Landforms unique to Mercury, hollows are shallow, flat‐floored irregular depressions notable for their relatively high reflectance and characteristic color. Here we document the range of geological settings in which hollows occur. Most are associated with impact structures (simple bowl‐shaped craters to multiring basins, and ranging from Kuiperian to Calorian in age). Hollows are found in the low‐reflectance material global color unit and in low‐reflectance blue plains, but they appear to be absent from high‐reflectance red plains. Hollows may occur preferentially on equator‐ or hot‐pole‐facing slopes, implying that their formation is linked to solar heating. Evidence suggests that hollows form because of loss of volatile material. We describe hypotheses for the origin of the volatiles and for how such loss proceeds. Intense space weathering and solar heating are likely contributors to the loss of volatiles; contact heating by melts could promote the formation of hollows in some locations. Lunar Ina‐type depressions differ from hollows on Mercury in a number of characteristics, so it is unclear if they represent a good analog. We also use MESSENGER multispectral images to characterize a variety of surfaces on Mercury, including hollows, within a framework defined by laboratory spectra for analog minerals and lunar samples. Data from MESSENGER's X‐Ray Spectrometer indicate that the planet's surface contains up to 4% sulfur. We conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low‐reflectance material relative to average surface material. Hollows may owe their relatively high reflectance to destruction of the darkening agent (sulfides), the presence of alteration minerals, and/or physical differences in particle size, texture, or scattering behavior.

Early during MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)'s orbital mission, the Mercury Dual‐Imaging System imaged the landform called hollows on the two craters Dominici and Hopper, using its Wide‐Angle Camera with eight narrowband color filters ranging from 433 to 996 nm. An absorption feature centered in the MDIS 629 nm filter is evident in reflectance spectra for Dominici's south wall/rim hollows. A different absorption feature found in photometry of Dominici's center hollows extends through the MDIS 828 nm filter. Hollows in Hopper exhibit a weaker spectral absorption feature than that observed in Dominici's center. At Dominici, we postulate that fresher hollows‐hosting material in the wall/rim was exposed to the space environment through a process of slumping of the overlying material. With time, local and global processes darken the hollows and change or mix the surface mineralogy, so that the spectral signature evolves. The hollows could contain low‐density MgS and an opaque component, potentially derived from background material.

An innovative broadband adaptive absorber, providing a reflection coefhcient lower than -10 dB in the frequency range from 7 to 22 GHz, is proposed. The new absorber is a two-period dielectric Salisbury, in which the outer lossy sheet is made of a graphene monolayer, and the inner one consists of a tunable graphene/dielectric laminate (GL). The two dielectric spacers are made of a commercial low-loss polymer. The total thickness is ~ 6 mm. The adaptive broadband response of the absorber is achieved tuning the effective sheet resistance of the GL through an applied electrostatic held bias. The sensitivity analysis of the reflection coefhcient of the absorber illuminated by a plane wave with normal incidence is carried out with respect to the spacer thickness and the graphene carrier charge mobility. By applying a dc-voltage source always below 15 V, the minimum reflection coefhcient of the adaptive absorber is centered around 14 GHz, and it is characterized by a bandwidth at -10 dB of ~15 GHz.