The atmosphere on Venus contains a good amount of sulfur dioxide. With all of that sulfur in the atmosphere, it rains sulfuric acid and when sulfur burns, it burns blue. (Though, rain on Venus doesn't actually reach the ground due to the heat and pressure of Venus' atmosphere.) But, the sulfur could condense into liquid at ground level and mix with the lava. Also, from the link that I left, you can see that this happens on Earth also. Although, it's because of the sulfur gas that erupts along with the lava, rather than being in the atmosphere. So, Bungie was thinking when they added this little touch to the Venusian landscape. It's not just a cool looking "Sci-Fi" aesthetic. They were using actual science. Also, Ishtar Terra is an actual continent on Venus. Though it's not surrounded by any water or liquid now, with work from the Traveler to Venus' atmosphere, I guess it could be. So, that's where the Ishtar name comes from. The one thing that I think Bungie may have gotten wrong with Venus is the rough, mountainous terrain. Though the volcanoes would be raised, the area around the volcanoes is pretty flat and smooth, due to all of the volcanic activity. Lava covers the landscape and continually resurfaces Venus and with no plate tectonics, there is no movement of the continents to raise mountain ranges. Also, there are no jungles on Venus, but then again, with the work of the Traveler, I guess that could change also.