Big news today about the red planet: Scientists have found the best evidence yet that water may exist during the spring and summer on the planet’s surface.

Researchers using the HiRISE instrument in orbit around Mars observed dark, finger-like features that appear and descend down some Martian slopes during the warmest months of the year.

The most plausible explanation for this is flowing, briny water.

The study in Science is not conclusive, but it’s certainly intriguing.

To gain a better understanding of what this all means, I reached out to Mark Lemmon, a planetary scientist at Texas A&M University who participates in day-to-day operations of the Spirit and Opportunity rovers on Mars (Spirit has now ceased to function).

Here’s my Q&A with Mark:

The paper posits that water exists on Mars today. How convincing is the evidence?

The paper is suggestive of liquid water, but the team has not found the smoking gun. A spectral signature of water or of the salts that are speculated would be a key step.

Prior to this publication, what was the thinking in the planetary science community on the possibility of present-day water on Mars existing?

There is debate over present-day liquid water, with brines as the suggested way to have a liquid and gullies as one possible expression of the water. But the evidence is, so far, weak. This paper does provide a fresh motivation and a new place to look. Water, of course, is present as vapor and ice around Mars in the atmosphere, surface, and sub-surface.

Will this paper move the debate?

Some.

Have the Spirit and Opportunity rovers found any significant evidence of present-day water on Mars?

No, not present-day liquid water. They have see ice clouds; Phoenix saw ice and measured water vapor in the atmosphere. Some have suggested evidence for liquid water brines, but the evidence was quite weak and not widely accepted.

How far are Spirit and Opportunity from the closest of the water-cadidate locations described in the paper?

Quite far. The equatorial region of Mars, where Spirit and Opportunity are and Curiosity is targeted, has few of these features. Most are much farther south. Flipping hemispheres, it is like having rovers in Guatemala and The Congo, and having these features in El Paso and Paris.

Would the existence of surface water suggest there might be springs or underwater sources of water on Mars?

This might suggest some sort of ground water flow, although the authors acknowledge some problems with that. The water may simply come out of the atmosphere with the aid of surface salts. But there is no smoking gun for where the water might come from.

Which of the probes scheduled to travel to Mars might be able to investigate these candidate locations more closely?

There may be options with future missions, but Curiosity will not have access to these.

Would briny water be of any use to humans traveling to Mars?

Yes, as would ice.

Mercury, the Moon and now maybe Mars. Is there anywhere in the solar system where there isn’t water (besides Texas this summer, of course)?

I was going to say, my backyard. It is though, important to note that water on Mars, now, has been definitely established for quite some time. And liquid water in Mars’ past is now well-established. The debate is over liquid water flowing on Mars in today’s climate–and there’s none of that on the Moon or Mercury.