Solar Power is Challenging on Mars
Key Points
Compared to the Earth, solar resources on Mars are poor, with an average irradiance only 43% that of Earth but with longer and more dramatic seasons that greatly exacerbate resource variability.
Orbital dynamics, atmospheric dust, red shifting, and other factors lead to low energy production outside of equatorial regions.
Many applications envisioned for Mars, from small robotic landers to large space settlements of tens of thousands of people, can nevertheless use solar power in the right circumstances, especially near the equator.
Better characterizing the Martian solar resource and developing Mars-specific solar technologies can improve the viability of Mars solar.
Large scale activities on Mars are likely to require integrated energy systems, including nuclear power, especially operations from the mid-latitudes to the poles.
Introduction
Energy resources are essential to crewed and robotic missions in outer space, including Martian surface activities. Indeed, energy supply is one of the primary constraints for mission planners, alongside mass, volume, cost, longevity, and harsh operating environments. Instruments, communications, life support, navigation, and other systems all require a sufficient level of reliable power. Chemical sources are primarily used for propulsion to get to Mars but, due to their relatively low energy density, cannot powering spacecraft for more than a few days. Subsequently, solar and radioisotopes are the power sources of choice for Mars surface missions.
Given the current high costs and limited availability of radioisotopes or fission power sources, a comprehensive understanding of the solar resource availability and technology is a primary factor shaping future Mars mission planning. In the last fifteen years, concerted efforts by national laboratories and other organizations have well characterized terrestrial solar resources. Some similar analyses have been published for Mars but, overall, Martian solar resource analysis and technology development is still in early stages.
The value proposition for solar energy on Mars is simple: the systems lack moving parts and have high mechanical reliability, they generate energy on site, they have achievable mass requirements, and they may be more economic or politically acceptable that space nuclear energy. However, the downsides include dependence on sunlight for production and a large area required for collection, which can be limiting for many applications.
In this post, I synthesize the current understanding of Martian solar resources. The three most important elements of Martian solar resources are:
Energy demand; the identity and needs of the energy user.
Solar irradiance; the nature of the relevant resource.
Solar technology; the ability to harvest said resource.
Generally, my analysis of these elements indicates that solar is a poor energy supply choice for many of the most interesting areas on Mars. Dust storms in particular threaten the ability to rely on solar for long-term crewed missions. Given the current limited availability of Plutonium-238, NASA’s radioisotope of choice, or other nuclear power alternatives, prospective Mars mission nevertheless will need to use solar power when possible.
So, lets dive into the details!
1. Demand profile
Before we can evaluate the solar resources on Mars, we first need to identify the potential demand sources in order to characterize solar’s ability to meet demand. Simply, the need for solar power can range from watts-electric to gigawatts-electric over the next fifty years. Everything from small robotic spacecraft, like Ingenuity helicopter to full scale cities have significant energy demands.
To date, the only missions to the Mars surface have consisted of landers or rovers. These missions can choose either solar or radioisotope power sources. Looking forward, we could see other types of power demand, particularly initial crewed missions and longer-term Mars settlement. This section describes each of these potential applications in more detail.
A. Robotic Probes, Landers, and Rovers
Since the beginning of the space age, there have been eleven successful landings on Mars, including six rovers. Solar energy has been the primary energy source for many of these missions, including Pathfinder, Spirit, Opportunity, and Insight. Such rovers and landers perform scientific missions targeted towards achieving a set of objectives with certain instruments in a limited geographic area.
Missions to Mars have a pretty scary failure rate. Source: ESA
The selection of solar power for a Mars mission can impose constraints on mission landing and operating locations. For example, Golombek et. al. (2003) describes how the constraint for near-equatorial landing areas for the Mars Exploration Rovers (Spirit and Opportunity) was heavily influenced by the need to maximize solar power.
Historically, NASA selected solar instead of radioisotope power sources due to the performance requirements of a specific mission, not the relative costs of each. However, radioisotope unavailability is becoming an increasingly important factor. To wit, the most recent Decadal Survey released by the National Academy of Sciences in early 2022 indicates that availability of Plutonium-238 supply for space probes is likely insufficient for all but flagship missions. The survey calls out solar for rover applications, yet it caveats that low power production remains an operational constraint:
“Solar arrays for landers and rovers present unique challenges. Power needs for these flight systems are generally high enough to need large arrays that cannot be readily accommodated, so smaller arrays are used, and operations are often limited by power cycling. But improvements in the accommodation and reliability of retractable arrays can make larger arrays a viable option for future missions.”
B. Small, crewed missions
By 2040, crewed missions to Mars are feasible for scientific and exploration objectives. There are various proposed architectures for these missions, as well as target locations. The current authoritative NASA document is the 2009 Design Reference Architecture 5.0. Notably, one of the largest power requirements is producing propellant for a return trip; the study prefers a nuclear fission reactor to solar because of solar intermittency and greater mass requirements (~three times larger).
Conversely, Cooper et. al. argues that a similar sized mission could be powered by solar, in part due to advantages related to policy considerations (namely reactor launch safety). Similarly, Rucker et. al. found solar could be competitive with nuclear for propellant production for small-scale human missions if located near the equator.
So are equatorial locations ideal locations for scientific missions? Perhaps.
In 2015, a workshop of NASA, industry, and other specialists identified dozens of “exploration zones” on the Martian surface that met scientific and space resource production criteria. The zones from this report, charted below, provide a decent proxy of potential areas of interest for future crewed missions.
For present purposes, zones are distributed relatively evenly across Mars, meaning global-level climate and resource data for solar energy is relevant for future technology selection. Many zones are equatorial, verifying Cooper and Rucker’s arguments regarding solar viability for certain scenarios. However, the workshop specifically excluded zones above 50 degrees latitude due to perceived mission complexity (landing, more delta-v for liftoff, and energy constraints like limited solar).
C. Long-term activities, including settlement
In the long-term, human habitation and settlement may be possible on Mars. Given the high energy requirements necessary to support such an endeavor, energy is likely to be a primary economic factor. Thus, solar power availability is likely to be a primary factor in site selection. There are limited studies of large-scale settlement activities on Mars and related energy requirements. For a decent proxy, the Mars Society had a contest to design a city with a million inhabitants on Mars. Of the 20 submissions profiled, most featured nuclear energy but many also included solar energy for primary, secondary, or emergency power sources.
Importantly, given the absolute necessity of energy for survival of large settlements and the varied types of energy demand for residential and industrial applications, a wide diversity of energy resources is likely to be preferable to maximize resilience and provide economic optimization. For most mid-latitude and even equatorial locations, I would expect a mix of solar, nuclear, and perhaps areothermal (i.e. Martian geothermal). What that mix looks like depends on the resources and technology available.
However, while beyond the scope of this blog post, I expect that many early settlements (let’s say around 2050), will prefer latitudes above ~40 degrees in the northern hemisphere. Water and other resources are likely to be better, while lower elevations may boost radiation protection and reduce landing delta-v due to greater aerobraking.
2. Solar energy resources on Mars
At the first principles level, solar energy resources are relatively easy to calculate. Take the distance from the sun at a moment in time to get the solar energy available (irradiance), then subtract out atmospheric losses (if any). In practice, however, changing orbital dynamics, different locations, seasons, and atmospheric conditions make solar production highly variable.
At a specific location, mission planners need to account for the variations in irradiance over the course of a whole year.
Although some relevant solar energy production factors on Earth, like the presence of seasons, likewise occur on Mars, the nature of seasons is fundamentally different. Additional environmental factors, such as dust storms, atmospheric density, radiation, temperature, and surface features like slope and roughness are all different. This section reviews the primary determinants of solar resource for a specific location: orbital characteristics and latitude, atmospheric dust, dust, panel degradation, and multi-factor synthesis.
A. Orbital characteristics and solar irradiance at top of atmosphere
Although Mars and Earth are both rocky planets in the inner solar system with relatively close masses, differences in their orbits and other physical parameters greatly impact solar resource availability:
Differences in Key Orbital Characteristics Between Earth and Mars
First, the good news. Unlike the Moon, where lunar nights last two Earth weeks which greatly limit the solar resource, Mars has an almost Earth-like day of just over 24 hours. This means that solar resources are relatively consistent and predictable. With energy storage, missions can generally manage short term variability. Beyond electricity, such frequency also means solar could be ideal for rover or mission thermal management.
Next, the neutral news. Mars is tilted on its axis by about 25 degrees, which is close to Earth and leads to seasonal variations in solar power. Mars has the equivalents to the Earth’s arctic/Antarctic circles and tropics, at ~65 degrees and ~25 degrees, respectively. These are relevant for mission planners as locations above ~65 degrees on Mars can experience multiple days without daylight, while everything between the tropics will have the sun directly overhead at least twice during a Martian year. Overall, axial tilt is neutral because it ensures everywhere has solar production for at least some part of the year but lacks consistency for any location.
Finally, the bad news. Martian solar resources are much worse than Earth’s. On average, NASA data indicates that average solar irradiance (W/m2) for the Martian orbit is 43.1% that of Earth orbit (586.2 vs 1361.0). This is because the solar irradiation power intensity from the sun falls by the square of the distance and Mars is that much farther out.
Worse, this is just an average value and there are large periods of the year where solar resources are much worse.
Why?
Orbital eccentricity.
Mars has a higher orbital eccentricity than Earth, meaning that its orbit around the sun is more oval shaped and the distance to the sun varies considerably. During its closest approach to the sun (perihelion) Earth is only 5 million kilometers (3.3%) closer than it is at the farthest point (aphelion). In practice this is so small that terrestrial solar developers can just use average irradiance values for project planning purposes. Comparably, the difference between Mars’ perihelion and aphelion in 42.6 million kilometer difference, which is almost 21%!
Accordingly, solar irradiance at Mars’ perihelion is 723.2 W/m2 while solar irradiance at Mars’ aphelion is only 497.2 W/m2! Solar irradiance at aphelion is only two thirds of that at perihelion, creating a large temporal difference in available solar resource, just at the orbital distance, let alone accounting for other factors.
Perihelion is relatively close to summer solstice in the winter hemisphere, meaning that the southern hemisphere gets the maximum solar irradiance compared to the Northern hemisphere. However, this portion of the orbit is shorter, meaning that the period of peak production is relatively brief. Further, Martian seasons are about twice as long as Earth’s.
Considering orbital distances, axial tilt, and other factors, multiple studies have looked at what these different orbital characteristics mean for available solar resources, particularly at the top of the atmosphere.
Solar irradiation at top of Mars atmosphere over course of year (cal per cm-2 per day-1)
Note that x-axis is note days, but degrees, each of which equals approximately 2 days. Note that 1 calorie per centimeter per day equals about 0.5 watts per meter squared. Source: Levine et. al.
The first major study, from Levine et. al., provides many important insights:
The poles get the most intense solar irradiation, but only for a limited duration near solstices and at low zeniths
The Southern summer is shorter than the Northern summer but with more intense solar irradiance and higher variability, especially at the south pole
Polar regions above ~70 degrees experience winter blackouts longer than 100 days and as much as 300 days nearest the poles
The equator features relatively constant irradiance (between 300 and 400 cal per cm-2)
Northern mid-latitudes generally have higher minimum irradiance levels than their Southern equivalents
These conclusions are supported by and expanded on in subsequent studies:
Atreya and Kuhn (1979) calculated the maximum daily radiation at 40 degrees is about 325 cal/cm-2, with notable wavelengths absorbed by the high carbon dioxide content in the atmosphere.
Hemelrijck (1983) evaluated how eccentricity, obliquity, and longitude of perihelion impacted seasonal insolation, particularly at high latitudes.
Appelbaum and Flood (1990) evaluated diurnal, hourly, and daily data on solar insolation, in part based on data from the Viking landers.
While a negligible factor, it is also a fun trivia note that eclipses by the tiny Maritan moon Phobos were picked up as slightly reducing InSight’s solar production.
B. Environmental factors on the solar irradiance received at the surface
a. Atmosphere and dust
Although the atmosphere of Mars is thin, it absorbs some incoming solar radiation, somewhat reducing solar irradiance on the surface of Mars compared to the top of the atmosphere. This is especially true for wavelengths below 200nm, which experience very high absorption from the relative high share of carbon dioxide in the Martian atmosphere.
Further, the atmosphere suspends dust, even in “clear” conditions, which scatters incoming sunlight. Optical depth is a key metric for Martian solar resources as it impacts the direct beam solar radiation and depends on the amount of dust in the atmosphere. In looking at representative samples, Levine et. al. found that surface radiation at the equator was about 15% lower, while almost 30% lower near the poles.
The 2001 global dust storm on Mars. Source: NASA/JPL
There are different mechanisms which lift dust in the atmosphere in normal conditions, with wind stress lifting dominating during the Southern summer and dust devils playing a larger role in Northern summer. Dust devils could play a very important role in local dust conditions and their impact on solar panels. In addition to reducing overall intensity, dust also shifts the spectrum of solar radiation hitting the surface of Mars, impacting solar technologies.
Mars also experiences periodic global dust storms, which greatly increases the optical depth of the atmosphere, reducing solar panel performance. These dust storms occur periodically, about every 3 Martian years or so. Such severe storms can last from several weeks, to several months. They are tied to Martian seasons and recent research has linked them to the planet’s overall energy budget, meaning they are a second order effect of Mars’ high orbital eccentricity.
There are differences in how dust impacts the mean seasonal daily insulations across latitudes, with less impact at the equator and more at the poles (reflecting atmospheric travel distance). In looking at a representative day from the 1970s Great Dust Storm, Levine et. al. found that the severe dust conditions would greatly reduce available solar irradiance on the surface, by more than 90% at the equator to more than 99% at the poles.
Dust and dust storms can greatly impact or even end solar-powered missions. During the first 800 sols of the Insight mission, dust coverage reduced solar power production by 0.2% per sol. Such an impact was expected and shaped mission plans, with landing dates and operations very mindful of global dust storm season.
Martian dust covers solar panels, as seen on Spirit. Spirit’s twin Opportunity eventually lost power during a dust storm and ended operations. Source: NASA/JPL
Dust coating can be periodically reversed by “clearing events” such as dust devils and wind. In part, InSight benefited from dust observations collected by Spirit and Opportunity, which were able to identify seasonal and annual trends in dust, including the impact on solar generation. More recent observations by Curiosity indicate sufficiently frequent clearing events to support a solar-powered rover at its (low) latitude and specific location.
b. Panel Degradation from Environmental Factors
Solar panels degrade over time and this degradation can be enhanced by environmental factors, with large implications for long-term missions. Jordan and Kurtz (2012) evaluated photovoltaic degradation rates for almost 2,000 terrestrial systems, finding a median value across technologies of 0.5%/year. However, they found significant variation across technologies and timeframes, with modern degradation rates as low as 0%/year and as high as 4.5%/year. Several studies have sought to identify degradation mechanisms:
Sanchez-Friera et. al. (2011) found that the most frequent defects in photovoltaics include glass weathering, delamination, and oxidation of the coating.
Boyd et. al. (2018) identified degradation mechanisms in perovskite photovoltaics, noting potential issues with moisture, oxygen, heat, light, and mechanical stress.
Kherici et. al. (2021) evaluated silicon solar cells in Algeria, a hot desert climate, finding the environmental conditions led to physical changes in the encapsulant, increasing the cell’s series resistance.
Just as Martian dust can block solar irradiance in the atmosphere, or directly coat solar panels, dust storms can accelerate degradation of solar panel systems. Estimated degradation could be as high as 2% per year.
For space solar systems, ionizing radiation is also a major degradation mechanism as nuclear particles strike a solar cell’s wafer, modifying the crystal structure and creating defects. Degradation depends on the type of radiation, its intensity, and the type of solar cell.
An early study on the solar panel degradation on the International Space Station found a total degradation rate of 0.2-0.5% per year, but it is worth noting the space station is in Earth’s protective magnetic field.
Due to its thin atmosphere, the radiation environment on the surface of Mars is intense, about 2.5 times greater than that at the space station. As surface radiation is tied to atmospheric height, the lowest elevations of Mars have half the radiation as the highest. Generally, the Southern hemisphere has higher altitudes and the Northern hemisphere has lower. Depending on the panel technology used and its location, radiation could shorten system lifetimes and impact economics.
Much like Mark Watney in the Martian, future astronauts may spend a lot of time cleaning dust off of solar panels. Source: The Martian
C. Synthesizing a solar environmental model for Mars
Combining solar radiation resources and other environmental conditions can provide a workable model for future missions. A white paper for the 2023-2032 decadal survey submitted by Schwartz et. al. (2021) succinctly describes the primary environmental challenges to the use of solar as “dust accumulation on solar panels, variations in optical depth through the atmosphere, and a red-shifted spectrum.”
Building on neural networks to estimate terrestrial solar resource, Khatib et. al. (2020) created a model that incorporated geography, time, temperature, altitude, pressure, dust, and clouds to predict mean solar radiation for every location on Mars. They found a “radiation-temperature belt” between 20 degrees south and 15 degrees north with optimal production conditions.
Generally, this aligns with my expectations. The solar radiance at these latitudes is relatively consistent and a solar system in these areas can produce relatively reliable power with battery storage throughout the Martian year. Given the length and frequency of global dust storms, alternative back up systems may nonetheless still be required for crewed operations.
3. Solar technologies and selection for Mars
This section reviews available solar power technologies, primarily focused on static systems. There are relative advantages and disadvantages for specific types of solar power panels. Depending on the specific solar technology selected, there are multiple strategies for maximizing solar radiation, including panel orientation, dust mitigation, and use of tracking technologies when available.
Solar photovoltaic panels produce power by converting light into direct current electricity using the photovoltaic effect. Different panel types use different materials and structures to best harness the solar resource (see figure below). Solar cells can be stacked to create multi-cell panels, enabling better overall light capture and conversion properties, while balancing advantages and disadvantages.
Research on record solar cell efficiencies by type since 1975
Several solar panel options are worth reviewing in depth:
High efficiency III-V multijunction solar panels are often the technology of choice for space given high efficiency and radiation resistance
Gallium arsenide (GaAs) solar cells have been favored by NASA for space applications for several decades due to high efficiencies and is one of the most common compounds in III-V panels
More recently, there has been increasing interest in the use of perovskite solar cells for both terrestrial and space applications as they have a better specific power, have sufficient radiation resistance, and may be cheaper
As discussed in Section 3, the solar spectrum on the Martian surface is different from that on Earth due to dust in the atmosphere. Given the red-shift of the spectrum, cells that produce the most electricity from the blue spectrum suffer performance losses in the Mars atmosphere while cells that produce more electricity from the red spectrum see improved performance. For example, Landis and Hyatt (2006) found that this spectrum shift can be especially important for multi-cell stacks as a poor outer cell, like GaInP, can limit certain incoming wavelengths whereas a GaAs cell can see enhanced performance. Approaches to optimize solar panels for Mars include redesigning multijunction cells, selecting for red-shifted subcells, and dust management.
As noted by Schwartz et. al., general technology development for solar photovoltaic technologies will improve technology options and performance for Mars systems, especially higher efficiencies for lighter weight systems. Similarly, technology development targeted towards other planetary bodies, such as techniques for dust mitigation on the Moon, could improve future solar options on Mars. Schwartz et. al. makes multiple recommendations for NASA’s investment in future space solar capabilities, including:
“Dust mitigation on lunar and Martian surfaces to enable long-term, reliable operations”
“Infrastructure for testing and qualification of solar array technology for unique environments”
“… leverage investments in high efficiency (~38% at AM0) solar cells enabling high power and lower mass arrays.”
Beyond solar panels, it is worth mentioning two other major types of solar power: concentrating solar power (CSP) and space-based solar power (SBSP).
CSP uses mirrors to concentrate the sun’s energy to heat water or another working fluid. Generally, utility-scale CSP has struggled to gain traction in global energy markets due to relative plant complexity or high cost. However, while typically not thought of as CSP, solar hot water is widely used in distributed applications globally. Utility or industrial scale CSP may be challenging on Mars given the harsh operating environment but, on the other hand, could be relatively to manufacture with local materials. Solar hot water systems could be used to support thermal management with much less complexity.
SBSP is a proposed technology solution for terrestrial energy markets that would use solar panels in space to generate electricity, regardless of atmospheric or nocturnal conditions, and beam it to the Earth surface with microwaves. While still in the R&D phase on Earth, SBSP may make sense for Mars, especially for high northern latitudes. By selecting optimal orbits, SBSP could enable year round solar use for most locations, although dust may still pose a challenge, especially for receiving equipment on the surface.
4. Conclusion: Martian solar resources are generally poor but sufficient for certain activities
Given relatively low costs, solar power could make sense for many activities on Mars, ranging from rovers to future cities. However, the poor and variable nature of the resource means that insufficient energy will be a primary mission constraint. Martian dust has already killed its first Martian rover and many more will follow. Ultimately, any complex activities are likely to benefit from integrated energy systems to achieve balance across reliability, crew safety, low cost, and resilience.
To make the maximum use of solar on Mars, there is a lot more research and technology development to be done. I believe further research is likely needed for the following topics:
Effects of variable irradiance and latitude on solar energy availability for constant-rate ISRU applications, including propellant production
Panel degradation mechanisms on Mars, including dust abrasion and radiation exposure
The nature and extent of local environmental dust conditions, particularly the frequency and intensity of clearing events
Design and reliability of integrated energy systems with solar power
Opportunities for non-power uses of Martian solar resources, including water heating, thermal management, and industrial applications
Relative advantages of specific solar technologies, tracking vs. non-tracking, orientation, and other factors at different latitudes on Mars
Development of solar panels specifically suited to the Martian environment, including the red-shifted spectrum, dust mitigation, and lower average temperatures
In-depth research on the ability of space-based solar power to deliver power to locations in upper latitudes
