Gas jets spawn dark ‘spiders' and spots on Mars icecap

August 15, 2006

Unlike anything that occurs on Earth,' says ASU scientist

Every spring, as the sun peeks above the horizon at the Martian south polar icecap, powerful jets of carbon dioxide gas erupt through the icecap's topmost layer. The jets climb high into the thin, cold air, carrying fine, dark sand and spraying it for hundreds of feet around each jet.

This dramatic scene emerges from new research by a team of Mars scientists that includes ASU's Phil Christensen. The research report, co-authored with Hugh Kieffer (U.S. Geological Survey, retired) and Timothy Titus (USGS), appeared in the Aug. 17 issue of the scientific journal Nature. The new work solves a longstanding Martian polar riddle. Download Full Image

“If you were there, you'd be standing on a slab of carbon dioxide ice,” Christensen says.

Looking down, the observer would see dark ground below the 3-foot-thick ice layer.

“All around you, roaring jets of carbon dioxide gas are throwing sand and dust a couple hundred feet into the air,” he says.

Visitors also would feel the vibration through their spacesuit boots, he says.

“The ice slab you're standing on is levitated above the ground by the pressure of gas at the base of the ice,” Christensen says.

Mystery markings

The team began its research in an attempt to explain what caused mysterious dark spots, fan-like markings and spider-shaped features on the icecap at the Martian south pole. The dark spots – typically 50 to 150 feet wide and spaced several hundred feet apart – appear every southern spring as the sun rises over the icecap. They last for three or four months and then vanish, only to reappear the next year, after winter's cold has deposited a fresh layer of ice on the cap. Most spots even seem to recur at the same locations.

“Originally, scientists thought the spots were patches of warm, bare ground exposed as the ice disappeared,” Christensen says. “But observations made with THEMIS on NASA's Mars Odyssey orbiter told us the spots were nearly as cold as the carbon dioxide ice, which is at minus 198 degrees Fahrenheit.”

That finding suggested the spots were just a thin layer of dark material lying on top of the ice and kept chilled by it.

THEMIS is the Thermal Emission Imaging System, a multiple-wavelength camera. Christensen, who is a Regents' Professor of Geological Sciences at ASU's new School of Earth and Space Exploration in the College of Liberal Arts and Sciences, designed THEMIS and is the instrument's principal investigator. The new school houses ASU's renowned Mars Space Flight Facility.

Using more than 200 THEMIS visible and infrared images, the team studied one area on the icecap, at 99 degrees east longitude and 86.3 degrees south latitude, from the end of southern winter through mid-summer. The spots began to appear when the sun was only half a degree high, then quickly became more numerous over several days.

“A few places remained spot-free for more than 100 days,” Christensen says. “Then they developed a large number in a week.”

The scientists saw that fan-shaped dark markings didn't form until days or weeks after the spots first appeared, yet some fans grew to half a mile in length. Even more puzzling was the origin of the “spiders,” grooves eroded into the surface under the ice. The grooves converge at points directly beneath a spot.

An icy greenhouse

“The key to figuring out the ‘spiders' and the spots was thinking through a physical model for what was happening,” Christensen says.

The whole process, he explains, begins during Mars' frigid Antarctic winter, when temperatures drop to minus 200 degrees Fahrenheit. That's so cold that the Martian air – 95 percent carbon dioxide – freezes out directly onto the surface of the permanent polar cap, which is made of water ice covered with layers of dust and sand.

This seasonal deposit begins as a layer of dusty carbon dioxide frost. Over the winter, the frost recrystallizes and becomes denser – a process called annealing. The dust and sand particles caught in the frost slowly sink. By spring, with the sun about to rise, the frost layer has become a slab of semitransparent ice about 3 feet thick, lying on a substrate of dark sand and dust.

Sunlight passing through the slab reaches the dark material and warms it enough that the ice touching the ground sublimates (turns directly into gas). As days pass and the sun rises higher, sublimation continues. Before long, the warmed substrate generates a reservoir of pressurized gas under the slab, lifting it off the ground.

Big blowouts

Soon after, weak spots in the slab break through, forming narrow vents, and high-pressure gas roars out at speeds of 100 mph or more. Under the slab, the gas erodes the ground as it rushes toward the vents, snatching up loose particles of sand and carving networks of grooves that converge on the vents.

“Once a ‘spider' becomes established, it affects the surface so that a vent will form in the same place the following year,” Christensen says.

As they erupt, the jets carry loose sand and particles high in the air. The largest and heaviest particles fall closest to the vent, piling up around it to make the spots. As lighter sand grains tossed out by the jet blow downwind, they create the fans, which can extend tens to hundreds of yards. The lightest particles, meanwhile, drift away on the wind to form a thin layer of dust.

“It's like separating wheat and chaff,” Christensen says. “The finest-grained materials are carried off by the wind, while coarser grains are sifted again and again, year after year.”

The vents and jets continue to erupt until the ice slab completely sublimates and vanishes. This mechanism “is unlike anything that occurs on Earth,” he says.

Robert Burnham

Science writer, School of Earth and Space Exploration


ASU joins effort to accelerate HIV vaccine

August 17, 2006

ASU's Biodesign Institute will embark on an international collaboration with Switzerland's Centre Hospitalier Universitaire Vaudois (CHUV) in an effort to ramp up the production pipeline of new HIV vaccine candidates for clinical trials.

It has been 25 years since the first cases of AIDS, the acquired immune deficiency syndrome, were first reported. Since that time, more than 40 million individuals have been infected by HIV in the worldwide viral pandemic. Despite many vaccine candidates that have been tested – and the progress in HIV research – an AIDS vaccine has remained an elusive goal. Download Full Image

As a result, a new, $287 million network of international research consortia involving 165 investigators from 19 countries has been assembled in the hopes of breaking through the AIDS vaccine development logjam. One such research team – Poxvirus T Cell Vaccine Discovery Consortium (PTVDC) – is led by CHUV, including 14 institutions and companies from Australia, Canada, France, Germany, the Netherlands, Spain, Switzerland, the United Kingdom and the United States.

CHUV lead investigator Giuseppe Pantaleo has been awarded a five-year, $15.3 million grant, one of 16 awards from the Bill & Melinda Gates Foundation, to create a research center devoted toward enlisting poxviruses in the global fight against HIV (the human immunodeficiency virus) and AIDS.

“Protective vaccines against a variety of infectious agents represent one of the most significant achievements of biomedical research during the 20th century,” Pantaleo says. “Yet no efficient vaccine exists against one of today's major infectious threats: HIV/AIDS. PTVDC is committed to collaborate with other initiatives of the Global HIV Vaccine Enterprise to increase the probability of success, and to ensure global access – particularly for the developing world – once a successful HIV vaccine is developed.”

ASU School of Life Sciences professor and pox virus expert Bertram Jacobs will play a pivotal role in the CHUV team effort, with a $900,000 research project to genetically engineer pox viruses to ward off HIV infection.

“Making an HIV vaccine is an incredibly daunting task,” says Jacobs, a researcher in the Center for Infectious Diseases and Vaccinology within the Biodesign Institute. “But we've got some of the best people in their respective fields working together on this project.”

Jacobs is one of the world's foremost experts on a pox virus called vaccinia, a cousin of the smallpox virus. Vaccinia virus first was used to eradicate smallpox. Now the research team wants to attempt a similar fate with HIV.

For the past 20 years at ASU, Jacobs has conducted basic research with the vaccinia virus. Jacobs has more than $3 million in federal research funding for projects that include producing a safer smallpox vaccine and a post-exposure vaccine to counter a bioterrorism incident.

“We are now taking the same technology that we are proposing to make a safer, better smallpox vaccine and using that technology to try to make a vaccine that will work against HIV,” says Jacobs, leader of an ASU team that includes assistant research professor Karen Kibler as co-investigator and a team of 20 undergraduate, graduate and post-doctoral researchers.

The road linking vaccinia together with HIV research may not seem inherently obvious, but for Jacobs it began with a trip he made a half-decade ago, when he attended his first international AIDS conference in Durban, Africa, in 2000.

“That's when I got my first idea of what the AIDS epidemic was really like in Africa,” Jacobs says. “When I went to that meeting, I made a commitment that we would try to use our technology to fight the epidemic. So, for me, this has been a long time coming – and now we are going to be able to test whether our technology will work.”

Sub-Saharan Africa has been especially hard hit by the AIDS epidemic, containing the majority (25.8 million) of the world's estimated 40 million cases of HIV. By comparison, there are 1.2 million cases in North America .

In the project, novel vaccinia vectors, or “carrier viruses” – batches of genetically altered vaccinia virus – will shuttle in different combinations of HIV genes to trigger a protective cellular, or T-cell mediated, immune response.

“You can think of it as a vaccinia virus ‘ship' to deliver the HIV cargo,” Jacobs says. “As the body's immune system is fighting the vaccinia virus, it will also be fighting the HIV proteins that are part of the cargo that is going in with the vaccinia vector.”

The use of pox virus-based vaccines is supported by extensive preclinical and clinical experience (one of the pox viruses that will be used is a modified version of the vaccinia virus that was used to eradicate smallpox), and evidence suggests that pox virus vector vaccines could be significantly improved in their ability to stimulate cellular immune responses.

“We've got a vaccinia virus that makes a better immune response and by putting the HIV genes in, we hope it will make a better immune response to HIV,” Jacobs says.

The consortium will focus on making improvements to three pox virus vectors that have been used in HIV vaccines: MVA, NYVAC and ALVAC. The investigators also will develop new immunologic tests and strategies to help better determine how the results of animal studies should guide decisions about which pox virus candidates are most promising to move forward into human clinical trials.

The ultimate goal of the consortium is to advance the most promising new poxvirus vaccine candidate into Phase I clinical trials by the end of the grant period.

“Will our vectors work? I can't guarantee that,” Jacobs says. “But we will get a very quick idea of whether or not they will work better than anything we have tried before. We want to go all the way and get something into a clinical trial – and so, clearly, if we don't have at least one vector into clinical trials by the end of the five years, we will have failed.”

Joe Caspermeyer

Manager (natural sciences), Media Relations & Strategic Communications