Time on Mars

INSApplies to the
InSight AN
MERApplies to the
MER AN
MSLApplies to the
MSL AN
PHXApplies to the
PHX AN

A number of measurement systems are used to reference time for Mars landed missions. The system used depends on the purpose.

Sol

A simple measure of time on Mars is the sol, that is, a solar day. Just like on Earth, a solar day on Mars is the period of time from one noontime to the next for a location on the planet. The number of seconds in a solar day varies throughout the year due to the planet's orbit eccentricity and obliquity (axial tilt). For general purposes, we can work with an average solar day on Mars being 24:39:35 (hours:minutes:seconds), just as we consider an average solar day on Earth being 24:00:00, even though we get leap seconds every so often.

Each landed mission uses a sol counter that starts with landing. Curiosity rover and the InSight and Phoenix landers began counting at sol 0 on the day of landing. Opportunity and Spirit rovers began counting at sol 1.

Because a sol (day on Mars) is only slightly longer than an Earth day, a 24-hour clock is a useful method for keeping local time on Mars. As humans, we already are familiar with 24 hours in a day, even if the length of an hour is not the same on both planets. We also benefit in many cases from being able to use computer time functions designed for 24 hour days. Certain tasks require higher precision values, of course, and we'll get there eventually. Local times are presented as day of mission following landing along with hour, minute, and second.

Local time

Mars lacks a time standard like Universal Time on Earth, leading landed Mars missions to reference local time relative to their landing site in addition to other time measurement systems. MSL Curiosity and MER Spirit and Opportunity rovers use a Hybrid Local Standard Time (HLST) for planning, based on the expected landing sites and mission duration. InSight lander use a Local Mean Solar Time (LMST) for planning, also relative to the expected landing site. The values are included in the Analyst's Notebook mission time table to assist power users. They will not be of use for the vast majority of users.

SCLK (Spacecraft clock count)

The Mars rovers and landers use a spacecraft clock that serves as the estimate of the number of ephemeris seconds past the J2000 epoch (roughly noon UTC on January 1, 2000). Data product metadata labels report observation start and stop SCLK values. Note that some instrument systems may have one or more internal clocks whose values also are reported in product labels.

UTC (Coordinated Universal Time)

Earth-based time values are presented as UTC values, that is, the mean solar time at 0° longitude on Earth. Data product labels commonly present UTC values for observation start and stop times. UTC times generally are presented as values of year, month, date, hour, minute, second. In some cases, fractional seconds also are given.

Solar longitude

The position of Mars in it's orbit about the sun can be measured as solar longitude. The 0° position is Mars' location at the vernal (spring) equinox and increases to 90° at the summer solstice, 180° at the autumnal equinox, and 270° at the winter solstice, all northern hemisphere seasons. Solar longitude is often written as Ls (or Ls) and is pronounced "L sub s". The seasons on Earth are about the same length because its orbit is nearly circular. Not so with Mars, with an eccentricity (0.0934) that is visible even in the scale of the figure below, created with SPICE-enhanced Cosmographia (https://naif.jpl.nasa.gov/naif/cosmographia.html ).

The solar longitude is not specific to a particular lander or rover. It can be useful for seasonal comparison studies, especially for missions that spanned multiple Mars years.

Ls Start of season, northern hemisphere Duration (sols) Duration (Earth days)
Spring 197 199
90° Summer

178

183
180° Autumn 143 147
270° Winter 153 158
    668 687

 

Solar azimuth and elevation

The sun's position in the sky at the time of an observation is denoted by the solar azimuth and elevation. The azimuth defines the sun's direction along the horizon, relative to the observer, with directly north being 0 degrees and increasing clockwise in a 360 degree circle. Thus, a solar azimuth of 90 degrees is directly east of the observer, and 180 degrees is directly south. The elevation defines the height of the sun in the sky along the vertical plane normal to the surface and including the sun and observer, with the horizon being 0 degrees and increasing to 90 degrees being directly overhead.

Be careful to differentiate between solar elevation, sometimes called solar altitude, and solar zenith. Elevation is measured from the horizon up to the sun's position, whereas zenith is measured from the vertical down to the sun's position. Elevation and zenith angles are complementary.

Sun distance

The distance between the barycenters (centers of gravity) of Mars and the sun. The Notebook reports this number in A.U. (astronomical units).