The measurement of time is intrinsically linked to a planet’s rotation and orbit. When exploring Mars, the rhythm of the planetary day shifts, creating a distinct temporal challenge. Because Mars spins at a different rate than Earth, the basic units of time—the day, hour, and minute—must be redefined. Scientists have developed unique timekeeping conventions to synchronize activities between Earth-based mission control and rovers operating millions of miles away.
Defining the Martian Day (The Sol)
The foundation of Martian timekeeping is the solar day, which planetary scientists refer to as a “Sol.” This term was officially adopted during NASA’s Viking missions in 1976 to prevent confusion with the Earth’s 24-hour day. A Sol marks the interval between the Sun’s return to the same position in the Martian sky, similar to how an Earth day is measured from noon to noon.
The precise duration of one Sol is 24 hours, 39 minutes, and 35.244 seconds in Earth time. This makes a Martian day approximately 2.75% longer than an Earth day, a small but significant difference that compounds over weeks and months. This close similarity exists because Mars’s rotational period is only slightly longer than Earth’s, and its axial tilt is comparable at about 25.2 degrees.
The slight extension of the Sol arises because Mars orbits the Sun while rotating on its axis. Since the planet moves forward in its orbit, it must rotate slightly more than a full 360 degrees for the Sun to appear in the same position again, adding the 39 minutes and 35 seconds to the total period.
The Martian year spans about 668.6 Sols. The consistent use of the Sol as the base unit is necessary for all surface operations, especially for planning activities around local light and temperature cycles.
Calculating the Martian Hour and Minute
To maintain a familiar structure, scientists divide the Martian Sol into 24 conventional Martian hours, mirroring the 24-hour clock used on Earth. This means a Martian hour is proportionally longer than an Earth hour. The exact length of a Martian hour in Earth time is 1 hour, 1 minute, and 39 seconds.
This calculation is derived by dividing the total duration of the Sol (24 hours, 39 minutes, 35.244 seconds) by 24. This is equivalent to a standard 60-minute hour stretched by about 2.75% to accommodate the longer planetary day.
The Martian minute and second are extended by the same proportion. A single Martian minute is equivalent to approximately 61.65 Earth seconds. Furthermore, a Martian second measures about 1.0275 Earth seconds.
A clock set to Martian time runs slightly slower than an Earth clock, but it completes a full 24-hour cycle aligned with the Martian day-night cycle. Engineers and scientists use specialized “Mars clocks” that incorporate this slower, 2.75% longer timing to track local solar time on the planet’s surface.
Operational Timekeeping on Mars Missions
The small daily difference between the Sol and the Earth day creates a unique logistical challenge for mission controllers. Ground teams working with rovers like Curiosity and Perseverance adopt “Mars time” to synchronize their schedules with the spacecraft’s local solar time. This alignment is necessary because most rover activities, such as driving and sampling, rely on daylight and optimal temperature conditions.
Operating on Mars time means the Earth-based team’s work schedule must shift approximately 40 minutes later each Earth day to keep pace with the rover’s sunrise and sunset. This continuous slippage requires mission controllers to periodically adjust to a nocturnal schedule on Earth for weeks, allowing them to command the rover during its daytime.
For long-term time tracking, each landed mission uses a Sol counter, beginning the count with either Sol 0 or Sol 1 on the day of landing. This simple numbering system provides a mission-specific calendar for operational planning and data logging.
While many missions rely on local solar time, a proposed universal system called Coordinated Mars Time (MTC) exists as an analog to Earth’s Coordinated Universal Time (UTC). MTC is defined by the mean solar time at Mars’s prime meridian, which passes through the crater Airy-0. Although MTC provides a standard reference, local solar time remains the most practical metric for coordinating the daily activities of surface assets.