A “sol” is the term space agencies use to define a single solar day on Mars, distinguishing it from the 24-hour day on Earth. This unit of time is fundamental to planning and executing all missions involving rovers and landers on the Martian surface. The difference in day length between the two planets results from their unique physical properties and their paths around the Sun. Understanding the duration of a sol is necessary for comprehending the daily life and operational challenges of Martian exploration.
The Exact Length of a Sol
The average duration of a Martian solar day, or sol, is precisely 24 hours, 39 minutes, and 35.244 seconds in Earth time units. This period represents the time it takes for the Sun to return to the same position in the Martian sky. The term “sol” was adopted during NASA’s Viking Lander missions in 1976 to prevent confusion between the two planets’ distinct day lengths.
A sol is nearly 40 minutes longer than the 24-hour mean solar day on Earth. Specifically, a sol is approximately 2.75% longer than an Earth day, meaning about 73 sols pass for every 75 Earth days. This slight difference accumulates over time, requiring a specialized approach to timekeeping for mission scientists and engineers.
The precise length of the solar day is not constant throughout the Martian year. Mars has a significantly more eccentric, or oval-shaped, orbit around the Sun than Earth does. This greater variation in orbital speed means the length of a sol can fluctuate more. The fluctuation causes the Sun to run up to 51 minutes slower or 40 minutes faster than a Martian clock at different points in the year.
The Astronomical Reason for the Difference
The length of a planet’s day is determined by two main factors: its rate of rotation on its axis and its orbital speed around the Sun. The time it takes for a planet to spin once relative to the distant stars is known as a sidereal day. On Mars, the sidereal day lasts 24 hours, 37 minutes, and 22.66 seconds.
The solar day (the sol) is longer than the sidereal day because Mars is moving in its orbit while it rotates. To complete a solar day, the planet must rotate slightly more than a full 360 degrees to bring the Sun back to the original position. This extra rotation accounts for the additional two minutes and 12.58 seconds that make the solar day longer than the sidereal day.
For comparison, Earth’s sidereal day is approximately 23 hours, 56 minutes, and 4 seconds. The solar day on Earth is nearly four minutes longer, averaging 24 hours, because the planet must rotate an extra amount to compensate for its orbital movement. Mars rotates at a rate nearly as slow as Earth’s, but its longer orbital period results in the final solar day length being only slightly longer than Earth’s.
Mars’s greater distance from the Sun means it travels a shorter distance along its orbit during one rotation than Earth does. However, the slower rotation rate of Mars is the primary reason the sol is extended to the 24-hour, 39-minute mark. The combined effect of the planet’s slower rotation and its orbital motion makes the Martian day just under 40 minutes longer than the day on Earth.
How Mission Scientists Use Sols
Mission control teams managing rovers and landers track time using sols to synchronize spacecraft operations with the Martian day-night cycle. Using sols allows for efficient scheduling of activities, such as driving, drilling, and taking photographs, which depend on Martian daylight and available solar power. By planning activities based on local solar time, scientists ensure a rover is active during the Martian day and resting during the Martian night.
This system requires mission personnel on Earth to adapt to “Mars Time,” where their work schedule shifts forward by about 40 minutes each Earth day. This continuous shift ensures the human team’s waking hours align with the rover’s daylight hours. Aligning schedules is necessary for timely command uplinks and data downlinks. Following a Martian schedule can be challenging for the human circadian rhythm but is necessary for maximizing the mission’s science return.
The count of sols begins upon the spacecraft’s successful landing. Historically, different missions have designated the landing day as either Sol 0 or Sol 1. For instance, NASA’s Curiosity and Perseverance rovers counted the touchdown day as Sol 0, while the earlier Mars Exploration Rovers used Sol 1. This standardized sol count provides an unambiguous, mission-specific calendar for tracking the cumulative duration of surface operations.