Those who follow my EPSILON Sci-Fi Thriller Series know it’s set in a feature on Mars called Hellas Planitia. To date, there have been twenty successful unmanned mission landings. Sites range from pole to pole, with most concentrated in craters and plains between 30 degrees North and 30 degrees South. Many have found evidence of the existence of past liquid water, or present-day ice. But none have explored the vast crater known as Hellas Planitia.
In some ways, Mars is much like Earth. Its axis of rotation is tilted 25 degrees with respect to the plane of its orbit around the Sun, similar to Earth’s axial tilt of 23.4 degrees. Like here, the Red Planet has distinct seasons. At low latitudes Hadley circulation dominates, comparable to the trade winds. At higher latitudes a series of high and low pressure areas dominate the weather, also like here. Katabatic winds affect both the northern and southern polar caps. The Greenland ice sheet is famous for its fierce katabatic winds.
But in many other ways the planet is, well, alien. It orbits the sun 60 million miles farther out than we do. It receives less than half the solar radiation per square meter that Earth does, 590 Watts/sq m compared to about 1370 Watts/sq m.
For a host of reasons, the Red Planet is colder than Earth. Much colder. A warm summer day at the equator barely rises above freezing, only to plunge to minus 150 degrees F at night.
Typical daily temperature swings are extreme. Away from the polar regions, diurnal differences are around 180 degrees F (100 degrees C)!
Mars’s mean surface pressure is about 600 pascals (Pa), roughly one-one-hundredth of sea level air pressure(101,000 Pa). 600 Pa is comparable to an Earth altitude of about 44 km (27 mi).Consider that at the summit of Mt Everest, about 5 miles high, most humans need supplemental oxygen to survive.
The low density of the Martian atmosphere means that winds of 18 to 22 m/s (40 to 49 mph) are needed to lift dust from the ground. But once airborne, the atmosphere is so dry small particulates can stay suspended far longer than on Earth, where it is soon washed out by rain.
Although the temperature on equatorial Mars can reach above freezing, the low atmospheric pressure is below water's triple point, 611.66 Pa. This makes liquid water unstable over much of the planet, meaning water ice sublimates directly into water vapor. Where brine breaks out of certain crater walls, it literally boils as it freezes, the flow never reaching the floor.
Polar winters on the Red Planet are considerably colder than the coldest winters on Earth, with temperatures dropping to a frigid minus 243 degrees F (minus 153 degrees C). This is cold enough to maintain ice caps of frozen carbon dioxide (dry ice). The north polar dry ice cap is about one meter (three feet) thick, and eight meters (26 feet) deep at the south pole. The water ice underneath Mars’s carbon dioxide ice sheath is gargantuan by contrast, two km (1.2 miles) thick in the north and three km (1.7 miles) thick in the south.
CO2 ice accumulates in the north polar region in winter only, sublimating completely in summer. But the south polar region’s cover is permanent. The south pole sits at a higher elevation than the north, and is therefore colder.
Spring dust storms are common on Mars. They are driven by the pressure gradient created when the CO2 ice cap sublimates during that hemisphere’s Spring. On the Red Planet, dust in the atmosphere absorbs sunlight, raising the air temperature another 7 degrees F (4 degrees C), making the winds even more energetic. The resulting feedback loop generates dust storms that obscure the entire planet.
A summer day on Mars may warm up to 70 degrees F (20 degrees C) near the equator, but at night it can plummet to about minus 100 degrees F (minus 73 degrees C). Gale Crater, situated at 5.4 degrees South latitude, sees an average summer high of 36 degrees F, an average summertime low of minus 105 degrees F. The winter average high is minus 9 degrees F, and average low is minus 126 degrees F.
Current research suggests that the Red Planet is in a warm (?!) interglacial period which has lasted more than 100,000 years. It is now thought that mid-latitude ice accumulated when Mars's orbital tilt was very different from what it is now. The planet’s rotational axis precesses (wobbles), so its angle changes over time. A few million years ago, the tilt of the axis was 45 degrees. During some previous ice ages Mars’s poles tilted as much as 80 degrees.
Studies have shown that when the angle of Mars's axis reaches 45 degrees, polar ice is no longer stable. Stores of solid carbon dioxide ice sublimate, thereby increasing the atmospheric pressure. This allows more dust to be held in the atmosphere, raising the temperatures even further. Moisture in the atmosphere falls as snow or as ice frozen onto particulates. Calculations suggest this material will concentrate in the mid-latitudes.
Scientists believe that these dusty snows may be the source of the midlatitude subterranean brines observed bursting out of the upper elevations of cliffs and crater walls. They may also be the source of the vast buried glaciers present in Hellas Planitia.
Why do I speculate in my series that the first commercial manned mission to Mars will occur in Hellas Planitia? Recall that Hellas Planitia is an impact crater, the largest confirmed in the solar system. The 7,000 km diameter basin is nine km deep, making it the lowest elevation on the planet.
It’s so deep that the summertime atmospheric pressure at the bottom reaches 1155 Pa, 1240 Pa during winter, when the air is coldest and at its highest density. This is above the triple point, so when the temperature exceeds freezing, liquid water can exist there. A Midsummer day can reach 32 degrees F or higher. The daily maximum soil temperature is about 75 degrees F. Water processing for drinking, irrigating crops, and electrolysis for hydrogen and oxygen won’t have to occur in pressurized facilities, as would be the case everywhere else on the planet’s surface.
The relatively mild Hellas summer daytime highs will require less energy expenditure for heating habitats and EVA suits. Not that the energy budget can be eliminated altogether. Nights are still bitter cold. The warmest Summer lows are minus 50 degrees F. Winters are much colder, dropping at night down to minus 140 degrees F with a daytime average of minus 60 degrees F.
But compared to equatorial locations, Hellas Planitia is a veritable garden spot. Now if only some ambitious company would start prospecting …
Like what you just read? Share this issue with friends and encourage them to
subscribe to receive free short stories, news about upcoming promotions and books by yours truly and other exciting Sci-Fi authors!
Want a deeper dive? Check out these sources, listed in the order of discussion.
https://solarsystem.nasa.gov/planets/mars/in-depth/https://en.wikipedia.org/wiki/Climate_of_Marshttps://en.wikipedia.org/wiki/Mars_landing#:~:text=There%20have%20also%20been%20studies,have%20conducted%20Mars%20landings%20successfully.
