Washington: NASA is funding studies that aim to understand more about Venus, Jupiter's moon Io and a unique and highly active icy moon of Neptune, Triton.
These selected studies will develop concept studies for new missions.
Although they are not official missions yet and some ultimately may not be chosen to move forward, the selections focus on compelling targets and science that are not covered by NASA's active missions or recent selections, the US space agency said on Friday.
"These selected missions have the potential to transform our understanding of some of the solar system's most active and complex worlds," said Thomas Zurbuchen, Associate Administrator of NASA's Science Mission Directorate.
"Exploring any one of these celestial bodies will help unlock the secrets of how it, and others like it, came to be in the cosmos," Zurbuchen added.
The four investigations were selected as part of NASA's Discovery Programme that invites scientists and engineers to assemble a team to design exciting planetary science missions that deepen what we know about the solar system and our place in it.
Each of the four nine-month studies will receive $3 million to develop and mature concepts and will conclude with a concept study report, NASA said.
After evaluating the concept studies, NASA will continue development of up to two missions towards flight.
Two of these investigations target Venus.