The Sun does far more than provide Earth with light and heat. Two NASA-funded studies suggest that the Sun’s ancient history and journey through the Milky Way may have influenced Earth’s climate in unexpected ways. The research explores how changes in the heliosphere may have contributed to ancient climate shifts and how powerful solar eruptions may have kept the young Earth warm enough for liquid water—and possibly life—to exist.
Research from the SHIELD (Solar Wind with Hydrogen Ion Charge Exchange and Large-Scale Dynamics) Center, one of NASA’s DRIVE (Diversification, Enabling, Integration, Venture, Education) Science Centers, examines how the heliosphere has moved through the Milky Way over billions of years. The heliosphere is a vast protective bubble created by the solar wind that surrounds the Sun and the planets. Scientists found that changes in the galactic environment surrounding this bubble may have affected conditions on Earth.
A second study led by NASA researchers investigates the “faint young Sun paradox”—the mystery of how Earth remained warm when the young Sun produced significantly less energy than it does today. The findings suggest that powerful solar eruptions may have triggered chemical reactions that produced greenhouse gases in Earth’s early atmosphere.
The Sun’s Journey Through the Milky Way
Earth’s climate has changed dramatically over tens of millions of years. Major ice ages temporarily reduced the planet’s average temperature by several degrees, while other periods experienced repeated cycles of warming and cooling.
Scientists have traditionally examined Earth’s orbit, atmospheric greenhouse gas concentrations, ice coverage, and other planetary factors to explain these climate changes. New research indicates that the cosmic environment surrounding the Sun may also have played an important role.
The solar system is enclosed within a protective region created by the Sun, somewhat like the way Earth is surrounded by an atmosphere. Known as the heliosphere, this region forms as charged particles in the solar wind stream continuously outward from the Sun in every direction.
Reconstructing the Sun’s Galactic Path
The heliosphere travels around the center of the Milky Way along with the solar system. During the Sun’s 4.6-billion-year history, the solar system has passed through a wide range of galactic environments.
In a study published August 21 in Annual Review of Astronomy and Astrophysics, researchers at NASA’s SHIELD Center used computer simulations to reconstruct the heliosphere’s past path through the galaxy. Their results suggest that some of the regions encountered by the solar system may have caused measurable changes on Earth.
Merav Opher, principal investigator of SHIELD at Boston University, and her colleagues simulated an encounter between the solar system and a frigid region containing dense gas and dust. Their research indicates that the Sun may have passed through similar environments at least three times during the past several million years.
During these encounters, enormous interstellar “cold clouds” may have pushed into the heliosphere with enough force to compress it dramatically. The simulations suggest that the heliosphere may occasionally have shrunk to a size smaller than Earth’s orbit, temporarily exposing our planet to the interstellar environment beyond the Sun’s protective bubble.
When Earth May Have Lost Its Solar Shield
The modeled encounters occurred approximately 2 to 3 million years ago, 6 to 7 million years ago, and 13 to 14 million years ago. If the heliosphere contracted as the simulations indicate, Earth’s atmosphere would have been exposed to a significantly different interstellar environment during those periods.
This timing is also consistent with geological evidence. Elements commonly associated with interstellar dust have been identified in deep-sea sediment cores, Antarctic snow, and lunar samples dating to similar periods.
These episodes of heliosphere compression may help explain some ancient climate patterns.
According to the simulations, exposing Earth’s atmosphere to dense, cold clouds of interstellar hydrogen could have increased atmospheric water vapor and altered conditions in the upper atmosphere. These changes may ultimately have affected the environment closer to Earth’s surface.
The findings raise the possibility that the solar system’s passage through colder regions of the Milky Way contributed to long-term climate changes on Earth, potentially including periods of glaciation.
Building a Digital Twin of the Heliosphere
NASA funds SHIELD as one of several centers advancing research in solar and space physics. As a DRIVE Science Center, SHIELD brings together researchers from multiple disciplines and scientific fields.
One of the center’s goals is to create detailed computer models, or “digital twins,” of the heliosphere. These models could help scientists understand how the Sun’s protective bubble responds when it encounters dense interstellar clouds and other galactic structures.
Studying the history and structure of our habitable solar system may also provide clues about how life evolved on Earth. In the future, this knowledge could help researchers identify other star systems capable of supporting habitable planets.
The Mystery of the Faint Young Sun
The second study focuses on another important question about the Sun’s early history.
Vladimir Airapetian of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues investigated how early Earth remained warm enough for liquid water to exist even though the young Sun produced far less energy than it does today.
Approximately 3 billion years ago, the Sun was only about 70% as bright as it is now. Based solely on this lower energy output, Earth should theoretically have been frozen solid.
Geological evidence tells a different story. Liquid water existed on Earth long before that time. The conflict between a relatively warm early Earth and a dimmer young Sun is known as the “faint young Sun paradox.”
A Violent Young Sun May Hold the Answer
Scientists may find clues by studying young stars similar to the early Sun elsewhere in the Milky Way.
These young stars are much more active than mature stars. Observations from NASA’s retired Kepler Space Telescope show that young Sun-like stars can produce enormous superflares every day, sending streams of energetic particles into space.
If the young Sun behaved in a similar way, Airapetian suggests that these high-energy particles could have triggered chemical reactions in Earth’s atmosphere and contributed to global warming.
To test this possibility, Airapetian’s team recreated conditions thought to resemble those of Earth’s early atmosphere in a sealed laboratory chamber. The researchers combined molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide.
They then bombarded the gas mixture with protons to simulate the high-energy particles produced by powerful solar superflares.
Superflares May Have Produced Powerful Greenhouse Gases
The simulated particle bombardment caused several chemical reactions, including the formation of nitrous oxide. Nitrous oxide is a greenhouse gas that is approximately 300 times more potent than carbon dioxide. The research was published in Astrophysical Journal Letters.
Nitrous oxide may have helped early Earth retain enough heat to support liquid water.
However, not all of the gas would have remained in the atmosphere. Intense ultraviolet radiation from the young Sun may have broken down some nitrous oxide molecules, separating them into nitrogen and oxygen.
Even so, the researchers found that relatively small amounts of nitrous oxide may have been sufficient to warm the early planet.
Computer simulations indicate that if approximately 10% of the nitrous oxide produced in the laboratory survived in the atmosphere, temperatures near Earth’s equator could have reached about 41 degrees Fahrenheit, or 5 degrees Celsius. That would have kept the planet’s surface above the freezing point of water.
Conditions That May Have Supported Early Life
A cold but unfrozen Earth may have provided additional benefits for the emergence of life.
If smaller quantities of nitrous oxide remained in the atmosphere, they could have supported prebiotic chemical reactions. Research suggests that temperatures just above freezing may be more effective than warmer conditions for assembling complex chains of amino acids.
This means the young Sun’s intense activity may have done more than prevent Earth from freezing. It may also have helped create environmental conditions favorable to prebiotic chemistry and the earliest stages of life.
How the Sun Helped Shape Earth
Together, the two studies highlight the deep connection between the Sun and Earth’s history.
The solar system’s movement through the Milky Way may have exposed Earth to changes in the surrounding interstellar environment, potentially influencing the planet’s climate. Billions of years ago, the young Sun’s powerful activity may have helped warm Earth when sunlight alone was not enough.
Earth is unusual in many ways, but it has never existed in isolation. The planet formed within a larger stellar and planetary system and has remained closely connected to the Sun’s changing activity and galactic surroundings.
Understanding this relationship could provide new insights into Earth’s climate history, the evolution of life, and the conditions that make other planetary systems potentially habitable.
Source: www.sciencedaily.com