Ice on Mars

Disappearance of mystery chunks hints at evaporation, and thus presence of ice

The Red Planet’s white stuff is ice after all.

BYE, BYE CHUNKINESS Before and after photos suggest that ice is on Mars. These Mars Phoenix Lander images are from a trench in the Martian northern plains. Lumps are visible in the lower left corner of the left image. In the right image (taken four days later), the lumps are gone. Scientists think the chunks disappeared through a process similar to evaporation, supporting the idea that they were ice, rather than salt. NASA, PL-Caltech, University of Arizona, Texas A&M University

ANOTHER HARD LAYER This image, acquired by Phoenix Lander’s Surface Stereo Imager on June 19, shows the trenches dubbed Snow White 1 on the left and Snow White 2 on the right. When digging in Snow White 2, the robotic arm suddenly stopped. It had hit something hard. The brightness at the lower right corner of the trench made scientists think they had uncovered another layer of ice. NASA, JPL-Caltech, University of Arizona, Texas A&M University, NASA Ames

A collection of crumblike stuff the Phoenix Mars Lander photographed one day was absent from photos a few days later, hinting that the bright chunks had disappeared through a process similar to evaporation. What’s more, the ice may have come from a layer beneath the soil that extends to the horizon, scientists reported during a June 20 press briefing.

“I am proud to announce that we have found proof that this material is water ice,” said the Phoenix mission’s principal investigator Peter Smith during the briefing. “I am absolutely overjoyed that we can make this statement based on image data.”

The lander has been exploring a site in the Martian northern plains since its safe landing there May 25. It began digging beneath soil with its robotic arm earlier this month.

For the past three weeks, the Phoenix science team has speculated about what the bright, hard material found under the lander and uncovered in the trenches could be. The stuff could have been ice, salt or something else, explained Texas A&M scientist Mark Lemmon, a coinvestigator of the Stereo Surface Imager that captured those images. But “the things are gone,” they melted from the trench, he says. “Salt does not behave like that, so we believe these things are water ice.”

The lander’s robotic arm dug up the crumbs, which were about a centimeter and a half in diameter each, on June 15. Looking at images from the next day — called Sol 21 for the lander’s 21 days on Mars — scientists saw that the crumbs had shrunk in size. The June 19 images showed the crumbs were gone. The crumbs cannot be ice of carbon dioxide, based on the temperature in the region, Smith said, so “indeed we were looking at water ice.”

Evidence for ice

The scientists are certain the chunks are ice because, in part, of the unique temperature and pressure of the Martian atmosphere. The water ice could not have blown away or been made of carbon dioxide. Carbon dioxide ice, or dry ice, would sublimate — go from solid to gas — much faster than the water ice appears to have evaporated in the thin Martian air.

That air is one-one-hundredth the pressure at sea-level on Earth and has an average temperature of about -55º Celsius during the Red Planet’s summer season. Solid chunks of CO2 ice would never form at that temperature. As for the winds, they are as gentle as the feeling of moving the back of one’s hand through the air and are at most about 20 kilometers per hour—not strong enough pick up the chunks and blow them out of the trench.

The real tests for water ice are already underway, though. Soil samples are being delivered to the lander’s three different scientific instruments, which will dissect the chemistry of samples and tell Phoenix researchers about the minerals and organic compounds in the water ice material.

To the horizon

On Sol 24, Phoenix also worked on two trenches, dubbed Snow White 1 and Snow White 2. Digging progressed until the robotic arm struck a firm layer. After three attempts to dig through the layer, the arm went into a holding position.

“The robotic arm stops when it detects something hard,” said Phoenix coinvestigator and “digging czar” Ray Arvidson from Washington University in St. Louis. The instrument basically thinks, “This is not good for me,” dumps the soil in a pile and waits for further instructions, he said.

When the data came down Thursday afternoon, the scientists saw that the arm had stopped digging, and they were concerned. But studying the initial findings from the new Snow White 2 trench showed that the arm had uncovered an ice layer at the same depth as the white material found in the other trenches, suggesting this layer spreads all the way to the horizon.

It is interesting to think that as “we look out into the horizon, which looks like merely rock and dirt, if we could take a big broom and sweep off the dirt, we would see we are on an ice sheet,” Smith said. It is one that in fact extends across 20 to 25 percent of the planet in the arctic and subarctic circle, he added.

“Now the goal is to find out what is mixed in with ice,” Lemmon said, “and find what else is there.”

The team hopes to study the surface soil and the dirt that lie just above the water ice layer to better understand the Martian soil chemistry. The goal is to identify whether or not the dirt has the minerals, salts and organic materials that would make this region a “habitable zone,” Smith said. “I am waiting on the edge of my chair for the TEGA [Thermal and Evolved Gas Analyzer] and MECA [Microscopy, Electrochemistry, and Conductivity Analyzer] instruments to start” doing their analyses to see whether those materials are in this region of the Red Planet.

“The big story is that we can reach out and touch the water ice,” Lemmon explained, and “that we can put it in the instruments and taste it and smell it.”

Tasting and smelling the combination of the soil and water ice material will tell the Phoenix team whether strings of carbohydrates, proteins or other energy sources or building blocks of life are present in the soil. The mission cannot say if there is life at the site, Smith said, only that the materials are there or that there could be life.

The team plans to begin delivering soil samples to its instruments for tasting and smelling over the next few days. Scraping and boring into the ice layer with a pinky-sized rasp will also be done to determine the ice thickness.

Answering the question of habitability will take weeks, though, so “I would encourage people to stay with us,” Smith said.

Ashley Yeager is the associate news editor at Science News. She has worked at The Scientist, the Simons Foundation, Duke University and the W.M. Keck Observatory, and was the web producer for Science News from 2013 to 2015. She has a bachelor’s degree in journalism from the University of Tennessee, Knoxville, and a master’s degree in science writing from MIT.

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