I have recently downloaded some publicly available ATAC-seq data. I aligned with BWA to reference genome, removed duplicate (in this instance 70% of library is duplicates), and then used picardtools to generate a fragment size distribution. However, I see a large peak at around 20bp? The library was sequenced with 75bp forward and 75bp reverse PEs. Does a 20bp insert length mean that the insert is just short? How can I check this? Presumably the reads have a lot of adapter sequence?