After doing some more research... let me answer the question.

Although Bob is correct that passports were not required of US citizens until 1941 (with the exception of some war years), it looks like if they were issued, they were only good for two years.

The family I'm researching loved passports, better to trace them. So it's likely that the two individuals I'm especially interested in might indeed have been in Europe for over two years and felt the need to renew their passports in 1839 (the first passport issued in 1836) from afar.