The Martian Year and Seasons
by Martin J Powell
Diagram showing the seasons on Mars and the Earth, together with opposition dates between 2012 and 2027 (click on thumbnail for full-size image, 246 KB). The outer ring of each orbit indicates the planets' Northern hemisphere seasons whilst the inner ring shows the Southern hemisphere seasons. The view of the Solar System is from above looking downwards, as if one was positioned approximately above the Sun's Northern pole. The axial tilt of the Earth (North Pole) is towards the left of the diagram (the Arctic region is shown bright white when in sunlight), whilst the axial tilt of Mars (Northern Polar cap) is towards the top of the diagram (the polar cap appears as a small white dot, illuminated when in sunlight).
The diagram allows one to determine the seasons experienced by both planets at the moment of all eight oppositions which take place between 2012 and 2027. For example, at the 2014 opposition (April 8th), Mars' Northern hemisphere is experiencing Summer (outer ring) and its Southern hemisphere is experiencing Winter (inner ring). On Earth, of course, it is early Spring in the Northern hemisphere and early Autumn in the Southern hemisphere. Viewed from the Earth, Mars will appear to be in the constellation of Virgo and the planet's Northern polar cap will be tipped in our direction.
The diagram shows that, during perihelic oppositions of Mars (when it is positioned in the vicinity of point 'P'), its Southern polar cap is tipped in our direction whilst at aphelic oppositions (when it is in the vicinity of point 'A') the Northern polar cap is tipped in our direction. The appearance of Mars, as viewed from the Earth at each opposition, is shown in the animation below. Opposition distances and other details for the period shown in the diagram can be found on the Mars Oppositions page.
The Monthly division of the year into equal 30° sectors forms part of the calendar scheme proposed by R. Todd Clancy and is described in the text below. An alternative calendar, proposed by Charles Capen and Donald Parker, has the Martian year begin with January 1st, this being positioned a few degrees counter-clockwise from the First Point of Aries.
The Martian year comprises 668.59 Martian days - or sols (equivalent to 687 Earth days or 1.88 Tropical Earth years). Each sol is 24.623 hours (24 hours and 37 minutes) in length, i.e. 37 minutes longer than a day on Earth. In one Earth day, therefore, Mars will rotate through about 350°.3 of longitude, or 9°.7 short of a full rotation. Consequently, an observer on Earth, viewing the planet through a telescope at the same local time on the following day, will see the Martian surface features shifted 9°.7 further West on the planet. An observer briefly viewing Mars at the same local time each night would only get to see a 'full rotation' of the planet after about 5½ weeks.
The Martian Seasons
The rotational axes of Mars and the Earth are tilted at very similar angles with respect to the planes of their orbits. This angle - known as the axial tilt - is 25°.19 for Mars and 23°.44 for the Earth. Hence both planets are similar in terms of their axial tilt and their rotation periods.
However, the axes of the two planets do not point in the same direction in space; there is a difference of about 95° in heliocentric longitude (symbol ) between the two (a little over one-quarter of an orbit). The result is that the Martian seasons and the Earth's seasons are out-of-step by one season. In effect, when the Earth and Mars are in the same quadrant of their orbits (e.g. at opposition), Mars is about one season ahead of that on the Earth. At the Earth's Spring Equinox, the Sun appears to be positioned in the constellation of Pisces (a little to the South-east of the Circlet of Pisces asterism) however at the Spring Equinox on Mars, the Sun appears to be positioned in South-eastern Ophiuchus (close to the border with Sagittarius).
It follows that the Northern rotational axis of Mars does not point towards Polaris ( UMi or Alpha Ursae Minoris, mag. +2.1) as does the Earth's Northern polar axis. On Mars the nearest bright star to the celestial pole is Deneb ( Cyg or Alpha Cygni, mag. +1.3) in the constellation of Cygnus, the Swan.
Areocentric Longitude of the Sun (Ls)
The areocentric longitude (symbol Ls, pronounced 'el-sub-ess') is the longitude of the Sun as viewed from the centre of Mars. It is the Martian equivalent of ecliptic longitude, which is the longitude of the Sun measured from the centre of the Earth. The word areocentric (literally 'Mars-centred') is derived from the name Ares, the ancient Greek God of War. Areocentric longitude is measured Eastwards (from 0° to 360°) from the point in the Martian sky where the Sun's apparent path crosses the celestial equator heading Northwards (i.e. the Martian Spring Equinox point); this point is defined as Ls = 0°.
Diagram showing the areocentric longitudes at the Martian Equinoxes, Solstices and Oppositions between 2012 and 2027 (click for full-size image, 123 KB)
Just like ecliptic longitudet as measured on Earth, the start of each subsequent season (in this case stated for the Northern hemisphere) is then Ls = 90° (at the Summer Solstice), Ls = 180° (Autumnal/Fall Equinox) and Ls = 270° (Winter Solstice). The seasons are of course reversed for the Southern hemisphere. The Martian seasons thus defined span the range of longitudes shown in the table below.
Table showing the range of areocentric longitudes applicable to each Martian season in both Northern and Southern hemispheres (click for full-size table, 11 KB).
Areocentric longitudes are often quoted in Mars apparition reports thus: '90° Ls' although they can be expressed simply as 'Ls = 90°' (as is the case in the current article).
Many older Martian apparition accounts used the heliocentric longitude (symbol ) of Mars to define the planet's position at any given time and, by deduction, its season. The heliocentric (Sun-centred) longitude of a planet is the angle, measured counter-clockwise (from 0° to 360°), from the First Point of Aries (Earth's Vernal Equinox direction in space, i.e. where = 0°). Heliocentric and areocentric longitudes are related by the simple formulae:
Heliocentric longitude () = Ls + 85°
Areocentric longitude (Ls) = - 85°
Since the 1990s the heliocentric longitude method has largely fallen out of favour and today, the majority of Mars apparition reports (as produced by the ALPO and the BAA, for example) use the areocentric longitude as the preferred method of defining the Martian seasons. Note that the heliocentric longitude is also used as the basis of the so-called Martian Date, which will now be discussed.
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Given the similarity between the orbits, axial tilts and rotational periods of Mars and the Earth, attempts to devise a Martian calendar which mirrors that of the Earth will come as no surprise. In fact, there is no internationally-established scheme defining a 'Martian calendar', although several proposals have been made over the years. Two of these will now be described.
The first scheme, proposed in 1980 by Charles F. Capen and Donald C. Parker of the ALPO, attempts to convert Mars' orbital position into an equivalent Earth date. The Martian seasons can then be compared directly with those on Earth. It uses the heliocentric longitude (symbol ) as its basis, such that the Martian Northern hemisphere Spring Equinox (when = 85°) is defined as taking place on March 20.8. The Northern Summer Solstice ( = 175°) is then defined as taking place on June 21.6, the Autumnal Equinox ( = 265°) on September 23rd and the Winter Solstice ( = 355°) on December 22nd. The Martian year, just like the Earth, is divided into 12 months and 365.25 days. The formulae used to calculate the Martian Date (MD) are shown in the table below.
Table showing how the Martian Date is calculated within each quadrant of heliocentric longitude, according to the Martian calendar proposed by Capen and Parker (click for full-size table, 10 KB). Data reproduced after Dijon, Dragesco and Néel, 1994.
Under this scheme, Martian dates are quoted just like an Earth date. Hence when Mars is positioned at = 187° the Martian Date will be June 34.05, i.e. July 4th.
The second scheme, proposed in 2000 by space scientist Dr R. Todd Clancy, is illustrated at the top of this article. It too divides the Martian year into twelve months, however in this case each month spans precisely 30° in angular width (as measured from the Sun). Because of the eccentricity of the planet's orbit, each month does not contain the same number of days (sols). When the planet is moving near perihelion (its closest point to the Sun) it is moving faster than when at aphelion (its most distant point from the Sun). Consequently the months around perihelion passage are shorter in length than those around aphelion passage. In fact, under this scheme the longest Martian month is 66.7 sols and the shortest is 46.1 sols - a difference of over 40% in length.
Martian Months are numbered from Month 1 through 12, with Month 1 commencing on the date of the planet's Northern Hemisphere Spring Equinox. Month 1 begins with Sol Number 1, the sols counting up through the year to Sol Number 669. However, because the Martian year spans 668.6 sols, this final day of the year (No. 669) lasts only 0.6 days(!)
The Martian Months, shown with their areocentric longitudes and durations, according to the Martian calendar proposed by R. Todd Clancy (click for full-size table, 33 KB). Data from the Mars Climate Database.
The Martian Year or Mars Year (MY) is numbered according to the number of Martian orbits that have taken place since April 11th, 1955 (a date on which Mars crossed the Spring Equinox, Ls = 0°). By definition, this is Martian Year 1 (MY 1). Hence the 1971 opposition of Mars took place eight orbits later in MY 9 and the 2003 opposition of Mars took place in MY 26. The concept of the Martian Year has been adopted by Mars scientists and The Planetary Society, among others.
Under this scheme, any given date on Mars can be defined by the Martian Year, Month and Sol Number. The relevant values for the Martian oppositions between 2012 and 2027 are shown in the animation below (an interface allowing one to convert Earth dates into Martian dates can be found at the Mars Climate Database).
Given the poor reception of any specific Martian calendar across the scientific world, most scientists and amateur astronomical societies, when describing the position of Mars at any given season, simply quote the areocentric longitude of the Sun (as described above), making no specific reference to a 'Martian date'. An internationally-recognised Martian calendar will most likely have to wait until mankind has established a permanent base on the Red Planet.
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The Appearance of Mars from the Earth
at Opposition from 2012 to 2027
The animation shows Mars as it appears from the Earth at each of the eight oppositions between 2012 and 2027 (click on the thumbnail for the full-size animation, 113 KB). It shows the changing apparent size and aspect of the planet from one opposition to the next, depending upon its distance from Earth and the relative position of the Martian polar axis in space. Note how the apparent size of the Martian disk increases from the 2012 opposition through to the 2018 opposition, after which it appears to shrink once more. By the 2027 opposition, it appears as small as it did in 2012.
The animation shows Celestial North up and East to the left. It was produced using image captures from the software 'Mars Previewer v 2.01' by Leandro Rios, available as a free download from the Sky & Telescope website.
Several physical parameters are listed beneath the image, which are as follows:
Constellation The Constellation in which the planet appears on the day of opposition, given by its three-letter abbreviation (listed here).
Dec The Declination of the planet (symbol ), i.e. the planet's position measured vertically on the celestial sphere to the North (+) or South (-) of the Earth's celestial equator. Mars normally lies within about ±3° of the ecliptic. When positioned exactly on the celestial equator, the declination of the planet is 0° ( = 0°).
The Heliocentric longitude of Mars - this is explained in more detail above.
CML The Longitude of the Central Meridian, an imaginary line passing through the poles of the planet, bisecting it into two halves. It defines the areographic longitude on the disk at a specific time (UT) during an observing run. The CML increases at a rate of 0.24° per minute (14.6° per hour) and is listed in annual publications such as 'The Astronomical Almanac' and the 'British Astronomical Association Handbook'. A diagram illustrating the concept of the CML, together with an animation showing a full rotation of Mars, can be seen on the Mars Through the Telescope page.
DE The Sub-Earth Point or Tilt, i.e. the tilt of the Martian polar axis towards or away from the Earth. Technically, it is the declination of the Earth (De) as seen from Mars. The value is positive (+) when the North pole is tilted towards Earth and negative (-) when the South pole is tilted towards Earth. The Martian Pole which is tipped towards Earth is indicated in the diagram by a short orange line.
Ls The Areocentric longitude of the Sun - explained in more detail above.
PA Axis The Position Angle of the Northern pole of the planet, measured Eastwards (counter-clockwise) from North (0°) through East (90°).
App. Diameter The Apparent Diameter of the planet in arcseconds, where 1 arcsecond (") = 1/3600th of a degree.
App. Magnitude The Apparent Visual Magnitude (brightness) of the planet, explained in more detail here.
Martian Yr / Mo The Martian Year (MY) and Month, according to the calendar proposed by R. Todd Clancy (2000) explained above.
Sub-divisions of the Martian Day
Gaining more acceptance amongst the scientific community has been the division of the Martian day into 24 'hours', i.e. 1/24th of a sol - each hour therefore being 2.7% longer than its equivalent on Earth (this system was used by NASA scientists during the Mars Pathfinder, Exploration Rover and Phoenix missions). Minutes and seconds are not expressly defined; the hour is given followed by the fraction of the hour thereafter. Hence 12:30 pm at a given location on Mars would be 12.5 hours on that particular sol.
The local time on Mars is related to the position of the Sun in the sky and therefore depends upon the areographic longitude of the observing site. It is thus given as the Local True Solar Time.
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Seasonal Surface Feature Changes
Perhaps the most interesting aspect of observing Mars telescopically is the changing appearance of its surface features with the Martian seasons; these are sometimes referred to as albedo changes. Successive oppositions take place in different regions of the Martian orbit, so to obtain a full assessment of Mars' changing features one should ideally observe the planet over the course of seven or eight successive oppositions, a period of around 15-17 years.
There now follows a brief overview of the typical feature changes which are known to be linked with the Martian seasons, based upon information gleaned from regular observers at the ALPO Mars Section (in the USA), the BAA Mars Section (in the UK) and the Société Astronomique de France (SAF). Some of these feature changes, originally only observed telescopically, were later verified or corrected by the findings of NASA's Viking lander missions (1976-82) and later, by Hubble Space Telescope images.
The darker regions of Mars known as maria ('seas') typically appear blue-grey through telescopes. During the Winter months in each hemisphere they appear pale, but during early Spring these regions appear to darken. Early observers thought that this effect was caused by the spread of vegetation as each hemisphere warmed up and water spread towards the equator from the melting polar caps. Since the Space Age, however, it is known that this darkening is a contrast effect caused by the brightening of adjacent desert regions as new layers of dust cover the ground during the early Spring. Regions of Mars which have shown significant seasonal changes in this respect are Syrtis Major (positioned at Martian areographic co-ordinates 300° W, 10° N), Pandorae Fretum (350° W, 25° S) and Hellespontus (340° W, 50° S).
Syrtis Major is the most prominent dark feature on Mars, projecting from the equatorial region into the Northern hemisphere. It often appears at its maximum width during the Northern hemisphere's midsummer, soon after aphelion (when Ls = ~145°) and its minimum width occurs in the Northern hemisphere early Winter, soon after perihelion (Ls = 290°). Curiously, variations to Syrtis Major of this nature have not been observed since 1990.
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Mars' famous dust storms are the single cause of the planet's long-term changes in dark markings, and a large number of them have been discovered by amateur astronomers. Before their true nature was fully understood, they were often referred to as yellow clouds or yellow veils. In fact, storms often do not appear yellow at first but white (owing to the increased presence of water vapour or ice crystals) only turning yellow at a later stage. The ALPO actively discourages the use of the term yellow clouds because the term is not an accurate description of what is being seen. When using coloured filters to view these clouds through telescopes, they actually appear brighter in red and orange light than they do in yellow light.
Dust storms can occur during any season, however they mostly take place soon after the Southern hemisphere's Summer Solstice (Ls = 270°) which itself takes place shortly after the planet's perihelion (Ls = 251°). The Southern hemisphere is then heated more than at any other time of the year, receiving about 40% more sunlight than at its aphelion position. The result is greater atmospheric convection currents and the subsequent formation of more powerful dust storms. Dust on Mars moves not only by near-surface winds (which vary in speed from 25-90 km/hr) but also by dust devils and geological processes called saltation and creep.
Dust storms most often start in the desert regions near Serpentis-Noachis (0°, 45° S), Solis Lacus (80° W, 20° S), Chryse (25° W, 10° N) or Hellas (292° W, 50° S). Analysis by the ALPO indicates that the majority of dust storms occur between Ls = 241° and Ls = 270°, with a peak at Ls = 255°, i.e. during the Southern hemisphere's midsummer period. There is a secondary dust storm 'peak' in the early Northern hemisphere Summer (Ls = ~105°). Long-term observations by amateurs suggest that, with the movement and subsequent deposition of dust around the planet, some areas of dust storm activity become dormant over time, whilst new regions of dust storm activity emerge.
Sketch by BAA Mars Section Director Richard J. McKim showing one of the primary emergence sources of the planet-encircling dust storm of 2001 (click for full-size image, 372 KB) (Source: BAA Journal, 2009)
Global (or planet-encircling) dust storms on Mars are rare - only eleven having occurred since 1873, nine of them since 1956 (see table below). They likewise originate in the Southern hemisphere (e.g. in the Hellas, Noachis or Solis Lacus regions) in the Spring and Summer and can encircle the planet within days. They typically occur within about 60° (in longitude) of the planet's perihelion. The ALPO concludes that the most likely areocentric longitude at which a global dust storm originates is around Ls = 315°. One of the most extensive global dust storms in history took place during the 1971 apparition, at which time NASA's American space probe Mariner 9 was scheduled to map the planet; its mission was subsequently postponed by a month until the storm had subsided. Two global dust storms in 1977 were observed by NASA's Viking landers, whilst the storm of 2007 threatened to cut short operation of its Martian rovers Spirit and Opportunity. Global dust storms cause the Red Planet's surface features to become almost completely obscured, the planet taking on a yellowish hue which can be detected even with the naked-eye.
Table showing the planet-encircling Martian Dust Storms which have been observed since 1909 (click for full-size table, 20 KB). Data from McKim (2008 and 2009).
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The planet's most famous features are the Northern and Southern polar caps, which can be glimpsed through even modest-sized telescopes. The Northern Polar Cap (NPC) is comprised of water ice with a thin layer of frozen carbon dioxide whilst the Southern Polar Cap (SPC) comprises only dry ice. Much like the polar caps on Earth, the Martian polar caps are greatly affected by the tilt of the planet's axis in space, so that for much of the Martian year they are either bathed in sunlight or hidden in shadow. As such, they are seen to shrink and expand in a predictable cycle, the NPC shrinking as the SPC expands, and vice versa. As one would expect, expansion takes place as the local temperature drops (during local Autumn) and shrinkage (recession) takes place as the region warms up (during local Spring).
French amateur astronomer Rene Jarry-Desloges carefully sketched the recession of the Martian Southern Polar Cap in 1909 (click for full-size image, 32 KB) Source: 2009 Paris/Meudon IWCMO Conference.
During the late Summer and Autumn in each hemisphere, a dull-grey haze forms over the polar region (known as a polar hood) which persists throughout the local Winter. The polar cap expands beneath the misty veil during this time. As Spring arrives and the hood dissipates, the ice cap begins to emerge from the gloom, appearing brilliant white and rapidly shrinking as Spring gives way to Summer. Because the planet's perihelion takes place during the Southern hemisphere's Summertime, the SPC shrinks to less than half the size of that of the NPC during its local Summer, though in neither hemisphere does the ice cap disappear altogether.
Shrinkage of the NPC takes place between = 145° (Ls = 60°) and = 175° (Ls = 90°), continuing through the Northern hemisphere Summer to a maximum recession at around = 250° (Ls = 165°). The NPC breaks into separate portions as it melts, the gaps between them being called rimae. The SPC melts at a faster rate than the NPC (owing to the proximity of perihelion), taking place between = 310° (Ls = 225°) and = 360° (Ls = 275°).
Polar ice sublimation - the process by which solid ice converts to water vapour without passing through the liquid stage - contributes significantly to the formation of clouds and hazes during each of the Martian hemisphere's Spring and Summer periods. They occur more frequently during the Northern hemisphere's Spring and Summer periods than at the same seasons in the Southern hemisphere. White clouds often form over the giant volcano Olympus Mons (133° W, 18° N) and along the adjacent Tharsis Ridge (100° W, 5° N) during the Northern hemisphere's Spring and Summer months. Discrete clouds are also seen over the Libya (265° W, 0°), Chryse and Hellas basins during the same period. The Syrtis Blue Cloud, first observed in the mid-19th century, establishes itself over the Libya basin and across Syrtis Major during the Northern hemisphere's late Spring/early Summer period, turning the colour of this already-dark feature to an intense blue. Morning and evening clouds and frosts appear close to the planet's limbs (edges) and usually dissipate by mid-morning, depending upon the season. These are often referred to as limb clouds and their formation and size appear to be related to the shrinkage of the NPC. They are prominent after aphelion (Ls = 70°) and begin to decrease rapidly in number once local Summertime has commenced. Limb hazes, on the other hand, increase in frequency during the Northern hemisphere Summer months.
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Copyright © Martin J Powell November 2013
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