Mysterious grooves on Martian moon Phobos explained by impacts

September 1, 2016

Some of the mysterious grooves on the surface of Mars' moon Phobos are the result of debris ejected by impacts eventually falling back onto the surface to form linear chains of craters, according to a new study.

One set of grooves on Phobos are thought to be stress fractures resulting from the tidal pull of Mars. The new study, published this month in Nature Communications, addresses another set of grooves that do not fit that explanation. Martian moon phobos Phobos, a ~20 km diameter potato-shaped moon that orbits Mars. Download Full Image

“The stunning thing about Phobos is all of its grooves and linear pitted chains,” said planetary scientist Erik Asphaug, of Arizona State University's School of Earth and Space Exploration, who co-authored the paper. “Because it orbits so close to Mars, we think it gets re-impacted by its own lost material whipping around the Mars system, like someone spitting in the wind.”

Phobos is an unusual satellite, orbiting closer to its planet than any other moon in the solar system, with an orbital period of just seven hours. Small and heavily cratered, with a lumpy non-spherical shape, it is only 9,000 kilometers from the surface of Mars (the distance from San Francisco to New York and back) and is slowly spiraling inward toward the planet.

Solving a half-century old puzzle

Since the first detailed images were obtained in the 1970’s, there have been many suggestions for how Phobos got its grooves, but none of them have been able to explain all the different orientations and structures of Phobos’ crater chains.

When a small meteoroid strikes Phobos, forming an impact crater, the material ejected from the surface by an impact easily escapes the weak gravity of this moon. This material remains in orbit around Mars, most of it moving either just slower or just faster than the orbital velocity of Phobos. Within a few orbits the material gets recaptured and falls back onto its surface, but not before it has been stretched out into strands.

Asphaug, and lead Author Michael Nayak of UC Santa Cruz, developed computer simulations showing how prominent crater chains could result from the debris falling back onto the moon’s surface. The simulations enabled them to track, in precise detail, the fate of the ejected debris.

They were able to simulate an impact at the 2.6-kilometer crater “Grildrig,” near the moon’s North Pole, and found that the pattern resulting from ejected debris falling back onto the surface in the model was a very close match to the most prominent actual crater chain observed on Phobos.

"A lot of stuff gets kicked up, floats for a couple of orbits, and then gets recollected and falls back in a linear chain, before it has a chance to be pulled apart and disassociated by Mars gravity." Nayak said. "The controlling factor is where the impact occurs, and that determines where the debris falls back."

A new look at the geology of Phobos

These findings would imply that many of the ~100 m to ~1000 m craters on Phobos could be the result of re-impacts of material ejected from other craters. If that's the case, then the common idea of using the number of craters in a region as a proxy for surface age, could go out the window. 

“These findings give us a fresh set of eyes for looking at the geology of Phobos. Most experts think Phobos' surface is around four billion years old, because it is heavily cratered. If so, it formed eons ago and just sat there, being peppered by meteoroids. But if those craters come from Phobos' other craters, then it's quite feasible that the surface could be only tens of millions of years old, continually resurfaced,” said Asphaug.

This difference is important because the plan for human voyages to Mars seem increasingly to go through Phobos as a 'stopping point', where we can find resources and spend time planning for the descent to the Martian surface. “This may be a very bad idea,” said Asphaug “if the surface geology of Phobos is young, and potentially active.”

Asphaug suggests that Deimos, the more distant moon, would be a more boring, but more suitable, stopping point.

This work was supported by NASA and the Department of Defense through the NDSEG fellowship.

Karin Valentine

Media Relations & Marketing manager, School of Earth and Space Exploration


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ASU launches women's triathlon program

Sun Devils launch new women's triathlon team thanks to donation, grant.
September 1, 2016

Sun Devils are the first Division I program in the Pac-12, thanks to donations and grant

Cliff English has spent eight months building Arizona State University’s women’s triathlon team from the ground up.

That involved recruiting the eight student-athletes and also buying the carbon racing bicycles — and then setting up storage. It meant not only setting a training schedule but also going out to buy a refrigerator for team snacks. And with a group of young athletes who are new to Tempe, it means English, the head coach, got on a bike himself so they wouldn’t get lost during training rides.

On Monday, English will take the newest Sun Devils team to its first competition, in Naperville, Illinois, against eight other university varsity and club teams. The NCAA teams compete in a “sprint” triathlon — a 750-meter swim, a 20-kilometer bike ride and a 5-kilometer run, with top finishing times around 65 minutes.

“After being able to start from zero, you realize how much work there was, but I was so excited to work with this age group,” English said.

ASU added the sport in October 2015 thanks to donations and a $140,000 “Women's Triathlon Emerging Sport” grant from the USA Triathlon Foundation. ASU was one of 10 NCAA schools to receive the grant and is the first Pac-12 school to have women’s triathlon as a Division I NCAA sport.

Before becoming the head coach, English was coach of the U.S. national team and a personal coach of 26 triathletes, ranging in age from their early 20s to late 30s.

“When I was national team coach, we always saw this gap in these years,” English said. “We had great junior programs in place and saw the athletes coming up from youth to ages 16 or 17 and then they went to college and maybe they ran or maybe they swam, and lot of times, they didn’t come back to our sport.

“We always saw the NCAA as an opportunity to continue to develop the athletes.”

Charlotte Ahrens

Charlotte Ahrens, 19, came from Germany to attend ASU and be part of its first triathlon team. Photo by Charlie Leight/ASU Now

One of the new Sun DevilsThe other team members are Delaney Bucker, Amy Darlington, Kathryn Gorcyza, Sarah Quintero, Lillie Robinson, Emily Wagner and Kendal Williams. is Charlotte Ahrens, 19, a freshman from Nuremberg, Germany, who has been competing in triathlons since age 6.

“After school, I had to decide whether to go to university or do a sport because in Germany, you can’t combine them,” said Ahrens, who reached out to university programs in the U.S. and made her decision after Skyping with English. She also was happy to come to ASU because she could major in kinesiology — a course of study not available in Germany.

“I only looked at ASU on the internet because I had no time to come to here before the year started, but it looked so good,” said Ahrens, who is excited to be part of the inaugural team.

“For everyone it’s new, and it’s so good to build a team and to say which direction it’s going in,” Ahrens said.

Training is intense. The women swim five times a week, bike three or four times a week and run three or four times a week. They work out at the Mona Plummer Aquatic Center on campus but during competitions will swim in open water.

“The first few hundred yards of the swim are a lot of contact, and you have to calm yourself,” English said. “We try to simulate that in training.”

They also work on the swim-to-bike and bike-to-run transitions.

“Any kind of lollygagging and it can be the difference between a Top 5 and a Top 15 finish,” he said.

The team has had only two weeks of training before the first competition, and English missed a few days of that with a hectic trip to the Olympics.

“I had two days of coaching, Aug. 15 and 16, and I flew overnight on the 16th to RioLeonardo Chacon of Costa Rica came in 30th, and Ashleigh Gentle of Australia came in 26th., where my male athlete competed on Aug. 18, and my female athlete on Aug. 20th. And then I flew out on the 21st, arrived here on the 22nd, and I’ve been at it ever since.”

This week, English got some help when Erin Densham arrived to start her job as assistant coach. Densham, a native of Australia, earned a bronze medal in the women’s triathlon in the 2012 London Games and came in 12th last month in the Rio de Janeiro Olympics.

The sport’s popularity dates to the early 1980s, with the Hawaii Ironman TriathlonIronman-distance races include a 2.4-mile open-water swim, a 112-mile bike race and a full marathon of 26.2 miles., which was broadcast on ABC’s “Wide World of Sports.” English competed in his first triathlon in 1989.

“I’ve been in this sport a long time. You’re always looking for that right combination of people and environment where you can do something special. I had never felt that until I got to ASU.

“This is an opportunity that none of us had seen before — to go to college and get a great degree and do the sport you really love.”

Top photo by Charlie Leight/ASU Now

Mary Beth Faller

Reporter , ASU News