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Headshot of Emily Lakdawalla

Martian timekeeping

Posted by Emily Lakdawalla

09-03-2011 12:23 CST

Topics: explaining science, Mars

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?"

Mars 3 arrives to a dust-shrouded planet

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, 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 yearSpring 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 DateMars
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?

 
See other posts from March 2011

 

Or read more blog entries about: explaining science, Mars

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