Martian timekeeping

Posted By Emily Lakdawalla

2011/03/09 12:23 CST

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While scanning through the talks scheduled for this week's Lunar and Planetary Science Conference I came across the following talk title: "Interannual and Seasonal Variability in the North Polar Region of Mars: Observations in Mars Years 29 and 30 by MARCI, CTX, and CRISM." My first thought was "hey, cool research spanning a long time period and across data sets." But my second was "Mars years 29 and 30? What does that mean?"

Russian Academy of Sciences / Ted Stryk

Mars 3 arrives to a dust-shrouded planet

When the Russian orbiter Mars 3 arrived at Mars late in 1971 (near the northern winter solstice late in Mars year 9, according to the convention described here), it found the surface of the planet to be almost completely obscured by an enormous global dust event. Several months later, the sky was finally clear enough to reveal surface features. The image on the left was captured on December 12, 1971, and the one on the right on February 28, 1972.

Clearly it's a way of marking the passage of time on Mars, but I didn't recall seeing such a thing used in an academic paper before. The agreed-upon method (where "agreed-upon" means "used by scientists publishing peer-reviewed papers") of marking time within a single Mars year is something called "Ls," read "ell sub ess," which is short for "solar longitude." Ls marks Mars' position in its orbit around the Sun, measured in degrees, from 0 to 360. As I explained on this webpage, Ls is 0° at the vernal equinox (beginning of northern spring), 90° at summer solstice, 180° at autumnal equinox, and 270° at winter solstice. Dust activity tends to peak a bit before the northern winter solstice, around 260°, though it can happen any time of the Martian year.

But that calendar resets with each of Mars' elliptical orbits around the Sun. Clearly "Mars years 29 and 30" refers to a way to count up the orbits, and it doesn't take much effort to deduce that it's counting up from some point in time near the start of the Space Age. A little bit of digging led me to a 2000 paper by Todd Clancy and several coauthors: "An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere," which states:

For the purpose of this comparison, we use the solar longitude range 0°-360° to define a Mars year and adopt April 11, 1955 (Ls=0°) as the beginning of year 1. In this arbitrary convention, the Mariner 9, Viking, Phobos, and Pathfinder missions occurred in years 9-10, 12-15, 19-20, and 23, respectively. By comparison, the 1992-1999 [Earth-based] millimeter observations extend over years 21-24, and the 1997-1999 [Mars Global Surveyor] TES observations extend over years 23 and 24.

The convention is described again in a 2010 paper by Bruce Cantor, Philip James, and Wendy Calvin: "MARCI and MOC observations of the atmosphere and surface cap in the north polar region of Mars," which includes a handy-dandy lookup table for Earth dates corresponding to Mars' seasons:

Martian years and start dates of northern hemisphere seasons
Mars year	Spring equinox Ls = 0°)	Summer solstice Ls = 90°)	Autumnal equinox Ls = 180°)	Winter solstice Ls = 270°)
01	04-11-1955	10-27-1955	04-27-1956	09-21-1956
02	02-26-1957	09-13-1957	03-15-1958	08-09-1958
03	01-14-1959	08-01-1959	01-31-1960	06-26-1960
04	12-01-1960	06-18-1961	12-18-1961	05-14-1962
05	10-19-1962	05-05-1963	11-05-1963	03-31-1964
06	09-05-1964	03-22-1965	09-22-1965	02-15-1966
07	07-24-1966	02-07-1967	08-10-1967	01-03-1968
08	06-10-1968	12-25-1968	06-27-1969	11-20-1969
09	04-28-1970	11-12-1970	05-15-1971	10-08-1971
10	03-15-1972	09-29-1972	04-01-1973	08-25-1973
11	01-31-1974	08-17-1974	02-17-1975	07-13-1975
12	12-19-1975	07-04-1976	01-04-1977	05-30-1977
13	11-05-1977	05-22-1978	11-22-1978	04-17-1979
14	09-23-1979	04-08-1980	10-09-1980	03-04-1981
15	08-10-1981	02-24-1982	08-27-1982	01-20-1983
16	06-28-1983	01-12-1984	07-14-1984	12-07-1984
17	05-15-1985	11-29-1985	06-01-1986	10-25-1986
18	04-01-1987	10-17-1987	04-18-1988	09-11-1988
19	02-16-1989	09-03-1989	03-06-1990	07-30-1990
20	01-04-1991	07-22-1991	01-22-1992	06-16-1992
21	11-21-1992	06-08-1993	12-08-1993	05-04-1994
22	10-09-1994	04-26-1995	10-26-1995	03-21-1996
23	08-26-1996	03-13-1997	09-12-1997	02-06-1998
24	07-14-1998	01-29-1999	07-31-1999	12-25-1999
25	05-31-2000	12-16-2000	06-17-2001	11-11-2001
26	04-18-2002	11-03-2002	05-05-2003	09-29-2003
27	03-05-2004	09-20-2004	03-22-2005	08-16-2005
28	01-21-2006	08-08-2006	02-07-2007	07-04-2007
29	12-09-2007	06-25-2008	12-25-2008	05-21-2009
30	10-26-2009	05-13-2010	11-12-2010	04-08-2011
31	09-13-2011	03-30-2012	09-29-2012	02-23-2013
32	07-31-2013	02-15-2014	08-17-2014	01-11-2015
33	06-18-2015	01-03-2016	07-04-2016	11-28-2016
34	05-05-2017	11-20-2017	05-22-2018	10-16-2018
35	03-23-2019	10-08-2019	04-08-2020	09-02-2020
36	02-07-2021	08-25-2021	02-24-2022	07-21-2022
37	12-26-2022	07-12-2023	01-12-2024	06-07-2024
38	11-12-2024	05-29-2025	11-29-2025	04-25-2026
39	09-30-2026	04-16-2027	10-17-2027	03-12-2028
40	08-17-2028	03-03-2029	09-03-2029	01-28-2030

I haven't seen this convention used very often, but then there are not very many people who publish research that requires referring to years beyond the span of a given mission. Really it's just people who care about Mars' year-to-year weather who actually need to apply this convention. But it's not just climatologists who care about interannual weather variability; those of us who follow the adventures of landed Mars missions care about variations in weather, too. So, adopting this convention for naming the Martian years, and using solar longitude as the most accepted method of marking time within a year, here's a Martian timeline of interesting events in Mars exploration. And thanks to Aymeric Spiga for pointing out this handy online Earth-to-Mars date converter.

Earth Date	Mars ear/Ls	Event
July 14, 1965	6.143	Mariner 4 flyby
August 1969	8.200	Mariner 6 and 7 flybys
November 1971	9.284	Mariner 9, Mars 2, and Mars 3 enter orbit
August 1972	10.64	Mars 2 and 3 shut down
October 27, 1972	10.102	Mariner 9 shuts down
February 1974	11.0	Mars 4 and 5 enter orbit
July 1976	12.88	Viking 1 Orbiter & Lander arrive
September 1976	12.116	Viking 2 Orbiter & Lander arrive
July 25, 1978	13.118	Viking 2 Orbiter shuts down
April 11, 1980	14.91	Viking 2 Lander shuts down
August 17, 1980	14.151	Viking 1 Orbiter shuts down
November 13, 1982	15.226	Viking 1 Lander shuts down
January 29, 1989	19.350	Phobos 2 enters orbit
March 27, 1989	20.18	Phobos 2 shuts down
July 4, 1997	23.142	Mars Pathfinder lands
September 1997	23.173	Mars Global Surveyor enters orbit; Mars Pathfinder shuts down
October 24, 2001	25.258	Mars Odyssey enters orbit
December 14, 2003	26.315	Nozomi flies past Mars
January 2004	26.325	Mars Express, Spirit, and Opportunity arrive
March 10, 2006	28.22	Mars Reconnaisance Orbiter arrives
November 5, 2006	28.130	Mars Global Surveyor shuts down
May 25, 2008	29.76	Phoenix lands
November 2, 2008	29.151	Phoenix shuts down
March 22, 2010	30.67	Last contact with Spirit
March 9, 2011	30.251	Today

So Mariner 4's images came from year 6, late in the northern summer. Mariner 6 and 7's images came a little more than two Mars years later, a little after the equinox had brought sunlight to the south pole and darkened the north. Mariner 9 and Mars 2 and 3 all arrived another Mars year later, year 9, right around the southern summer solstice; Mars 2 and 3 lasted through the next equinox, and Mariner 9 through the next solstice. Mars 4 and 5 arrived right at the northern vernal equinox in year 11.

The Vikings came in year 12, around the northern summer solstice, so they saw a very differently lit Mars to what Mariner 9 imaged, at least at first. Of course those missions also lasted a long time. Viking 2 Orbiter lasted almost exactly one Mars year; Viking 2 Lander and Orbiter about two Mars years; and Viking 1 lander an amazing three and a half Mars years, most of the way through year 15.

There followed a long drought in the Mars program. Phobos 2's brief mission came around the equinox beginning year 20, but that was the only mission at Mars in the period from year 16 through year 22.

The resurgence of Mars science began midway through year 23, very late in northern summer, when Mars Pathfinder landed. It survived until close to the northern autumnal equinox, when Mars Global Surveyor arrived. That orbiter was still active near the southern summer solstice in year 25, when Odyssey joined the party; and both were active late in the southern summer of year 26 when a flotilla of spacecraft (Nozomi, Mars Express, and the Mars Exploration Rovers) flew past, entered orbit, or landed. Spirit lasted through at least three and a half Mars years. All the rest of the orbiters except Mars Global Surveyor are still operating now, late in the southern spring of Mars year 30.

What do you think? Is this a valuable way of looking at Mars time?