Scientists continue to analyze the data returned by the Phoenix mission to Mars. On May 25th, 2008, when Phoenix touched down on Mars, the lander’s thrusters blasted away as much as 18cm of Martian soil underneath the probe and exposed a layer of ice. While the discovery of the subsurface ice generated a great deal of excitement, some scientists were puzzled, because Phoenix’s relatively weak rocket engines, with 30 percent less total thrust than those of the Viking Mars Landers in the '70s, shouldn’t have excavated that much dirt.

In a new study led by Manish Mehta of the University of Michigan, scientists recreated Mars-like conditions in a laboratory to understand what happened when Phoenix landed. Their study led to a discovery of a new phenomenon in which a rocket plume excavates underlying soil explosively. It was published in this month’s issue of planetary science journal Icarus.

Previously, scientists knew of four ways a rocket plume could cause soil erosion. The first is the sweeping of top soil caused by the blowing air, just like a leaf blower kicks up dust in a garden. The second is when the plume’s pressure force depresses the soil, kind of like stepping on a soft garden soil and leaving a footprint. Third, the pressure exerted by the plume can pump the gas into the ground and move the soil particles in the process. When the rocket engine is shutdown, the gas can now seep out of the soil and lift dust, which is the final option. However, none of these could have been effective enough to explain the extensive digging done by Phoenix.

Mehta’s research team used facilities at NASA’s Ames Research Center in Mountain View, California, to simulate Phoenix’s landing on Mars. Through their experiments, they found that the Lander’s use of pulsed thrusters caused much more extensive digging than a conventional non-pulsed engine. When the experimenters blasted simulated Martian soil with pulsed rocket plumes, they discovered that they created shock waves that traveled through the soil and allowed much more gas to be pumped into the ground than the non-pulsed versions. As a result, the rocket plumes flowed through the soil and fluidized it. When the plume flowing through the soil reached an area outside of the engine’s blast zone, it explosively erupted out of the surface, carrying away the soil and causing extensive erosion.

The scientists named this new process the Diffusive Gas Explosive Erosion. The new erosion process went undiscovered on Earth mainly because the comfortable atmospheric pressure of our planet prevents the explosive eruption process even when rocket engines pump gas into the soil. The new experiment demonstrates that it can be a powerful excavation mechanism under the tenuous atmosphere of Mars.

Having identified the process responsible for the extensive digging done by Phoenix, the scientists were also able to determine the properties of the soil at the Phoenix landing site. They repeated their experiments with various soil types, and excavations similar to those of Phoenix happened when the soil contained a mixture of fine sand and silt particles. Their report notes that, if the soil consisted of only silt, Phoenix would have dug a crater six times bigger; if the landing site was covered with coarse sand grains, it wouldn’t have made much of a hole at all.

Even though accidental discoveries such as the subsurface ice layer uncovered by Phoenix are part of the excitement of planetary exploration, it is also a bit scary when things behave unexpectedly and make us realize that our understanding is a bit off. These misunderstandings could have real impacts in a planetary mission.

For example, Phoenix Lander was equipped with instruments to study the soil surrounding its landing site. The mission designers expected, based on what they knew, that the rocket engines would not disturb the surrounding soil too much, so the probe's robot arm was long enough to reach an area with undisturbed soil. The wrong type of soil could have been disturbed so much that the digging device couldn’t reach any in a natural state, or worse, the lander could have dug a grave for itself when it touched down (fortunately, these were not problems in the end).

Even when an exciting discovery pops up in an unintended way, it is often important to examine the process of the discovery itself, because it is a way to maximize the return from such scientific investments and improves the planning of future missions. In science, scrutinizing an unexpected outcome is a good way to figure out what’s missing in our understanding so that we can further refine what we know. The new study is an example of a discovery that emerged out of things behaving unexpectedly.

Icarus, 2011: DOI: 10.1016/j.icarus.2010.10.003