T h e M a r t i a n C h r o n i c l e November 1990 No. 6

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                    T h e   M a r t i a n   C h r o n i c l e

November 1990                                                            No. 6

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     The electronic newsletter by the International Mars Patrol (I.M.P.), 

   an observing program of the Association of Lunar and Planetary Observers   

                                   (A.L.P.O.)

                             Edited by: J.D. Beish

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                         A DUST CLOUD IN NOVEMBER 1990

     While using the University of Hawaii's 24-inch at Mauna Kea  Observatory,

Beish received a call from Don Parker with an alert of yet another dust storm. 

Beish was there during 30 October through 7 November 1990  photographing  Mars

for  Lowell Observatory's    International  Planetary  Patrol.   On  4  and  5

November he recorded in color three dust  clouds  moving  across  the  Aurorae

Sinus-Solis Lacus regions.

                          METEOROLOGY OF MARS IN 1990

     The ALPO/IMP has over 20,000 observations of Mars and many  of  the  late

Chick Capen's scientific papers covering his  meteorological  studies  of  the

Red Planet.  Results of Capen's work and more recent studies by the IMP, Beish 

and Parker published "Meteorological Survey of Mars, 1968-1984" in the Journal 

of Geophysical Research, (95, B9, August 20, 1990) that can be used to predict 

the times when clouds, hazes, and other atmospheric phenomena are most  likely

to occur on Mars.

     As we draw nearer to closest approach (20 NOV 04UT)  and  opposition  (27

NOV 21UT) the orbit of Mars will pass through  336  degrees  and  340  degrees

Planetocentric Longitude of the Sun (Ls), respectively.  These  periods corre-

spond to late Martian southern summer and observers should  begin  to  see  an

increase in cloudiness on Mars.  Already clouds have been seen forming in  the

evening in Libya, an area just east of Syrtis Major.

     Approaching local Martian noon, discrete white orographic clouds, identi-

fied as water clouds by the Mariner 9 spacecraft, are seen in  Martian  spring

and summer forming on the upper slopes of the large  volcanoes  (Olympus  Mons

[113W, 18N], Ascraeus Mons [104W, 11N], Pavonis Mons [112W,  00],  Arsia  Mons

[120W, 09S], and Elysium Mons [212W, 25N]), and  between  Tharsis  Tholus  and

Valles Marineris (80-100, 04N).  These seasonal clouds ("W-clouds") were  well

observed during northern Summer (120 - 160 Ls) in 1984 and in 1986 after rapid 

thawing of the North Polar Cap (NPC) began (204-209 Ls).  In  the  future,  it

will be interesting to learn if  these  orographic  clouds  occur  twice  each

Martian year, because they originate in the equatorial region of  the  planet.

Look for them after the large southern basins, Hellas and  Argyre,  have  lost

their whiteness and have returned to a dark-ocher hue, and  during  the  rapid

retreat of the South Polar Cap (SPC) from  southern  mid-Spring  until  Summer

(235- 270 Ls). Because of the observed appearance and seasonal behavior of the 

great southern basins, the authors suspect  that  they  act  as  "cold  traps"

during Mars' southern Autumn and Winter, thus controlling the water  vapor  in

that hemisphere.  Because orographic clouds are best  seen  through  blue  and

violet filters, they are well-elevated and are probably generated  by mechani-

cal uplift, and growing by convection.

     A faint veil of wispy white clouds with variable  shapes  and  levels  of

opacity, known as the equatorial cloud band (ECB), is  occasionally  seen  ex-

tending across Mars' disk.  Because the ECB is detected  best  in  ultraviolet

and violet light, it resides at a chilly high altitude, and  is  probably com-

posed of Carbon Dioxide ice crystals.  Because the  ECB is equatorial, it  too

may occur twice each Martian year, appearing with the sublimation of each cap.  

     Limb haze appears as a bright misty arc of light on the sunrise or sunset 

limb of Mars, and is caused by the observer's oblique view through the equiva-

lent of several Martian atmospheres of aerosols, which may consist  of  Carbon

Dioxide ice crystals, fine dust, cirrus-type water clouds,  or  a  mixture  of

these. Consequently, the observation of the global location, color, and densi-

ty of limb haze is a very sensitive method of diagnosing the global system  of

Martian weather and unusual polar phenomena.  Sometimes, the color and density 

of the hazes help to detect dust storms that have begun on the other  side  of

the planet.

     Limb haze may be seasonal or nonseasonal and, since it  does  not  rotate

with the planet, disappears around local 8 or 9 A.M., Mars time.  It  is  best

seen in violet light if at high altitudes, or in blue  light  if  at mid-alti-

tudes.

     A most delicate and challenging feat of observation is the  detection  of

volatiles at the boundary between the Martian atmosphere and its surface.   In

this volatile regime, ice-fogs and frosts, often called bright patches, can be 

distinguished from elevated  clouds  by  means  of  comparing  their  relative

brightnesses and boundary definitions as seen  with  the  aid  of  blue,  blue

green, green, and yellow filters.   If  the  suspect  bright  feature  appears

brighter in blue light than it does in green or yellow light, it is  an atmos-

pheric cloud.  If it is brighter and better defined in blue-green  light  than

in blue or yellow light, it is probably ice-fog contiguous to the surface.  If 

the patch appears brighter with sharp boundary in green and yellow  light  and

is not well seen in blue light, it can be  identified  as  surface  frost.   A

boundary-layer volatile's diurnal behavior and location also helps  to distin-

guish it from clouds and limb haze.  Fogs and frosts form in the chill of  the

Martian night, rotate with the planet, dissipate in the morning sunlight,  and

usually disappear by local noon.  Fogs normally form  in  valleys,  in  fossae

(linear depressions), basins, and on upper slopes.  Frosts are  usually  noted

on cool, light albedo features,  plana  (plateaus),  montes  (mountains),  and

floors of large craters.  Because  these  volatiles  are  topographically con-

trolled the discovery of their  locations  and  seasonal  occurrence  is  most

important to the study of Martian weather patterns and areography. 

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