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THE INTERNATIONAL MARSWATCH ELECTRONIC NEWSLETTER
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Volume 3; Issue 6
October 26, 1998
Circulation: 1479
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Dear Marswatch participant,
Enclosed is the third installment of the series of Marswatch emails
dedicated to preparations for the 1998-1999 observing season. The
series was written by Jeff Beish and other experienced Mars-observing
colleagues who are involved with the Association of Lunar and Planetary
Observers (A.L.P.O.). A calendar of important Mars observing dates is
also enclosed. Now is the time to start preparing to observe the
Red Planet through your telescope...
We are still trying to figure out how and where Marswatch images from
the 1998-1999 apparition can be uploaded to and downloaded from via a
web and/or ftp site. More on this soon, hopefully...
--Jim Bell
Cornell University
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1998-1999 APHELIC APPARITION OF MARS: PART 3
By: Jeffrey D. Beish
With Donald C. Parker, M.D., Daniel Troiani, and Daniel Joyce
SURFACE FEATURES OF MARS
The dark surface markings were once thought by some
astronomers to be great lakes, oceans, or vegetation, but space probes
in the 1970's revealed the markings to be vast expanses of rock and
dust. Windstorms sometimes move the dust, resulting in both
seasonal and long-term changes.
Among the areas where yearly variations have been recorded
are Trivium-Elysium, Solis Lacus, Syrtis Major, and Sabaeus-
Meridiani. The Syrtis Major is the planet's most prominent dark area.
Classical observations have revealed seasonal variations in the breadth
of this feature: maximum width occurring in northern mid summer
(145 deg Ls), and minimum during early northern winter, just after
perihelion (290 deg Ls) [Antoniadi, 1930, Capen, 1976]. However,
recent observations by ALPO astronomers and by the Hubble Space
Telescope (HST) suggest that no such variations have occurred since
1990 [Lee, et al., 1995. Troiani, et al., 1997].
Solis Lacus, the "Eye of Mars", is notorious for undergoing
major changes. In 1977 amateur observers discovered a new dark
feature in the Aetheria desert at longitude 240 west, 25 north,
between Nubis Lacus and Elysium. It was subsequently found on
Viking Orbiter photographs taken in 1975, apparently undetected by
Viking scientists. This is an example of the importance of ground-
based observations of the Solar System.
Another feature that is of great interest to professional Mars
researchers is the Trivium-Cerberus, on the southern rim of the
Elysium shield. A classically dark feature 1300 x 400 km in size, it
has all but disappeared during the 1990's [Moersch et al., 1997.
Troiani et al., 1997].
MARTIAN METEOROLOGY
Clouds and Hazes. -- The Martian atmosphere is ever-
changing. White water ice clouds, yellowish dust clouds, bluish limb
hazes, and bright surface frosts have been studied with increasing
interest in the past two decades. Clouds seem to be related to the sea-
sonal sublimation and condensation of polar-cap material. An
intensive study of Martian meteorology has been conducted by the
ALPO Mars Section using visual data and photographs from
professionals and amateurs around the world. The first report,
published in 1990, analyzed 9,650 IMP observations submitted over
eight Martian apparitions between 1969 and 1984 [Beish and
Parker, 1990]. This study has now been expanded to include 24,130
observations between 1965 and 1993. Statistical analysis
indicates that discrete water ice crystal cloud activity and near-surface
fogs occurrence is significantly higher in the spring and summer of the
Martian Northern Hemisphere than the same seasons for the Southern
Hemisphere.
For inclusion in this unique study, it is essential that ALPO
astronomers employ blue filters when making visual, photographic, or
CCD observations.
Discrete clouds -- have been observed on Mars for over a
century. In 1954, a remarkable W-cloud formation was found to be
recurring each late-spring afternoon in the Tharsis-Amazonis region.
A decade later, C.F. Capen proposed that the W-clouds are oro-
graphic (mountain-generated), caused by wind passing over high
peeks. Indeed, in 1971 the Mariner 9 spacecraft probe showed them
to be water clouds near the large volcanoes Olympus Mons (longitude
133 west, latitude 18 north), Ascraeus Mons (104W, 11N),
Pavonis Mons (112W, 0N), and Arsia Mons (120W, 9S). The W-
clouds should be active during the 1999 apparition at least until
opposition (129 deg Ls) and, perhaps, late in the apparition, during
southern spring. Although often observed without filters, they are best
seen in blue or violet light when they are high in altitude and in yellow
or green light at very low altitudes. Other orographic clouds are
observed over the Elysium Shield.
In addition to the dramatic orographic clouds, Mars exhibits
many localized discrete clouds. These rotate with the planet and are
most often found in northern spring-summer in Libya, Chryse, and
Hellas. One remarkable example of a discrete topographic cloud is the
"Syrtis Blue Cloud," which circulates around the Libya basin and
across Syrtis Major, changing the color of this dark albedo feature to
an intense blue. Originally named the "Blue Scorpion" by Fr.
Angelo Secchi in 1858, this cloud usually makes its appearance
during the late spring and early summer of Mars' northern
hemisphere It has been prominent during the 1995 and 1997
apparitions and is best seen when the Syrtis is near the limb. Viewing
this cloud through a yellow filter causes the Syrtis to appear a vivid
green (yellow + blue = green).
Limb brightening, or "limb arcs" are caused by scattered
light from dust and dry ice particles high in the Martian atmosphere.
They should be present on both limbs often throughout the apparition
and are also best seen in blue-green, blue or violet light. When dust is
present, these arcs are often conspicuous in orange light.
Morning clouds -- are bright, isolated patches of surface
fog or frosty ground near the morning limb (Mars' western edge as
seen on Earth's sky). The fogs usually dissipate by mid-morning,
while the frosts may persist most of the Martian day, depending on the
season. These bright features are viewed best with a blue-green, blue,
or violet filter. Occasionally, very low morning clouds can be seen in
green or yellow light.
Evening clouds -- give the same appearance as morning
clouds but are usually larger and more numerous than morning
clouds. They appear as isolated bright patches over light desert
regions in the late Martian afternoon and grow in size as they rotate
into the late evening. They are best seen in blue or violet light.
The size and frequency of limb clouds appear to be related to
the regression of the northern, rather than the southern, polar cap.
Both limb arcs and limb clouds are prominent after aphelion (70 deg Ls),
but limb clouds tend to rapidly decrease in frequency after early
summer, while limb hazes become more numerous and conspicuous
throughout the northern summer.
Equatorial Cloud Bands (ECB's)-- These features
appear as broad, diffuse hazy bands along Mars' equatorial zone and
are difficult to observe with ground-based telescopes. HST has
revealed that these clouds may be more common than we have
suspected in the past. Their prevalence during the 1997 apparition led
some conferees at the Mars Telescopic Observations Workshop-II
(MTO-II) to postulate that many limb clouds are simply the limb
portions of ECB's. ALPO astronomers are encouraged to watch for
these elusive features during the 1999 apparition. Are they really more
common, or are our improved technologies merely allowing us to
detect them more easily?
ECB's are best detected visually through a deep-blue (W47
and W47B) Wratten filters and may be photographed or imaged in
blue or ultraviolet light.
New technologies, such as CCD cameras, sophisticated
computer hardware and software, and large-aperture planetary
telescopes have given rise to a virtual explosion in advanced
techniques of studying our Solar System. Never before have we been
able to readily detect the delicate wispy Martian Equatorial Cloud
Bands so well as we do now with CCD imaging.
Dust storms -- Recent surveys, including our Martian
meteorology study, have shown that dust events can occur during
virtually any season [Martin and Zurek, 1993. Beish and Parker,
1990]. The main peak (285 deg Ls) occurs during Mars' southern
summer, just after southern summer solstice, but a secondary peak
has been observed in early northern summer, around 105 deg Ls.
Classically, the storms occurring during southern summer are larger
and more dramatic: they can even grow rapidly to enshroud the whole
planet. It should be remembered, however, that these global dust
storms are quite rare--only five have been reported since 1873, and
these have all occurred since 1956. Much more common is the
"localized" dust event, often starting in desert regions near Serpentis-
Noachis, Solis Lacus, Chryse, or Hellas. During the 1997 apparition,
CCD and HST observations revealed localized dust clouds over the
north polar cap early in northern spring.
Identifying the places where dust storms begin and following
their subsequent spread is most important to future Mars exploration
missions. The following criteria apply in the diagnosis of Martian dust
clouds:
1. The sine qua non of Martian dust clouds is movement with
obscuration of previously well-defined albedo features. Absence of
this criterion in the present study disqualified a candidate from
inclusion under dust clouds.
2. They must be bright in red light. In the past, astronomers have
identified Martian dust clouds and/or obscurations as "yellow clouds."
It is incorrect to describe the color of Martian dust clouds as "yellow."
While they may appear yellowish when observed without the aid of
color filters, they are in fact brighter in red and orange light than they
are in yellow light. Dust clouds brighten faintly in yellow filters and
display well-defined boundaries through orange and red filters.
During the initial stages of formation, they often appear very bright in
violet and ultraviolet light, suggesting the presence of ice crystals.
We vigorously discourage the use of the term "yellow clouds"
to describe dust. If a suspect cloud is not bright in red light, it is not to
be considered a dust cloud.
3. There are numerous reports of anomalous transient albedo
features appearing near dust clouds, especially when the solar phase
angle was reasonably large. When these clouds reach heights of
several kilometers, they may cast shadows that are observable from
Earth.
Dr. Richard McKim (BAA) has written an excellent review of
Martian dust storms [McKim, 1996].
Blue Clearing -- Normally the surface (albedo) features of
Mars appear vague through light blue filters, such as the Wratten 80A.
With a dark blue (W47) or violet (380-420 nm) filter, the disk usually
appears featureless except for clouds, hazes, and the polar regions.
When a little-understood phenomenon known as the "blue clearing"
occurs, however, Martian surface features can be seen and
photographed in blue and violet light for periods of several days. The
clearing can be limited to only one hemisphere and can vary in
intensity from 0 (no surface features detected) to 3 (surface features
can be seen as well as in white light). The Wratten 47 filter or
equivalent is the standard for analyzing blue clearing.
Recently there has been renewed professional interest in blue
clearing. We encourage ALPO Mars observers to watch for this
phenomenon during the 1999 apparition.
CALENDAR OF EVENTS -- MARS 1998-1999
DATE POINTS OF INTEREST
1998 Dec 16 Ls 70 deg. Aphelion. Mars at 5.7" apparent diameter
Views of surface details well defined. NPC Rima Tenuis may appear.
Northern hemisphere mid-spring. NPC beginning rapid retreat? Are
limb arcs present? NPR clouds increasing in frequency, intensity.
Use filters! Antarctic hazes, hood? Cloud activity increases. Watch
for "Aphelic Chill" in NPR (usually between 60 deg and 70 deg Ls).
1998 Dec 25 Ls 75 deg. Mars at 6" apparent diameter. Apparition
begins for observers using 4-inch to 8-inch apertures telescopes and
up. Begin low resolution CCD imaging. Blue Syrtis Cloud? Is Hellas
brightening?
1999 Jan 29 Ls 90 deg. Northern Summer/Southern Winter Solstice.
Mars at 7.8" apparent diameter; Orographics over the Tharsis
volcanoes -- W-Cloud? Look for orographic clouds (violet filter and
blue-green filter).
1999 Feb 02 Ls 96 deg. Mars at 8" apparent diameter. Quality
micrometer measurements of NPC possible.
1999 Feb 26 Ls 102 deg. Mars at 10" apparent diameter. Continue
NPC measurements. Is North Cap fairly static or entering rapid retreat
phase? South polar regions becoming difficult to observe. Any signs
of SPH? Some photography now possible. Begin high resolution
CCD imaging.
1999 Mar 16 Ls 110 deg. Mars at 12" apparent diameter. Begin
high resolution. Visual observations and high quality photography
begins. Hellas bright? Watch for limb clouds. Blue Syrtis Cloud.
1999 Apr 24 Ls 128.7 deg. Opposition. Mars at 16.02" apparent
diameter. Dec -11.5. Distance 0.58435 AU (54,313,968 miles)
1999 May 01 Ls 132 deg. Closest approach, Mars at 16.18"
apparent diameter., Dec. -11.0, Distance 0.57846 AU (53,771,107
miles). Polar clouds. NPC remnant. Is Syrtis Major broad?
1999 Jun 25 Ls 160 deg. Mars drops below 12" apparent diameter.
Is north polar hood forming? Look for NPC remnant in red light.
1999 Jul 20 Ls 173 deg. Mars drops below 10" apparent diameter.
Are both polar hood visible? Decrease in discrete clouds.
1999 Jul 31 Ls 180 deg. Northern Autumn/Southern Spring. Watch for
increase in dust clouds. Is south polar cap visible free of its hood?
1999 Aug 28 Ls 195 deg. Mars drops below 8" apparent
diameter. Early southern spring. Possible W-clouds reforming. Is
Syrtis Major narrowing?
1999 Nov 08 Ls 240 deg. Mars drops below 6" apparent
diameter. Mars near perihelion. Watch for dust. De = -9 deg. Late spring
south polar cap visible.
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Part 1 of this series, an introduction and description of the A.L.P.O.
and International Mars Patrol observing programs, can be found at:
http://astrosun.tn.cornell.edu/marsnet/imw/imw3.4.html
Part 2 of this series, a description of the Marswatch project and an
introduction to observing Mars can be found at:
http://astrosun.tn.cornell.edu/marsnet/imw/imw3.5.html
The next newsletter will include an article by Jeff Beish on the
measurement of the Martian polar caps from CCD images.
For more information, the A.L.P.O. WWW home page can be found at:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
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Other Useful WWW sites:
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Latest MGS images:
http://www.msss.com/mars/global_surveyor/camera/images/index.html
Main MGS Home Page:
http://mars.jpl.nasa.gov/mgs/index.html
Pathfinder Home Page:
http://mars.jpl.nasa.gov/default.html
JPL Mars Missions Page
http://www.jpl.nasa.gov/mars
Mars-98 MVACS Science Payload Home Page:
http://mvacs.ess.ucla.edu/index.html
Mars-01 Athena Science Payload
http://astrosun.tn.cornell.edu/athena/index.html
A.L.P.O. Mars observations:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
1996-97 Marswatch highlights:
http://mpfwww.jpl.nasa.gov/mpf/marswatch.html
1996-97 Marswatch ftp site:
ftp://marsnt3.jpl.nasa.gov
MarsNet:
http://astrosun.tn.cornell.edu/marsnet/mnhome.html
Mars (and other) Educational Resources Page
http://marswatch.tn.cornell.edu/marsidea
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I will continue to maintain the email distribution list as well
as the various Cornell and JPL Marswatch-related WWW archives. If
you are receiving duplicate copies of this mailing, or you want
your name removed from the distribution list, please send me email.
--Jim
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Jim Bell
Cornell University
Department of Astronomy
Center for Radiophysics and Space Research
402 Space Sciences Building
Ithaca, NY 14853-6801
phone: 607-255-5911; fax: 607-255-9002
email: jimbo@marswatch.tn.cornell.edu
WWW: http://marswatch.tn.cornell.edu
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