Is it likely that life ever existed on either Venus or Mars? Justify your answer in each case.

Mars:

The possibility of life having existed on Mars is a topic of significant interest. The reason for this is the discovery of certain key elements on the planet that are necessary for life as we know it. These include water, carbon, and nitrogen. In 2015, NASA announced the discovery of liquid water flowing intermittently on Mars' surface. Moreover, several missions have found evidence of ancient riverbeds and polar ice caps. The Mars rover Curiosity discovered that Gale Crater, where it landed in 2012, was once a lake with conditions that could have been suitable for microbial life. However, no direct evidence of past or present life has been found yet. The harsh conditions on Mars today, such as its thin atmosphere and the presence of perchlorates in the soil, make it unlikely for life as we know it to exist currently.

Venus:

Venus is often described as Earth's "evil twin" due to its similar size but vastly different conditions. It has a thick, toxic atmosphere filled with carbon dioxide and it's the hottest planet in our solar system with surface temperatures high enough to melt lead. These extreme conditions make it unlikely that life as we know it could have survived on Venus. However, in 2020, scientists detected phosphine, a gas associated with life on Earth, in Venus' atmosphere. This has sparked renewed interest in the possibility of life in Venus' past or in its cloud decks, where conditions are less extreme. But this discovery is still under debate and more research is needed to confirm whether this is a sign of life or if there's a non-biological explanation for the phosphine.

In conclusion, while there is some evidence to suggest that conditions suitable for life may have once existed on both Mars and Venus, there is currently no definitive proof that life ever did exist on these planets. The search for past or present extraterrestrial life continues to be a major focus of planetary science.