar caps and 3s illustrated in Figure 8. We 
have plotted the man latitude of the north polar cap boundaxy against 
the lairtian season, expxssed in tern of Ls. This diagram spans two- 
thirds of a Martian year, but the data were obtained from the h l l  
plate collection covering more than 60 years (Fischbacher, Martin, and 
Barn, 1969), plus more recent data from Patrol films; and the solid 
dots represent means for different years. Since the scatter of the 
dots is not significantly greater then the estimated emrs, the 
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I 
regressim of the surface a.p evidently runs on a very predictable 
schedule. 
Now, what does this diagram say? Until a few weeks af ter  the 
vernal equinox (until a b i t  past Ls - 0') , the boundary is vague, 
rapidly variable, and visible mainly cn blue or ultraviolet images. 
It ccsmKnly readzes d m  t o  latitudes as low as 40' north. We identify 
this as the north polar hood, - and we use the crosses here to  represent 
approximate long-tern means. As the hood clears away s m  after the 
vernal equinox, a new and different looking boundary becanes visible 
- 
further north. I t  is sharp, it is seen in a l l  colors, and its 
latitude is 65 t o  66 degrees north. Ne identify it as the surface 
and we suspect that it never extends much below 65', because it 
0 
is not ini t ia l ly  receding. 
unti l  80 days later. The boundary again becames almost tationary 
when it reaches the edge of the permanent cap at 82 t o  83 degrees north. 
About 150 days llfter that, a new polar hood s tar ts  t o  obscure the cap. 
In Figure 9, our result is again represented schematically by 
In fact, it does not s t a r t  receding rapidly 
the bent ,solid line. 
cap regression ,curves published previously by others. 
cums do not give any clear indicatim of the approximate st i l lstand 
that we find near 65' latitude, and the ovlerall disagreement amongst 
the curves is large. We believe that the differences arise fran the 
method of measurement. We superimpose images optically onto an 
orthographic grid and read the latitude of the bomdary of the cap 
near the central meridian of the image. Others ham instead meashred 
For canparism, the three dashed lines show north 
These dashed 
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the apparent angular width of the cap near h e  polar limb, where the 
hood would be harder t o  distinguish fma a surface cap and where any 
atmospheric haziness would bias the readings. 
The situatim in the southern hemisphere shown in Figure 10 is 
mch the same, except that the cap is mors eccentric with respect t o  
the pole. When the south surface cap first becanes visible, its 
boundary runs fran about 56's latitude on me side to'60'S on the 
other. Like the north cap, it appears to  hE-,-e been a t  a sti l lstand, 
but it starts receding sooner. 
when is about 280'. There is no evidence to  support the widely 
I t  reaches the eccentric permanent cap 
- 
held belief that the suuth cap dissipates Mmpletely. 
REFERENCES 
Baum, W. A. (1973). 'The Internaticnal Planetary Patml Program: An 
Assessment of the First Three Years." Planetary and Space ScienceAl, 
1511. 
Baun, W. A., and Martin, L. J. (1973). %havior of the Martian Polar 
Caps Since 1905." Bull .  Amer. Astrm. SOC. 4, 296. 
Capen, C. F. , and Martin, L. J. (1971). The Developing Stages of the 
Martian Yellow Storm of 1971." L o u e l l  'as. Bull. No. 157. 
Capen, C. F., and Martin, L. J. (1972a). lMars' Great 'Storm of 1971." 
Sky and Telescope 43, 276. 
Capen, C. F., and Martin, L. J. C1972b). llSurvey of Martian Yellow 
Storms." Bull. h e r .  Astron. SOC. 4, 374. 
Fischbacher, G. E. , Martin, L. J., and Barn, W. A. (1969). Wars 
Polar Cap Bcnmdaries.l' Report unhr JPL Cartract No. 951547, May 1969. 
Thompson, D. T. (1973). "Time Variatim of Martian Regional'Cantrasts." 
IcaNs, in press. 

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Results of current Mars studies at the IAU Planetary Research Center

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