The last year has been an interesting one. In this time, I’ve graduated from York University with a honors bachelor in physics and astronomy, space science stream. This major taught me not just astrophysics and planetary science, but also how to design interplanetary science missions. So I got the best of both worlds – physics and space engineering. During my last summer at York (2014), I had the opportunity to work with the veteran planetary scientist Dr. John Moores on the feasibility of solar-heated lifting gases for Mars balloons. After I submitted in my final paper on this topic, he asked if I would take this further into a Mars mission design to be peer-reviewed for a journal.
I, of course, said yes. The resulting work, done over a little more than a year, cumulated in a 7000-word paper that outlines how this mission would work. It has been submitted for peer review. Even though it was Moores’ idea, I gave it its own acronym – Hydrolyzed Polar Terrain Ice Aerobot, or HYPATIA. In Part 1 of this post, I’d like to summarize this paper’s scientific rationale and why NASA or ESA or JAXA should at least consider it. Part 2 will be on the engineering.
Mars is the only other planet in the solar system besides Earth that has its own extensive polar caps. Besides the permanent water ice caps, there are dry ice caps that form every winter, as 25% of the atmosphere freezes out in the polar nights. That reason alone has piqued scientific interest in the polar caps, because of the hypothesis that the ice layers may contain clues pertaining to what the Martian atmosphere was like thousands to millions of years ago.
A few orbiters has mapped the Mars polar caps with ground-penetrating radar. They found that the ice masses are a few kilometers thick, and built over older, deeper layers that may be at least 5 million years old. These old deposits are exposed in some places on the northern polar cap, thanks to the katabatic wind, which flows downwards and outwards from the high center to the low rim. Similar winds are found elsewhere on Mars and in some places on Earth, especially Antarctica. Katabatic winds are caused by radiational cooling of the air on high slopes, which then increases in density and flows downhill. The winds on the Martian northern polar cap can range from 6 to 12 meters per second (22 to 43 kilometers per hour).
Figure 1 shows how the katabatic winds behave. As they flow on a slight downward slope, they ablate material from the leeward (equatorial-facing) slopes and deposit it on the windward (pole-facing) slopes. This, coupled with Coriolis forces due to the planet’s rotation, gives rise to spiral troughs on the northern polar cap. These troughs are typically a few kilometers wide and a few hundred meters deep. An orbital view is shown in Figure 2.
The southern polar cap is different. CO2 ice does not completely disappear from this area due to its higher altitude and Mars’ orbital eccentricity, which leads to shorter summers in the south than the north. HYPATIA is not targeted there (for reasons to be expanded on in the next sections).
Investigating the Spiral Troughs
Because of the exposed ice layers (Figure 3) there is an opportunity to investigate the deepest parts of the polar cap at multiple sites at low cost, without the power and time expense of deep drilling.
An alternative to an deep-drill lander is to send a rover to one of the spiral troughs, where it can take core samples of the eroded slopes, where the ice layers are exposed. This idea has merit, but the slow speed of Mars rovers precludes the investigation of more than one trough before the polar night cuts off all solar power.
Why is this important? From radar readings, we know that the northern polar cap has at least 3 historical caps, each exposed in a different area. A mission capable of investigating several spiral troughs in one launch would be able to shed some light on how each historical cap differs from each other. A rover is not fast enough to do this, but a balloon vehicle would be more than capable of accomplishing this task.
To understand the relationship of water to atmosphere and climate, scientists rely on isotope ratios found in water and trapped gases from ancient ice cores. One such isotope is deuterium. Deuterium is a heavy isotope of hydrogen, consisting of one proton and one neutron (regular hydrogen only has one proton). Water molecules can have zero, one, or two deuterium atoms (H2O, HDO, and D2O).
Deuterium shows up in tiny but detectable amounts in every water sample in the cosmos. HDO and D2O molecules are heavier than H2O molecules, so they are less likely to sublimate/evaporate into the atmosphere. Therefore, an ice sample with a high D/H ratio would suggest high evaporation rates at the date when it was formed. Knowing the D/H ratio as a function of depth in the ice layers would help us understand how water is exchanged with the polar cap over time.
A small spectrometer capable of measuring the D/H ratios from sublimated ice would also be able to detect minuscule amounts of gases trapped in the ice, such as methane or carbon dioxide, providing a more complete record of how the atmosphere interacted with the polar cap up to 5 million years ago.
For how the balloon vehicle is designed to achieve its scientific objectives, stay tuned for Part 2!