Two Yale University researchers have found a potential shortcut in sampling Venus’ ancient surface. Instead of sending a probe on a costly and extraordinarily challenging Venus sample return mission, they propose simply finding a Venusian meteorite on our own Moon.

There’s never been a bona fide detection of a Venusian meteorite on Earth. For one reason, that’s because in the last several hundred million years at least, Venus’ atmospheric pressures have been so intense that even a catastrophic impactor could not dislodge any Venusian rocks into space. 

But before Venus underwent a runaway greenhouse and morphed into the climatic hellhole it is today, it may have had liquid water oceans as late as 700 million years ago. If so, its atmosphere would have been thin enough for surface rocks to have been dislodged by massive impactors and possibly have found their way to both the Earth and our Moon. 

Due to weathering here on Earth, Venusian meteorites on Earth wouldn’t survive long. But because our Moon has no atmosphere, the authors of a paper accepted by The Planetary Science Journal posit that the Moon may have be the ideal spot to preserve Venus meteorites. 

Lead author Samuel Cabot and co-author Gregory Laughlin investigated the amount of material ejected from Venus when it suffered past impacts from asteroids and comets, and then traced the orbits of the rocks throughout the Solar System. They found that a small (but still significant) fraction of rocks ejected from Venus will be swept up by Earth’s Moon. 

Today, due to Venus’ thick atmosphere, even a catastrophic impactor would not be capable of ejecting material with a force to overcome drag from Venus’ thick atmospheric pressures.

But even if they found a Venus meteorite on the Moon or in Apollo rock samples, would researchers be able to offer a high degree of confidence that it originated on our sister planet? That is, in the same way that researchers have high confidence that the Allan Hills meteorite ALH84001 from Antarctica actually originated on Mars?

For an Allan Hills Martian meteorite, we can compare its composition to measurements made by Martian rovers, Cabot, a graduate student in Yale’s Department of Astronomy, told me.  A Venusian meteorite would lie in a completely new category from the rest, but require some extra work, he says. Several samples, plus some comparison to Venus’ current atmosphere, would help single out Venus as the place of origin, Cabot explains.

However, the degree of confidence in a putative Venus meteorite would not be as high as for Martian meteorites, As Laughlin, a Yale planetary scientist, told me. That’s because inclusions in the Martian meteorites contain gases that exactly match Mars’ known atmosphere, he says.

As to where on the Moon to look?

The lunar highlands might be a good place to look since they host some of the oldest rocks, says Cabot.

Once identified, Laughlin says an analysis of grains in the rock would likely yield chemical evidence of existence of oceans. Similar strategies have been carried out on zircon minerals in ancient Earth rocks to date an early existence of oceans, he says. Radioactive dating would give an accurate age for a Venusian sample, which would place constraints on the presence or absence of oceans at a particular point in time, Laughlin notes.

How could we go about looking for Venus meteorites on our Moon now? 

Upcoming Moon missions in NASA’s Artemis program are a great opportunity to search for Venusian meteorites, says Cabot.  In situ analysis might be possible, he says, with astronauts or robots picking out rocks to bring back to Earth for laboratory analysis.

Yet as the authors note, the largest and earliest Venusian rock fragments may lie in a zone beneath the regolith, known as the megaregolith. Finding such fragments will likely require excavating several meters below the lunar surface, they say.

Might there be microfossils in such Venusian surface fragments?

I don’t think the odds are good for that, says Laughlin.  Most Venusian rocks would have undergone some shock during the initial Venus impact, and then again later during the lunar impact, he says. We know that even with an extraordinary sample —- such as ALH84001 from Mars —- that identifying microfossils is fraught with controversy, says Laughlin. With Venus, it would almost certainly be far more controversial and far less clear-cut, he notes.

But would a Venus meteorite look like any other Moon rock? 

The authors write that it would likely be made up of igneous rocks such as basalt, andesite and granite. But you would not be able to tell from looking at it, says Cabot, since a piece of Venus could be any type of rock. He maintains that a chemical test in a laboratory is probably going to be needed to pick out a fragment that is definitely from Venus.

As for how many pounds of lunar rocks researchers would need to sort through before hitting an actual Venus fragment?

Several hundred pounds of lunar rocks were returned from the Apollo missions, and there is a small chance a piece of Venus is within that collection, says Cabot. But based on our simulations and depending on when Venus lost its thin atmosphere, one would probably need to search 10 to 100 times that amount to find a fragment of Venus, he says.

Why not just go to Venus and do a sample return?

“Sample return from Venus or Mars is an extremely difficult and expensive undertaking,” Stephen Kane, a planetary geophysicist at the University of California at Riverside, who did not take part in this research, told me. So, if samples from these bodies can be found in our locality then that is an enormous advantage, he says.  

We could possibly acquire samples of Venus that predate the resurfacing of the planet, thought to have occurred 700 million to 1 billion years ago, at a time when Venus may have had temperate surface conditions, says Kane.

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