Beginners Astronomy: Mars Part 2- Mars and its Weather

By Duncan Lunan

Mars: Credit NASA

When Mariner 9 approached Mars in 1971, to become the first probe to orbit the planet, Mars was at its closest to the Sun and in the grip of a dust storm more severe than any we’ve seen since.   On approach the only features visible were four dark spots.   The first was identified with Nix Olympica, as it then was, but the other three, in a straight line running diagonally across the Martian equator, were labelled ‘North Spot’, ‘Middle Spot’ and ‘South Spot’.   As the dust settled each of them revealed a huge collapse caldera, and then, to general disbelief at first, each of those turned out to be at the summit of a gigantic shield volcano – the three ‘Spots’ standing on the Tharsis Ridge, a huge bulge in the crust of Mars which may have been formed by shockwaves from the enormous Hellas impact basin on the far side of the planet, or possibly by the formation of all five of the largest basins, along a great circle arc, due to the breakup and fall of an asteroid  (‘Ancient Impact Craters Reveal Mars’s First Equator’, Universe Today, 18 Apr 2005).   Its formation changed the rotational axis of Mars and helped to create the climatic conditions Mars has today.   (“Planet Mars may have ’tilted’ to keep its balance, say scientists”,  Princeton University News Release, August 25, 2006.)

The ‘Spots’ were identified with the features formerly known as Ascraeus Lacus, Pavonis Lacus, and Arsia Silva, which then lost their designations as ‘lakes’ and ‘wood’ and each became ‘Mons’ = mountain.   [Gavin Roberts’s painting for “Man & the Planets”, ‘SIOS  (Standard Interplanetary Operations Spacecraft)  over Middle Spot’, was painted before the new names were finalised.]    Pavonis Mons is right on the Martian equator and becomes the site of a space elevator in “Red Mars”, the first of the Mars settlement trilogy of novels by Kim Stanley Robinson.  From north to south the Tharsis volcanoes are Ascraeus Mons  (formerly Ascraeus Lacus), Pavonis Mons  (A1008)  and Arsia Mons.  

The highest mountain in the known Solar System is the shield volcano Olympus Mons.   Although 20 miles in height it’s more than 300 miles across, so its slopes are actually very gentle.   The landslip material around it is more than three billion years old, and its five summit calderas formed at 20 million year intervals 100-200 million years ago.  It was thought to be a mountain even before the space age, because the clouds that frequently surround it could be seen from Earth as a white spot.   It was then called Nix Olympica, ‘the Snows of Olympus’, on the assumption that it might be ice-capped.   Arthur C. Clarke used that title for his book about the possibility of making Mars habitable  (Gollancz, 1994), and made the mountain the focus of his study on terraforming Mars, taking 2000 years for conditions within the caldera to become fully habitable, although the summit lake was formed by 2500 AD and hosted the Martian Winter Olympics.   But photographs from Mars Express reveal glacial deposits at the base of the mountain, around four million years old, suggesting that it has been capped with snow and ice in the past and there may even be a cap now at high altitude, under an insulating layer of dust.   (Peter Bond, ‘Recent Volcanism on Mars’, Astronomy Now, February 2005.)

There’s a lot of argument about whether there’s currently any active volcanic activity on Mars.   There are some impact craters with light or dark streaks downwind which suggest that some active process in them is generating the dusty material.   Until recently it seemed that the huge Martian shield volcanoes like Olympus Mons, Elysium and the three giants of the Tharsis Ridge have apparently been quiet for up to 10 million years.   The caldera of Ascraeus Mons is 100 million years old, but results from the Mars Express orbiter suggest that the lava cascades over the western scarp of Olympus Mons are only 2.4 million years old  (Peter Bond, op cit).   It’s possible that Mars Express has even found active vents within the Olympus Mons caldera  (Eric F. Freeman, ‘Are There Active Volcanoes on Mars?’, Astronomy Now, November 2005).   In 2004 orbiting spacecraft found features near the south pole resembling the ‘ice pinnacles’ which build up over vents on the sides of Antarctic volcanoes, and recently formed volcanic cones have been found near the north pole  (David Powell, ‘Active Martian Volcanoes?’, Astronomy Now, November 2005.).   The orbiters and rovers Spirit, Opportunity and Curiosity, subsequently found seasonal methane in the Martian atmosphere in quantities indicating volcanic activity or possibly even life.  

One of the great surprises with Mars, however, was that the planet is not covered in dunes as Chesley Bonestell envisaged in “The Exploration of Mars”, for example  (see above).   The discovery of cratering on Mars was a major shock in the early 1960s, but it was suggested that it might be there just because Mars was close to the Asteroid Belt.   There are very small dues at the Viking 1 landing site, but the only counterpart to the huge dune fields of Earth’s deserts consists of localised patches of very dark material.   These were discovered in Phaethontis and other locations by the Mariner 9 orbiter in 1971 and by the time of the Viking missions of 1976 they had moved, in the directions of the prevailing winds – but as small discrete units, as if each patch of dark material was physically stuck together.   One very remarkable photo from Viking Orbiter 1  (P-116950, 12A52)  showed a dark dune field 30 miles long and 12 miles wide, which had been migrating up Ganges Chasma away from Valles Marineris when the wind dropped.   “The wind appears to have been blowing from the west, and the leading dunes to the east are starting to climb the canyon wall”  [my emphasis].   Could they be carpets of living matter?   By far the largest concentration of them surrounds the north polar cap, where the concentration of water vapour is highest under the polar hood in the Martian spring.   In Endurance crater, the Opportunity lander approached a small field of these dunes in 2004 and to my untutored eye, they showed what looked very like a root system!   But the rover then turned away, presumably to avoid the wheels getting stuck in soft sand – if that’s what it really is.   There’s a middle-distance photo of the whole dune field in Astronomy Now, February 2005  (David Hawksett, ‘Focus:  Mars One Year On’, p. 60-61).  Curiosity has since scaled dunes in Gale Crater, and the Perseverance rover is scheduled to examine more.

When Mariner 9 reached Mars in 1971 the planet was covered by an impenetrable dust storm, in which drifts of sand were seen blowing across the summit of Olympus Mons at 200 mph.   Much more recently it’s been revealed that the Soviet lander Mars 3 carried a TV camera and a mini-rover, and on touchdown it began to transmit a picture.   But it landed on the edge of the Hellas basin during the storm, probably the most dangerous place on Mars at the time, and went dead before sending more than a few lines of the picture, showing the line of the horizon and nothing else.

The “Man and the Planets” discussions assumed that such conditions were typical, at least when Mars was at its closest to the Sun  (there had been huge storms at the close opposition with Earth in 1956).   We gave a lot of thought to protecting surface installations, and it was pointed out that only such storms could justify the extra, external dome with retracting panels, over the lunar landing pad in 2001, A Space Odyssey.   But although there are many signs of wind erosion on Mars, in the Viking landings and the rover missions which began twenty years later weather conditions on Mars have proved surprisingly mild.   Viking Lander 1 survived a planet-wide dust storm without noticing any major changes, even in the dune field nearby.  

In 2007 Spirit and Opportunity survived a planet-wide dust storm, though threatened by low light levels which provided insufficient power for their solar panels.   But something strange happened to Opportunity during its stay in Endurance crater.   The power generated by its solar panels had gradually been diminishing due to accumulated dust, but on three occasions in October-November the panels were cleared overnight in mysterious circumstances.   Dust devils passing over the rover were suggested as a possible explanation, though at first they left no trails – but both dust-devils and trails have been photographed since.   One was photographed from orbit by Mars Global Surveyor and appeared in Astronomy Now, November 2002.   From such photographs of the trails they leave it’s estimated that if you stood beside the Spirit Mars rover at noon on a typical day, up to six dust devils would be visible at any given time.   Eventually the rover cameras captured images of them and they were shown on The Sky at Night, 3rd October 2005, and in the December 2005 issue of Astronomy Now.  They were seen again by the Phoenix polar lander, but none have turned up to clear the solar panels of the Insight lander, so an artificial wind was created to do it by dropping soil samples nearby.

Back in the 1950s there was an alternative idea that the Martian caps were carbon dioxide, because both caps were seen to shrink in spring, and the southern one sometimes seemed to vanish altogether.   It wasn’t visible at the time of the Mariner 6 and 7 flybys in 1969, though the site was occupied by a strange spiral pattern of ridges.   When Mariner 9 arrived in 1971 during a huge dust storm, the south polar cap was one of the few things visible.   It shrank as spring took hold and was announced to be carbon dioxide;  then it stopped shrinking and the main body was revealed to be, beyond doubt, water ice.   Its ‘disappearances’ had been due to temporary overlays of dust.   The picture sequence that proved the point was published in “Mars and the Mind of Man”  (Bruce Murray, ed., Harper & Row, 1973).

Mariner 9 photographed the north polar cap only briefly, late in the mission, and only limited coverage was obtained between the clearing of the north polar hood and Mariner 9’s running out of attitude control gas.   (We could have had another year of operation, but NASA had saved $30,000 on the valves, to Carl Sagan’s fury when it became an issue.)   Because the orbit of Mars is more elliptical than Earth’s, and the axis of Mars precesses like Earth’s, but with a period of 50,000 years, Sagan proposed a ‘long winter’ model of the Martian climate in which each pole would in turn become a ‘cold trap’ in which most of the atmosphere froze out – in between, as the late Mel Adam put it, “It does rain on Mars, but only once every 25,000 years…”   It was suggested that we could bring the warm season forward and make Mars habitable, by disintegrating a carbonaceous asteroid over the north cap so that it would absorb more solar heat and evaporate.

Near the Martian equator, long sinuous channels were discovered by Mariner 9, apparently formed by water action, and the Martian atmosphere is far too thin now for that.   Since there’s clear evidence of water flows on the surface in the past, and the present atmosphere is too thin for water to be liquid, the mystery of the missing atmosphere remains unresolved.   Meanwhile the idea that the southern cap was carbon dioxide re-established itself in the literature, and each time it’s disproved, at roughly two-year intervals, it’s hailed as unexpected.   It was repeatedly announced by Patrick Moore as dramatic news. 

The Viking Orbiters proved that the north polar cap consists almost entirely of water ice, like the southern one.   But the dark dunes which surround the north polar cap follow the patterns of the winds over the cap and may have moved off it fairly recently, because the lack of craters on them makes them younger than 10,000 years.   In “New Worlds for Old” I suggested a ‘forward spring’ model in which a carbonaceous asteroid really did disintegrate over the polar cap, like the one in the film Robinson Crusoe on Mars  (1964), releasing the denser atmosphere out of season, and as a result Martian life had a binge, converting all the carbon dioxide and releasing oxygen which then escaped into space because Mars isn’t massive enough to hold it.   That would leave Mars as a permanent cold desert, and life would have destroyed its environment much as we’re threatening to do to Earth’s.   The quotation is from Richard III, in regard to the precocity of the Princes in the Tower:  “Short summers often show a forward spring…”

There seem to be clear evidence that there has been water action on Mars in the past, and in 2004 the Spirit and Opportunity rovers found clear evidence of actual oceans, followed by Curiosity’s discovery of a lake bed in Gale Crater.   Liquid water might perhaps survive on the face of Mars under a crust of ice, especially if it was very salty, but it would still need to be heated by volcanic activity below.   In July 2005 ESA’s Mars Express photographed a frozen lake 35 km across and up to 2 km deep overlying a dune field, inside a crater on Vastitas Borealis, the plain which covers much of Mars in northern latitudes.  Whether water exists in liquid form below the surface, remains to be seen.  Analysing the Viking Orbiter results, in 1979, Dr. Robert Huguenin of the University of Massachusetts claimed to have discovered a 400-mile wet area in Solis Lacus, which Percival Lowell thought was the capital of Martian civilisation, and a 720-mile one in Noachis-Hellespontus  (Adrian Berry, ‘Water Found on Mars’, Daily Telegraph, 9th June 1979).  His findings didn’t win general acceptance, but more recently orbiting spacecraft have found features near the south pole resembling the ‘ice pinnacles’ which build up over vents on the sides of Antarctic volcanoes, as water vapour freezes, and Mars Express has detected possible lakes below the ice-cap.  

The Viking mission turned up many surprising and controversial findings concerning the planet Mars.  Probably not many people realise that Viking Lander 1 remained active on the surface of Mars for six years, until 1982 when we lost contact due to a computer problem, while the two Orbiters lasted nearly as long and returned huge amounts of data.  One surprise was the colour of the sky.  Chesley Bonestell had painted it in deep blue for “The Conquest of Space” and “The Exploration of Mars”, but that was when the density of the atmosphere was thought to be like Earth’s at the summit of Everest.  Mariner 4’s flyby in 1965 had shown that the pressure at the surface was only 1% of Earth’s, like ours twenty miles up, and many artists then showed the sky as black, with stars.  Viking showed that the sky was pink with floating dust, tinged with green higher above the horizon, and that blue-green tint was confirmed by Mars Pathfinder in 1998.  It’s not certain whether the stars can be seen on Mars even at night, except perhaps in one of the occasional ‘Blue Clearings’ when the atmosphere is unusually still and the dust settles out.  

But few could have anticipated the stir which would be caused by a photo in ‘Mission Status Bulletin No. 37’, published on August 4th, 1976, exactly a month after the US bicentennial when Viking 1 would have landed, were not the terrain too rough at the originally chosen landing site.   (It did land in Chryse, but at a different location from the one originally chosen.)   In Cydonia, from which Viking 2 had been diverted for the same reason, “An eroded mesa seems to be keeping a watchful eye on the Viking Orbiter nearly 1200 miles above.   The mesa is like others in the area, except that the low Sun angle has formed shadows that give the illusion of a human head – including eyes, nose and mouth.   The formation is approximately one mile across.”

Two scientists named DiPietro and Molinar were sufficiently interested to hunt for other images of the same region, and found a second one, at a different Sun angle, in which the appearance of a face was preserved.  Their paper on the subject sparked more active investigation which turned up other features in the region, possibly the ruins of an ancient city.  These ideas were gathered together by Richard C. Hoagland in a book called “The Monuments of Mars”  (North Atlantic Books, 1987), subtitled ‘A City on the Edge of Forever’ by analogy with the Star Trek episode of that title written by Harland Ellison, and including a brief mention of me.  I had quoted one of Hoagland’s earlier papers on life on Mars in “New Worlds for Old”, and we met at Brian O’Leary’s Phobos-Deimos workshop in the Case for Mars II Conference in Boulder, Colorado, in 1984.

In 1998 Mars Global Surveyor photographed the Cydonia region, and at high resolution the Face no longer appeared to be one.   (David Whitehouse, ‘Close-ups unveil the ‘face on Mars’, Astronomy Now, June 1998.)   But the interpretation of the image was challenged because apparently a spurious ‘spike’ had been added on the forehead, and has since equally mysteriously disappeared.   Even so the feature still looks natural, with its right-hand side formed by slumping rather than by artificial carving.   

Meanwhile, at the maiden launch of the Space Shuttle in 1981, my friend and colleague Chris O’Kane had come across the Mission Status Bulletin in question, and decided to investigate further.  He became UK representative on an international Mars Study Group to study the features, and set up a group of students at North Kelvinside School in Glasgow, where he worked as a technician.  They discovered the first 4-sided pyramid on Mars, like the Egyptian ones but about a mile on a side, and presented a report on it at the ‘Heresies in Archaeoastronomy’ seminar which I organised for the Edinburgh International Science Festival in 1996.  There are some very interesting links between Mars and ancient Egypt, which Chris continues to work on.

In “Mapping Mars”, describing a visit to the Smithsonian Air and Space Museum, Oliver Morton writes:

“On the floor is a Viking lander, a flight spare almost identical to the two that sit on Chryse and Utopia;  on its side is a plaque that identifies it as the Thomas Mutch Memorial Station, named in honour of the geologist in charge of the lander science programme, who died in 1980.   Displayed with the plaque is the explanation that while the lander is the property of the museum, the plaque is merely on loan;  one day NASA will take it away and mount it on the lander’s dusty twin, a few hundred kilometres south of the mouth of Kasei Vallis.  There is no date;  there is no policy;  there is no guarantee.  But the things we say on gravestones have a special value, a requirement for commitment.  You’re not meant to touch the plaque.  I did.  It meant more about Mars than a meteorite.”

You may also like:

Beginners Astronomy: Mars Part 1

Also by Duncan Lunan: The Sky Above You – July 2021

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