How planetary maps are made; counting rhyme

First we display spacecraft data. The image shows a digital elevation model (DEM) made from elevation points measured by the MOLA laser altimeter. Gaps are filled with averaged, interpolated values. Grey-coded elevations are converted to colours: high ground is brown, middle elevations yellow and then white, and low ground grey. Blue and green are avoided so the map does not suggest Earth’s water and vegetation. Hill shading makes the map appear three-dimensional. Shadows can come from photographs, but computer-generated shadows based on the DEM look better. On every map, across the whole planet, the Sun shines from the northwest—impossible in reality. Lines and point symbols are added from geologists’ mapping. Each catalogue of feature types results from research mapping photographs, published as maps or tables in scientific papers over the preceding decade or so. Finally we add names. The International Astronomical Union and US Geological Survey publish approved names in a shared database. A few names requested by researchers are added each year.

Ready to calculate the temperature on Mars and understand why it differs from the simplest estimate? Practise on Earth first. At Earth’s distance, the solar constant is 1,370 W/m². Distribute this energy across a sphere and subtract the reflected fraction: clouds, snow and sand reflect 30%, giving an albedo of 0.3. E = solar constant × (1 − albedo) / 4, where E is the average incoming energy per unit area in W/m². Divide by four because a sphere’s area is four times the area of a circle of the same radius. For Earth, E = 240 W/m². The same amount must be radiated to space to prevent continuous heating. T = (E/σ)^0.25, where T is in kelvin and σ, the Stefan–Boltzmann constant, is 5.670373 × 10⁻⁸ W/m²K⁴. This gives Earth 255 K, or −18 °C using °C = K − 273. Measurements instead give an average of +14 °C. The greenhouse effect accounts for the difference: the atmosphere retains some energy. Question: How large is the greenhouse effect on Mars? Repeat the calculations using a solar constant of 593 W/m², an albedo of 0.24 and a measured average temperature of 218 K (−55 °C). Compare the calculated temperature with the measurements. Check your answer beside the woodlice on page 188.