The Spacecraft Era Observations of the Polar Caps of Mars

Mariner 4 - the first spacecraft to flyby Mars

Reacting to the shocking news of the launch of Sputnik in 1957, President Eisenhower used existing National Aeronautical research labs to form NASA in 1958 with the task of matching the Soviets in Space. Motivated by Kuiper's telescopic observations, the fledgling space agency directed its Mariner missions to Venus, Mars and Mercury. Mariner 1 and 2 went to Venus. Mariner 3 and 4 would flyby Mars. Mariner 3 failed soon after launch. In November 1964, Mariner 4 launched for Mars, and arrived seven months later in July 14, 1965. The spacecraft was equipped with a digital camera which took 20 images of the surface, and conducted the first radio-occultation measurement of the Martian atmosphere. By measuring the decreasing intensity of the radio signal as the spacecraft traveled behind the planet, the density of the Martian atmosphere was estimated at around 7 mbar, less than 1% that of the Earth, confirming the 200 year old hypothesis of Herschel.

The Martian CO2 ice cycle

The year after Mariner 4 flew by Mars, Bruce Murray and Robert Leighton of Caltech published a paper suggesting that CO2 was the major constituent of the polar caps, based on a 1 dimensional heat model of the atmosphere, taking into consideration the thin atmosphere, and the solar radiation heating the surface (Leighton and Murray, 1966). Five months later, using a different approach with a two dimensional numerical model, Conway Leovy inferred that the Martian polar caps should consist of CO2 ice (Leovy, 1966). Murray and Leighton suggested that there was a CO2 cycle that enabled the deposition of CO2 in the cold winter cap. Let's look at what they found.

Leighton and Murray calculated the rate at which both CO2 and H2O would evaporate from the polar caps, and compared this to observations of the south polar retreat made by Earl Slipher at Flagstaff in Arizona. Their plot shows that H2O evaporates too fast to explain the observed retreat. This plot is shown below.

Mariner 6/7 - first spectrometers in space confirm CO2 composition

Mariner 5 was sent to Venus, but Mariner 6 and 7 were sent to Mars. They launched one month apart in February and March of 1969 and both flew by Mars in quick succession in July and August of 1969. Motivated by the results of Leighton and Murray and Leovy, these spacecraft carried the first spectrometers in space, and investigated the atmospheric composition up close. They carried an IR spectrometer that covered from 1.9 to 14.3 microns. In this spectral range, the instrument was sensitive to thermal heat emitted by the Martian surface and the temperature can be calculated.

While flying over the Martian south polar cap, the temperature readings dropped off abruptly (see p. 10 of NASA Mariner 1969 results) and leveled off at 150K. That value is higher than the value expected for pure CO2 ice. NASA scientists put this discrepancy down to a mixture of materials being measured by the instrument, and the south polar cap was interpreted as "predominantly CO2 ice".

Coming weeks after the Apollo missions, the stunning results of the dual Mariner 1969 were largely unheralded. However, one can get a sense of the excitement of these results at the time from this presentation of the Mariner results by Patrick Moore on the BBC in 1969. The interested reader should note that Patrick Moore translated Antoniadi's book "The Planet Mars".

Mariner 9 instruments and findings

NASA's hugely successful Mariner 9 mission was launched in May 1971 and was the first spacecraft to enter Martian orbit in November 13, 1971. Mariner 9 carried two primary spectroscopic instruments with them, and because they went into orbit around Mars, they were able to study both north and south polar caps and return imagery of the entire Martian surface.

Polar Caps controlled by Obliquity

In 1973, just prior to the Viking mission, Bill Ward produced a model for Martian obliquity and identified for the first time that the obliquity of Mars (essentially this is the angle of the planet axis with the Sun) varied between 14.5 and 35.5 degrees due to the influence of other planets (Ward, 1973). In contrast, the Earth's obliquity changes are limited to 2 degrees due largely to the stabilization effects of our large moon. This was immediately recognised as a potential driver for long term climate change on Mars (e.g Toon et al, 1980).

These orbital histories allow us to infer the amount of sunlight that was hitting Mars in the past, and they are commonly used by modellers as a huge part of efforts to infer the history of recent Martian climate. For example, they are used in General Circulation Models (GCMs) and other studies of the waxing and waning of the polar caps, such as those developed by Jim Pollack (e.g. Pollack et al. 1981 and Pollack et al. 1993) and Bob Haberle and Bruce Jakosky at NASA Ames (Haberle and Jakosky, 1990) and more recently by Ralf Greve during his PhD studies (Greve 2000).

In later years, the numerical models of the Martian orbit improved remarkably. However, a limit on their accuracy was first recognised by Jaques Laskar, when he developed an orbital model of the solar system and found it displayed chaotic evolution (Laskar, 1989). He followed this observations with a paper that identified the chaotic nature of the obliquity cycle of the planets (Laskar and Robitel, 1993). That paper was accompanied by another independent study by Jihad Touma and Jack Wisdom at MIT that suggested the history of the obliquity of Mars is completely unpredictable prior to 4 million years before the present (Touma and Wisdom, 1993).

The Martian spin state results were updated more recently by Laskar et al. (2004), and have most recently (2019 LPSC abstract) been re-analysed by Dr. Bruce Bills and James Keane at JPL/Caltech . Bills and Keane suggest that rather than being chaotic, the Martian obliquity state has been damped and the damped nature of the orbit obscures the true Martian obliquity history. This story is still evolving, as surely as Mars orbits the sun.

Wait just a minute ... that's not dry ice ... Viking instruments and findings

In the last half of the 70's, NASA launched the hugely ambitious Viking mission. Two Viking landers and two Viking orbiters would travel together to learn about the surface and atmosphere at the same time.

Dr. Hugh Kieffer was the PI of the Infra Red Thermal Mapping (IRTM) instrument on the Viking orbital components. The IRTM instrument was designed to measure the temperature of the surface of Mars and create temperature maps of the surface. They were intending to map the super cold temperatures of the expected CO2 ice in the north and southern poles of Mars.

The first temperature maps that they got back, however, were not consistent with CO2 ice, and instead they observed the north polar caps were too warm to be CO2 ice. However, the imaging observations of the southern pole were consistent with CO2 ice, as reported in Kieffer et al. (1976). This is an extremely cool dichotomy (residual water ice in north, dry ice in south) in Martian climate that is still evolving and being better understood to the present day, as we will discuss further below.

Soon after the IRTM dataset results were announced, James et al. (1979) used Viking imagery to look in detail at the recession of the south polar cap during Mars Year 12. On comparing their data to previous observations of the recession, they found that the Viking recession was slower during MY12, and they attributed this to a dust storm early in the Mars Year. They used a 1 dimensional Monte Carlo model to examine the effects of dust from the dust storm on the sublimation of CO2 at the south pole. James et al. suggested that the IRTM observations of the south polar cap were too low for pure CO2, and posited that this could be a lower atmosphere boundary effect. They were able to conclude for the first time (with the caveat that imagery could not distinguish ice types) that their results were consistent with CO2 ice being able to survive throughout the Martian summer.

A simplified one dimensional model of the polar cap processes was first tested against the Viking pressure curves by Stephen Wood and David Page (1992). They used a diurnal and seasonal model based on the work of Toon et al. (1980), with fixed albedo and emissivity for the north and south polar caps, and only included the effects of radiation, latent heat of the polar caps (essentially this means how much heat is required to melt the CO2 ice) and heat conduction to a small number of soil layers beneath the polar ice. They found that the model was quite sensitive to heat conduction to the soil layers, and their emitted radiation was artificially small.

In an effort to improve the Wood and Paige model, Haberle et al. (2008) used a the NASA Ames GCM to study the effects of ground ice and found that the best models of the subsurface required an ice table below the soil. The inclusion of ground ice has the effect of raising the thermal conductivity of bare soil, and increasing the amount of summer radiation absorbed by the ground, which is later released in fall and winter, thereby retarding the condensation rate. They found a best fit of the average depth to the water ice of 8cm in the north and 11cm in the south. The following year, Guo et al. (2009) used the MarsWRF GCM with a subsurface water ice table and examined variations in 5 parameters (albedo of north and south, emissivity of north and south and the total CO2 mass) while the Viking pressure curve. They concluded that (as with previous studies) it was not possible to find a satisfying fit to the Viking pressure curve, particularly in the north pole summer where the Viking curve oscillates in a manner that is not matched by GCMs.

Layering of the Polar Caps

In analogy to terrestrial ice sheets, it was long suspected that the Martian polar regions held clues in the form of layers that formed a perennial deposit and were built up over time as layers of dust and ice. These suspicions were confirmed when the two Viking orbiters took over 700 visible camera images of the north polar region, and found light and dark layering, as reported in Cutts et al. (1976). The images also confirmed the presence of at least one unconformity in these layers, hinting at a complex geological history. No craters were identified in these images, which lead the Viking team to suggest the north polar region is very young and is either undergoing relatively rapid deposition or removal of material.

Mars Observer

Following the hugely successful Mariner and Viking missions, NASA sought to take a quantum leap in Mars exploration with their next mission. Called "Mars Observer", it was launched on September 1992, and just prior to inserting itself into orbit around Mars in August 1993, all contact with the spacecraft was lost, probably due to a fuel and oxidiser leak to the common propellant system. I can still remember being very sad about this, and seeing a display dedicated to the mission on my first visit to the US in 1994 at the Kennedy Space Flight Center.

Surveyor-Odyssey-Reconnaissance

However, the instruments of Mars Observer were put on Mars Global Surveyor (MGS - arrived at Mars 1997), Mars Odyssey (ODY - arrived at Mars 2001) and the Mars Reconnaissance Orbiter (MRO - arrived at Mars in 2006).

The MGS was equipped with an instrument called the Thermal Emission Spectrometer (TES), which was built by Phil Christensen at Arizona State University. This instrument was somewhat better than the first TES instrument that was onboard the lost Mars Observer. Hugh Kieffer was part of the TES team and immediately went to work using it to track polar phenomena with his student Tim Titus.

Martian modern day climate and the Seasonal Polar Caps

As we discussed earlier, the seasonal polar caps of Mars had been remotely observed by telescopes, and their retraction was remotely photographed by Earl Slipher in the 1950s. As soon as spacecraft arrived in orbit, the recession of the caps during springtime, when the pole was increasingly well illuminated, became even more important as a record and indicator of changes in the current day climate.

Hugh Kieffer used more accurate observations from the TES instrument to map the seasonal CO2 ice retreat and also used it to discover the "cryptic region" in the south, which he termed the Cool and Bright Anomaly (CABA) due to its observed thermal behaviour in Kieffer et al. (2000). One year later, Tim Titus led a corresponding TES paper covering the recession of the north polar cap in Titus et al. (2001).

Mars Global Surveyor was also equipped with a laser instrument called the Mars Orbiter Laser Altimeter (MOLA) which was able to map deposition of CO2 ice seasonally by measuring changes in the height of the ice cap. The LIDAR instrument operated from 1997 to June 2001. The figure below is a representation of the results of the MOLA investigation into the polar regions as published by Smith, Zuber and Neumann in 2001. It shows variations in the seasonal caps in the northern and southern regions by latitude. This amazing portrait of volatile exchange gives an astonishing look at the power of active sensors to effectively interrogate the surface of Mars.

Is that ... Buried ice?

In one of the more exciting findings of recent times on Mars, a radar observation was made by the SHARAD instrument on MRO that located buried CO2 ice beneath the south polar cap by sending EM waves whizzing into the icy snow pack. The paper by Roger Phillips et al. (2011) estimated that the CO2 released from that deposit would be equivalent to almost doubling the current thickness of the Martian atmosphere. This result is very exciting, since we now begin to understand how buried volatile deposits in the polar caps might have participated in global warming on a relatively recent timescale, and may be available as a resource for future global greenhouse type scenarios, such as those proposed by McKay, Toon and Kasting in 1991.

In 2018, another radar instrument, MARSIS, an Italian instrument on the Mars Express spacecraft, reported another interesting buried water deposit buried deep in the south polar region Orosei et al. (2018). The authors of the paper suggested that due to the nature of the return, the presence of liquid water was required to explain the observations. This surprising result was followed up in 2019 by Mike Sori and Ali Bramson, who calculated that for a deeply buried liquid water deposit, a sustained heat source such as a magma chamber was required and implied by the observation Sori and Bramson (2019).

Northern springtime recession and Asymmetry of the "Houben Effect"

Northern springtime recessions have been observed in the infrared using orbiting instruments by Kieffer and Titus [2001] using the Thermal Emission Spectrometer (TES) and Apperé et al. [2011] using the OMEGA VNIR spec- trometer. Spring recession visible albedos have been tracked by spacecraft using Viking cameras [James, 1979, 1982], Mars Orbiting Camera [Bass et al., 2000; James and Cantor, 2001; Benson and James, 2005], Mars Orbiting Laser Altimeter [Byrne et al., 2008] and MARCI [Cantor et al., 2010]. The retreating CO₂ north polar cap is surrounded by a H₂O ice annulus that is deposited by H₂O ice carried onto the cap by baroclinic eddies and cold trapped on top of the CO₂ ice cap in what has been termed the “Houben process” [Houben et al., 1997; Bass and Paige, 2000; Schmitt et al., 2006; Wagstaff et al., 2008; Apperé et al., 2011]. Apperé et al. discovered that this process was not symmetrical. In [Brown et al. 2012] we were able to show using CRISM maps of the springtime recession that the Houben process (where H₂O ice is cold trapped on the edge of the retreating CO₂ ice cap by on-cap winds [Houben et al., 1997]) is not symmetrical around the cap and from L_s = 25 –62 water ice is deposited on and obscures more than half of the remaining CO₂ ice cap. We presented a new model as a possible explanation of this intriguing asymmetrical H₂O ice deposition, and this is shown in the figure below. The central idea of this model is that as the CO2 ice cap retreats, it exposes meter scale rough deposits of surficial water ice that is then entrained in the huge on-cap winds generated by the recession of the cap, while the CO2 cap is still present. Since it had not been possible to observe this effect, our paper was the first to describe it.

Water Ice Grain size variations

Langevin et al. 2005 first reported variations in the grain size of water ice in the North Polar cap over the summer period. Using the Observatoire pour la Minèralogie, l’Eau, les Glaces et l’Activitè (OMEGA) visible to near infrared hyperspectral instrument, they were able to see absorption bands due to water ice at 1.25, 1.5 microns.