On Mother’s Day of 2007, Candice Hansen asked her family to indulge her by picking up some dry ice from the grocery store before driving to the local sand dunes.
Hansen is a Planetary Science Institute senior scientist studying seasonal polar ice caps on Mars and a member of the High-Resolution Imaging Science Experiment, or HiRISE, team. And she had an idea.
She had recently come across an image of a large sand dune near the Martian north pole. She noticed what appeared to be carbon dioxide ice clinging to the dune top and vertical gullies lining the gentle slope face. She hypothesized the blocks of ice were breaking off and sliding downslope, carving channels.
Her trip out to the dunes that Mother’s Day had two purposes. One was to informally test her hypothesis about how the dune’s gully channels formed, and the other was to encourage herself to propose her idea to her teammates for formal investigation.
Her test was successful, so she took her results to her next team meeting.
“The heat from sand sublimates the dry ice and slightly lifts it,” Hansen said. “It’s like playing air hockey. This process is called the Leidenfrost effect, and was discovered about 300 years ago. I’m sure Johann Leidenfrost, who discovered it, never thought to apply it to Mars. Then we came along with Daisy.”
“The question was, can a sliding block of ice create a channel?” she said. “So, I showed them the video of me, my husband and my daughter at the dunes, and no one laughed. My HiRISE teammate Serina Diniega (of the Jet Propulsion Laboratory) was enthusiastic, so we did two days’ worth of experiments with different size blocks, different slopes, comparisons of water ice and dry ice in 2012. A few years after that, Jim McElwaine wrote a proposal to study the process in detail.”
In 2015 a crew hiked out to the coral pink sand dunes in southern Utah with a little explorer in tow named Daisy – a plastic duck dressed like an astronaut about two inches all. The HiRISE team placed her on a four-inch square piece of dry ice and released her on the dune, which was sloped only about two or three degrees. Daisy rode the sandy wave so quickly that the person recording the experiment had to ride an all-terrain vehicle to keep up, and she left a miniature gully in her path.
Hansen and her co-authors recently published a paper in Icarus weaving together past and present research to explain why the north and south seasonal ice caps at the Martian poles are so different. In it, they discuss physical processes, such as this, that shape the landscape in different ways depending on location.
Daisy has awaited her debut to the scientific community for years.
