Introduction
After a decade-long search, NASA’s MAVEN (Mars Atmosphere Volatile Evolution) mission has directly observed for the first time a mysterious atmospheric escape process called cathodic sputtering, which may help answer ancient questions about Martian water loss.
Background on Martian Water Loss
Scientists have long known, thanks to abundant evidence, that water was present on Mars’ surface billions of years ago. However, the critical question remains: “Where did the water go and why?”
Early Martian Atmosphere and Water Loss
In the early history of Mars, the planet’s atmosphere lost its magnetic field and became directly exposed to solar wind and storms. As the atmosphere began to erode, liquid water became unstable on the surface, causing much of it to escape into space. But how did this once-dense atmosphere disappear? Cathodic sputtering may provide an explanation.
Understanding Cathodic Sputtering
Cathodic sputtering is an atmospheric escape process where atoms are expelled from the atmosphere by high-energy charged particles.
“Imagine shooting a cannonball into a swimming pool,” said Shannon Curry, MAVEN’s principal investigator at the University of Colorado Boulder’s Laboratory for Atmospheric and Space Physics and lead author of the study, in a press release. “Here, the cannonball is the heavy ions colliding with the atmosphere at high speeds, ejecting neutral atoms and molecules.”
Previous Indications of Cathodic Sputtering
Scientists had previously found hints that cathodic sputtering was occurring, but they had never directly observed it. Previous evidence came from analyzing light and heavy isotopes of argon in Mars’ upper atmosphere. The lighter isotopes were found at higher altitudes than their heavier counterparts, and it was discovered that there were far fewer light argon isotopes than heavy ones in the Martian atmosphere. These lighter isotopes can only be removed through cathodic sputtering.
“It’s like finding ashes from a campfire,” said Curry. “But we wanted to see the actual fire, in this case cathodic sputtering, directly.”
MAVEN’s Observations
To observe cathodic sputtering, the team needed simultaneous measurements at the right time and place using three instruments aboard the MAVEN spacecraft: the Solar Wind Ion Analyzer, Magnetometer, and Neutral Mass Spectrometer. Additionally, measurements on the planet’s day side and night side at low altitudes were required, which takes years to observe.
Combining data from these instruments allowed scientists to create a new type of argon sputtering map relative to the solar wind. This map revealed argon presence at high altitudes in the exact locations where energetic particles impacted the atmosphere and ejected argon, showing cathodic sputtering in real-time. Researchers also discovered that this process occurs at four times the previously estimated rate and increases during solar storms.
Implications for Early Martian History
The direct observation of cathodic sputtering confirms that this process was a primary source of atmospheric loss during Mars’ early history when solar activity was much more intense.
“These results establish the role of cathodic sputtering in the loss of Mars’ atmosphere and in determining the history of water on the planet,” Curry stated.
Key Questions and Answers
- What is cathodic sputtering? Cathodic sputtering is an atmospheric escape process where high-energy charged particles expel atoms from the atmosphere.
- Why is cathodic sputtering important for understanding Mars’ water loss? Cathodic sputtering is a significant mechanism for atmospheric loss in Mars’ early history, when solar activity was more intense.
- How did scientists directly observe cathodic sputtering on Mars? Scientists used data from three instruments aboard the MAVEN spacecraft to create a map of argon sputtering relative to solar wind, revealing cathodic sputtering in real-time.
- What are the implications of this discovery for understanding Martian habitability? This finding contributes to our scientific understanding of conditions that allowed liquid water to exist on Mars’ surface billions of years ago and its implications for past habitability.
