1998-1999 APHELIC APPARITION OF MARS
By: Jeffrey D. Beish
With Donald C. Parker, M.D., Daniel Troiani, and Daniel Joyce
INTRODUCTION
Recent Spacecraft missions into our Solar System have sparked renewed
public interest in planetary sciences. The Mars Global Surveyor space
mission will bring the first close-up surveillance of the Red Planet
Mars since the Viking I and II visits during the mid-1970's and early
1980's.
While the past accomplishments of U.S. space missions throughout the
Solar System has yielded extensive volumes of scientific information we
never the less ponder many questions about the Earth-like planet Mars.
Some of those questions remain unanswered today: Are the polar climates
static or are they changing over long periods of time? Can surface wind
directions be inferred from cloud formations and movements? Are
equatorial water ice crystal clouds seasonal? If so, can their
appearance and locations be predicted? What causes the secular
(long-term) changes in dark albedo features? Are their locations
topographically controlled or result from unseasonable winds? These
are just a few of the important questions remaining in the Martian
mystery.
Still an intriguing world, Mars offers the casual and serious observer
both alike many challenges and delights. This planet offers
astronomers a free laboratory for the study another planet's
atmosphere: the behavior of condensates and effects on its atmosphere.
Mars is similar to Earth in that it has four seasons, exhibits global
climates, changing weather patterns, annual thawing of polar caps,
storm clouds of water ice, howling dusty winds, and a variety of
surface features which predictably change with color and size and move
around the surface over long periods of time.
THE A.L.P.O. AND THE I.M.P.
The International Mars Patrol (I.M.P.) is an international cooperative
effort between individual observers and members of observing groups
located around the world. Established in the late 1960's by the late
Charles F. Capen, the IMP has contributed more than 30,000 observations
of Mars. Contained within the archives of the Association of Lunar and
Planetary Observers (A.L.P.O.) Mars Section library are the records of
fifteen apparitions of Mars covering a span of 35 years.
From the late 1960's, interested amateur and professional astronomers
located in 47 foreign countries and U.S. territories cooperated in a
24-hour watch on the Red Planet Mars. Additional support is provided
by the British Astronomical Association (B.A.A.), the Arbeitskreis
Planetenbachter (Germany), and Japan's Oriental Astronomical
Association (O.A.A.).
THE A.L.P.O./I.M.P. OBSERVING PROGRAM FOR MARS
The I.M.P. coordinates and instructs the cooperating observers in using
similar visual, photographic, photometric, and micrometric techniques
employing color filters and standard methods for reporting their
observations, which results in homogeneous sets of observing data that
have good analytic value.
Each apparition the A.L.P.O. Mars Section receives thousands of
individual observations consisting of visual disk drawings made with
the aid of color filters, black-&-white and color photographs,
intensity estimates of light and dark albedo features, color contrast
estimates, and micrometer measurements of polar caps, cloud boundaries,
and variable surface features during the 10 to 12 month observing
period. The chronological filing of this large quantity of data
requires the observation information obtained for each night Universal
Date be recorded on one or two standard observing report forms!
Observational data consist of color filter photography, visual disk
drawings, visual photometry (intensity estimates on the standard ALPO
scale: 10 = polar brightness, 8 = desert mean brightness, 0 = night
sky), micrometry, and CCD imaging. Great emphasis is placed on quality
photographs in red, blue, and violet light, full disk drawings using
standard color filters, polar cap measurements made with the
astronomical micrometer, and with modern observing techniques such as
full disk photometry and CCD imaging.
It is highly recommended that all observers, visual as well as
photographers and CCD camera users, use at least a basic set of
tricolor filters according to the following guide: Red or Orange (W-25
or W-23A); Green (W58); Blue-Green (W-64); Blue (W-38A or W-80A); and
Violet (W-47). Observers with smaller telescopes, such as 3 to 6-inch
apertures may find a Yellow (W-15) useful and may provide better
performance than the deep red filter (See Table 1). Those employing
larger instruments, such as 8 to 16-inch apertures, will find the deep
Red and Blue filters most useful for fine surface details or
atmospheric cloud detection [Capen, et al, 1984].
Table 1. Eastman Kodak Wratten Filters used by A.L.P.O. observers.
Characteristics for Mars Observations.
Yellow (W12, W15) to brighten desert regions, darkens bluish and
brownish features.
Orange (W21, W23A) further increases contrast between light and dark
features, penetrates hazes and most clouds, and limited detection of
dust clouds.
Red (W25, W29) gives maximum contrast of surface features, enhances
fine surface details, dust clouds boundaries, and polar cap
boundaries.
Green (W57) darkens red and blue features, enhances frost patches,
surface fogs, and polar projections.
Blue-Green (W64) helps detect ice fogs and polar hazes.
Blue (W80A, W38, W38A) and deep blue (W46, W47) shows atmospheric
clouds, discrete white clouds, and limb hazes, equatorial cloud bands,
polar cloud hoods, and darkens reddish features. The W47 is the
standard filter for detection and evaluation of the mysterious blue
clearing.
Magenta (W30, W32) enhances red and blue features and darkens green
ones. Improves polar region features, some Martian clouds, and surface
features.
THE MARS WATCH OBSERVING PROGRAM
The Marswatch program was initiated in electronic form in 1996 through
the collaboration of astronomers at Cornell University, the JPL Mars
Pathfinder Project, and the Mars Section of the A.L.P.O. as a vehicle
through which Mars astronomers worldwide can upload their observations
to a WWW home page and archive site at JPL.
MarsNet is the WWW arm of the International Mars Watch, a group founded
by professional astronomers interested in Mars to facilitate better
communication between the amateur and professional Mars observing
communities. At those Internet sites, you will find images of Mars
contributed by amateurs and professional, tools to aid you in planning
your own Mars observations, current and past issues of the
International Mars Watch Electronic Newsletter, and links to other
Mars-relevant sites on the Internet. The primary purpose of this
project is frequent CCD imaging of Mars using B,V,R or other standard
filters and visual drawings and photos in order to monitor the planet's
atmospheric dust and cloud activity.
Secondary goals include imaging or spectroscopic characterization of
the surface color and mineralogy, characterization of the growth and
retreat of the polar caps, and analysis of atmospheric water vapor
content. Because Mars rotates at nearly the same rate as the Earth and
it also has a dynamic atmosphere that exhibits hourly, daily, and
seasonal changes, frequent observations from observatories spanning the
widest possible range of longitudes are desired.
The upcoming apparition (1999) is particularly important because the
U.S. Orbiter (Mars Global Surveyor) will start regular imaging during
this time. In addition, the orbiter will be in a low sun-synchronous
polar orbit, so it will only "see" the surface of Mars around 2 a.m.
and 2 p.m. local time (the rest of the planet is over the horizon), so
quality ground-based observations are needed in order to place these
single-time-of-day orbiter views of the planet as well as the
single-location lander data, into a global context.
The project will maintain a WWW home page and archive site in
association with the Mars Pathfinder mission. The goal will be to have
participants submit one or more of their images (or entire data sets if
they like) to this site for dissemination to NASA Project personnel,
professional astronomers, amateur astronomers, news and print media,
educators and schoolchildren, and the general public. Another general
project goal is to post at least one new CCD image of Mars on the Web
every day between December, 1998 and December, 1999. Even better would
be one "daily global view" per day, composed of 2 or 3 Mars's images
taken on the same night but from observatories widely separated in
longitude. To make this a reality will require a dedicated and
geographically-diverse network of observers.
The current web site address for Marswatch is
http://mpfwww.jpl.nasa.gov/mpf/marswatch.html
The images from both the 1995 and 1997 apparitions may be viewed there.
When it becomes available, the address for 1999 will be announced in
the Martian Chronicle and on the ALPO Home Page.
MARS IN 1998-99
Mars has an average 15.8-year seasonal opposition cycle, which consists
of three or four Aphelic oppositions and three consecutive Perihelic
oppositions. The 1998-99 apparition will be considered an Aphelic
apparition because opposition occurs only 58 degrees after aphelion
(70( Ls). Mars will reach opposition on 24 April 1999 (128( Ls) and
be closest to Earth on 01 May 1999 (132( Ls) with an apparent
diameter of 16.2 seconds of arc. Mars will be at a distance of 0.57846
A.U or 53,771,107 miles from Earth.
For observers located in Earth's Northern Hemisphere, Mars will not be
positioned as favorably during the upcoming apparition as it was in
1997, since it will be placed south of the celestial equator throughout
the entire apparition.
Mars' North Pole will be tilted earthward during the entire 1998-1999
apparition, permitting study of the planet's Northern Hemisphere during
Martian late spring, summer, and autumn.. Thus astronomers can again
investigate the regression of the NPC and follow Martian arctic
meteorology. This apparition should also allow careful scrutiny of the
summer NPC remnant.
DAYS AND SEASONS ON MARS
The Martian solar day, also called a "sol" by space scientists, is
about 40 minutes longer than a day on Earth. Thus Mars rotates through
only 350( of longitude in 24 hours. An astronomer on Earth, who
observes a particular surface feature on Mars, on a particular night,
sees the same feature 10( further to its west (closer to its morning
limb) the next night.
Mars and Earth have four comparable seasons because their axes of
rotation are each tilted at about the same angle to their respective
orbital planes. In describing Martian seasons, scientist use the term
"Ls" which stands for the Areocentric longitude of the Sun along Mars'
ecliptic. ("Areo-" is a prefix often employed when referring to Mars
or "Ares.") Mars' axial tilt is 25.(2 as compared to 23.(5 for that of
the Earth. The Martian year is 687 Earth days, nearly twice as long as
ours, so that the Martian seasons are similarly longer. While Earth's
are nearly equal in duration, the length of a Martian season can vary
by as much as 52 days because of the greater eccentricity of its
orbit.
The axis of Mars does not aim at our North Star, but is displaced about
40( towards Alpha Cygni. Because of this celestial displacement the
Martian seasons are 85( out of phase with the terrestrial seasons, or
about one season in advance of ours. Consequently, when you observe
Mars next spring and summer you will be seeing summer and autumn,
respectively, in the Martian Southern Hemisphere.
MAKING OBSERVATIONS OF MARS
The ancient art of visual observation at the telescope is still a most
useful tool to the modern astronomer, and is the forte of the amateur
astronomer. The authors, attending various professional meetings over
the past few years, were pleasantly surprised to find that carefully
made amateur drawings were considered to be useful sources of data by
Mars's professionals.
Even at its best, Mars is challenging to observe. The disk is tiny and
its markings are blurred by the Earth's atmosphere. A telescope for
planetary work should provide sharp images with the highest possible
contrast. A long-focus refractor is generally considered the best,
followed by a long-focus Newtonian or Cassegrain reflector. Telescopes
with large central obstructions do less well.
Anyone who observers Mars will find it rewarding to make a sketch of
whatever is seen, both to create a permanent record and to help train
the eye in detecting elusive detail. Start with a circle 1-3/4 inches
(42 mm) in diameter. Draw the phase defect, if any, and the bright
polar caps or cloud hoods. Next shade in the largest dark markings,
being careful to place them in exactly the right locations on the
disk. At this stage, record the time to the nearest minute. Now add
the finer details, viewing through various color filters, starting at
the planet's sunset limb. Finally, note the date, observer's name, the
instrument(s) used, and any other relevant information.
The Martian Central Meridian (CM) is an imaginary line passing through
the planetary poles of rotation and bisecting the planetary disk and is
used to define what areographic longitudes are present on the disk
during an observing session. It is independent of any phase that may
be present--if Mars presents a gibbous phase the CM will appear to be
off center. The CM value is the areographic longitude in degrees which
is on the central meridian of the disk as seen from Earth at a given
Universal Time (U.T.). It can be calculated by adding 0.24/min., or
14.6 degrees /hr., to the daily CM value for 0h U.T. as listed in The
Astronomical Almanac.
The terminator (phase defect) is the line where daylight ends and night
begins. The terminator phase, or defect of illumination, is given in
seconds of subtended arc on the apparent disk, or in degrees (i) or the
ratio (k), to define how much of the geometrical Martian disk is in
darkness. The sunset terminator appears on the east side, or evening
limb, before opposition; and after opposition, the terminator becomes
the sunrise line on the morning limb on the west side. At opposition
there is no perceptible phase defect.
The axial tilt. The declination of the planet Earth (De) as seen from
Mars defines the axial tilt of Mars relative to Earth. The De is also
equal to the areographic latitude of the center of the Martian disk,
which is known as the subearth point. The latitude is (+) if the North
Pole is tilted toward Earth and (-) if the South Pole is tilted toward
Earth. This quantity is an important factor when drawing Mars or when
trying to identify certain features.
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( Ls), and
minimum during early northern winter, just after perihelion (290( 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's
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 seasonal sublimation and
condensation of polar-cap material. The ALPO Mars Section using visual
data and photographs from professionals and amateurs around the world
has conducted an intensive study of Martian meteorology. 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 orographic
(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 (104(W, 11(N), Pavonis Mons (112(W, 0(N), and
Arsia Mons (120(W, 9(S). The W-clouds should be active during the 1999
apparition at least until opposition (129( 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( 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(
Ls) occurs during Mars' southern summer, just after southern summer
solstice, but a secondary peak has been observed in early northern
summer, around 105( 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's 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( 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( and 70( Ls).
1998 Dec 25 Ls 75( 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( 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( Mars at 8'' apparent diameter. Quality micrometer
measurements of NPC possible.
1999 Feb 26 Ls 102( 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( 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( Opposition. Mars at 16.02'' apparent diameter. Dec
-11.5. Distance 0.58435 AU (54,313,968 miles)
1999 May 01 Ls 132( 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( Mars drops below 12'' apparent diameter. Is north
polar hood forming? Look for NPC remnant in red light.
1999 Jul 20 Ls 173( Mars drops below 10'' apparent diameter. Are both
polar hood visible? Decrease in discrete clouds.
1999 Jul 31 Ls 180( Northern Autumn/Southern Spring. Watch for increase in
dust clouds. Is south polar cap visible free of its hood?
1999 Aug 28 Ls 195( Mars drops below 8'' apparent diameter. Early
southern spring. Possible W-clouds reforming. Is Syrtis Major
narrowing?
1999 Nov 08 Ls 240( Mars drops below 6'' apparent diameter. Mars near
perihelion. Watch for dust. De = -9(. Late spring south polar
cap visible.??
For
more about this background