Satellite-based magnetic measurements of Mars indicate complex and very
strong magnetic anomalies, which led to an intensive and long-lasting
discussion about their possible origin. To make some progress in the
investigation of the origin of these anomalies the MOURA vector magnetometer
was developed for in situ measurements on Mars. In this work we propose the
utilisation of such an instrument for future planetary on-ground surveys. The
proof of its suitability is seen through testing it on various terrestrial
analogues characterised by the distinct magnetic anomalies of their basement
rocks: (1) a magnetite body of EL Laco (up to
Mars' magnetic field has been exhaustively measured between 100
and 440 km altitude by Mars Global Surveyor (Acuña et al., 1998;
Connerney et al., 2005; Morschhauser et al., 2014). These data show that the
magnetic anomalies of the martian crust are up to 20 times higher than those
of the Earth (Scott and Fuller, 2004). However, a profound understanding of
the magnetic signature of the martian crust would require mapping at
different altitudes and consequently with a different magnetic zoom apart
from further petrological analyses. Since Mars presents a very low dense
atmosphere, aeromagnetic surveys are not achievable in the short term. Thus,
on-ground magnetometry with landers and rovers seems to be the most immediate
feasible technology to complement the satellite measurements. The MOURA
magnetometer was developed by Instituto Nacional Técnica Aerospacial
(INTA) in the context of MetNet Precursor Mission to perform vector
magnetometry and gradiometry during on-ground surveys prior to rover-based
surveys on extraterrestrial planets, such as Mars. The instrument has a very
low mass (72
The first objective of the present work is to demonstrate the capability of the miniaturized MOURA instrument in a real context of terrestrial analogue surveys by means of the inter-comparison with the data of a scalar caesium reference magnetometer (Diaz-Michelena and Kilian, 2013). To do this, four different sites with a wide variability in the intensity of their magnetic signatures have been selected. A second objective is the magnetic investigation of these sites and their implication as terrestrial analogues of Mars. A further objective is the potential of high-resolution mapping to show the lowest magnetic contrasts in the terrain and their correlation with the distinct magnetic carriers responsible for their signatures (Acuña et al., 1998; Connerney et al., 2005; Lillis et al., 2013).
Our selected sites include magnetic anomalies from complex geological environments, where, e.g., noise factors, geometrical characteristics of non-exposed rock units and effects of the terrain relief can partly obscure the interpretations concerning the kind and magnetic effects of non-exposed rock units. Thus, a modelling of the magnetic anomalies is desirable in general to improve the interpretation of geometrical and compositional effects of non-exposed rocks (e.g. Eppelbaum, 2015; Eppelbaum and Mishne, 2011; Ialongo et al., 2014). Since we are aware of the problematics, primarily we focus on the interpretation of the effects of distinct types and compositions of exposed rocks concerning the observed magnetic anomalies. Future more detailed studies of the investigated sites should include the above-mentioned magnetic modelling, which is outside the scope of this magnetometer demonstration.
Two different magnetometers have been used for the present surveys: a conventional caesium scalar magnetometer: model G-858 MagMapper by Geometrics and the MOURA vector magnetometer designed and developed by INTA MetNet team for Mars exploration.
G-858 is taken as the reference magnetometer because it is a well-established
hand-held instrument (8–9
The MOURA is a vector magnetometer with two three-axes magnetic sensors of
anisotropic magnetoresistance (AMR) by Honeywell to build up a compact and
miniaturized instrument (72 g mass and 67.5
The characteristics are designed for Mars' surface environment (
Summary comparison between the MOURA and Geometrics 858 magnetometers.
The tracks have been defined to cover most of the relevant geological features of the selected areas. Continuous and discrete modes have been selected depending on the characteristics and heterogeneity of the sites. For example, the continuous mode has been applied in extended areas in order to have more flexibility and speed to move. In areas with very small-scale heterogeneities the discrete mode has been preferred. In these cases between 5 and 7 measurements have been taken and averaged per point. An advantage of this mode is that it can be measured directly on-ground or at a fixed distance above ground.
The positions of measuring points and tracks have been georeferenced by a
Garmin 62s GPS. The GPS tracks have been used to derive the orientation. For
some relatively small mapping areas, like the Bahía Glacier (site 4;
Fig. 1), a grid of
Selected sites: 1 – El Laco, 2 – Pali Aike, 3 – Monturaqui, and
4 – Bahía Glaciers located in South America between latitudes 20 and
52
It has to be taken into account that the different data have been obtained
from multiple instruments individually without an automatic synchronism.
Therefore, all the acquisition units have been manually synchronised, and
the sequence of measurement has been achieved systematically as follows:
marking selected measurement point; GPS measurement with time stamp (with removal of the GPS from measurement point to avoid magnetic contamination
by this device; magnetic measurement with time stamp (for G-858); magnetometer (three axes), accelerometer (three axes), temperature
measurement with time stamp (for the MOURA).
The data files have been pre-processed manually (preliminary corrections of the GPS data with the above-described information). Ad hoc software has been performed to include the temperature and tilt angle correction of the MOURA data, to subtract the Earth geomagnetic field with respect to total intensities in the case of G-858 and each vector component in the case of the MOURA, and to plot the different magnitudes of the processed data.
Local magnetic field anomalies have been calculated with respect to the
International Geomagnetic Reference Field (IGRF) averaged for the month of
the surveys. At single sites the surveys were performed during less than
2
In all the surveys, the correlation parameters between the scalar data of the two magnetometers (G-858 and MOURA) as well as between the vector data of the two separate sensors of the MOURA have been systematically calculated.
The selected test sites for the on-ground survey are situated in or near the
southern Andes between latitudes 20 and 52
Required general site characteristics are that (a) exposed rocks are relatively unaltered, (b) high-resolution grids with scales of metres to centimetres can be performed, and (c) exposed rocks are representative for a large number of martian surface rocks.
Four large magnetite bodies with a total estimated ore resource of 500 million tons crop out around El Laco volcano in the central Andes (Fig. 2a; Alva-Valdivia et al., 2003; Naranjo et al., 2010). Together with the iron ore deposits of Kiruna (e.g. Jonnsson et al., 2013) they represent worldwide unique examples for very strong local magnetic anomalies, which may be comparable to that observed in some areas of the southern Noachian highlands of Mars (Connerney et al., 2005; Lillis et al., 2013).
The selected area with an extension of
Sernageomin (Servicio Nacional de Geología y Minería of Chile;
Naranjo et al., 2010) performed aeromagnetic surveys and constructed maps of
the anomaly. They reflect a dipolar anomaly according to the isodynamic lines
of the map with intensities of the order of
Fission-track dating of apatite grown within the magnetites gave an age of
The origin of the magnetite bodies has been strongly debated. A combined
magmatic and hydrothermal origin was proposed by Alva-Valdivia et al. (2003),
Sillitoe and Burrows (2002), and Velasco and Tornos (2012). This is based on
the fact that field and petrograhic evidences suggest that some magnetites
have a primarily magmatic texture, whereas others show features that indicate
a formation during a hydrothermal triggered re-emplacement of andesitic lava
flows. Trace element compositions of the latter magnetite type are not
compatible with a magmatic origin. For example, Dare et al. (2014) documented
that these magnetites are characterized by high-Ni
Microscopy studies under reflected light as well as temperature-dependent
susceptibility measurements and isothermal remanent magnetization (IRM)
acquisition show that low Ti-magnetite and/or maghemite are the magnetic
carriers (Alva-Valdivia et al., 2003). Sometimes ilmenite–hematite minerals
appear in significant amounts. Grain sizes range from a few microns up to
several millimetres. Hysteresis measurements of Alva-Valdivia et al. (2003)
of seven ore samples from El Laco Sur point to pseudo-single-domain status
and show a large range of Koenigsberger ratios (
Palaeomagnetic data show distinct local declinations indicating a complex crystallization history, probably during different geomagnetic field orientations (Alva-Valdivia et al., 2003).
Lava sheets with volcanic spatter cones represent a common feature in many areas of the surface of Mars (Kereszturi and Németh, 2012; Robbins et al., 2013). On Earth such volcanic rocks often exhibit distinct magnetic anomalies (e.g. Bolós et al., 2012; Urrutia-Fucugauchi et al., 2012). However, only few examples have been mapped with high resolution (e.g. Cassidy and Locke, 2010).
Thus, an agglutinated spatter cone of 170 m diameter and surrounding
Quaternary lava sheet of the Pali Aike volcanic field in southernmost Patagonia (Figs. 1, 3; Skewes and
Stern, 1979) has been selected as potential martian analogue. The
well-preserved morphology and stratigraphy indicates an age of approximately
1.0
The mapping site (52
Impact craters represent a very frequent feature on the Mars' surface (Lillis
et al., 2013). Depending on, e.g., size, target rocks, impactite composition,
and possible hydrothermal processes, they can be characterised by distinct
and complex magnetic signatures (e.g. Osinski and Pierazzo, 2013). On Earth,
large impact craters are strongly eroded. In addition, some of them are
covered by vegetation or modified by anthropogenic influences. A Late
Pleistocene simple type impact crater in the Atacama Desert of northern Chile
was selected for this case study (Fig. 1). The crater was discovered in 1962
from aerial photographs and firstly described by Sánchez and Cassidy
(1966). It is located at latitude 23
Plutonic rocks and layered intrusions form significant parts of the martian
crust (e.g. Francis, 2011) and analogues on Earth (McEnroe et al., 2004,
2009). These rocks may have the capacity to store remanent magnetic
signatures that can be used to distinguish between different magmatic rock
types during future rover-based magnetic surveys. The Patagonian
Batholith in the southernmost Andes
provides a good example of continental crust formation on Earth and other
planets (Kilian and Behrmann, 2003; Diaz-Michelena and Kilian, 2015). A small
mapping area of
The magnetic field surveys have been complemented with other rock analyses to improve the interpretation of the magnetic signatures of the surveys. Even though the detailed analysis is outside the scope of this work, the types of measurements are briefly described because they support partially some of the conclusions of the work.
Firstly, a macroscopic description of the rock types and mineral components has been done at each site. Representative rock samples were collected along the tracks for macroscopic investigation and future analyses in the laboratory. For instance, the samples from Pali Aike (site 2) and Bahía Glacier (site 4) have been analysed with polarization and refracted light microscopy of thin sections of the rocks. Texture and grain sizes of samples from El Laco (site 1) have also been investigated with a scanning electron microscope (Leo 435 VP, Geology Department, Trier University). The mineral composition of granites and amphibolitic dykes of the Bahía Glacier (site 4) have been analysed by an X-ray diffractometer (Siemens D500, Geology Department, Trier University).
Hysteresis properties of representative samples from Pali Aike (site 2) and Bahía Glacier (site 4) have been characterized magnetically at room temperature by means of a vibrating sample magnetometer at the Space Magnetism Laboratory of INTA, Spain. Magnetic susceptibilities have also been measured with a MS-2 susceptometer by Bartington along the transects at site 4.
Magnetic and geological features of El Laco Sur.
The comparative performance and results of the magnetic surveys with the MOURA and G-858 magnetometers are described below. In all cases with identical single point measurements the correlation between the two instruments and the two sensors of the MOURA has been analysed. The number of differences among the distinct sites has made it possible to demonstrate the versatility of the MOURA. Thus, in each study case some of the individual capabilities of the instrument will be highlighted and discussed.
The surveys were performed with both instruments using continuous and
discrete measurement modes. During the surveys the temperature was ranging
from 5 to 27
The magnetite-bearing outcrops exhibit very high positive magnetic anomalies
from 30 000 to
During G-858 surveys, the magnetometer became often saturated when high local
magnetic anomalies were reached (
Magnetic and geological aspects of a Pali Aike crater and its
surroundings.
Since the MOURA magnetometer provides vector magnetic data, it is possible to determine the orientation of the field in the area. This is shown in the rosette of Fig. 2i together with the palaeo-declinations of other rock samples from El Laco Sur determined by Alva Valdivia et al. (2003).
A dense grid was performed over the depicted surface with G-858 magnetometer
(Fig. 3a). In this case, the MOURA measurements have been performed with the
discrete mode (Fig. 3b) to obtain well-referenced vector data. During the
survey, the temperature was oscillating between 7 and 15
Figure 3b compares an interpolated magnetic anomaly map measured with G-858
with discrete points from the MOURA that are illustrated with a colour code.
Both data sets match very well (
Vector information obtained with the MOURA magnetometer at the individual points
along the crater rim and crater infill is illustrated in Fig. 3e. In general
the predominant declination in all the measurements taken on consolidated
lava blocks and the sedimentary infill is around 355
Magnetic and geological features of Monturaqui impact crater.
The Bahía Glacier mapping area with mafic dykes in a granite:
The MOURA has two magnetometers with a small vertical distance of 10
For, all these data the tilt angle of the MOURA has been taken into account apart from the deviation with respect to the north taken with the GPS. This has been possible due to the fact that the MOURA has a tilt angle sensor to measure the deviation from the horizontal. This sensor has been used in this example to derive a gravity contrast along the transect within the crater and along its rim, which is illustrated in Fig. 3f. Highest values occur along the eastern and western crater rim whereas lowest values are measured at the western eolian sedimentary crater infill. This relationship is shown in the W–E transect of Fig. 3d.
In this example we performed a dense grid of the centre of the crater as well
as the northeastern, eastern, and southern part of the crater rim. An
interpolated map shows very slight magnetic anomalies (
A higher-resolution mapping was performed at a local area of around
The local mapping area at the north-eastern crater rim is characterised by
pronounced local topography changes in the range of
A
Figure 5e shows one of the high-resolution transects where the magnetic
signatures have been measured with both magnetometers and a M2 Bartington
device for susceptibilities. The dykes exhibit very clear but weak positive
anomalies with respect to the granite in the range from
Detailed petrographic studies including electron microprobe and X-ray
diffraction (XRD)
analysis show that both granites and amphibolitic dykes do not contain
magnetite, but instead include ferromagnetic monoclinic C4 pyrrhotite as
magnetic carrier. Areal microscopic mapping of pyrrhotite in thin sections of
the different dykes and the granite and XRD analyses of the different rocks
indicate a content of 1–4 vol % pyrrhotite with grain sizes ranging
from
Magnetic effects of inclined dykes in perpendicular transects.
(Left) Asymmetry of the D1 dykes, which is caused by its 70
The results of the different ground magnetic surveys of both magnetometers are discussed with respect to the appropriateness of the different instruments and the relationship to mineralogical and magnetic properties of the exposed rocks. In particular the potential of high-resolution detection of weak magnetic contrast between different surface rock types is considered.
Final processed data from both instruments show a very good correlation in
intensity of magnetic anomalies (in all cases
The stability of both instruments has been appropriate for the different
surveys. G-858 MagMapper shows a better thermal stability, which can be
observed during faster temperature variations during dawn and dusk, when the
transducer may experiment thermal variations up to
0.1
Regarding the dynamic range, both magnetometers have also casted appropriate results in most of the cases. The limitation in this feature affects in a different way the response of both instruments. G-858 is influenced in the measured modulus of the field, while the MOURA is affected separately in every axis. Apart from the extension of the range in modulus, this is an advantage since it could provide useful data in two directions in spite of saturation in the other axis. For example, at site 1, the huge intensity of the anomalies makes it impossible to map them with G-858, while the MOURA can measure them in the auto mode, when the maximum offset is applied.
At the El Laco site, the MOURA surveys were performed with discrete and
continuous modes. The continuous mode enabled a higher resolution and an
easier performance but it may include a shifting by slight variations of the
distance between sensor and the ground. The extreme high gradient at this
site causes a pronounced fluctuation of around
Surface rock alteration processes, which modify the magnetic signatures, have influence on limited areas. In particular, the related mineral transformations processes are a direct consequence of the contact of the rocks with the hydrosphere and atmosphere, and their influence depth is limited to several tens of metres. This fact together with the exhumation processes often offers the possibility to correlate the measured direction of the magnetization with the coetaneous palaeomagnetic field. In the case of Mars, where the main source of field is the remanent magnetization, oriented measurements does not only contain information on the carriers and the possible alteration effects suffered by them, but also record the palaeomagnetic field orientation. This possible tool has been applied to remanence dominated sites, which is further discussed in Sect. 4.2.
The sensor head orientation is also a matter for discussion. Despite the good signal-to-noise ratio presented by the scalar magnetometer, it is affected by the relative orientation of the head and the magnetic field vector. This is highly improved in a three-axes magnetometer like the MOURA.
Another consideration is the gradient immunity and capability to derive a
gradient of the field of the instruments. On the one hand, the MOURA
instrument presents a better gradient immunity, which makes it very suitable
to map areas with high frequency patching of the signatures. For example, it
is very appropriate to perform decimetre-scale resolution mappings like in
the cases of El Laco, Monturaqui, and Bahía Glacier. G-858 presents
troubles with moderate gradients
(
On the other hand, the inclusion of a second head (and therefore having two
three-axes magnetometers) in the MOURA instrument offers the capability to
better understand the characteristics and depths of the sources. This cannot
be applied to deep and extended magnetic sources, because the distance
between the two magnetometers is very small (10
High-resolution ground surveys may indicate compositional variations in soils and/or uppermost crustal rocks, depending on the magnetic contrast between different exposed rocks and the intensity of active magnetic field (Gobashy and Al-Garni, 2008; Hinze et al., 2013). Despite the fact that it is not the primary goal of the MOURA instrument, which is part of the instruments suite of a lander, due to its potential in future exploration missions, the capacity of the commercial G-858 and the MOURA magnetometer for extraterrestrial high-resolution mapping is discussed in the following for the different investigated sites.
At this site the intensities in the magnetic anomalies range from 0 to
In areas where andesitic lavas are exposed, field surveys show only low fluctuations of the positive anomalies (Fig. 2c, d). This let us to hypothesize that there are no underlying local ore bodies and the lava flows have relatively homogenous compositions.
Measurements with the MOURA vector magnetometer show a clear northward
declination between 350 and 10
Magnetic surveys have been performed at some volcanoes worldwide with
different spatial resolution, e.g. from Australia (Blaikie et al., 2012), New
Zealand (Cassidy and Locke, 2010), and Italy (Okuma et al., 2009). In general
these case studies show more and less positive magnetic anomalies (up to a
few thousand nT) depending on the composition of the volcanic rocks, its
cooling history, and the single versus multi-domain status of their magnetites
(Clark, 1997). Our example of a small crater (170 m diameter) and its
surroundings at the Pali Aike volcanic field was performed with both
magnetometers and with a higher spatial resolution than the previous studies.
The transect across the Pali Aike crater shown in Fig. 3c and d has a spatial
resolution of 30–50
Despite the relatively high intensity of the IGRF at Pali Aike
(
Figure 3e shows arrows for the declination calculated from the vector data of
the MOURA. The values of all measurements on consolidated lava blocks (white
arrows in Fig. 3e) range from 352 to 360
The present field, which has a declination of 12
Planetary impact craters can be characterised by a variety of magnetic anomalies, which are related in particular to distinct magnetic carriers of the target rocks (e.g. mafic versus felsic or sedimentary) and the sedimentary crater infill as well as the compositions of the impactor, impact-induced melt/glass and/or impact-related hydrothermal mineralization and/or demagnetization (e.g. L'Heureux et al., 2008; Langlais and Thébault, 2011; Osinski and Pierazzo, 2013; Pilkington and Grieve, 1992; Prezzi et al., 2012).
At the relatively small Monturaqui crater, a coarse grid of magnetic mapping
with spacings of approximately 70
The very pronounced negative to positive anomalies from
Mafic dykes within felsic to intermediate crustal rocks often produce pronounced local positive magnetic anomalies since they include frequent tiny magnetites (e.g. Hinze et al., 2013). However, in our case study the petrographical investigations indicate that the investigated dykes have not preserved their original magmatic mineral textures and the mineral assemblage point to an emplacement and later equilibration of the dykes under upper greenschist to amphibolite facies conditions (Bucher and Grapes, 2011; Philpotts and Ague, 2009). Granites and dykes do not contain magnetite, but both contain monoclinic pyrrothite as magnetic carrier (Dekkers, 1988, 1989; Clark, 1984). This mineral appears disseminated and along veins and has been formed during hydrothermal mineralization together with Cu and Au enrichments during the exhumation (Díaz-Michelena and Kilian 2015; Nelson, 1996; Schalamuk et al., 1997).
Despite the lower potential of pyrrhotite to produced magnetic anomalies both
magnetometers (MOURA and G-858) clearly show high resolution and slightly
positive magnetic anomalies (
Hinze et al. (2013) show examples of mafic dykes in felsic rocks where
different dyke geometries cause distinct shapes of local magnetic anomalies
across dykes. For the investigated Bahía Glacier site, Fig. 6
illustrates asymmetric behaviour of the magnetic anomalies along lines, which
have been measured across dykes that dip between 50 and 80
Several sites with a huge variability in magnetic anomalies have been analysed. As a first conclusion it can be said that the surface measurement of the sourced field often gives direct information on the composition, petrogenesis, and alteration processes of the surface rocks.
For the study, two different magnetometers have been used. On the one hand,
the MOURA vector magnetometer and gradiometer (
According to the comparison with the reference instrument, it has been
demonstrated that the MOURA magnetometer is not only appropriate for the
static measurement of the absolute value of the magnetic field
and its temporal variations, but
also suitable for the prospection measurements in the range of sourced fields
from
The particular conclusions for the four case studies are the following:
El Laco magnetite-bearing ore deposits in the northern Andes of Chile
represents a worldwide unique example with extremely high on-ground
anomalies ranging from In this case the MOURA enabled better results than G-858 due its larger range
of operation (130 000 A crater in the Pali Aike volcanic field, in southern Chile, shows very
high positive magnetic anomalies (up to 12 000 Since these rocks, like that of many other comparable volcanic rocks on Earth
and other planets, have high Koenigsberger ratios ( The small Monturaqui impact crater in the Atacama Desert of northern
Chile represents an analogue for many other simple type craters, like
Bonneville crater on Mars. The granitoid and rhyolitic target rocks have few
magnetic carriers and only week magnetic anomalies. Pronounced anomalies
along the crater rim indicate metre-sized unexposed remnants of the
iron-bearing impactor (octahedrite). Local mapping with a decimetre resolution, with intensities ranging from
A site within the Patagonian batholith of the southernmost Andes
provides a window into deeper planetary crustal magnetic signatures. The
exposed rocks include granites and mafic dykes that have been partly
equilibrated at lower amphibolite facies conditions, where all primary
magmatic magnetites have been transformed to iron-bearing silicates, and a
later hydrothermal mineralization produced pyrrhotite as only magnetic
carrier. The freshly exposed transitions between these granites and mafic dykes have
been mapped on a decimetre scale. Despite the very low magnetic contrast from
20 to 80
Authors acknowledge all the MOURA MetNet team; in particular, the payload electronics engineering laboratory for their work with the magnetometer and V. Apéstigue for the technical support. This work was supported by the Spanish National Space Programme (DGI-MEC) through the project AYA2011-29967-C05-01 and the Spanish National Space Program of R&D Externalization through the project PRI-PIBUS-2011-1105. Edited by: L. Eppelbaum