Lunar surface potential and electric field

The Moon has no significant atmosphere, thus its surface is exposed to solar ultraviolet radiation and the solar wind. Photoemission and collection of the solar wind electrons and ions may result in lunar surface charging. On the dayside, the surface potential is mainly determined by photoelectrons, modulated by the solar wind;while the nightside surface potential is a function of the plasma distribution in the lunar wake. Taking the plasma observations in the lunar environment as inputs, the global potential distribution is calculated according to the plasma sheath theory, assuming Maxwellian distributions for the surface emitted photoelectrons and the solar wind electrons. Results show that the lunar surface potential and sheath scale length change versus the solar zenith angle, which implies that the electric field has a horizontal component in addition to the vertical one. By differentiating the potential vertically and horizontally, we obtain the global electric field. It is found that the vertical electric field component is strongest at the subsolar point,which has a magnitude of 1 V m-1. The horizontal component is much weaker, and mainly appears near the terminator and on the nightside, with a magnitude of several mV m-1. The horizontal electric field component on the nightside is rotationally symmetric around the wake axis and is strongly determined by the plasma parameters in the lunar wake.

Detection and Classification of Potential Caves on the Flank of Elysium Mons,Mars

Martian caves have revived interest in the field of subsurface exploration because they are the potential destinations for future human habitats and astrobiological research.There are many pits on Mars,but some of them look like collapsed cave roofs.These special pits are formed by the collapse of surface materials into the subsurface void spaces.The signature of life is probable in a subsurface cave on Mars as the subsurface environment can protect life from the harsh and dangerous radiation environment of the surface.In a cave,there may be an abundance of minerals,fluids,and other key resources.Therefore,locating the access point of the subsurface cave is essential and crucial for formulating plans for robotic/human explorations of the Red Planet,Mars.We have used remote sensing data from Mars Reconnaissance Orbiter(MRO;NASA),Mars Global Surveyor(MGS;NASA),and Mars Odyssey(NASA)for identifying,mapping,and classifying selected special pit candidates on the flank of Elysium Mons,Mars.A total of 32 special pit candidates has been identified and classified based upon morphology and geological context.Out of these,26 are newly discovered ones.The thermal behavior of 23 special pit candidates confirms that the special pits are radiating heat energy at nighttime,similar to potential caves.Also,cave entrances have been detected in nine candidates using data from the Hi RISE camera onboard MRO.These sites could be important destinations for future robotic/human exploration and the search for life on Mars.

Evolution of Subkilometer-scale Impact Craters on the Lunar Maria as Constrained from Mini-RF Data and Topographic Degradation Model

Physical properties(e.g.,ejecta size and distribution)of impact craters are crucial and essential to understanding the ejecta excavation and deposition process,estimating rock breakdown rate,and revealing their evolution characteristics.However,whether these physical properties are scale-dependent and how they evolve in different radial regions needs further studies.In this study,we first investigated the physical properties and evolution of subkilometer(D≤800 m)craters on lunar maria based on the radar circular polarization ratio(CPR).In addition,we estimated the periods over which rocks and blocky ejecta are exposed and buried in the shallow subsurface layer(termed as exposure time)in different radial regions and assessed the retention time and degradation states for potential radar anomalous craters.We found that in the central region of craters,the largest median CPR occurs after an 80 Myr delay following crater formation.In the rim region,there is no obvious CPR peak in the first100 Ma,whereas in the upper wall region,an evident CPR peak occurs beyond 100 Ma and could last over one billion years.In addition,the probable exposure time of rocks and blocky ejecta is estimated to be~2.0 Gyr(central region),~2.7 Gyr(upper wall region),~2.1 Gyr(rim region),and~0.6 Gyr(continuous ejecta blanket region).We also propose that the retention time of radar anomalous craters depends on the crater size,whereas their degraded states are independent of crater size.