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

Return to Mars Circulars for 1998-99

Return to reports of past oppositions of Mars 

Return to the Mars Section home page