Telescopic Observations of the Polar Caps of Mars

The Dry Ice of the Polar Caps of Mars

As soon as Herschel had invented the spectroscope, humans were thenceforth able to tear apart light photons of different energies, and we could use their variations to study and observe the caps of Mars. Using telescopes of the 18th century that Herschel had at his disposal, the results were ambiguous. The chief difficulty for most telescope spectrometers is how to avoid terrestrial contamination in your spectra.

It took until the 20th century for telescopic spectroscopic measurements of the polar cap to determine the composition of the caps. The reason for this is obvious with hindsight, and that is that in actual fact the caps consist of a delightful mixture of two ices, interacting and interpenetrating each other, that leads to a dizzying cornucopia of features, and frustrated the scientists of the 20th century.

In the closing days of the 19th century, G. Johnstone Stoney published a paper in the Astrophysical Journal that discussed his dynamical model for the molecules that would be kinetically stable on the terrestrial planets, based on the known planet masses and molecular weights (Stoney, 1898). He had first communicated this model in 1867 to the Royal Society. Using this model he predicted water would be unstable in the Martian atmosphere and that instead carbon dioxide would be the most condensible gas in the Martian atmosphere and would, due to its heavy mass, create fogs of CO2 gas in valleys and would precipitate out onto the polar regions during winter, creating the observed polar caps.

Ten years after Stoney's original communication, in 1877, Mars went through a "Grand opposition", and Giovanni Schiaparelli (1835-1910), an Italian Astronomer and science historian, observed Mars from the Bresa Observatory in Milan. He created a map of the surface and first named the surface features he saw after locations in the Mediterranean (e.g. Hellas and Arabia, although these features are upside down with Arabia being over Hellas on Mars). Schiaparalli reported seeing "canali" which ran between the regions he identified. In his 1893 book "Life on Mars", he suggested that "these channels are probably the main mechanism by which water (and with it organic life) can spread on the dry surface of the planet".

The French Connection

Nicolas Camille Flammarion (1842-1925) was a French astronomer and prolific author who owned his own observatory and published scientific investigations into reports of the paranormal. He built his own private observatory in Juvisy-sur-Orge and in 1892, (inspired by Schiaparelli's map reporting canali), he published the results of his observations of Mars and his thoughts on the astrobiological potential of the Red planet. His book was entitled "The Planet Mars and its Hability Conditions". In it, he interpreted the lines previously observed by Schiapareli as being due to a past civilization on Mars, which had straightened the watercourses to create canals.

In the same year his book was published, Flammarion recruited the Greek astronomer Eugene Michel Antoniadi (1870-1944) to work with him in France in 1902. Antoniadi stayed with Flammarion for nine years and then left to join the Paris Observatory in Meudon where he used an 83cm refracting telescope to make maps of Mars and Mercury. When Antoniadi moved to work with Flammarion, he supported the idea of canals, but following his observations during the favourable opposition of 1909, he came to believe they were optical illusions. The image below is a THEMIS/MOLA map which shows Flammarion, Antoniadi and Jezero (landing site of the Mars2020 rover) craters, all lying in a row north of the Syrtis Major.

Antoniadi published a paper on the retreat of the south polar cap in 1916 paper entitled "Mars, On a probable relation between the changes in solar radiation and the melting of the polar snow caps", he reported (for example for the years 1872-79) that for the periods of maximum solar activity, rapid melting occurred and very slow melting occurred during minimum solar activity.

Little did Antoniadi know at the time, but a Russian astronomer named M. Shajn had had the same idea 4 years earlier and published a correlation between sunspot activity and rate of cap retreat in the Bulletin of the Society of Russian Astronomers in Russian. In 1922, the editors of the Royal Astronomical Society let Eugene know about this paper. He published a mea culpa in 1922 and explained that he did not even know Russian, and did not therefore copy the idea from Shajn, however he was happy to acknowledge his precedence in the matter.

In 2012, Giovanni di Giovanni of the Teramo Observatory published a more recent study investigating the possible relationship of cap retreat rate with solar activity, including modern results to supplement Antoniadi's. He used a measurement of the radiation at Mars in an equation first published by Cross (1971).

I(phi,delta)=I_o (r_o/r)^2.sin(phi)sin(delta) (0.1)

where phi is the latitude on Mars, delta is the declination of the Sun at Mars, I_o is the insolation at the equator when the distance to Mars is r_o and r is the current distance to Mars.

He then derived the "rapidity" or rate of retreat of the polar cap per degree of solar longitude (L_s), using the following equation:

dR/dLs=-alpha.n(1-A)I_o(r_o/r)^2.sin(phi)sin(delta) (0.2)

where n is the number of Martian days the planet takes to cover a degree of L_s and A is the cap albedo (reflectivity) of the surface. He suggests integration of the equation to get:

R(Ls) = R_o exp[-b sin(kLs+phi)

He uses this equation for extrapolation of years with poor data. His dataset covers the time period 1798-2010. He calculated the moving average (3 point) for each observation, and then compares this to the number of sunspots observed, as an indication of solar activity. He fits this data with a third order polynomial and shows in broad terms a correlation between the two measures on a century timescale. He then presented the trends of these data, and this is shown below. He did a further linear trend fit to a scatterplot of these data concluded there is a possible correlation (he found the likelihood of such a correlation happening randomly would be 3-4%).

Kuipers kontribution

Gerhard Kuiper was a founding father of Planetary science - a huge figure in the planetary landscape, as a Dutch spectroscopoist in the last days of World War 2, he was charged with collecting German lead sulfide detectors from German laboratories for the western allies. When he returned to the United States, he founded the Planetary Sciences Laboratory at the University of Arizona, and turned these Nazi spectrometers to the planets to measure their spectra. In 1947, he took a remarkable series of spectra of the planet Mars, proving for the first time that there was CO2 in the atmosphere, and also that there was water ice near the polar regions. He edited the founding text of planetary atmospheres to describe his discoveries, published in 1952 as "The Atmospheres of the Earth and Planets", and in his contribution to this book, he showed the figure below.