http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== http://www.es.ucl.ac.uk/research/planetary/undergraduate/dom/magrev/marsmag.htm ** *10. Magnetic Fields at /Mars./* Back to the Table of Contents. For non-frames browsers. Given the number of space-craft that have either flown past Mars or orbited the planet (five carrying magnetic field experiments) it seems perverse that we should know so little about the Martian magnetic environment. Although Mars is much smaller than the Earth (1R_M = 3394km) its core is believed to be roughly the same size as the core of Mercury. Given that it rotates in a little over one Earth day it was expected, prior to space-craft visitations, that Mars should have an intrinsic field midway in moment between the Earth's dipole moment and Mercury's dipole moment. In fact, applying the somewhat unconvincing Magnetic Bode's Law (Blackett's Hypothesis) to scale the Martian magnetic field yields a moment of ~10^-2 that of the Earth. We have two sources of information, one concerning the modern magnetic field, and another relating to palaeofield intensities. Information on the present day field is available from data returned by number of American and Russian space-craft, most recently the Phobos 2 orbiter in 1989. The first indications that Mars might have an intrinsic field came from the solid detection of a bow-shock signature by the Mariner 4 flyby probe shortly after 1:30am (GMT) on July 15^th 1965. The shock was seen as a four-fold increase in field intensity and corresponding jumps in the temperature and density of the surrounding plasma. A conic section fit to this observation yielded a sub-solar shock position ~1.5R_M in front of the planet and ~2.4R_M either side of the dawn/dusk terminators. The most recent observations confirm the sub-solar location but place the position of the shock over the terminators at ~2.7R_M : This difference may well be due to differences in solar wind pressure. Further observations of the position of the bow-shock were recorded by the (then) soviet space-craft Mars 2,3, and 5 in 1971 and 1974. So called Gas-Dynamic models were applied to the position of the bow-shock in order to determine the size of the obstacle creating the shock. For the observed solar wind density and velocity outside the bow-shock the obstacle was calculated to be ~400km above the surface, and was thus assumed to be the martian ionosphere. However, radio occultation experiments by Mariner 9 and the two Viking orbiters demonstrated that the ionosphere had only a negligible density at 400km altitude. Based on this result it was thought that an intrinsic magnetic field was creating the obstacle which the detected ionosphere, on its own, could not apparently do. The dipole moment sufficient to balance the average solar wind dynamic pressure at 400km altitude was calculated to be ~1.4x10^12 Tm^3 (compare with the ~8x10^15 Tm^3 for the terrestrial dipole). This calculation is made surprisingly easily. Scaling the solar wind pressure to 1.52 AU using the relation; P = n_E mv_¥ ^2 /2R^2 Where n_E is the solar wind particle number density (~10cm^-3 ) m is the mass of a proton (1.66x10^-27 kg) v_¥ is the solar wind velocity (400,000m/s) R is the distance of Mars from the Sun (1.52 AU) and so P = *5.75x10^-9 dyne cm^-2 *. In order to balance this solar wind dynamic pressure at an altitude of 400km (a radial distance of 1+^400 /_3394 = 1.118R_M ) requires a magnetic pressure of; (M^2 /8)(1/1.118)^6 where M is the martian dipole moment. Equating this expression to the value of the solar wind pressure and rearranging to find M gives; M = Ö 8 x [5.75x10^-9 /(1/1.118)^6 ] = 2.997x10^-4 This answer, it should be noted, is in Gauss R_M ^3 . Multiplying by 3394^3 and by 10^-5 yields an answer in Tesla m^3 : 1.1x10^12 Tm^3 , very close to the published value quoted above. Further magnetometer data from Mars 2 (which entered Mars orbit on November 27^th 1971) and Mars 3 (December 2^nd 1971) were interpreted as into an intrinsic magnetosphere. Simple data inversion of the field intensities inside this magnetosphere yielded a moment of 2.4x10^12 Tm^3 . Subsequent study of this 'magnetopause' has led to the suggestion that, rather than being the boundary of an intrinsic magnetosphere, the boundary might instead be a 'foreshock' where interplanetary magnetic field lines pile up in front of the planet's conductive ionosphere. Models based on Mars 5 data, assuming that encounters downwind of Mars were with an induced magnetotail (as opposed to an intrinsic magnetotail), gave moments of as little as 10^11 Tm^3 . Encounters with the induced magnetocavity of Venus at around the same time were lending a good deal of weight to the supposition that the solar wind shock around Mars was the result of induced fields in the ionosphere. As at Venus, ions such as O^+ are produced by photoionization of that part of the neutral exosphere that extends /above/ the obstacle boundary. Impact ionization of this neutral population, and charge separation between solar wind protons and neutral atoms, also add to the ion supply in the martian magnetosheath. The addition of this plasma to the magnetosheath plasma flow is significant in the venusian magnetocavity and is also believed to be responsible for the shape of the martian solar wind obstacle. Plasma analyzers on the Mars 2 and 3 probes detected a region of slow moving plasma and increased magnetic field strength at roughly the location that an obstacle would need to be to stand off the bow-shock to its observed position. Mars 5 also reported the detection of heavy ions (possibly O^+ ) in a tail boundary layer extending approximately 20,000km downwind of Mars. Phobos 2 has since made a definite detection of O+ ions in the inner magnetosheath. All of these observations are consistent with an ion-laden mantle, threaded by bunched field lines, as the obstacle (now referred to as the Planetopause) which creates the bow-shock, and _not_ an intrinsic magnetic field. At most, therefore, Mars can have an intrinsic field with a moment no greater than ~1.5x10^12 Tm^3 , and is currently considered to be much closer to 10^11 Tm^3 - over 100,000 times weaker than the terrestrial dipole moment. But has the martian magnetic field always been this weak? Though none of the vehicles to have safely reached the martian surface have carried magnetometers, we are still able to study the palaeomagnetic field in martian rocks thanks to the very small supply of meteorites that have originated on Mars. These meteorites are igneous rocks (with ages of ~1.3Ga) which acquired a natural thermal remanent magnetization as they cooled below their blocking temperature in an ambient martian palaeofield. They /could/ also have become magnetized by the shock which liberated them from the martian surface. The former remanent magnetization is known as 'hard' magnetization and is not easily erased. Shock magnetization, on the other hand, is 'soft' and quite easily removed to reveal the harder underlying thermoremanent field. This 'hard' component has been detected in some of the Shergottite and Nakhla meteorites: The original Shergotty meteorites itself is estimated to have cooled in a palaeofield with an intensity of 250-1000nT. Other Shergottites, such as EETA79001, yield palaeofield estimates as high as 10,000nT. Though these remanent fields could have been induced by a pre-existing remanent field in the martian crust (acquired during an active dynamo period 3-4 billion years earlier) it is considered mored plausible that the martian meteorites picked up their remanent magnetization at the time they were erupted from an active dynamo with a moment of ~10^13 Tm^3 (around 1/1000^th the strength of the terrestrial field). The resulting surface field strengths of 250-1000nT are more in keeping with the present day Hermean magnetic field. The more rapid cooling (relative to larger bodies such as the Earth and Venus), and subsequent solidification of the martian core appears to explain the presence of an intrinsic field at some time in the distant past and the apparent lack of an intrinsic field today. A more detailed investigation of the remanent magnetization of surface rocks in situ, and the deployment of a network of sensitive seismic stations across the martian surface is one of the next steps forward in elucidating the geophysical history of Mars. That much of the emphasis of the current martian exploration programs is on the detection of life (past or present) has led to a degree of neglect of some of the more 'hardcore' geological investigations necessary to make comments about Mars' long term magnetic field and possible areodynamo. Mars' magnetosphere and tail, following studies by the Phobos 2 space-craft in 1989, and comparison with the Cytherean magnetocavity, appears to be characterised by variations that are controlled by solar wind activity and the orientation of interplanetary magnetic field lines. This is typical of an induced magnetotail formed from draped interplanetary field lines. Planetary ions are observed sweeping downwind from Mars as a cold plasma sheath around the tail. These ions are accelarated along field lines over the day-side exosphere, their large gyroradii (comparable to the diameter of Mars) allowing them to escape permanently. This erosion of the upper atmosphere - also referred to as scavenging - is believed to be responsible (partly) for a significant degree of atmospheric evolution over time. Estimated of oxygen ion loss rates from Phobos 2 data are of the order of 100g/s. In addition, direct sputtering of the base of the exosphere (altitude ~200km) by the solar wind could also be the cause of the loss of other neutral atmospheric species over time, such as nitrogen which is observed to be strongly fractionated with respect to its heavier isotope. In conclusion, Mars does carve out a cavity in the solar wind as result of bunched field lines over the ionosphere forming a magnetopause-like obstacle. The cavity downwind of the planet is free of solar wind electrons and protons, but exhibits variations in polarity with the orientation of interplanetary field lines that are not seen in the magnetospheres of planets with their own intrinsic magnetic fields. Back / Next E-mail the author. © A.D. Fortes. 1997.