The Sun’s History is Buried Beneath the Moon’s Surface

Stellar outflows were both frequent and violent in the Sun’s early history. A record of these outbursts, which could be responsible for where life occurs in the solar system, may be held in perpetuity beneath the surface of the Moon.

Early in the Sun’s history — four billion or so years ago — our star erupted, frequently ejecting violent outbursts of intense radiation. These scorching, high-energy clouds and particles were dispersed across the solar system. New research implies that these outflows may have both enabled the development of life on early Earth by igniting chemical reactions that kept our planet both warm and wet and prevented it from occurring elsewhere.

An artistic conception of the early Earth, showing a surface pummeled by a large impact, resulting in extrusion of deep-seated magma onto the surface. (Simone Marchi)

Outflows could have paradoxically prevented life from developing on other planets on by violently stripping them of their atmospheres thus causing the evaporation of nourishing chemicals.

These conditions for habitability were boiled away at different rates determined to how rapidly the infant Sun was rotating on its axis. The faster the rotation, the more quickly the primordial outbursts did their destructive work.

Prabal Saxena, an astrophysicist at NASA’s Goddard Space Flight Center, studies how space weather, variations in solar activity and other radiation conditions in space, interacts with the surfaces of planets and moons. He and other scientists are beginning to realise that the Moon may well hold clues vital to decoding this behaviour of the Sun, and their critical links to the development of life, or not, as the case may be.

Saxena says: “We didn’t know what the Sun looked like in its first billion years, and it’s super important because it likely changed how Venus’ atmosphere evolved and how quickly it lost water. It also probably changed how quickly Mars lost its atmosphere, and it changed the atmospheric chemistry of Earth.”

Saxena began his investigation into the Sun’s rotation and its connection to the Earth-Moon system mystery whilst contemplating a seemingly unrelated conundrum. That being, the fact that both the Moon and Earth are made of mostly the same material — yet the soil on the Moon contains significantly less sodium and potassium than Earth’s soil does.

Samples collected from the Apolo missions first alerted scientists to the disparity between element dispersion in Earth soils and Moon soils (NASA)

This puzzle first came to the attention of scientists through analysis of samples collected from the Moon during the Apollo program and lunar meteorites found on Earth. Not only has this problem defied explanation for decades, but it has also challenged the leading theory of how the Moon formed.

This theory suggests that the Moon formed when a Mars-sized object smashed into Earth — approximately 4.5 billion years ago — sending materials spewing into orbit, where they cooled coalesced into our natural satellite.

Rosemary Killen, a planetary scientist at NASA Goddard who researches the effect of space weather on planetary atmospheres and exospheres, elaborates: “The Earth and Moon would have formed with similar materials, so the question is, why was the Moon depleted in these elements?”

Saxena and Killen suspect that these two puzzles are intrinsically linked, and therefore, the history of the Sun is buried in the Moon’s crust.

Past research points the way forward

Back in 2012, Killen performed research that helped simulate the effect solar activity has on the amount of sodium and potassium that is either delivered to the Moon’s surface or knocked off by a stream of charged particles from the Sun — solar wind, or by powerful eruptions — coronal mass ejections. It was this previous work that laid the groundwork and pointed the new investigation in the right direction.

The ESA and NASA Solar Heliospheric Observatory (SOHO) captured these images of the sun spitting out a coronal mass ejection (CME) on March 15, 2013, from 3:24 to 4:00 a.m. EDT. This type of image is known as a coronagraph since a disk is placed over the sun to better see the dimmer atmosphere around it, called the corona. (ESA&NASA/SOHO)

The team coupled this with the relationship between a star’s rotation and its outburst cycle —namely; the faster a star spins the more powerful and violent its ejections. This insight was derived from observing the activity of thousands of stars discovered by NASA’s Kepler space telescope.

Saxena says: “As you learn about other stars and planets, especially stars like our Sun, you start to get a bigger picture of how the Sun evolved over time.”

Using sophisticated computer models, Saxena, Killen and colleagues think they may have finally solved both mysteries.

Their computer simulations, which they described on May 3 in the journal Astrophysical Journal Letters, suggests that the early Sun rotated slower than 50% of baby stars. According to their estimates, within its first billion years, the Sun took at least 9 to 10 days to complete one rotation.

They determined this by simulating the evolution of our solar system under a slow, medium, and then a fast-rotating star. And they found that just one version — the slow-rotating star — was able to blast the right amount of charged particles into the Moon’s surface to knock enough sodium and potassium into space over time to leave the amounts we see in Moon rocks today.

Saxena continues: “Space weather was probably one of the major influences for how all the planets of the solar system evolved, so any study of habitability of planets needs to consider it.”

Mystery solved? How Earth’s atmosphere survived.

The rotation rate of the early Sun is partly responsible for life on Earth — but for Venus and Mars — both rocky planets similar to Earth — it may have precluded it.

Earth’s atmosphere was once very different from the oxygen-dominated one we find today. When Earth formed 4.6 billion years ago, a thin envelope of hydrogen and helium clung to our molten planet, within 200 million years outbursts from the young Sun had stripped away that primordial haze.

As Earth’s crust solidified, volcanic activity gradually created a new atmosphere — filling the air with carbon dioxide, water, and nitrogen. Over the course of the next billion years, early bacterial life consumed carbon dioxide, in turn releasing methane and oxygen into the atmosphere.

Earth also developed a magnetic field — the magnetosphere — shielding it from the bombardment of charged particles, thus allowing the atmosphere to become oxygen- and nitrogen-rich.

As charged particles hit the Earth’s magnetosphere they pass down the field lines and move behind the planet safely. (Ningchao Wang)

Vladimir Airapetian, a senior Goddard heliophysicist and astrobiologist who studies how space weather affects the habitability of terrestrial planets, adds: “We were lucky that Earth’s atmosphere survived the terrible times.”

It’s all about spin

If the Sun had been a fast rotating star, it would have erupted with super flares 10 times stronger than any in recorded history, at a frequency of least 10 a day. Even the magnetosphere wouldn’t have been enough to protect it from such a bombardment.

The Sun’s blasts would have decimated the atmosphere, reducing air pressure so much that Earth wouldn’t retain liquid water — thus making the changes of life prospering slim to none.

Although the Sun rotated at an ideal pace for Earth — which thrived under the early star — Venus and Mars weren’t so lucky.

Venus was once covered in water oceans and may have been habitable — solar activity and the lack of an internally generated magnetic field resulted in it being stripped of hydrogen — a critical component of water. This resulted in its oceans evaporating within approximately 600 million years.

The greenhouse effect as it occurs on Venus (ESA)

As a result, the planet’s atmosphere became thick with carbon dioxide — a heavier molecule than hydrogen making it harder to blow away. This led to a runaway greenhouse effect that keeps Venus a sizzling 462⁰ C, far too hot for life.

Mars, despite being further from the Sun than Earth suffered a similar stripping of atmosphere — partly due to the Red Planet’s weak magnetic field and low gravity. Thus, the early Sun gradually was able to evaporate away its atmosphere and water in a similar way to that of Venus. As a result — about 3.7 billion years ago — the Martian atmosphere had become so thin and nebulous that liquid water immediately evaporated into space — even though frozen water still exists on the planet in the polar caps and in the soil.

After influencing the course for life or its absence on the inner planets, the ageing Sun gradually slowed its pace and continues to do so. Now completing a revolution once every 27 days — three times slower than it did in its infancy. The slower spin renders the Sun much less active, with occasional violent outbursts and a less violent stellar wind.

The Moon: Key witness in the evolution of the Solar System

Exploring the Moon — a witness of Solar System evolution — is key to learning more about the early Sun and its behaviour. This is because the Moon is one of the most well-preserved artefacts from the young solar system.

Saxena elaborates: “The reason the Moon ends up being a really useful calibrator and window into the past is that it has no annoying atmosphere and no plate tectonics resurfacing the crust.

“So as a result, you can say, ‘Hey, if solar particles or anything else hit it, the Moon’s soil should show evidence of that.’”

The Moon’s southern pole could provide important information about the Sun’s early activity. (NASA)

Despite being a great starting point for probing the early solar system, the aforementioned Lunar samples are they are only small pieces in a large and mysterious puzzle. One significant drawback is that as the Apollo samples are from a small region near the lunar equator, and scientists can’t tell with complete certainty where on the Moon meteorites came from — making them hard to place them into geological context.

Since the South Pole is home to the permanently shadowed craters where we expect to find the best-preserved material on the Moon — including frozen water — NASA is aiming to send a human expedition to the region by 2024.

Should astronauts succeed in retrieving samples of lunar soil from the Moon’s southernmost region, it could offer more physical evidence of the Sun’s early rotation rate.

Airapetian suspects that solar particles would have been deflected by the Moon’s erstwhile magnetic field 4 billion years ago and deposited at the poles: “So you would expect — though we’ve never looked at it — that the chemistry of that part of the Moon, the one exposed to the young Sun, would be much more altered than the equatorial regions.

“So there’s a lot of science to be done there.”