The principal gas in the Venusian atmosphere is carbon dioxide (CO2). It's also a minor component of the atmospheres of Earth and Mars.

Here's a breakdown:

* Venus: The atmosphere of Venus is almost entirely composed of carbon dioxide (96.5%), making it incredibly dense and hot.

* Earth: While nitrogen (78%) is the most abundant gas in Earth's atmosphere, carbon dioxide makes up about 0.04% of the atmosphere.

* Mars: While Mars' atmosphere is thin, carbon dioxide makes up the largest percentage (95.32%).

So, the answer is carbon dioxide.