http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== * 6. The Intrinsic Magnetic Field of /Mercury/. * Back to the Table of Contents. For non-frames browsers. Only one flyby probe has ever visited Mercury, Mariner 10, whose last pass of the diminutive planet occurred over 22 years ago. Mariner 10's orbit allowed three close flyby between March 29^th 1974 and March 16^th 1975 of these, only two (the first and third) were suitable for probing the magnetic moments of the Hermean field. The Mariner 10 flyby of March 1974 came within 723km if the nightside of Mercury and detected changes in field strength and direction associated with entry to , and wait from, a magnetospheric cavity. During the pass field lines were dominantly directed away from the sun and above the ecliptic plane. A maximum field intensity of ~100nT was detected near closest approach. This observed field was considered to be too string to be induced as a result of polar interaction with an ionosphere and is now believed to represent an intrinsic field. Bow shock and magnetopause observations during the nightside pass were used to infer a bow shock strand off and magnetopause stagnation point at just under 2R_M and just under 1R_M respectively, along the planet-sun line. An interesting feature of the Mariner 10 magnetic field data is the observation of disturbed field lines during the outbound portion of the encounter. These disturbance were not recorded during subsequent passes, and were considered at the time based on energetic particle detections to be the signature of magnetic disturbances, common to the terrestrial magnetosphere, known as a substorm. The third pass made by Mariner 10 just under a year later, again over the nightside, though covering a greater range of latitudes, came much closer to Mercury (327km) and observed a correspondingly higher magnetic flux density. Approximately thirty minutes prior to closest approach the onboard magnetometer detected magnetic disturbances in the (~20n/t) ambient field. At 22:22 UT a sharp increase in field strength signalled the bow shock crossing. The magnetopause crossing was recorded as a 180° switch in direction of the field intensity vector, after which the observed field strength increased to a maximum of just over 400nT at 22:39UT respectively. The data from the two nightside passes are woefully inadequate to the task of determining the field geometry or even deriving an accurate dipole moment. The dipole moment computed initially was derived by calculating the field strength necessary to yield a sunward magnetopause at 1.6R_m against a given solar wind ram pressure. Values determined in this way varied with the solar wind density (a factor deduced from a rather inaccurate measurement of electron number densities by Mariner 10) and are between 3.1 x 10^12 Tm^3 and 5.5 x 10^12 Tm^3 . When data from the third flyby became available, more complex model fits were attempted, incorporating external field terms (due to ring currents) and higher degree axial harmonics. These models rendered equally wide ranging estimates of the planetary dipole moment from a high of 5 x 10^12 Tm^3 for a model using a uniform ~50nT external field, to a low of 2.4 x 10^12 Tm^3 for a model composed of a dipole plus current sheet. Computations involving higher degree terms invariably yield lower dipole moments than those models which assume harmonic terms greater than degree /l/=1 are zero. John Connerney and Norman Ness found a correlation between the calculated dipole and quadruple moments and subtracted these models from the observed field measurements to derive root mean square residuals. The strength of the dipole varies by a factor of two, between 200 and 400nT R_M depending on the strength of the quadruple contribution. The polarity of this field is of the same sense as the terrestrial field, and the proposed tilt approximately 14°. In order to better characterise the field strength and harmonic spectrum a low altitude polar orbiter is required. Studies of long term secular variation in this field demands the return of sample from sites where temperatures have not exceeded the Curie temperature of local materials - most likely the polar region. Though there are no plans for a sample return mission at the present time, there are rival plans, one from NASA, the other from ESA , to send spacecraft back to Mercury. The NASA mission is simply a flyby; ESA's mission, named Hermes, is a polar orbiter. Though placed in this orbit ostensibly to facilitate radar mapping of hypothesised polar water deposits, it is also ideal for studying the magnetic field in three dimensions. The source of the /Hermean/ field. The strength of the measured field is too low to be the result of external induction mechanisms. An internal field can then be attributed to either remanant magnetisation of the crust or an active core dynamo. Studies of the possibility that the crust could retain an imprint of an earlier field imposed upon it and then give a signature matching the observed field concluded that it is unlikely that the Hermean field has an origin anywhere but in the core. From the outside Mercury gives every indication that it is an inert world with a ancient surface similar to the Moon's. However Mercury has a tenuous atmosphere (10^-9 millibar) containing sodium and potassium vapour which has been detected spectroscopically from the Earth and in the UV by Mariner 10. These substances cannot have been delivered by the solar wind and, in conjunction with tentative spectroscopic evidence for lava flows, is taken as evidence of volcanic activity. If Mercury is still active than a liquid outer core does not seem unreasonable. However were the dynamo in Mercury's interior operating in the same way that the terrestrial dynamo operates then the field strength ought to be greater than it is. Models in which the dynamo is energy limited or powered only by thermal gradients along the core-mantle boundary have been suggested but must await detailed seismological investigation in order to be tested. Click here for a noon-midnight slice through the magnetic field of Mercury. Back / Next E-mail the author. © A.D. Fortes. 1997. http://www.es.ucl.ac.uk/research/planetary/undergraduate/dom/magrev/mercmag.htm