Susceptometer for fast and in-situ determination of the complex magnetic susceptibility. Field demonstration in Cerro Gordo volcano, a Martian analogue.

INTRODUCTION

The characterization of the magnetic field and the magnetic properties are useful tools to understand the composition, structure and geological history of the surface rocks. Additionally, this has important implications on the determination of magnetic fields present in the early history of the planets.

Determining the complex magnetic susceptibility (both real and imaginary parts) of rocks is important to obtain their complete magnetic characterization [1]. Real and imaginary susceptibility provide complementary information, such as: how much magnetization can acquire a rock in the presence of an external field, how this magnetization behaves under alternating current (AC) and the magnetic energy losses during the magnetization by different mechanisms (induced currents, hysteresis, etc.), which helps in the identification of minerals, magnetic carriers and their phases.  This information could be used in the selection criteria of rocks for sample return missions or for the in-situ scientific studies of the magnetic properties during planetary missions.

SUSCEPTOMETER INSTRUMENT

The Space Magnetism Area at INTA has developed a magnetic instrument, named “NEWTON susceptometer”, for the determination of the complex magnetic susceptibility, to characterize planetary soil and rocks during in situ exploration [2]. The instrument design is based on AC - inductive methods, reaching a resolution of about c = 10⁻⁴ (S.I. Vol. Susceptibility) and a dynamic range between c’ = 10⁻⁴ S.I. and c’ = 10¹ S.I. for the real susceptibility, which are representative values  for the rocks of the Earth, Moon and Mars [3, 4, 5]. The imaginary susceptibility measurement procedure is currently under calibration, with an expected resolution in the order of c” = 10⁻⁶. Such resolution is adequate for most natural rocks characterization and competitive with that of larger and widely proven laboratory instruments (like a Vibrating Sample Magnetometer – VSM). Additionally, the sensor size, power consumption and portability make it suitable to be placed on board rovers, or to be used as a portable device during field campaigns and by astronauts in manned space missions. Furthermore, this sensor provides a great advantage compared to available commercial susceptometers, given that it does not require sample preparation, but only a minimum sample size of 50 x 20 x 20 mm approximately.

The instrument has successfully passed vibration and thermo-vacuum tests representative of interplanetary missions, and is being used in scientific campaigns in terrestrial analogs to characterize in situ the real part of the susceptibility. In particular, the instrument has been used during field campaigns in Cerro Gordo volcano, in Spain

CERRO GORDO VOLCANO – MARTIAN ANALOGUE

Cerro Gordo is a volcano located at 38° 50’ 1.353’’ N, 3° 44’ 20.068’’ W, by Granátula de Calatrava, Ciudad Real (Spain), with a greatest axis of 1000 meters and a relative height of 90 meters. Previous studies date the volcano between 4.7 and 1.75 million years ago, during the Pliocene and Quaternary periods. Throughout its history, it went through several volcanic phases, i.e. strombolian and phreatomagmatic. It has been proposed as a Martian analogue for the similarities of the structure with other volcanoes on the surface of the Red Planet [6]. Cerro Gordo area hosts rocks of different geological origins, from quartzite (box rock) to phreatomagmatic and strombolian phases along a transversal line of the volcano, comprising different minerals with a high range of magnetic susceptibility values. These characteristics make Cerro Gordo an excellent scenario for a demonstration campaign of the susceptometer prototype.

RESULTS AND CONCLUSIONS

In Cerro Gordo, we have determined the magnetic susceptibility in-situ in order to mimic the operation of future exploration missions. The study includes the measurement of ten different outcrops on the volcano, which are representative of the volcanic phases and the field basement (Figure 1). The susceptibility measurements have served successfully to distinguish between the rocks from a spatter deposit and the box rock.

Figure 1A shows the susceptibility, measured in situ, of 13 rocks along a transect of the volcano. Rocks 15_1, 13_1, 13_2, 12_1 and 5_1 corresponds to the box rock and scoria from strombolian eruptions. They cast low values of susceptibility in good correlation with the nature of the quarzites (box rock) and the fast cooling history of the surface scoria. Five other samples (22_1, 10_2, 10_3, 10_4 and 9_1) with heterogeneous composition, show medium susceptibility values, four of them corresponding to non-cohesive rocks of the top of a spatter deposit. The highest susceptibility values of Figure 1A corresponds to the bottom part of the spatter deposit: rocks 23_1 and 24_1: a compact rock associated to a low speed cooling history.

In Figure 1B we study the spatter flow in more detail, dividing this structure into three deposits: a very compact layer (1) with the highest values of susceptibility, friable rocks with some levels of fluidity (2) with low values of the susceptibility, and some cohesive rocks (3) with medium values of susceptibility and supposedly more heterogeneous.

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