Scientists reported on Monday definitive signs of liquid water on the surface of present-day Mars, a finding that will fuel speculation that life, if it ever arose there, could persist to now.

“That’s a direct detection of water in the form of hydration of salts,” Dr. McEwen said. “There pretty much has to have been liquid water recently present to produce the hydrated salt.”

In 2011, Dr. McEwen and colleagues discovered in photographs from the Mars Reconnaissance Orbiter dark streaks descending along slopes of craters, canyons and mountains. The streaks lengthened during summer, faded as temperatures cooled, then reappeared the next year.

They named the streaks recurrent slope linae, or R.S.L.s, and many thousands of them have now been spotted. “It’s really surprisingly extensive,” Dr. McEwen said.

Many mysteries remain. For one, scientists do not know where the water is coming from.

“There are two basic origins for the water: from above or from below,” Dr. McEwen said. The perchlorates could be acting like a sponge, absorbing moisture out of the air, but measurements indicate very low humidity on Mars — only enough for 10 microns, or about 1/2,500th of an inch, of rain across the planet if all of the wetness were wrung out of the air.

That idea cannot be entirely ruled out if the lower part of the atmosphere turns out more humid than currently thought.

Christopher P. McKay, an astrobiologist at NASA’s Ames Research Center in Mountain View, Calif., does not think the R.S.L.s are a very promising place to look. For the water to be liquid, it must be so salty that nothing could live there, he said. “The short answer for habitability is it means nothing,” he said.

He pointed to Don Juan Pond in Antarctica, which remains liquid year round in subzero temperatures because of high concentrations of calcium chloride salt. “You fly over it, and it looks like a beautiful swimming pool,” Dr. McKay said. “But the water has got nothing.”

Earthly life adapts to many hostile environments, but Don Juan Pond is lifeless.

Others are not so certain. David E. Stillman, a scientist at the Southwest Research Institute’s space studies department in Boulder, Colo., said water for the streaks might be different in different regions. In some, they form only during the warmest times, suggesting that those waters might not be too salty for microbes.

In other news, Donald Trump says the darndest things and will build walls but only if you are paying for them.

“I like Mexico, I love the Mexican people, I do business with Mexico,” he said. “But you have people coming through the border who are from all over and they’re bad, they’re really bad … We have people coming in and I’m not just talking Mexicans, who are killers, they’re rapists, they are people we don’t want in this country.”

“Mexico has not treated us well,” Trump said. “Mexico treats us as though we are stupid people, which of course our leaders are. I don’t blame them. China’s even worse.”

Mexico will not pay for the wall. Throughout the course of his presidential bid, Trump has assured voters that his plan for a wall would be subsidized by the Mexican government. ”Trust me, Mexico will pay,” the billionaire said on July 31.

But Mexican Presidential spokesperson Eduardo Sánchez called Trump’s assertion that Mexico would pay for a wall “false.” Sánchez said: “It reflects an enormous ignorance for what Mexico represents, and also the irresponsibility of the candidate who’s saying it.”

The idea that Mexico will pay for a blatant monument to anti-Mexican hostility defies all logic.

Mexican immigration has declined. Trump has called the immigration situation “completely out of control” and has promised “to take our country back.” But Pew Research Center’s  data shows that Mexican unauthorized immigrants living in the U.S. declined by 1 million, from the 6.9 million 2007 peak to 5.9 million in 2012. Driven partly by economic and demographic factors, net migration from Mexico reached zero in 2010, and since then more Mexicans have left the U.S. than have arrived.

“The U.S. has become a dumping ground for everyone else’s problems,” Trump thundered. But actually, the world has become a dumping ground for U.S. economic policy. And few countries feel this more than Mexico, where our free trade agreements opened the border to cheap American-grown corn — cheap because it’s subsidized by our tax dollars — which flooded the Mexican market and killed local agriculture.

By 2001, just seven years after the North American Free Trade Agreement took effect, one out of every three tortillas in Mexico was made with imported corn, one report suggests. And a similar dynamic affected other crops and industries. We’ve literally been dumping our cheap subsidized crops and low-wage factories in Mexico — not the other way around.

In reality, such a project would face major practical obstacles.

Aside from the physical challenge presented by the building of a wall through thousands of miles of isolated and rugged southwestern terrain, much of that terrain is environmentally sensitive.

Under George W Bush, the last Republican occupant of the White House, the federal government explored the concept. It found that the construction of mere fencing along even sections of the border presented an array of legal issues. Many were due to the presence of vital animal habitats.

Much of the land along the US-Mexico border is also privately owned, which would increase costs. Large stretches of the border also runs through Indian reservations, which would present additional legal obstacles as well.