There is a special "imaginary line" that astronomers like to draw between Mars and Jupiter (not the asteroids). What is that line, and how might it explain why terrestrial planets are much less massive than Jovian planets?

The special imaginary line is called Frost Line. Jovian Planets do not have solid surfaces and are made up of mostly gases. This is the reason why they are called gas giants. Jovain planets are located very far away from the sun d hence they re cooler.

Explanation

Terrestrial planets are made up of solid surfaces while Jovin planets re mostly made up of gas. From sie perspective, gaseous surface planets or jovian planets are much bigger than terrestrial planets. The terrestrial planets are located very close to the sun hence they are extremely hot. High temperature is very much suitable for ensuring rocks and metals to condense, however, jovian planets are formed very far away from the sun where temperatures are low enough for the condensation of ice. There is a special imaginary line that astronomers like to draw which is called the frost line. These jovians planets are formed outside this frost line. This is one of the reasons why terrestrial planets are much less massive than jovian planets.