Current Science

J. P. Pabari ¹, D. Banerjee ¹

Affiliations
1 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, IN

Abstract

The surfaces of airless, non-magnetized bodies like the Moon are directly exposed to solar wind and ultraviolet radiation, causing surface dust grains to be electrically charged and levitated, whenever electric fields exceed the surface forces and gravity. For an improved understanding of the lunar dust environment, we study the surface charging processes using electrostatic modelling and present the results here. We apply Gauss's law to examine the dust levitation and compare the implications with those obtained using freespace capacitance of the particle. Calculating grain charge on surface by assuming its free-space capacitance is erroneous and is therefore inapplicable. The daytime surface potential during high solar activity is estimated to be ˜20 V, while the nighttime potential can be as high as -3.8 kV. The maximum radius of levitating particles is greatly affected by the method used to model the dust levitation. Using Gauss's approach, it comes out to be in the picometre range near the terminator, in contrast to existing calculations which estimate it to be in the nanometre to micrometer range. The LDEX provided no indication of 0.1 μm-sized particles near the terminator, as suggested previously from Apollo observations. This result is not inconsistent with our predictions based on Gauss's law. Hence, it still remains an open question whether dust levitation occurs on the Moon or not, and experiments are necessary on future lunar lander mission which provide direct measurement of surface potential and near-surface charged dust particles to confirm the same.

Keywords

Dust, Levitation, Lunar Environment, Photoemission, Plasma.

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J. P. Pabari ¹

Affiliations
1 Physical Research Laboratory, PLANEX, Navrangpura, Ahmedabad 380 009, IN

Abstract

Recent observations of MAVEN find dust to be present at altitudes from ~150 to 1000 km from the Mars surface. It is expected that it could be interplanetary in nature, based on assumption of particle velocity. Existence of dust at orbital altitudes on Mars could be mainly due to two plausible sources, viz. interplanetary dust and Phobos/Deimos-originated dust. Dust devils prevailing near the surface can lift the dust to a few tens of kilometres and at present, no physical process can explain dust transport to high altitudes (>50 km) from the dust devils. Another possible source of dust around Mars could be interstellar in nature; however, its possibility is rare. Dust originating from Phobos/Deimos could be either due to secondary ejecta created by continuous bombardment of micrometeorites or due to grain levitation. Though dust levitation on the Moon is yet to be confirmed by in-situ measurements, it is expected that it should occur on the airless bodies. The velocity of secondary ejecta and that of levitated dust can exceed the escape velocity of Phobos/Deimos and cause the dust to escape into outer space. Such escaping particles can form dust ring/torus around Phobos/Deimos and therefore also around Mars. The dust ring/torus is yet to be studied and fully understood. Observations of dust, whether originating from Phobos/Demos or interplanetary dust particles, are necessary for finding its origin, abundance and distribution around Mars. This article discusses the existence of high-altitude (>100 km) dust around Mars, and techniques for the detection of these dust particles using an impact ionization detector in a future Mars orbiter mission.

Keywords

Dust Devils, Mars, Natural Satellites, Torus.

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J. P. Pabari ¹, Trinesh Sana ¹

Affiliations
1 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, IN

Abstract

Dust Devils (DDs) prevail near the Martian surface during the Southern hemisphere summer. Their whirlpool effect give rise to smaller particles in the atmosphere, which subsequently affects optical depth and decreases ion concentration. Presence of dust affects atmospheric conductivity and permittivity, which in turn affect electromagnetic wave propagation. An understanding of the underlying physics of electrical discharges due to dust is critical for future missions. Low atmospheric pressure and arid, windy environment suggest that dust is more susceptible to triboelectric charging. This article presents a study of Schumann Resonance (SR) on Mars, whose presence indicates the possibility of a lightning. We have extended our previous work for variable dust mixing. A random dust mixing is chosen and finally, an inverted cone-shaped DD is considered for effective permittivity. It is found that SR modes essentially depend on the shape of DDs, which consequently determines effective permittivity of the medium. Also, SR does not depend much on the conductivity. At present, InSight magnetometer is searching for the presence of SR on Mars. Our results could be useful for future missions to carry out in situ measurements of SR, the most promising detection related to electrical activity on Mars

Keywords

Atmospheric conductivity, devils, dust, lightning, permittivity, triboelectric charging

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