That feature, called recurring slope linae – Latin for “line” – refers to long, dark flows that point downhill and, more importantly, contain salts that harbor crystals of liquid water.
One of the instruments the team used for their recent discovery was the HiRISE camera on board the Mars Reconnaissance Orbiter, which has taken photos of all the different locations on Mars where recurring slope linae appear, and consequently, where liquid water recently flowed on the Martian surface.
Because you need relatively warm temperatures for liquid water, all of these places are near the Martian equator. Here’s where scientists think there is liquid water on Mars:
Here, in Coprates Chasma, you can clearly make out recurring slope linae (RSL) against the bedrock. HiRISE images like this one often have false colors that highlight distinct features like sand dunes and these linae flows.
One of the most active places on Mars is the mountainous bedrock in Hale Crater. Here, you can see RSL sliding down the bedrock’s steep slopes.
Juventae Chasma is an enormous canyon on Mars that scientists suspect could have formed from powerful flowing rivers billions of years ago. Shown below is a region of the canyon with evidence of much smaller water flows today.
One of the largest geographical changes on Mars ever observed is located in a crater on the floor of Melas Chasma. The giant dark streak is 59 feet wide — almost 4x larger than the average RSL. Scientists aren’t sure what this feature is, but they’re monitoring it closely to see if it behaves similarly to an RSL.
The HiRISE camera has taken multiple photos of this spot in Palikir Crater during different seasons. From the photos, scientists learned that RSL change over time, growing larger in later spring and early summer, and disappearing entirely throughout winter.
The RSL in this photo are almost invisible, suggesting that this region of Mars, in Raga Crater, is either entering or in the midst of winter. The linae will reappear the following spring and summer.
RSL flow down Newton Crater in this shot. Since its arrival to Mars in 2006, the HiRISE camera has recorded how these RSL change each time the reappear. Interestingly, they take on the same shape as the year before. Why that happens remains a mystery.
During the Martian winter, carbon dioxide ice carves ravines, shown below, into the Martian surface. Then, when temperatures warm up, RSL has been seen flowing out of some of these ravines.
At the center of the 65-mile-wide Porter Crater is a peak with shallow gullies and possibly RSL. Scientists only have this one photo of the crater, so they will need more to determine whether what they’re seeing is actually RSL or something different.
Asimov crater — named for American biochemist and author Isaac Asimov — is a mysterious spot. Certain regions of the crater are rich with RSL while other similar locations are devoid of any. “RSL activity must be controlled by something that HiRISE can’t detect, such as the presence of salts or groundwater,” the HiRISE team says on the site.
Here’s another shot of Asimov Crater and its mysterious deep troughs. Scientists don’t know how these troughs formed, but they’re typically a great spot to see RSL. There are no RSL in this photo because it was taken during winter.
Though RSL are linked to flowing water on Mars, scientists aren’t sure where this water is coming from or how much of it exists. To better understand this, they’re seeking out more locations on Mars where RLS appear, like in gullies shown in bright blue below.