What was the first probe that successfully landed on Mars?

Viking 1 was the first of two spacecraft (along with Viking 2) sent to Mars as part of NASA's Viking program. On July 20, 1976, it became the first spacecraft to land successfully on Mars and perform its mission. Viking 1 held the record for the longest Mars surface mission of 2307 days until that record was broken by Opportunity on May 19, 2010.