Although layer-specific molecular and morphological diversity among cortical astrocytes is increasingly well established, how these distinct states are linked to specialized calcium signaling and maintained in the adult cortex remains unclear. Here, combining super-resolution structural imaging, two-photon calcium imaging, and analysis of public single- cell and spatial transcriptomic resources, we identify Layer 1 (L1) astrocytes in mouse primary motor cortex as a distinct superficial astrocyte program. These cells occupy compact territories yet contain dense synapse associated loop-like structures and display frequent, fast, broadly spreading calcium events that engage a large fraction of the territory. Transcriptomic analyses identify Id1 and Id3 as enriched components of this superficial program. CRISPR-Cas9 deletion of Id1 and Id3 in adult astrocytes selectively disrupts L1 and superficial Layer 2/3 (L2/3) astrocytes, expanding territory size, reducing fine process complexity, and suppressing calcium activity. Thus, adult layer-specific transcriptional programs maintain specialized astrocyte structure and signaling.