Standard Primary Habitability (SPH)


The SPH is a multiparameter measure of habitability for primary producers (i.e. plants) as a number between zero (nominal environment) and one (best environment). It is correlated with the distribution of vegetation and net primary productivity (NPP). The most important application of the SPH is for evaluating the global habitability of Earth (or exoplanets) using primary producers as the standard. We used QH Theory to define the SPH starting with two environmental variables that are easy to measure from ground or orbital observations: temperature and relative humidity. Therefore, the current formulation of the SPH can be considered as a measure of climatic habitability. The SPH helps establish terrestrial baselines for planetary habitability assessments and evaluating the evolution of the global terrestrial habitability from paleoclimates to global change. The SPH is given by

where SPH is the Standard Primary Habitability, Hx are the habitability functions for temperature T and relative humidity RH within some defined cardinal values and a weighing exponent w (Table 1). Similar SPH formulations can be extended to other biological relevant environmental variables (i.e. pCO2) and to the oceans starting with temperature and nutrient concentration. Figure 1 shows the SPH applied to landmasses using AIRS sensor data. Although, based on global monthly mean data, the SPH can be used to calculate the habitability for plants for a specific region by the hour. Also, the formulation works for any ectotherm (cold blooded), including microbial life, given their characteristic parameters in Table 1.

The annual mean global terrestrial SPH today is 0.335 with strong oscillations between seasons (Figure 2). Area corrected, the south hemisphere is more habitable (0.432) than the north hemisphere (0.184). The fact that these number are not 1.0 tells that our planet is not optimized for its vegetation today, there are too many deserts and seasonal ice. Even that plants adapted to many environments, they have their limits. Planets with higher values will be more evenly covered with vegetation and less seasonal changes (Pandora?). Earth probably had a few good times in its past where its habitability was much higher, maybe close to 0.7, therefore, Earth globally not only had a better environment for plants, but also for the rest of the food chain. The SPH can also be calculated for exoplanets from their mean surface temperature and some estimates of their potential surface humidity based on their land/ocean ratio.

Table 1. Cardinals values and weight exponents for an SPH global model based on temperature and relative humidity of land areas. These values are characteristic of the mean requirements of most terrestrial plants and were derived from an extensive analysis of data from NASA MODIS and AIRS sensors.

Figure 1. Global seasonal habzones maps of land masses showing the best regions for photosynthetic activity (greener) based on a Standard Primary Habitability (SPH) of temperature and humidity. Regions were divided for convenience in six habitable zones denoted with letters. Contrary to classical static zones such a Holdridge Life Zones, the SPH habzones are dynamic zones that shift with seasons.

Figure 2. Mean monthly Standard Primary Habitability (SPH) for the north hemisphere (red), south hemisphere (blue), and globally (black). These values are area-averages and show that the north hemisphere has the largest seasonal variation in habitability. Globally, from July to August is the most habitable period of our planet (highest plant productivity).


  • Méndez, A. (2011) Measuring Terrestrial Habitability. [in preparation]

  • Méndez, A. (Apr. 26-29, 2010) Evolution of the Global Terrestrial Habitability During the Last Century on the AbSciCon 2010: Sixth Astrobiology Science Conference in Houston, Texas. [extended abstract]

  • Méndez, A. (Mar. 23-27, 2009). Standard Planetary Habitability (SPH) of Global Land Areas on the 40th Lunar and Planetary Science Conference in The Woodlands Texas, LPI Abstract #2333. [extended abstract]