Tann asked you to look for leads on the various missing arks.
This mission is acquired automatically after speaking to Director Tann during the Nexus Reunion.
Find leads on the Salarian ArkEdit
Find Leads on the Asari ArkEdit
At some point, you will receive an e-mail from Cora Harper telling you that asari refugees have landed on Eos. Conversing with one of them reveals that the Leusinia jettisoned its transponder over Voeld. After landing on Voeld and acquiring the transponder, wait for Cora to decipher it and learn the Ark's present location (the Valay system).
Find Leads on the Turian ArkEdit
At some point, you will receive an e-mail from Tiran Kandros titled "Turians on Havarl". Read the mail to acquire Turian Ark: Not Dead Yet. Landing on Havarl to help them will reveal they are survivors from the Natanus. Speak with Avitus Rix to acquire Turian Ark: Lost But Not Forgotten. The mission will be on hold, but Rix will eventually contact you via vidcomm to tell you he has a lead on the Ark. After accepting his request to rendezvous, you will find the Natanus in the Remav system, where H-047c (Habitat 5) is located.