Molecules with ALMA at Planet-forming Scales (MAPS) III: Characteristics
  of Radial Chemical Substructures
  The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program
provides a detailed, high resolution (${\sim}$10-20 au) view of molecular line
emission in five protoplanetary disks at spatial scales relevant for planet
formation. Here, we present a systematic analysis of chemical substructures in
18 molecular lines toward the MAPS sources: IM Lup, GM Aur, AS 209, HD 163296,
and MWC 480. We identify more than 200 chemical substructures, which are found
at nearly all radii where line emission is detected. A wide diversity of radial
morphologies - including rings, gaps, and plateaus - is observed both within
each disk and across the MAPS sample. This diversity in line emission profiles
is also present in the innermost 50 au. Overall, this suggests that planets
form in varied chemical environments both across disks and at different radii
within the same disk. Interior to 150 au, the majority of chemical
substructures across the MAPS disks are spatially coincident with substructures
in the millimeter continuum, indicative of physical and chemical links between
the disk midplane and warm, elevated molecular emission layers. Some chemical
substructures in the inner disk and most chemical substructures exterior to 150
au cannot be directly linked to dust substructure, however, which indicates
that there are also other causes of chemical substructures, such as snowlines,
gradients in UV photon fluxes, ionization, and radially-varying elemental
ratios. This implies that chemical substructures could be developed into
powerful probes of different disk characteristics, in addition to influencing
the environments within which planets assemble. This paper is part of the MAPS
special issue of the Astrophysical Journal Supplement.
