A celebration of the opposition of 2003
This summer (2003) Mars will be in opposition on 28 August, on the meridian at midnight, at a magnitude of -2.9, as bright as it ever appears. Since it is at perihelion on 30 August, it will be almost as close as it ever comes, so 2003 will see a very favorable opposition, and this will be an excellent time to observe Mars and to review its lore. In a small telescope, it will be seen as a definite disk about 25" of arc in diameter, larger than Saturn, and more than half the size of Jupiter. The rusty red color is the color of ferric iron, which has absorbed all the oxygen produced by erstwhile Martian life. The life is gone, and what water has not evaporated into space is frozen beneath the surface. Mars varies in brightness from +1.8 when it is near aphelion beyond the sun, to -2.9 when it is near perihelion opposite the sun, a range of 4.7 magnitudes. The light received changes by a factor of 76, equivalent to a brightness difference comparable to that between the faintest stars visible in a constellation to the brightest (the eye responds logarithmically to energy). The distances from the earth to Mars are 2.66 AU and 0.38 AU in the two cases, so the angular size varies by a factor 7, and the inverse-square law gives a factor of 49. [I do not know why this differs from 76, at the moment.] The angular size at an unfavorable conjunction is only 3.5", while at a favorable opposition, like this one, it is 24.7". Last year, Mars was a relatively inconspicuous object in the dawn or twilight skies; this summer it will be a brilliant object, brighter than Jupiter, in the midnight sky. The photo at the right is from the Hubble Telescope.
Although Galileo observed Mars, he could not even determine clearly if it exhibited the expected gibbous phases. Huygens observed a dark marking, the Syrtis Major, and from it determined the rotational period. He also noted the polar caps, and the expected gibbous phase. It is very difficult to discern surface markings from the Earth, both from the distance and the Martian atmospheric haze, but now, of course, we have photographs from the surface and know what it looks like. The surface shows seasonal changes that are now known not to be due to vegetation or lichens, as was once believed. The canal illusion was originated by Schiaparelli in 1877, and carried on extensively by Percival Lowell at his Flagstaff Observatory. There are, of course, no canals, and they were not the deep valleys now observed.
The state of our knowledge of physical conditions on the surface of Mars has grown rapidly since the Mariner 4 fly-by in 1965. Conclusions from remote observations before this time are reviewed in de Vaucouleurs (see References), and make an interesting contrast with what is now known. Then, the "canals" favored by visual telescopic observers were still in question, the white polar caps were supposed to be water ice, primitive life was a possibility, and the atmosphere was considered to be mainly nitrogen with some argon, a little water vapor and less carbon dioxide, but no oxygen, at a surface pressure of about 46 mmHg. We now realize that the canals were an illusion, the polar caps are solid carbon dioxide, or maybe carbon dioxide on some ice, and the atmosphere is mainly carbon dioxide with a little nitrogen and argon, and at a pressure of only some 8 mmHg. There is supposed to be some water frozen beneath the surface, but it has been difficult to detect, and no evidence of life has yet been found. Further information will be found below.
The orbital period of Mars is 1.88071105 years, or about 697 days, corresponding to its mean distance of 1.5236793419 astronomical units (1 AU = 1.49597870 x 1011 m). The eccentricity of the orbit is 0.0934006199474, and it is inclined to the ecliptic by 1° 50' 59.01532". The longitude of the node is 49° 33' 29.13554", and the longitude of perihelion is 336° 03' 36.84233". On 2000 January 1.5, its mean longitude was 355° 25' 59.78866". These figures can be used to find the position of Mars at any time to a fairly good accuracy. See Orbits for details. The orbital eccentricity causes the distance to the sun to vary by a factor of 0.186, or nearly 20%, which means that the distance from the earth to Mars at opposition (when Mars is exactly opposite the sun) varies by an even larger amount, from 0.38 AU to 0.67 AU. Oppositions when Mars is near perihelion (as in 2003) correspond to the shorter distance, and are called favorable or perihelic oppositions, often occurring at times separated by two years, such as the perihelic oppositions of 2001 and 2003. On the average, Mars is seen as a disc about 18" in diameter, but at this opposition it will subtend 25".
The last opposition of Mars was on 13 June 2001, 806 days before the current opposition. This interval is called the synodic period of Mars, the time interval between similar planetary phenomena (opposition, conjunction, quadrature). The calculation of synodic period is explained in the diagram at the right n1 = 2π/T1 and n2 = 2π/T2 are the mean daily motions of planets 1 and 2, which are assumed to move in circular orbits. When planet 1 has moved from 1 to 1', the slower planet 2 has only moved from 2 to 2' in the same time. Planet 1 moves ahead of planet 2 by Δn = n1 - n2 more each day. When this amounts to 2π, planets 1 and 2 are again lined up, but not usually in the same direction. Planet 1 catches up with planet 2 in a time equal to the synodic period Ts, where 2π/Ts = n1 - n2, or 1/Ts = 1/T1 - 1/T2. This is the formula for finding the synodic period in terms of the sidereal periods.
For the earth, T = 365.25636 days, and for Mars, T = 686.94167 days, so the mean synodic period for Mars is 779.985 days. This is close to, but not quite equal to, the 806 days separating the oppositions of 2001 and 2003. The discrepancy is a result of the nonuniform elliptical motion of Mars and the earth. We must perform exact orbital calculations to find the actual dates of opposition. The 2003 opposition comes 76 days later in the year than the 2001 opposition, which is also expected, since 780 days is 2.1354 years, two years plus 49 days. Again, the difference is due to nonuniform motion. At the 2001 opposition, Mars was 15° east of Antares, near θ Ophiuchi, on the boundary of Scorpius and Sagittarius. At the 2003 opposition, Mars will be about 14° north of Fomalhaut, in the southern reaches of Aquarius, after Scorpius has set in the southwest. It will be halfway between the Water Jar and Fomalhaut.
Mars will pass through its descending node on 28 February, reach perihelion on 30 August, and pass through its ascending node on 29 December. Throughout 2003, Mars will be in the half of its orbit below the ecliptic. Mars will be stationary on 30 July and on 29 September. On 30 July, its normal eastern motion will stop, and the planet will turn around and move westward, or retrograde. The retrograde motion will continue through opposition, until it ceases on 29 September and Mars again picks up it regular eastward motion. The path of Mars is sketched from June to November in the figure at the left. The action takes place in Aquarius, between 22h and 23h right ascension and -8° and -17° declination. On 2 June, Mars passes within 15' of δ Capricorni, a third-magnitude star just to the right of the area in the figure (at that time it is also halfway between Uranus and Neptune). It reaches δ Aquarii two months later, just after the easternmost stationary point. Locate these two deltas, and σ Aquarii, to mark out the theatre of action. The point of opposition is marked "O", and it is very close to the perihelion as well. Note the stationary points when the motion in right ascension is reversed. Can you picture the earth speeding by, as the slower Mars goes through perhelion below the ecliptic? A few stars of Aquarius that are visible in this region are also marked. Use them as reference points for plotting the path. This summer will be an excellent time to observe the retrograde loop that Mars will make against the stars.
Jupiter and Saturn, of the visible outer planets, also describe retrograde loops near opposition. Jupiter's opposition occurred on 2 February, and Saturn's will happen on 31 December. Since these orbits are less inclined than Mars's, and further away, the loops are flatter, as seen almost on edge. They are, however, nearly as wide, Jupiter's retrograde motion starting in December 2002 at 9h 23m, and ending in April 2003 at 8h 43m. After April, Jupiter will retrace his path very closely, proceeding steadily eastward, and passing the old stationary point in late June. In comparing the retrograde loops of Mars and Jupiter, remember that Mars is moving in the same direction as earth, but more slowly, while Jupiter is practically a fixed point.
The equatorial radius of Mars is 3 397 000 m, about half that of the earth, and its dynamic flattening f is 1/192, so it is more oblate than the earth, but still an approximate sphere. The geometric flattening was quoted as 1/77, but the discordance and the reason for it has not been mentioned lately. The geometric flattening is currently given as 1/154. The coefficient of the second-order zonal harmonic in its gravitational potential is J2 = 1.964 x 103, nearly twice that of the earth. The dynamic flattening and J2 were determined from the precession of the line of nodes and the perihelion of its satellites. Its period of rotation is 1.02595675 mean solar day, about 37 minutes longer than the earth's. Its equator is inclined 25.19° to its orbit, so Mars experiences seasons as the earth does. White polar caps form in the winter seasons and shrink in the summers, alternately at the two poles. These polar caps are solid carbon dioxide, crystallizing and subliming without intervention of a liquid. All in all, Mars is more like the earth than any other planet.
There are recent reports (Science, 14 February 2003) that Caltech workers think the poles are mainly water ice, covered with solid carbon dioxide that evaporates in the local summer, while the ice is largely unaffected. Now, they haven't actually detected any ice, but think it could be there, which is enough for firing off a press release in case it happens to be true. They do not explain why the water ice has not all evaporated, since there is certainly not enough water in the atmosphere to be in equilibrium with it. They also fear for a shortage of carbon dioxide, though without a shred of evidence of how much is in rocks, as in the earth. Speculation is the theme of today's science!
Mars has two tiny satellites, Phobos ("fear") and Deimos ("terror"), that were discovered during the favorable opposition of 1877 by Asaph Hall. They were, remarkably, mentioned by Voltaire in Micromegas, and by Jonathan Swift (1667-1754) in Gulliver's Travels (1726). Swift said one revolved in 10 hours, the other in 21-1/2 hours, and were 3 and 5 diameters away from the center of the planet. The actual figures are 0.31891023 days or 7.65 hours, and 1.2624407 days or 30.3 hours for the periods, and 9378 km (1.38 diameters) and 23,479 km (34.6 diameters), for the mean distances. Phobos is 13.5 x 10.8 x 9.4 km, and Deimos is 7.5 x 6.1 x 5.5 km, both apparently captured asteroids. When Phobos passes overhead, its largest dimension subtends 7.8' of arc, so it would be visible as a bright speck in the sunlight, moving from west to east, since its orbital period is less than the rotational period of the planet. The more conventional but smaller and more distant Deimos would be just on the limit of sharp vision, subtending about a minute of arc.
These satellites can be used to confirm Kepler's third law, and to find the mass of Mars. In a circular orbit of radius a, GM/a2 = v2/a, from which a3/T2 = GM/4π2, where G is the Newtonian gravitational constant, 6.67259 x 10-11 m3/kg-s2. Using our figures, a3/T2 is 1.0863 x 1012 for Phobos and 1.0879 x 1012 for Deimos, showing excellent agreement with Kepler's third law. Using an average value of 1.0871, we find M = 6.432 x 1023 kg. 6.419 x 1023 is the official figure, so we came out pretty well (within 0.2%). The average density of Mars is then 3.92, about what one might expect from a ball of olivine without a metallic core. The surface gravity on Mars is 0.38 that of earth, or 373 cm/s2. The visual albedo (fraction of light reflected) of the light areas of Mars is about 0.15, typical of sandy deserts. Mars has no general magnetic field originating in its interior, only a weak field caused by currents in its ionosphere.
The atmospheric pressure at the surface is reported as between 8 and 15 mmHg. In the absence of better data, I shall assume for argument that it is 8 mmHg or 11 mb. This is the pressure in the earth's atmosphere at an altitude of 16 km, where the temperature is about -56°C, not too different from Martian conditions. The surface temperature on Mars seems to vary between about 20°C as a maximum, down to a more typical -88°C, during the day. The atmosphere is 95% CO2, 2.7% N2, 1.6% Ar, 0.6% CO and 0.15% O2, with a small and variable amount of water vapor, perhaps 0.03%, the same as the percentage of carbon dioxide in the earth's atmosphere. The CO and O2 come from photochemical decomposition of CO2 by ultraviolet light. There is no ozone in the Martian atmosphere, since there is no oxygen to make it from, and so ultraviolet penetrates to the surface. The small amount of nitrogen probably reflects loss to space, since the escape velocity is less on Mars than on the earth. The molecular weight of the Martian atmosphere is 43.41 compared to 28.96 for earth, so the density at the surface is about 0.25 kg/m3, equivalent to an altitude of about 13 km in the earth's atmosphere, in the vicinity of the tropopause. An atmosphere of this density will have fierce winds, and is dense enough to support the observed giant clouds of dust that sometimes obscure a large part of the visible surface.
Mars has long been known to have three kinds of clouds: yellow, blue and white. The yellow clouds are low dust clouds, stirred up by the strong winds. Blue clouds are high, and may be crystals of carbon dioxide, analogous to the cirrus clouds of earth. They are probably responsible for the "violet layer" that was reported by visual observers. Visual observers have also reported white clouds, but their nature is not known, and they may simply be blue clouds with larger particles. The blue is, of course, due to Rayleigh scattering, and the particles have been estimated to be about 100nm in diameter. Because of Mars's lower gravity, its upper atmosphere is denser that the earth's above a certain level. The triple point of CO2 is at -56.062°C and 3.7809 mmHg. At -88°C, the vapor pressure of solid carbon dioxide is only 0.3357. It is easy for the gas to sublime, and the white polar caps are of solid carbon dioxide. No liquid water has yet been observed on the surface, but water is diligently sought and is supposed to be here and there under the surface.
Mariner 4 flew by Mars in 1965, Mariners 6 and 7 in 1969, and Mariner 9, the first to orbit the planet, did so in 1971. The Viking landers touched down in 1976. Twenty years later, Mars Pathfinder and its little Sojourner crawler reached the surface on 4 July 1997. Global Surveyor was launched for Mars in 1997, and is still returning good pictures. The 1993 Mars Observer mission was unsuccessful, as was the 1996 Russian Mars expedition. The 1999 Mars Climate Orbiter and Mars Polar Lander missions also ended in failure (the first over the difference between feet and metres). Currently, the Mars Odyssey orbiter is examining the planet. Two 300-pound, 6-wheeled surface rovers will be launched in May and June 2003, to land in January 2004 near the equator. They will be able to wander about 1 km away from the landing site, at about 100 m per day under solar power, and scrabble about taking pictures and making spectroscopic analyses for several months. At about the same time, the European Space Agency Mars Express mission will be launched, with its 10-kg lander Beagle 2 to probe for life signs. The JPL-NASA and ESA websites mentioned in the References are probably good places to keep up to date on these things. It is wonderful that these probes have existed, but the amount of real scientific information returned seems trifling for all the effort, which was not all that much effort, and what there was, seems misdirected. NASA has been much more concerned with its political, military and entertainment objectives, such as the space station and Space Shuttle, whose scientific returns are almost negligible. Venus and Mercury probes are not even under consideration by NASA. The USSR made some attempts to study Mars, but recent politics and poverty have called an end to that. A permanent "weather station" is needed on the Martian surface, and thorough seismological investigation, not prospecting for water and relics of life. The image at the left shows the caldera of the huge volcano Olympus Mons, 22 km high. The depth of the caldera is as large as 3 km. The image is from the ESA Mars Express, by way of BBC.
During the favorable opposition of 1877, when Phobos and Deimos were discovered, G. V. Schiaparelli thought he saw a network of lines on the Martian surface, which he called canali. Later enthusiasts, among them Percival Lowell, thought they were actual canals, and imagined them carrying irrigation water from the melting ice caps, to cause a general seasonal greening. In 1938, a Martian invasion caused panic, but it was just Orson Welles's radio dramatization of H. G. Wells's War of the Worlds. All this speculation evaporated when the fog of ignorance finally lifted, and now there are different fables to be feared and fought with duct tape [alleging to the homeland security panic], which would be of no use against Martians.
The information received from space probes beginning in 1965 and continuing fitfully since then have clarified many things about the planet Mars. They have also given rise to many speculations, not all of which should simply be accepted as fact, because they are very probably not. It is true that no typical plate-tectonic structures have been noticed, so Mars probably does not have mantle convection. Olivine has been detected spectroscopically in the walls of the Ganges Chasm. Mars probably is heated by radioactivity, and the scattered large volcanoes show that hot active fluids can be created in the interior. A large part of the planet is, indeed, covered by what are probably basaltic flows. No evidence of acidic volcanism exists, and it is not known how far the planet has been sorted by density. Much more knowledge is necessary before these things can be discussed with any confidence. Our information now must come from probes, not from favorable oppositions.
The three surface probes of winter 2003-2004 have now landed (as of January 2004). The ESA Beagle II was uncommunicative and presumed lost, but the orbiter is working well. Spirit has had computer problems indicating less than expert programming, and Opportunity has sent back little information so far. There are some nice pictures, but very little additional scientific information and no surprises.
The meteorite ALH-84001, found in Antarctica, is supposed to have come from Mars, and contains microscopic magnetite crystals that have been claimed to be biological products. This may be true, but is very hard to believe on such tenuous evidence. It would be better to find a rock on Mars, and even better to confirm the existence of any other kind of life there. I'd like to see a stromatolite or two. This is an ingenious speculation, but it is only a learned, wild speculation of the sort that has become so common lately in the struggle for social recognition. It is a question why magnetite has a biological role, if the role is a magnetic one, and why magnetite would be favored on a planet without a magnetic field. I think Mars had life, but this is only another wild speculation that I hope may be confirmed before too long. I doubt that it had a magnetic field, but there are even speculations that it did, and it was later lost. This, too, would be very interesting. The demise of the canals has not reduced the mysteries of Mars.
The Martian surface is very unfriendly to life, with its lack of water and oxygen, abundance of radiation, and cold temperatures. The radiation dose appears to be about 1.2 millisieverts/day, about three times that on the International Space Station. There is little hope that Mars could ever be even the temporary home of fragile humans, and even less reason why it should be. Humans cannot survive long in the weightlessness of space, and their support even for short intervals is extraordinarily expensive. Every increase in the gigantic human family, with its drain on resources and ever more mouths and growing poverty, makes any venture off the home planet less and less likely. Any manned flight to Mars would, at the present time, be a one-way journey.
P. K. Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valley, CA: University Science Books, 1992).
The Astronomical Almanac for the Year 2003 (Washington, DC: U.S. Government Printing Office, 2001).
I. Ridpath, ed., Norton's 2000.0 Star Atlas and Reference Handbook, 18th ed. (New York: John Wiley & Sons, 1989). Map 3.
M. A. Seeds, Foundations of Astronomy (Belmont, CA: Wadsworth Publishing Co., 1990). pp. 464-475.
The JPL Mars website, with many pictures: Mars Odyssey. There are pictures, but no scientific information, on this site. There is a Fun Zone, however.
Scientific American, 288(4), April 2003, p. 76. Notes on meteorite ALH-84001.
Discover, 24(5), May 2003, pp. 34-43. The ESA Mars Express mission.
The ESA gives us the ESA Mars Express.
G. de Vaucouleurs, Physics of the Planet Mars (London: Faber and Faber, 1954). An interesting look at the state of knowledge of Mars before the planetary probes.
Composed by J. B. Calvert
Created 16 March 2003
Last revised 14 February 2004