Molecules with ALMA at Planet-forming Scales (MAPS) I: Program Overview
  and Highlights
  Planets form and obtain their compositions in dust and gas-rich disks around
young stars, and the outcome of this process is intimately linked to the disk
chemical properties. The distributions of molecules across disks regulate the
elemental compositions of planets, including C/N/O/S ratios and metallicity
(O/H and C/H), as well as access to water and prebiotically relevant organics.
Emission from molecules also encodes information on disk ionization levels,
temperature structures, kinematics, and gas surface densities, which are all
key ingredients of disk evolution and planet formation models. The Molecules
with ALMA at Planet-forming Scales (MAPS) ALMA Large Program was designed to
expand our understanding of the chemistry of planet formation by exploring disk
chemical structures down to 10 au scales. The MAPS program focuses on five
disks - around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 - in which dust
substructures are detected and planet formation appears to be ongoing. We
observed these disks in 4 spectral setups, which together cover ~50 lines from
over 20 different species. This paper introduces the ApJS MAPS Special Issue by
presenting an overview of the program motivation, disk sample, observational
details, and calibration strategy. We also highlight key results, including
discoveries of links between dust, gas, and chemical sub-structures, large
reservoirs of nitriles and other organics in the inner disk regions, and
elevated C/O ratios across most disks. We discuss how this collection of
results is reshaping our view of the chemistry of planet formation.
