NEW YORK • Here is an interplanetary botany discovery that took college students and not scientists from America's National Aeronautics and Space Administration (Nasa) to find: Hops - the flowers used to add a pleasant bitterness to beer - grow well in Martian soil.
"I don't know if it's a practical plant, but it's doing fairly well," said Professor Edward F. Guinan of astronomy and astrophysics at Villanova University.
Last semester, 25 students took his class on astrobiology, about the possibility of life elsewhere in the universe.
For the laboratory part of the course, the students became farmers, experimenting to see which crops might grow in Martian soil and feed future travellers there.
"I was trying to come with a project for the students to do, a catchy project that would be fairly easy," Prof Guinan said. "I kept telling them, 'You're on Mars, there's a colony there and it's your job to feed them. They're all depending on you.'"
He presented the findings last Friday at a meeting of the American Astronomical Society in Washington, DC.
But - soil from Mars?
Of course, no one has yet brought back anything from the red planet, but spacecraft such as Nasa's Phoenix Mars lander have analysed Martian soil in great detail. Based on those measurements, scientists have come up with a reasonably good reproduction on Earth - crushed basalt from an ancient volcano in the Mojave Desert.
It is available for purchase, and Prof Guinan bought 45kg.
Martian soil is very dense and dries out quickly - perhaps better for making bricks than growing plants, which have trouble pushing their roots through. That includes potatoes, the saviour food for the fictional Mark Watney in The Martian, the book by Andy Weir and later a 2015 movie starring Matt Damon about a Nasa astronaut stranded on Mars.
For the most part, the students chose practical, nutritious plants such as soya beans and kale in addition to potatoes. Some added herbs such as basil and mint so that astronauts could enjoy more flavourful food on the solar system's fourth world.
And one group chose hops. "Because they're students," Prof Guinan said. "Martian beer."
For the experiments, the students had a small patch of a greenhouse, with a mesh screen reducing the sunlight to mimic Mars' greater distance from the sun.
What did "fabulous" in pure Martian soil was mesclun, a mix of small salad greens, even without fertiliser, Prof Guinan said.
When vermiculite, a mineral often mixed in with heavy and sticky Earth soils, was added to the Martian stuff, almost all the plants thrived. Because astronauts would likely not be hauling vermiculite from Earth, but might have cardboard boxes, Prof Guinan also tried mixing cut-up cardboard into the Martian soil. That worked too.
One group of students hypothesised that coffee grounds could similarly be used as a filler to loosen up the soil. They figured the astronauts would be drinking coffee anyway and coffee would also be a natural fertiliser.
"Also, it may help acidify Martian soil," said Ms Elizabeth Johnson, a Villanova senior who took the class.
Mars soil is alkaline, with a pH level of 8 to 9, she said, compared with 6 to 7 on Earth.
"We think the coffee has a lot of potential," she said.
Her team's carrots, spinach and scallions sprouted quickly in the mix of coffee grounds and Martian soil, initially growing faster than even plants in a control planter full of Earth potting mix.
Prof Guinan is not the first to try growing plants in Martian soil. Five years ago, scientist Wieger Wamelink at Wageningen University and Research in the Netherlands had the same idea, a way to combine his work - ecology research - with his interest in science fiction.
The first round of experiments grew 14 types of plants including rye, tomatoes and carrots in Martian soil, simulated lunar soil and Earth soil. This semester, two Villanova astronomy students will perform follow-up experiments. That includes attempting to grow barley, the other essential ingredient for future Martian beer.