The technological system ICT (Information and Communication Technologies) generates the electronic space, digital objects and a distributed memory. The development of the Information Society carries out the problem of preservation of digital memory, and mainly of objects "born digital", which only exist on line. This paper analyses the principles promoted by the UNESCO in its Charter for the Preservation of Digital Heritage (2003) and states that public heritage should be preserved using free software, in order to provide an open and universal access to those new cultural goods. The problem of the sustainability of digital heritage is also analysed, because of the strong rhythm of technological change of the system ICT. That problem implies a new modality of digital divide, the mnemonic divide.