Marketo tracks “Visited Web Page” and “Click Link on Web Page” by default over non-secure HTTP protocol. If you want to have your tracking links secured with their own certificate, you need to have Marketo build a separate non-shared server to enable it. To secure all aspects of a contact's interaction with you typically means securing both landing pages and tracking links.
For more detail on adding SSL to your landing pages, see this article.
If you would like SSL for your Marketo landing pages, contact [email protected].