A space geologist's Reddit Ask Me Anything session totally rocked. Here's why

IE organized a Reddit AMA with Dr. Gretchen Benedix, a professor at Curtin University who studies meteorites to understand the formation of planets.
Deena Theresa
Representational image of asteroids in outer space.
Representational image of asteroids in outer space.


Meet Dr. Gretchen Benedix, a cosmic mineralogist and astrogeologist who has the coolest job - studying space rocks.

A professor at Curtin University, Benedix uses the chemistry, mineralogy, and spectroscopy of meteorites to understand the formation and evolution of asteroids and planets.

On August 6, 2022, Interesting Engineering spoke to Benedix about her fascinating work with meteorites and the Australian Desert Fireball Network (DFN). This interdisciplinary research group is investigating the mysteries surrounding solar system formation. The story paved the way for a Reddit AMA (Ask Me Anything) with IE on Wednesday, where we received around 2.6k upvotes and a hundred questions from inquisitive Redditors on asteroids, meteorites, and planetary formation.

Here's what we gathered from the session.

1. The last meteorite that fell to earth...is probably right now

It's true. Benedix confirmed the same to a Redditor who wondered if such events were common. But, the important point is that such material is very small, and larger impacts occur less often. "I know that earlier in November, a fireball was seen over Ontario that probably dropped some rocks, but none have been found yet. There is an inverse relationship between the size of the object and the frequency of impacts," wrote Benedix.

She then highlighted the asteroid that impacted the Earth 65 million years ago. That was about 10km in diameter. Objects that massive are "statistically" expected roughly every 100 years. "Objects that are about a meteor across are likely to hit every year - but that doesn't mean they will be a meteorite either," she added.

A space geologist's Reddit Ask Me Anything session totally rocked. Here's why
Professor Gretchen Benedix holds a sample of the Bunburra Rockhole meteorite, the first meteorite recovered by the Desert Fireball Network.

2. We can mine in space but it needs to be cost-efficient

The main cost when it comes to extraterrestrial mining is getting off the Earth. "The moon's main resource at the moment is water ice - which can be converted to rocket fuel. This will make exploration of the solar system far cheaper overall," wrote Benedix.

3. It's not impossible to make a completely man-made planet. But

"it would be very very hard," responded Benedix to a Redditor with a valid question - What steps must one take to design a planet from early formation? "If you ever made snowballs or a snowman you've basically done the planet-building mechanism. Planets in our solar system form when tiny bits of dust start sticking together when they collide. The pieces get bigger and bigger and bigger until you have asteroid-sized objects. Then you can stick those together and form planets," said Benedix.

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The Redditor was also curious about the astrogeologist's personal design choices if she ever built a planet.

"I think Earth is pretty much the ideal version of a planet - the liquid water and plate tectonics means we have a huge amount of variety," wrote Benedix. "No other planet in the solar system appears to have more than one geologic plate - and therefore, there isn't much variety on the surfaces. Earth is a good size, we have access to a lot of elements that help us build things, and the earth's natural processes keep things mostly in equilibrium," she explained, adding that she couldn't really think of a better design for a planet.

4. Gases in a meteorite play a key role in identifying the rock's origin

There are several ways we can determine with certainty the whereabouts of a meteorite - if one found in Antarctica originated on Mars. "Sometimes when rocks form, they have little bubbles in them that can hold onto the gas that they were forming in. In the case of the meteorite, this gas had the exact composition of the atmosphere on Mars, which was measured by the Viking lander while it was on the surface of Mars," explains Benedix.

There is also indirect evidence that helps one link the meteorite. The first is the formation stage. "A lot of Martian meteorites are young - geologically speaking. Most meteorites in our collections are in the 3 to 4.5 billion-year-old range. A large number of martian meteorites are 180 to 600 million years old. This is really young for rocks and means they have to come from a body big enough to have active volcanism for a long time," said Benedix.

The second is an association with oxygen isotopic composition. "This is like DNA for rocks. Meteorites can actually be grouped by their oxygen isotopic composition, which was determined long before we thought about meteorites coming from Mars," added Benedix.

5. Would the asteroid belt be as resourceful as claimed?

Asteroids contain high concentrations of rare earth elements, "but that's only because they haven't fully melted. So if we can find ways to extract the cost-efficient elements, it could be worth it," wrote Benedix.