Venus atmosphere consists of more than 96% carbon dioxide with no free oxygen. The atmospheric pressure at the planet's surface is 92 times that of Earth, or roughly the pressure found 900 m (3,000 ft) underwater on Earth. Venus is by far the hottest planet in the Solar System, with a mean surface temperature of 735 K (462 °C; 863 °F).
Mars' atmosheric pressure is 6⁄1000 that of the Earth's. The atmosphere is over 95% carbon dioxide and the mean surface temperature is about 210K (-63°C; -82 °F).
You might want to figure out how to modify the humans to live under such abnormal conditions before worrying about processing cod liver oil. If the aliens are smart enough to genetically alter a different creature than themselves (humans) I question their lack of knowledge to terraform Mars (why terraform if the have the capability of such drastic changes in genetics of other creatures than themselves, why not alter themselves to live on Mars if that is what they want?) through mechanical means (processing facilities, robots, etc.).
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