Sketch of the filtration mechanism in the PDS 70 disk. (a) The disk has a gap between the inner and outer disks. The inner disk is initially enstatite-rich, and the outer disk is forsterite-rich. (b) While enstatite is depleted in the inner disk due to accretion onto the central star, small dust that penetrates the pressure bump travels through the gap to the inner disk. (c) The small dust supplies the inner disk, and the inner disk eventually becomes forsterite-rich.