|
Mars
Section Circular No. 8 1999 July 16–August 31 |
|
General This
Circular summarises the period 1999 July 16 (Ls = 171 deg, D = 10.3
arcsec, decl. = –14 deg.) to August 31 (Ls = 197 deg, D = 7.9 arcsec,
decl. = –21 deg.). Mars remained an increasingly difficult evening object,
and the last UK observation received is that by the Director on August 10,
observing from Truro on the eve of the total solar eclipse. The planet has
been easier to view from the USA, Italy and Japan, etc. Since the last Circular
I have received observations from Parker, Foulkes, Crandall, Di Sciullo,
Melillo, Wasiuta, Topping, Teichert, Niechoy, Meredith, Johnstone, Hendrie,
Heath, Haas, Grego, Colombo, Adamoli and Frassati, six of whom had not sent
observations earlier this apparition. Especially praiseworthy among the
visual observations are the series of later drawings by Haas, Frassati and
Adamoli, which continue to document the changing polar regions and white
clouds. Mars
Global Surveyor observations, 1999 June MGS
observations of dust activity in the N. polar region on June 30 were released
onto the Web on August 10, in other words since the last Circular. The
images are dated June 30, 0652h, 0850h, 1047h and 1245h UT, showing the
hemisphere of Mare Acidalium. A dust cloud curls southward from near the NPC
remnant, concealing Hyperboreus Lacus. (See Extract B from the MGS web site,
below.) The Director was not aware of this event at the time, so he could not
alert observers. But the region was on the wrong side of the planet to be
viewed from the UK at that time. Japanese observers may have viewed some
indications of the event in early July. It will be
recalled from Circular No. 6 that of the BAA
observers, Cidadao and McKim viewed dust activity in the Mare Boreum region
on June 2–3, not far from the event of June 30. This area, adjacent to
and NW of the large dark area Mare Acidalium, has therefore produced several
small dust events and white ‘cyclonic’ clouds (see an earlier Circular)
during the apparition. Historically the region has produced other
‘cyclonic’ clouds during the 20th Century. But what about dust
storms? The large albedo and thermal inertia contrasts between Acidalium and
the neighbouring classical deserts of Tempe, Chryse, Xanthe has occasionally
led to dust initiation in the latter desert regions, but I cannot recall dust
activity starting over Mare Boreum during the period reviewed in the Dust
Storm Memoir (i.e., up to 1993). This all goes to show that Mars can
still surprise us. Past records of a yellowness about the polar regions
should therefore not be dismissed immediately as observational error! In the
Mars Memoir (see later) I have called attention to such polar dust
records, and briefly describe my results. But after the martian polar storms in
the summers of 1997 and 1999 it may be prudent to further re-examine some
historical data. Extracts
from the Mars Global Surveyor web site Herewith
some exciting extracts from the MGS web site, with links to images. I have
chosen the comments most relevant for ground-based observers, and have added
a few of my own comments in square brackets. Extract A : Wind
Action – The Dust Devils of Amazonis Planitia, MGS MOC Release No.
MOC2-171, 10 August 1999 Dust
devils result from spinning vortices of air that lift dust from a
planet’s surface. They look something like a miniature tornado. The
dust devils shown here were observed in mid-May 1999 in northern Amazonis
Planitia (northwest of the Olympus Mons volcano). Dust devils are common in
this region and were seen there even during the Viking orbiter missions in
1976–1980. The first two
pictures (A and B [link]) show a colour composite view of the Amazonis dust
devils as they appeared to the Mars Global Surveyor (MGS) Mars Orbiter Camera
(MOC) red and blue wide angle cameras; the white arrows in MOC2-171b (B,
above) point to each individual dust devil. The third picture (C, above) is a
GIF ‘movie’ (Click on the Icon) that shows dust devil occurrences
on two different dates in May 1999. The scene in this ‘movie’ is
about 88 kilometres (55 miles) across. The fourth picture (D, above) is a
diagram that compares the typical heights of dust devils and tornadoes on
Earth with dust devils on Mars. Click on the ‘cartoon’ icon to
see the entire diagram, including a comparison with the heights of the
tallest mountains on Earth (Himalayas) and Mars (Olympus Mons). The heights of
dust devils in MOC images can be estimated from the length of the dark
shadows that they cast. The shadows in these pictures all point toward the
northeast (toward upper right). The largest dust devil in these pictures
towers nearly 8 kilometres (5 miles) above the martian surface, and has a
lower basal plume of dust that suggests substantial surface flow of wind and
dust into the rising column. In the MOC images shown here, north is up, and
the sun’s illumination is from the lower left. The 40-kilometre scale
bar also indicates a distance of about 25 miles. Additional MOC images
regarding these and other dust devils: ‘Large Martian Dust Devils
Caught in the Act,’ July 1, 1999. ‘SUV Tracks on Mars? The
‘Devil’ is in the Details’, July 30, 1998.’ Extract B : Late
Summer Storms Over the North Polar Region, MGS MOC Release No. MOC2-172, 10
August 1999 Storm
clouds have been brewing over the north polar cap of Mars since the last week
of June 1999. During the month of July, summer was ending; autumn began at
the start of August. The wide angle cameras of the Mars Global Surveyor (MGS)
Mars Orbiter Camera (MOC) have been documenting the changing weather patterns
of the red planet nearly every day since the Mapping Phase of the mission
began in March 1999. These images are showing many more details about martian
weather than had been previously recorded. Mars is a dynamic planet, with
weather systems as complex and exciting as the Earth’s. The four
still-frame images (above) show the evolution of a storm system that
developed over the martian north polar region on June 30, 1999. Each picture
was taken approximately 2 hours later than the previous. The north polar ice
cap is the white feature at the centre of each frame. Clouds that appear
white consist mainly of water ice, clouds that appear orange/brown contain
dust. This particular
storm system lasted well into the next day – July 1, 1999. A total of
23 red and 23 blue camera images were used to create a time-lapsed
‘movie’ that displays the development and evolution of this storm
over the two-day period. Of great interest are the ‘curling’ of
the clouds behind the largest of the storms – this indicates a flow
vortex that follows the storm front that is moving toward the top/upper right
of the frame – and the correlation of white water-ice clouds with
orange/brown dust clouds. High surface winds must have raised dust and mixed
it with water vapour in the air over the summer-time polar cap to create this
effect. To view the ‘movie,’ click or download to your desktop
the following 2.2 MByte MPG file. Storms similar
to those shown here were observed to continue throughout the month of July
and into August [unfortunately there are no details given on the Website
– RJM]. Over the next several months, the north polar cap will grow
dark as the region transitions through autumn and into winter. When northern
winter begins in December 1999, this region will be dark and obscured by
clouds.’ [This is not so: the NPH will be a permanent feature well
before 1999 December! – RJM.] Extract C (part) :
Mars: An Active Planet, MGS MOC Releases MOC2-166 to MOC2-172, 10 August 1999 Among
the goals of the Mars Surveyor program are to characterize the planet’s
climate and the interaction of the atmosphere with the planet’s
surface. Both the Mars Global Surveyor (MGS, presently in Mars orbit) and the
Mars Climate Orbiter (MCO, which reaches Mars in September) address these
goals in part by using cameras to observe martian weather and changes on the
surface that occur from season to season. Mars Global
Surveyor has been orbiting the Red Planet for just over 1 martian year (687
days). Although the spacecraft only recently attained its final design orbit
after 18 months of aerobraking and other orbit phasing activities,
‘snapshots’ of Mars acquired during this period by the MGS Mars
Orbiter Camera (MOC) – when it has been on – and more recent
views from the present mapping orbit have captured a unique record of
seasonal and meteorological events that demonstrate that the planet is quite
active and dynamic today. Evidence for
present-day activity comes in two forms: weather, and surface changes.
Detailed weather observations include the tracking of dust devils and the
daily mapping of cloud and storm patterns. Other changes on the planet have
been seen among frost-covered sand dunes. These changes are connected to the
passage of martian seasons and the retreat of polar ice as winter draws to a
close and spring begins. As the winter ice begins to sublime, dunes develop
small dark spots that grow and eventually coalesce until the frost
disappears. Some dunes show evidence that wind and gravity are actively
moving the dune sands, as well. The images
shown below were presented at a Space Science Update briefing at NASA
Headquarters, Washington, DC, on 10 August 1999. For the corresponding JPL
Press Release: see http://www.jpl.nasa.gov/news/ 1.
The Mystery of the Spotted Dunes MOC2-166,
‘Bushes’ On Dunes, August 1999 MOC2-167,
Snow Leopard Dunes, August 1999 MOC2-168,
Changes In 26 Days, August 1999 MOC2-169,
Wind Streak Dunes, August 1999 Every
year, Mars grows two large, seasonal frost caps (one during each
hemisphere’s winter) out of a combination of atmospheric carbon dioxide
(the major component of the cap) and water vapour. At the end of each winter,
these caps recede in latitude as the sun moves across the equator and into
the spring-time hemisphere. This annual cycle of frost deposition and
sublimation (i.e., the process by which ice transforms when heated directly
from solid to gas without first ‘melting’ to form a liquid) is
one of the defining elements of the martian climate. There is much that is
unknown about this process, but recent observations of sand dunes within the
polar regions are providing new information about the seasonal retreat of the
polar ice caps. Observations
made in 1998 at the end of the north polar winter showed some evidence that
dark spots develop on sand dunes as the winter frost begins to disappear.
This year, 1999, similar features have been observed in the southern hemisphere
as winter has proceeded into spring (southern spring began August 2, 1999).
The evidence from the dunes suggests that defrosting begins more or less
simultaneously at many small, localized sites. Each site then grows radially
from the initial spot, enlarging and eventually merging until all of the
frost is gone. The rate of growth of the defrosted surfaces, and the details
of their appearance, indicate that the frost is probably repeatedly sublimed
and locally re-deposited, with this local cycle acting to
‘refrigerate’ the ground and moderate or slow the rate of polar
cap retreat. Each picture shown above (MOC2-166 to MOC2-169) provides
examples of the dark spots that develop on dunes as they ‘thaw
out,’ including comparisons over a 26-day period and an example of
local wind transport of sand exposed in the spots. 2.
Recent Dune Activity MOC2-170,
Proctor Dunes, August 1999. Since first seen in Mariner 9 images of Mars, the
isolated dune fields within large impact craters have been of great interest,
as their dark colour indicates that the light dust that covers much of the
planet does not accumulate on the sandy surfaces. This indicates that the
dunes must be active – moving – and that we might, with time,
eventually see evidence of changes that allow us to measure the effectiveness
of wind erosion on Mars. The dune field in the picture above shows evidence
of recent activity, as dark sand has been mobilized and transported across
surfaces covered by the late-winter remains of seasonal frost.’ BAA
and other observations (mostly concerning the polar regions), 1999 July
16–August 31 North Polar Region The
OAA (Japan) has reported observations of both the hood and ground cap during
July 1–15, the hood predominating, with the situation changeable from
day to day and especially with CM longitude. Since the last Circular,
further supporting BAA data have come to hand for the first half of July.
Observations for the second half of the month continue to show the NPH. Minami writes
in OAA Circular No. 221: ‘This apparition provided a rare
opportunity to watch the moment we had after these 15 years since 1984... We
will meet 160 deg. Ls again in mid-May 2001 with apparent diameter 16 arcsec,
but the sub-Earth point will be [in] the southern hemisphere.’ The
Director agrees. It is rare to have this opportunity to watch the NPC to NPH
transition with favourable presentation and disk size. A well-documented
apparition for the transition from ground cap to hood is that of 1905. After
re-examining Percival Lowell’s published sketchbook Drawings of
Mars, 1905 in my library, I was struck with the enormous energy of Lowell
for examining the planet so frequently. If we ignore the stylised
representation of the albedo features, the polar region seems to be very
accurately portrayed. The transition from cap to hood that year is also
recounted in one of Lowell’s popular books. But despite his 24-inch
Clark OG, Lowell could only watch at one terrestrial longitude. South Polar Region The
S. limb has remained bright, though not always at all longitudes. While I am
sure that the ground cap must have already formed, it may be overlaid with
the hood sometimes, and it is not easy to see with severe foreshortening and
bad seeing on the increasingly tiny disk. Some observers refer explicitly to
a ground cap. In the longitudes of Argyre and Solis Lacus the SPH was
especially bright in July (e.g., Parker’s images of July 21 and 24),
but more data are needed for August. Again, I do not intend to report upon
white cloud activity on the planet generally. |
|
|
|
The
BAA martian dust storm Memoir By the time this Circular
reaches you, the delivery of the Mars Memoir will be just a few weeks
away. The Memoir will be advertised in the December 1999 BAA Journal,
and I am hoping Sky and Telescope will mention it. (For details, see Memoir) Circulars This
is the last Circular for the current apparition. Although further data
will be much appreciated, I do not intend to issue further Circulars
for the 1998–99 apparition. If something exceptional happens I will try
to report it in the Journal. A
big thank you to all those who supported the Section this apparition. My
congratulations upon your fine observations, which I am now working to get
into print! See you all in 2001. |
|
Richard McKim, Director 1999 September 24 |
|
|
