Using the Keck II telescope on Mauna Kea, in Hawaii, scientists from the University of California, Berkeley, and the WM Keck Observatory captured a high-resolution picture of both spots on July 20.
Red Spot Jr, which is about the same size as Earth, formed from the merger of three white spots very recently, some time between 1998 and 2000. It turned red only last December, the astronomers said in a statement.
The Great Red Spot is nearly twice its smaller companion's size and has been circling Jupiter for at least 342 years. But the two are located in the same area and appear to be racing each other around the planet.
The two spots are about the same colour when seen in visible light, but Red Spot Jr was much darker when viewed at infrared wavelengths, the scientists said.
That difference could mean the smaller storm's cloud tops are lower than the big storm's.
More information and images are available online at http://www.keckobservatory.org/article.php?id=88