But something happened 3.7 billion years ago that severely changed the Red Planet's climate and, over time, left the surface dry, desolate, and frozen - a lifeless shell of its former self.
For years, planetary scientists have wondered where all of the surface water and atmospheric carbon dioxide (for possible plant growth) went.
Now, these questions are one significant step closer to being answered thanks to NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission that recently completed one year in orbit around Mars.
Today, scientists announced that Mars is currently losing about 1/4 pound of atoms in its atmosphere to space every second. One of the main causes of this extreme loss is the solar wind - fast-moving charged particles ejected from the sun.
When the solar wind is especially powerful - after a particularly powerful event like a solar flare or coronal mass ejection - the amount of atmospheric loss is 10 to 20 times more. Here's a scary animation showing a cascade of atmospheric loss from Mars:
"This [discovery] is exciting to me to think that a [powerful solar] event like this increases escape because solar storms were more common and more intense earlier in solar system history [so] ...we can expect escape to happen all the time [on an earlier Mars]," said David Brain, a co-investigator for MAVEN, during a NASA teleconference.
These answers will not only shed light on just how habitable Mars was in its distant past, but also the potential habitability of similar worlds in distant solar systems with a star much like our sun.
MAVEN is a school-bus-sized spacecraft equipped with eight instruments that study the different layers of the Martian atmosphere as well as the space weather that bombards it. It takes about 4.5 hours for MAVEN to complete one orbit around Mars.
A panel of MAVEN scientists presented their results from four scientific papers, published in
A diagram of MAVEN's eight instruments is below. Some of them measure space weather while others sniff out different molecules in the Martian atmosphere, especially during deep dips just 78 miles above Mars' surface - three times closer than the International Space Station floats above Earth.
In order to figure out what happened to Mars' luscious, potentially life-giving climate, MAVEN had three science goals, explained Stephen Bougher, who is a co-investigator on the MAVEN team and lead author of one of the Science papers:
- Find out what Mars' upper atmosphere looks like today and what forces control it.
- Determine how fast the atoms in Mars' atmosphere are currently leaving the planet and escaping into space.
- Use the current rate of atmospheric loss to calculate how much atmosphere Mars had in the past.
"We can't actually go back in time," Bougher said during a presentation at the University of Michigan last year. "As you lose hydrogen and oxygen, you'll eventually lose water. As you measure that in the current time and get those loss rates ... you can integrate that loss over time ... and figure out what might be the volume and depth of water covering the whole surface of Mars that might have been lost over its history."
Bougher and his team published their paper on how a very powerful solar event, like the one shown below, affected these loss rates. In fact, it was the most powerful solar event ever recorded around Mars, the team reported.
On March 8 of this year, a giant mass of magnetically charged gas from the sun - called an interplanetary coronal mass ejection (ICME) - struck Mars. These solar events are the most energetic explosions in the solar system and threaten not just Mars, but Earth as well.
Luckily, Earth's magnetic field shields us from this high-energy radiation - what does penetrate our magnetic field we usually see as the aurora borealis, or Northern Lights. But Mars' magnetic field is weak and can only deflect a small amount of the CME compared to Earth.
Yes, that means that Mars can have spectacular auroras, like in the illustration below. And, in fact, another Science paper published today discusses results from MAVEN when it dipped down near the surface during one of these auroras. They found evidence to suggest that the auroras are likely the result of interactions between the solar radiation and the remnant magnetic field of Mars' crust.
Artist's conception of MAVEN's Imaging UltraViolet Spectrograph (IUVS) observing the "Christmas Lights Aurora" on Mars.
"The observations and the model results suggest there are substantial enhancements in the ion [charged particles] loss rates during ICME events," the team stated in their paper.
The team's results contradict another paper, published earlier this year in the Journal of Geophysical Research, that used data from the European Space Agency's Mars Express satellite to measure a decrease in overall atmospheric ion loss rates when the solar wind, generated by solar activity, is especially dense.
The reason could be different data sets or different instruments, the team wrote, or it could be that different kinds of events induce a different response in Mars' atmosphere. More data will be needed to constrain "how ICME events influence ion escape, [which] is an important component for understanding escape rate from early Mars," Bougher and his team concluded.